MUON FLUX MEASUREMENT WITH SILICON DETECTORS IN THE …

CERN 83-06 Experimental Physics Facilities Division 21 July 1983

ORGANISATION EUROPÉENNE POUR LA RECHERCHE NUCLÉAIRE

CERN EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

MUON FLUX MEASUREMENT WITH SILICON DETECTORS IN THE CERN NEUTRINO BEAMS

E.H.M. Heijne

GENEVA 1983

© Copyright CERN, Genève, 1983

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ABSTRACT The neutrino beam installations at the CERN SPS accelerator are

described, with emphasis on the beam monitoring systems. Especially, the muon flux measurement system is considered in detail, and the calibration procedure and systematic aspects of the measurements are discussed- An introduction is given to the use of silicon semiconductor detectors and their related electronics. Other special chapters concern non-linear phenomena in the silicon detectors, radiation damage in silicon detectors, energy loss and energy deposition in silicon and a review of energy loss phenomena for high energy muons in matter.

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TABLE OF CONTENTS Page

INTRODUCTION AND SUMMARY 1

PART I SEMICONDUCTOR DETECTORS USED FOR THE MEASUREMENT OF FLUX

SEMICONDUCTOR PARTICLE DETECTORS 5

1.1 Generation of free charge by radiation 5 1.2 Diode structures for silicon particle detectors 9

1.2.1 Diffused detectors 10 1.2.2 Surface barrier detectors 10 1.2.3 Ion implanted detectors 11 1.2.4 Microstrip detectors 12 1.2.5 Si(Li) and Ge(Li) lithium drifted detectors 12

1.3 The utilization of semiconductor detectors 13 1.3.1 Nuclear physics 13 1.3.2 High energy physics 14 1.3.3 Flux measurement 15

1.4 Fast amplifiers and noise 16 1.4.1 Voltage sensitive preamplifier 19 1.4.2 Current sensitive preamplifier 20

MINIMUM IONIZING PARTICLES IN SEMICONDUCTOR DETECTORS 23

2.1 Energy loss of high energy particles 23 2.1.1 Excitation and ionization 23 2.1.2 Delta electrons 26 2.1.3 Plasmon generation 27 2.1.4 Radiative energy loss 28

2.2 Fluctuations of energy loss in a thin silicon layer 29 2.2.1 The Landau distribution 29 2.2.2 Escaping energy 32

2.3 Measurements of energy deposition in silicon detectors 32 2.3.1 Measurements at CERN with muons, pions and protons 3 2 2.3.2 Comparison with other data for silicon 36 2.3.3 Measurements with electrons 37

MUON ENERGY LOSS AND FLUX MEASUREMENT IN A SHIELD 39

3.1 Introduction and review of shielding calculations 39 3.2 Energy loss of high energy muons 40

3.2.1 Bremsstrahlung, pair production and nuclear interaction 41 3.2.2 Production of secondary electrons and photons 45 3.2.3 Absorption of electrons and photons 47 3.2.4 Calculation of the electron density in an absorber 49

3.3 Measurements to determine the influence of the secondary radiation 52 3.3.1 Cascades by high energy electrons

in a tiny counter telescope 53 3.3.2 Muon fluxes in the earth shield behind M2 57 3.3.3 Coincidence counting in the neutrino shielding 59

3.4 Secondary radiation in integrated charged measurements using absorbers 63 3.4.1 Influence of secondary radiation on the muon flux profiles 64 3.4.2 Secondary radiation as a function of muon energy 66 3.4.3 Secondary radiation as a function of absorber thickness 67

3.5 Conclusions 70

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TABLE OF CONTENTS (Cont'd) Page

NON-LINEAR RESPONSE OF SILICON DETECTORS 71 4.1 Signal formation and charge collection time 71 4.2 Charge collection efficiency 74

4.2.1 insensitive layers 74 4.2.2 Carrier trapping and detrapping 76 4.2.3 Pulse height def-sct for heavy ions 79

4.3 Anomalous charge injection 80 4.3.1 Signal multiplication for fission fragments 81 4.3.2 Charge injection in microstrip detectors 86 4.3.3 Charge funneling in partially depleted structures 87 4.3.4 Charge injection in a pulsed flux of muons 88 4.3.5 Charge injection in a pulsed flux of electrons 92 4.3.6 Discussion 95

4.4 Conclusion 97

RADIATION DAMAGE IN SILICON DETECTORS 99 5.1 Defects in silicon introduced by irradiation with muons 100

5.1.1 Introduction 100 5.1.2 Experimental procedure 102 5.1.3 Thermally stimulated current measurements 103 5.1.4 Comparison with electron and neutron irradiation 108 5.1.5 Discussion 110

5.2 Degradation of silicon detectors 111 5.2.1 Radiation test of prototype detectors 113 5.2.2 Diode reverse current and minority carrier lifetime 113 5.2.3 Change of diode capacitance 119 5.2.4 Energy resolution 120 5.2.5 Long term muon irradiation in the SPS 122

5.3 Conclusion 123

PART II MUON FLUX MEASUREMENT FOR THE SPS NEUTRINO BEAMS THE SPS NEUTRINO BEAMS 125 6.1 The West Area Neutrino Facility WANF 125 6.2 The production of neutrinos and muons 128

6.2.1 The proton beam 128 6.2.2 Proton intensity monitors 128 6.2.3 Neutrino parents 132 6.2.4 Decay kinematics 133

6.3 The Narrow-band Neutrino Beam (NNB) 135 6.3.1 Sign and momentum selection of parents 135 6.3.2 Monitoring of the parent beam 136 6.3.3 Monitoring in the muon shield 137

6.4 The Wide-band Neutrino Beam (WNB) 137 6.4.1 Focusing with a magnetic horn 137 6.4.2 Timing of the extraction 139

6.5 The Muon Shield 140 6.5.1 The shielding magnet 140 6.5.2 The proton beam dump 142

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7. THE NEUTRINO FLUX MONITORING SYSTEM (NFM) 143 7.1 The Muon Detectors 143

7.1.1 Detector specifications 143 7.1.2 Prototype testing 144 7.1.3 Detector performance and detector degradation 145

7.2 Layout of the Monitoring System 147 7.2.1 Detector positions 147 7.2.2 The calibration boxes 154 7.2.3 The absorber box 157

8. DATA ACQUISITION AND ELECTRONICS 159 8.1 Signal processing 159

8.1.1 The direct coupling 159 8.1.2 Charge sensitive preamplifier with programmable gain 163 8.1.3 ADC and data acquisition 165 8.1.4 Special treatment for very low signal 166

8.2 Amplifier control 166 8.2.1 Setting of bias voltage and gain 167 8.2.2 The offset of the amplifiers 168 8.2.3 Leakage current compensation 169

8.3 Movements control 171 8.4 Data links to users 171

8.4.1 NEC data link 171 8.4.2 Off-line data 172 8.4.3 Bubble chambers and counter experiments 173

8.5 Organization of OPCOM 174

9. DETECTOR CALIBRATION 177 9.1 The relative calibration 177

9.1.1 Calibration boxes 179 9.1.2 "Calbox effect" on fixed detectors 181

9.2 The absolute calibration with nuclear emulsions 183 9.2.1 The exposure of emulsions 183 9.2.2 Counting of tracks in the emulsion 184 9.2.3 Delta electron contribution in the emulsion 187 9.2.4 Angular distribution of tracks in the emulsion 188

9.3 Results 190 9.3.1 Results of absolute calibration in NNB 190 9.3.2 Results of absolute calibration in WNB 194

9.4 Absolute calibration with counting detectors 198 9.4.1 Scintillator telescopes 199 9.4.2 Measurement of muon angular distribution 199

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10. THE MUON FLUX 10.1 Beam monitoring "on-line"

10.1.1 General description 10.1.2 Initiation of the program 10.1.3 Display of muon flux 10.1.4 Periodic checking by the operator 10.1.5 NFM errors

10.2 Data analysis "off-line" 10.2.1 Neutrino fluxes from muons fluxes 10.2.2 Flux measurement if the beam is not centred 10.2.3 Typical flux distributions

201 201 201 202 203 204 207 208 208 209 210

11. MUON FLUX MEASUREMENTS AT VARIOUS BEAM MOMENTA 11.1 The narrow band beam at various momenta 11.2 Results of the measurements 11.3 Muon range straggling

215 215 215 220

Acknowledgements 223

APPENDIX A The layout and distances in the West-Area Neutrino Facility (WANF) 224

APPENDIX B Detailed drawing of the installations in the neutrino cave 227

APPENDIX C

APPENDIX D

List of silicon detectors in the NFM monitoring system Glossary of abbreviations and mnemonics

235 247

REFERENCES 249

Some tables of general interest: - Table 1.1 Properties of silicon - Table 2.2 Energy deposition measurements in silicon - Table 3.1 Mean energy loss of a muon in several metals - Table 9.1 SPS neutrino experimental runs 1976-1982

8 37 44 176

INTRODUCTION AND SUMMARY Muon flux measurement has been a distinctive feature of the CERN

neutrino beams ever since 1964. It contributed to the discovery of the linear rise of the neutrino cross section with neutrino energy. It gives a basis to the calculations of the neutrino flux and the neutrino energy spectrum, which have as second ingredient the pion and kaon production spectra. Finally it provides the means to steer the neutrino beam and to monitor its performance during the experiments.

in the beginning, gas-filled ionization chambers were employed, but from 1969 onwards these were replaced by silicon particle detectors. Descriptions of the early systems and their results were given by Bloess et al. [1.1] and Eichten [1.2].

The present work is mainly describing the "Neutrino Flux Monitoring" system (NFM) , which has been built for the 400 GeV Super Proton Synchrotron (SPS) neutrino beams. Alternatively a wide-band neutrino beam, with the highest possible acceptance to parent particles, or a narrow-band neutrino beam, with momentum selected parent particles, can be produced to serve a number of experiments in the West Area Neutrino Facility WANF. The parent particles, mainly pions and kaons, are allowed to decay to neutrinos and muons. These muons are stopped in a massive iron shield, but still serve physics by providing the means to measure indirectly the neutrino flux. Preliminary design considerations of the SPS neutrino beams and the requirements for the neutrino flux monitoring via the muons in the shield were set out by Venus and Wachsmuth [1.3].

in part I a treatment is given of some general subjects related to the utilization of silicon detectors and the properties of high energy muons. Chapter 1 is a short introduction to semiconductor detectors and their use for the muon flux measurement. Note that "semiconductor detector" is a more precise description than the often used "Solid State Detector" or "SSD" , which may include also solid track detectors and solid light emitting devices like Cerenkov detectors, scintillators and transition radiation detectors.

Energy loss of minimum ionizing particles, which has to be distinguished from energy deposition in the detector, is treated in chapter 2. Some new measurements are reported here and compared with the scarce published data. In chapter 3 the special aspects of energy loss by high energy muons are discussed and recent formulae are used to calculate contributions of pair production and bremsstrahlung in several materials. Secondary radiation, also called "spray", consisting of "delta rays" and other cascade products, is shown to play an important role in the muon flux measurement inside a shield, especially for muons of high energy

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(> 100 GeV) . An approximate calculation is presented of the number of secondary electrons which accompany a high energy muon inside a shield. A number of experiments is described which provide a mainly qualitative understanding of the behaviour of the secondary radiation.

In chapter 4 a study is made of the conditions under which a nonlinear response of the silicon detectors is obtained, using measurements in the PS neutrino beam, and fission fragments spectra. In chapter 5 the practical subject of radiation induced damage in the detectors, which determines the long term performance, is discussed and some results of current measurements are presented, both for test samples and complete detectors, irradiated with muons, electrons or neutrons.

In part II the NFM system is described. Also some details are given concerning the layout of the SPS neutrino area, beams, monitoring, etc. These general features are the subject of chapter 6. A more detailed technical description of the muon flux detectors and their layout in the muon shield are given in chapter 7, whereas in chapter 8 the detector electronics and signal processing are described.

An essential part of the work has been to determine the relation between the detector response and the real muon flux. Chapter 9 is crucial because it contains the results obtained in this effort of absolute detector calibration. The main uncertainty arises from the presence of the secondary electrons, already mentioned.

In chapter 10 the use of the NFM system for on-line beam monitoring is described. Also a few typical results of muon flux distributions are illustrated. Finally, chapter 11 describes measurements of the muon range at various energies, for which the NFM system constitutes a nearly ideal apparatus.

In general, physical quantities are expressed in units of the CGS system, except particle energies which are given in Mev or GeV. Further exceptions are clearly indicated, where necessary.

Occasionally a comparison is made with the muon flux measurement in the neutrino experiment with Gargamelle in the 24 GeV neutrino beam at the Proton Synchrotron PS. The muon flux measurement system there was in many aspects different from the NFM, but also used silicon detectors.

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The subject matter of this work is a mixture of high energy physics, solid state physics, silicon technology and electronics. It may appear as a heterogeneous assembly of experimental data, observations and some theory. It was not possible, and maybe not desirable, in the present context, to treat single details more completely and rigorously. But it is hoped that the neutrino beam user can find here what he needs to understand the measurement of the muon flux.

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SEMICONDUCTOR PARTICLE DETECTORS

1.1 Generation of free charge by radiation

The characteristic property of a monocrystalline semiconducting

material is the small separation between the electronic conduction band

and the valence band. A small amount of energy, a few ev only, is

therefore sufficient to excite an electron into the conduction band. The

hole left in the valence band behaves like an independently moving

positive charge carrier and has a mobility which is comparable to that of

the free electron. At a given temperature there is equilibrium between

the generation and recombination of free electrons and holes. Impurities

and crystal defects act as generation or recombination centres and reduce

therefore the mean free carrier lifetime. Intrinsic semiconductors have

equal concentrations of electrons and holes. The introduction of

electrically active donor or acceptor impurity atoms ("doping") results in

a n-type resp. p-type conductivity semiconductor with an excess of

electrons resp. holes. The product of the concentrations of electrons n

and holes p remains however constant

n.p = N N e c c v ïïf (1-1)

where E is the semiconductor bandgap, k is Boltzmann's constant, T is

the absolute temperature, N and N are the numbers of allowed energy

levels in the conduction band, respectively the valence band.

The doping of the semiconductor material determines its resistivity,

p, which is the reciprocal of the conductivity a

P = ô = q(uen + yhpT ( 1 - 2 )

where y , u h are the mobilities for electrons and holes and q is

the unit charge. The highest resistivity which can technically be

obtained in silicon is 20 kftcm, corresponding to about 10 x l doping

atoms per cm 3. In germanium, controlled doping to a level of 10 9

electrically active atoms per cm3 has become possible.

Free charge can be generated in excess of the thermal equilibrium

value by electromagnetic or particle irradiation. The photoelectric

effect in which a photon excites an electron to the conduction band,

provides a precise measurement of the bandgap, in silicon 1.12 eV,

corresponding to the electromagnetic wavelength of 1110 nm. For energetic

particles the excitation process is more complicated, and it was found

that on the average 3.62 eV is deposited in the silicon for each generated

electron-hole (e-h) pair [1.4]. This value is nearly independent of the

type of particle, as long as the density of the ionized charge is not too

high. Then non-linearity may occur, which is the subject of chapter 4.

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The statistical fluctuation in the number of charge carriers per unit

energy loss is smaller than the value expected on the basis of Poisson

statistics. This is due to the correlation in the formation of

consecutive e-h pairs. The ionizing events cannot be regarded as

completely independent. The Fano factor F is the ratio of the observed

variance and the Poisson-predicted variance, and although its measurement

is made difficult by the influence of other effects, F has been determined

to be "v .1 for silicon.

The study of low energy electron transmission through thin foils

revealed that a collective excitation of the electrons, called "plasmon",

with a well defined energy can occur in Si; this plasmon energy is 16.7 eV

[1.5]. The plasmon generates several e-h pairs, but also couples to other

excitations like phonons. Of course, the plasmon causes a correlation in

the energy deposits leading to the e-h generation. Several authors

developed models to explain the difference between the bandgap energy and

the average energy required for an e-h pair, see e.g. [1.6]. Essentially,

this energy difference reappears as lattice kinetic energy. Energy loss

of relativistic particles will be discussed further in the next chapter.

The excess charge concentrations resulting from the irradiation will

return to thermal equilibrium with a time constant equal to the lifetime

of the minority charge carrier. Defects and impurities limit this

lifetime, but in very pure silicon or germanium crystals a value of a few

milliseconds is achievable. The radiation-produced charge can then be

integrally collected by applying an electrical field to the semiconductor.

Clearly this charge must be detectable above the thermally generated

charge. Therefore one must choose material with a large bandgap (e.g.

diamond) or cool the material (e.g. germanium) . Silicon can be used at

room temperature but if a collecting field is applied to a piece of Si,

via ohmic contacts, a continuous charge injection takes place. This

leakage current is much reduced when a diode structure is created, by

joining an n-type region with an excess of free electrons to a p-type

region, with more free holes. Around the junction the diffusion of the

carriers establishes a space charge region, which becomes depleted of free

charge carriers, and which blocks the current.

The width of this depletion layer x^ can be found by solving the

Poisson equation for the charges present in the diode, and depends on the

doping density n (or p) and the externally applied reverse bias voltage

V B

*D = V ë ( V° + V = V 2 £ U e p ( V o + VB> (1'3)

Vo is the "built-in" voltage created by the space charge in the junction,

e is the permittivity (for Si e = 1.054 x 10" 1 2 Fern"1), formula (1.2) is

used to obtain the second expression.

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To obtain a thick depletion layer, a very high resistivity is necessary. In the case of n-type silicon, with a resistivity of 20 000 ftcm, a depletion layer of 1 mm thickness can be obtained with a moderate bias voltage V = 300 V.

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depletion layer x.. with an area A has a capacitance CL , such that

C°~t-*H eP(V 0 + V B)

From a measurement of C as function of V B

(1.4)

the resistivity p can be determined, as shown in fig. 1.1. Above a certain voltage the capacitance tends to a constant value, and the complete thickness of silicon is depleted. The electrical field in the diode then extends from the rectifying front contact to the ohmic rear contact. In a reverse-biased diode a leakage current or dark current exists which is the major source of noise in the charge collection. The different components of this leakage current will be further discussed in sect. 5.2.2.

150

T

DETECTOR PBD 4

T

100

50

3 xlO" cm-3 p = 14 kiicm

V. - 2 0 0 V depl J y

.S

:34 pF

0,1 0,2 0,3 0,4 0,5

INVERSE BIAS VOLTAGE V •1/2

Fig . 1.1 Detector diode capacitance versus the inverse square roo t of the bias vol tage Vg for a de tec tor with surface area A = 2 cm 2 . From the s lope of the s t r a i g h t p a r t of the curve the doping dens i ty n = 3 x 1 0 1 1 cm"3 can be determined. Total deplet ion occurs above Vg "v. 200 V when the capaci tance becomes nea r ly cons tan t . C0 i s the p a r a s i t i c capac i tance , from leads and mounting, which has to be subtracted from the depleted capaci tance to find the t r ue capacitance of the diode.

The r e v e r s e b i a s v o l t a g e V g c r e a t e s an e l e c t r i c a l f i e l d E i n t h e d i o d e , w h i c h i s h i g h e s t a t t h e j u n c t i o n and d r o p s t o w a r d s t h e e d g e of t h e

x , d e p e n d i n g on t h e s p a c e c h a r g e d i s t r i b u t i o n . The depletion layer free charge carriers are collected by this field, which gives a drift velocity v, to both electrons and holes, proportional to their mobilities y and y. e.g. for electrons

'd = V e( E ' T ) ' E (1.5)

For high f i e l d s t he d r i f t v e l o c i t y becomes n e a r l y c o n s t a n t , around 6 x 1 0 6 cm s " 1 . The m o b i l i t y i n c r e a s e s for lower t e m p e r a t u r e s , and a t 30 K i t i s n e a r l y 20 t imes h i g h e r than a t room t empe ra tu r e [ 1 . 7 ] . However,

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the s c a t t e r i n g of the charge c a r r i e r s by impur i t i es in the c r y s t a l becomes dominant a t low temperature, and leads to a reduction of t h i s mobi l i ty . Some re levant proper t ies of Si are summarized in tab le 1 .1 .

TABLE 1.1

P rope r t i e s of s i l i c o n [1 .6 , 1.8, 1.26]

Atomic number 14 Atomic weight 28.09 Stable isotopes, natural abundance 28-29-30 (92.212-4.702-3.09%) Density (300 K), g cm"3 2.328 Atoms cm" 3 5.0 x 1 0 2 2

Crystal structure Diamond type, space group Fd 3 m Lattice constant (300 K) cm 5.43 x 10"» Melting point °C 1410 Linear thermal expansion coefficient °C"' 2.6 x 10" 6

Specific heat (25°C) J kg"1 "C" 1 760 Thermal conductivity (25°C) J m" ' "C" 1 84 Dielectric constant 11.9 Permittivity F cm"1 1.054 x 10" 1 2

Band gap (300 K) eV 1.12 Band gap (0 K) eV 1.16 Intrinsic carrier density (300 K) cm"3 1.5 x 10'° Intrinsic resistivity (300 K) !2cm 2.3 x 10 5

Electron diffusion coefficient (300 K) cm 2s"' 35 Hole diffusion coefficient (300 K) cm2 s" ' 12 Electron mobility (300 K) cm 2 V s " ' 1500 Hole mobility (300 K) cm2 V ' s - 1 600 Carrier saturation velocity (300 K) cm s"1 8 x 10 6

Breakdown field V cm"1 % 3 x 10 s

Electron affinity V 4.05 Work function V 4.8 Effective density of states N in conduction band (300 K) cm" 3 2.8 x 1 0 1 9

Idem N in valence band (300 K) cm"3

v 1.04 x 10 1 9

Effective mass m /m 0 Elee trons m* = .98 m* = .19 Holes

Raman phonon energy eV m„, = .16 m* = .49 ih hh .063

Vapour pressure Torr 10" 8 at 930"C Energy required per electron-hole pair

(300 K) eV 3.62 (77 K) eV 3.76

dE/dx minimum MeV g"1 cm 2 1.66 Nuclear collision length g cm"2 69 Radiation length g cm"2 21.8

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1.2 Diode structures for silicon particle detectors Silicon detectors were first fabricated by the diffusion process.

The necessary high temperatures usually affect the quality of the resulting detector in that carrier lifetimes are much reduced. The surface barrier or "Schottky type" diode is made by evaporation of a suitable metal, like gold, on a properly prepared Si surface, and no high temperature is needed. This process yields the best quality room temperature nuclear detector but as it depends on the state of the surface, it is a very delicate instrument. A more rugged detector can be obtained with the ion-implantation process. Detectors of arbitrary geometry can be fabricated if one uses oxyde masking ("planar") technology in conjunction with ion-implantation. Special care is needed, however, to avoid the lifetime degradation. With the previous processes, silicon detectors of up to 1 or 2 mm thickness can be fabricated, the limitation coming from the lack of homogeneity in very high resistivity silicon. If a thicker detector (> 2 mm) is required, the lithium drift process can yield a homogeneously compensated volume of nearly intrinsic silicon ("i-type") of which a "PIN diode" can be made (p-type, i-type, n-type) . Extremely thin detectors (2 um) can be constructed from an epitaxially grown layer of high resistivity Si on a low resistivity substrate, which subsequently is etched away locally, using the difference in etch rate. In fig. 1.2 these diodes are schematically represented and in the following subsections they are discussed in somewhat more detail.

Ol > ,Zfj.m PHOSPHORUS DIFFUSION

p-TYPE Si

.2 fin BORON DIFFUSION

Diffused d e t e c t o r

3 0 ran GOLD EVAPORATION

100 nm ALUMINIUM

Surface barrier detector with epoxy edge protection

>. l f tm BORON-IMPLANTATION 'l^Z

As IMPLANTATION . 2 u m

Lon implanted detector with oxide passivation

GUARD RING

Surface barrier guard ring structure

BORON IMPLANTATION

— i i r ~ i v i i r ~ i

AS IMPLANTATION .2 / j .n x " Ion implanted microstrip detector

Li DIFFUSED n CONTACT

IL

Li COMPENSATED

INTRINSIC Si

p CONTACT

GOLD EVAPORATION

Lithium d r i f t ed N-I-P de tec to r

Fig . 1.2 Schematic cons t ruc t ion of var ious types of s i l i c o n d e t e c t o r s . The r e c t i f y i n g contact i s always drawn at the top . Regions of high doping a re indicated with - - - (not to s c a l e ) . Depleted region when b iased i s indicated with:

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1.2.1 Diffused detectors

On wafers of high resistivity Si one diffusion is required to form the diode, and a second one for an ohmic back contact. Either of these can be made shallow, to reduce the "window" thickness for the particles entering the detector, but the minimum possible thickness is .2 urn. If the detector serves for detection of minimum ionizing particles, the window thickness is not important but for e.g. heavy ion spectrometry diffused detectors are for this reason of little use.

(* ) For example a diode can be formed by a phosphorus diffusion at 1000°C through an etched oxide mask, grown at 1100°C under steam, on a p-type boron doped substrate; the back contact is made by a short, shallow boron diffusion, and this detector should then be operated at a bias voltage V B such that the depletion region (formula (1.3)) extends all through the substrate until the back contact, the "total depletion voltage". Particles enter through this thin back contact. Around the diode a ring structure of grooves and additional oxide serves to reduce the surface leakage current. A different type of diffused detector***' in use is cut ultrasonically from the big previously diffused wafer, ,so that a well defined sensitive area can be obtained. For a low leakage current one relies on the blocking effect of the epoxy resin used for mounting the chip on the glass fiber printed circuit board (PCB). Detectors of this type which became noisy, with much increased current, could be stabilized again by curing the epoxy in a 12 hours bake out at 110°C.

1.2.2 Surface barrier detectors

Although the principle of the surface barrier is described by Schottky's model for a metal in contact with a semiconductor [1.9] in practice it does not account for all phenomena, observed in surface barrier detectors. Partly this is due to the unusually high resistivity of the silicon employed for detectors, partly it comes from the influence of surface treatments (see e.g. Turner and Rhoderick [1.25]) . Siffert and Coche [1.10] showed that the barrier is formed only after oxygen is admitted to an evaporated gold layer. Also with other metals a useful diode can be obtained, and ponpon and Siffert, [1.11] found that a low heat of oxidation (the metal is then not easily binding the oxygen) favours the barrier formation. The charge present in the metal-semiconductor interface layer is the determining factor, and therefore the etching prior to the evaporation is the most important step in the process

(*) This example corresponds roughly to the construction of SIMTEC Industries "Slim Line" dE/dx silicon detectors.

(**) Quantrad Corporation, Los Angeles.

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because it creates the surface states. It should be kept in mind that the surface state density on Si is 1 0 1 2 cm"2, which has to be compared to an electrically active impurity content in the volume of the Si of 10' 10 1 J cm"3

All surface barrier detectors used in our experiment are Au on n-type Si, mounted with epoxy resin in an isolating ring, with an Al evaporated rear side and pressure contacts on the ring. The sensitive volume of the detector is not only determined by the irregular shape of the epoxy lining but even more by the extension of the depleted region under the epoxy. In a crude approximation, a lateral extension equal to the depletion thickness is assumed, which leads to a reasonable prediction of the sensitivity [1.12]. In sect. 3.3.2 measurements will be discussed which show that this method of evaluation of the effective volume gives agreement between scintillators and silicon detectors for flux counting.

Some surface barrier detectors do not withstand a prolonged over-depletion voltage and show current injection via the back contact. Others have been treated such that they are not stable in air, but in a sealed nitrogen atmosphere they stabilize.

F ig. 1.3 Picture of some detectors which were used for muon flux measurement. The smallest has an area of 1 mm 2, the biggest 200 mm 2.

1.2.3 Ion Implanted Detectors A diode can be formed by the process of ion implantation, e.g. B

ions are accelerated and the beam is "scanned" over the surface on a n-type Si subtrate, so that a homogeneous dose of ions is implanted over all the surface. The lower the ion energy, the thinner the window. Generally, 15 keV is used, resulting in a junction depth of i< 80 nm, when the ions are not implanted along a channeling direction. Hofker [1.13] determined the effect of varying the parameters: ion-energy, ion-dose and post implantation annealing temperature. 30 min annealing at

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600°C results in activation of 10 to 50% of the implanted B atoms, the rest still occupies interstitial sites in the lattice and does not contribute to the shallow acceptor level. The annealing also repairs much of the radiation damage which was introduced in the surface layer by the ions. Using the thermally stimulated current of the diode itself, Muller [1.14] showed that trapping levels, typical for radiation induced centres, disappear after heating. The same method was also used to assess the radiation effects of the muon flux, and will be discussed in detail in chapter 5.

Recently it was recognized that irradiation with a high power laser can accomplish the activation and surface annealing, even without a significant rise of temperature in the substrate [1.15].

A special application of low dose ion implantation is in the production of "position sensitive" detectors, which have a contact with high sheet-resistivity [1.16]. Using a charge division readout technique the coordinates of the particle can be measured.

1.2.4 Microstrip detectors A different type of position sensitive detector, the "microstrip

detector" , can be obtained by dividing the detection volume in separately sensitive elements, each with its proper read-out. A detector with 200 urn wide strips, obtained with the surface barrier technique, is described in [1.17], where also references to earlier work can be found. Much finer strips can be obtained if the planar technology is used, because photolithography directly on the silicon wafer is then possible [1.18, 1.19] .

Microstrip detectors have very well defined, small sensitive volumes, and could therefore be very suitable for counting of high fluxes.

1.2.5 Si(Li) and Ge(Li) Lithium Drifted Detectors The small Li + ion can easily be drifted through crystals of Si or

Ge and it compensates B doped p-type Si or Ge, so that the resistivity increases, near to the intrinsic limit. Thick depleted layers are then obtained at relatively low bias voltages. If a silicon detector thickness of 1-5 mm is needed, there is practically no alternative to this Si(Li) structure. Precipitation of Li at radiation induced lattice defects takes place, which may unbalance the compensation. Like all high resistivity detectors, the Si(Li) detectors are apt to radiation damage (see chapter 5) . For this reason they are only used for flux measurement, where a very big detection volume is required at low signal levels.

- 13 -

1.3 The Utilization of Semiconductor Detectors 1.3.1 Nuclear Physics In nuclear physics the semiconductor radiation detectors are used

primarily for energy spectroscopy ("E-measurement") and particle dE identification via the specific ionization ("73-? measurement"). For

dE -j— a t h i n , well-defined detect ion volume i s needed, and for E the de tec tor must have a su f f i c i en t volume to stop the ion or contain the electromagnetic cascade of an e lec t ron or a photon. Compared to gas f i l l e d ionizat ion chambers, 5 to 10 times more charge is produced for the same energy l o s s , in a much smaller volume of high stopping power, and not only the e lec t rons but a lso the holes are col lec ted with nearly 100% ef f ic iency . Because of the larger number of charge c a r r i e r s and the smaller Fano f ac to r , the r e l a t i v e s t a t i s t i c a l f luctuat ion in the number of charges per mean energy loss i s smaller in semiconductor d e t e c t o r s , which r e s u l t s in superior energy r e s o l u t i o n . The response of the semiconductor de tec to r s i s fas t because of the small dimensions and the r e p e t i t i o n r a t e i s not impeded by ion feedback or space charge accumulation.

Detectors are normally used in "pulse mode" and the charge pulse i s measured with a charge sens i t ive preampl i f ier , shaped and amplified, converted to a d i g i t a l s ignal and s tored in a d i g i t a l multichannel ana lyzer . A spectrum of the a - p a r t i c l e s emitted by a 2l*1Am source, and measured with a surface ba r r i e r d e t e c t o r , i s shown in f i g . 1.4. The fu l l spectrum width at half the peakheight (FWHM) is a measure for the energy resolut ion of the de tec to r , and i s specif ied for each detector by the manufacturer. The ca l i b r a t i on i s car r ied out by means of ions pene t ra t ing a t the rear s ide of the d e t e c t o r . The reso lu t ion measured in t h i s case might be i n f e r i o r , if the de tec tor has incomplete deplet ion or presents charge c o l l e c t i o n problems.

F i g . 1.4 Example of 2 , , 1 Am alpha pulse he igh t spectrum recorded with de t ec to r PBA26 a t 90V b i a s . The energy of the main peak i s 5.486 MeV. The channel number corresponding to t h i s main peak i s ind ica ted on the spectrum disp lay by x and the number of counts in t h i s channel i s given as y . The l e f t p i c tu re r e p r e s e n t s the spectrum for the alpha p a r t i c l e s inc iden t on the front of the de tec to r (gold e l ec t rode ) and the r igh t p i c t u r e i s obtained if the p a r t i c l e s en t e r the de tec tor on the rear s ide (aluminium ohmic c o n t a c t ) . The s i g n a l height for r e a r incidence i s lower (channel 270, ins tead of 358) due to a th icker dead layer at the r ea r (see s e c t . 4 . 2 . 1 ) . The FWHM noise width i s 20.1 keV for p a r t i c l e s inc iden t on the front and 21.1 keV on the r e a r s i d e .

- 14 -

1.3.2 High energy physics

In high energy physics the range of the particles is so large, that practically it cannot be contained in the small semiconductor detectors. A large fraction of the cascade, initiated by a high energy photon, also may escape from the detector. Energy measurement, as carried out in low energy physics, therefore becomes impossible. Fluctuations in the signal height of minimum ionizing particles are much more determined by the physical energy loss phenomena than by the energy resolution of the detector. A more detailed discussion will follow in chapters 2 and 3. Nevertheless, ionization measurements on individual particles may be meaningful. The detector surface area is relatively small, a few cm 2, although in principle monocrystalline silicon wafers up to 200 cm 2 can be obtained. An advantage is, that signals with good energy resolution can be obtained with extremely thin detectors, which do not need a gas circulation system or bulky light guides and photomultipliers. Applications in high energy physics experiments are limited to the following categories:

(a) Detection of the presence of particles under extreme conditions, e.g. at cryogenic temperatures, where some scintillators cannot operate (trigger for polarized target), or in a high magnetic field.

(b) Measurement of event multiplicity close to the interaction region, from the height of the signal collectively generated by the particles.

(c) Particle flux measurement by counting individual particles ("pulse mode"). In comparison with scintillators, the pulses are rather long due to the long drift time of the charge carriers. Moreover the detector surface area is not well-defined, but the thickness of the detector can be ;< 300 urn. A better defined and much smaller area can be obtained with the microstrip detector structure (1.2.4).

(d) High precision ( 10 um) position sensitive detection of particles near the interaction region, with these silicon microstrip detectors, is a new technique, now under development.

(e) Measurement of the ionization from individual particles.

(f) Flux measurement in "current mode", by the integration of all charge produced by particles passing through the detector.

This last category, the "current mode" of operation, is used to measure the high flux of muons. The current is integrated over the muon pulse length, which can have a duration between 1 us and a few ms. The highest flux values correspond to 5 x 1 0 1 3 cm" 2 s" 1 , but some non-linearity was observed at this very dense flux, as will be discussed in chapter 4.

- 15 -

w

1.3.3 Flux measurement For a flux up to 10 1 2 cm"2 s" ' Si detectors deliver a charge

hich is perfectly proportional to the energy deposited by the particles in the detecting sensitive volume. For long integration times the thermally generated charge ("dark current") may be much more important than the radiation-produced charge and this dark current must be determined and subtracted from the signal, if the detector is directly coupled to the amplifier {"DC coupling") . For long integration times the usual AC coupling via a coupling capacitor cannot be used, as will be explained in sect. 8.1.1.

The electronic circuit designed for this "current mode" operation will be described in chapter 8, in the framework of the whole measurement system. It responds with an output voltage V to a charge from the detector, and the proportionality factor G is the amplifier gain (V.pC"'). The offset 0 is the signal obtained when no particles go through the detector. The particle flux N per cm2 is then

N p = S D x ¥=£• [cm"2] (1.6)

S is the reciprocal detector sensitivity, which is determined by the sensitive detector volume and the charge collection efficiency. For a good detector, this efficiency is near to 100%, and if the detector is totally depleted, the sensitive thickness is equal to the thickness of the Si wafer. The reciprocal sensitivity S D then has a constant value, and can in principle be determined from geometry. For the sensitive area the lateral extension of the depleted zone must be taken into account, as discussed in 1.2.2. Taking the mean energy deposition of a "standard minimum ionizing particle" as 3.5 MeV per cm of Si (sect. 2.3.2) and using the proportionality of 3.62 eV per e-h pair, we define this reciprocal sensitivity S as the number of "standard" particles per cm2 required to produce one picocoulomb of charge in the detector

l

S, 3 . 5 x 1 0 6 1 . 6 x I P ' 1 9 , , . , , 2 x •— x d x i r ( r + d) 3.62 10~ 1 2

[cm"2 pC"1] (1.7)

d detector thickness, r radius of sensitive area of circular detector, in cm.

In practice, one finds a difference between the true and the nominal detector volume. Therefore more precise calibration procedures had to be adopted for determining individual S values for all detectors. In appendix C the nominal S of the detectors are listed and compared to calibrations. A systematic difference of 25-35% is found for most detector types. This indicates that formula (1.7) is approximately correct as far as detector geometry is concerned. The energy deposited in the detector must be different from 3.5 MeV cm . In chapter 3 it will be shown that this discrepancy can be attributed to secondary r adiation.

- 16 -

1.4 Fast amplifiers and noise First, the charge sensitive preamplifier will be discussed, because

it is generally used in spectrometry with semiconductor detectors. Only in a situation of high particle rate, the voltage sensitive or current sensitive preamplifiers are preferable. A charge sensitive preamplifier generally has a Junction Field Effect Transistor (JFET) as input element. This configuration is schematically shown in fig. 1.5(a). The charge Q which is generated by the particle in the detector with capacitance C n

d gives rise to a signal on the input capacitance C. = C, + C. + C» (where C. is the amplifier capacitance and C_ the sum of all other stray capacitances)

Vin = a 2" <l-8> in

The charge s e n s i t i v e a m p l i f i e r s e n s e s t h e i n t e g r a t e d cha rge Q through the feedback c a p a c i t o r C f ( r a t h e r than t h e v o l t a g e V. as does a v o l t a g e s e n s i t i v e c o n f i g u r a t i o n , ( f i g . 1 . 5 ( b ) ) . The o u t p u t v o l t a g e i s g iven by

V o u t = " A V i n = " A C i n + (A + D C " ( 1 - 9 >

I f one assumes t h a t t h e gain A>>(C^ n + Cf) /Cf

v s - 2_ v o u t C

The output signal is thus insensitive to variations in the input capacitance c-;n- T n e time constant RfCf °f t n e feedback network must be much longer than the duration of the input pulse, and this may cause a pile-up of pulses, increasing the potential at the output. Instead of discharging this potential via R f, the reset can be obtained by a light induced discharge on the FET (optoelectronic feedback) and in this way the noise from R f [1.20] is completely eliminated. With a cooled JFET a noise of 100-150 eV FWHM has been obtained, but there is still an appreciable deadtime and basically the charge sensitive configuration is not very suitable for high counting rate.

In the special case of counting of minimum ionizing particles, the precision in pulse height measurement is of less importance and only small detectors with a capacitance below 50 pF are used. Therefore it is preferable to use fast amplifiers with the current sensitive or voltage sensitive configuration, illustrated in fig. 1.5. Such a voltage sensitive amplifier was designed and used for the muon flux counting. In a later stage, also a current sensitive preamplifier has been built with a bipolar microwave transistor as input element instead of the usual JFET. This solution leads to a very simple device with a low power dissipation, which also is well suited to the needs of the multi-element microstrip detectors.

- 17 -

In high energy applications the fluctuation in signal height is mainly caused by the energy loss process, following the Landau distribution. It is nevertheless needed to achieve a low noise amplification because the signals of particles in thin detectors are rather low, 2-5 fC (12000-30000 electron-hole pairs), corresponding to 50 keV-120 keV of energy loss. The noise sources at the input of the preamplifier (fig. 1.6) can be divided in two categories, those which are described in terms of series noise resistance R (equivalent to a series voltage generator) and those described as a parallel noise resistance R. (equivalent to a parallel current generator). The noise

V o u t ^ " V 0 u t - • V o u » - R f I i n

Cin "out

[N v i n

L> V,

i

out

( Q ) ( b) (c)

Fig. 1 • 5 Simplified diagrams of the various configurations, which can be used as preamplifier and impedance adapter for silicon detectors. (a) The usual charge sensitive configuration with feedback capacitance Cf (b) A voltage sensitive configuration, more conventional in other electronic applications (c) The current sensitive configuration also known as "current to voltage converter"

Fig. 1.6 (a) The electrical noise sources in series with the signal source at the input of an idealized preamplifier can be represented as a series voltage generator v s and a parallel current generator i p. R s is the effective signal source noise resistance.

signal ^ . noise [ ) source ^ r (b) In an equivalent representation the noise

is attributed to a series noise resistance R v and a parallel noise resistance Rj.

- 18 -

. B . . S &

VOLTAGE SENSITIVE PREAMPLIFIER

PULSE SHAPmG AMPUFIE

T>-i'V

Fig . 1.7 C i r c u i t diagram of the vol tage s e n s i t i v e preampl i f ie r ( l e f t ) and the shaping a m p l i f i e r / t r a n s m i t t e r ( r i g h t ) .

Fig . 1.8 The vo l t age s e n s i t i v e p reampl i f i e r i s i n t eg ra t ed with the d e t e c t o r mounting on the same pr in ted c i r c u i t board.

(a)

("a) Signals from muons passing through a 480 urn thick silicon detector, processed by the voltage sensitive preamplifier and shaping amplifier (10 ns/div).

(b) The energy deposition spectrum as obtained by the pulse height analysis shows the typical form of a Landau distribution, except for a low energy tail. The horizontal axis gives the energy, the vertical axis is the number of events for each energy channel.

- 19 -

figure F of the amplifier, which is the Signal to Noise Ratio (SNR) of input, divided by SNR of output, is found to be (see e.g. [1.21])

R R F = 1 + + (1.10)

i s where R is the effective signal source noise resistance. The minimum noise figure can be found for

R. >> R >> R l s v

and has t he v a l u e

'R F m i n = 1 + 2 \ i T ( 1 - l D

In practice, it is also useful to express the noise as the equivalent noise charge (ENC) , which is the amount of charge (in electrons) that gives rise to an output voltage, equal to the RMS level of noise. If the signals from a "noiseless" detector are monoenergetic, the noise causes a broadening of the peak in the spectrum, and the FWHM is 2.35 times the ENC. Again the ENC can be represented as the sum of a parallel noise ENC and a series noise ENC P s

ENC2 = ENC2 + ENC2 (1.12)

These correspond respectively to a mean square current noise ip2 and a mean square voltage noise v 2. The series noise is more important in comparison to the signal, the bigger the detector capacitance C,. The parallel noise on the contrary is not influenced by C,. The noise performance of an amplifier can be optimized for a given C, by equalizing the contributions of parallel and series noise.

1.4.1 Voltage sensitive preamplifier The voltage sensitive preamplifier [1.22] is AC coupled, with an

input impedance of 1 MSÎ in parallel with C. , and uses a FET cascode amplifier configuration, which drives an emitter follower, as illustrated in the circuit-diagram (fig. 1.7(a)). It has a rise time of i 2 ns, a voltage gain one and the noise equivalent in a 100 MHz bandwidth is 15 keV FWHM. When a main amplifier with .3 us RC shaping time is used to process the output signal, a noise of a few keV can be obtained, which is then due mainly to the input resistance. The sensitivity (gain) depends on the capacitance presented at the input, and is •v 5 yV per keV, for a 20 pF detector. To obtain stable operation, only totally depleted detectors should be used. As shown in fig. 1.8, the detector mounting is integrated with the electronics on the same printed circuit board, which also reduces capacitance variations. This preamplifer can be used in

- 20 -

connection with commercial NIM standard main amplifiers, but to obtain

full profit of the speed, a differentiating shaping amplifier

(fig. 1.7(b)) with 3 dB gain and a bandwidth between 10 MHz and 100 MHz

was built. This amplifier drives a 100 m long cable. Fig. 1.9(a) is a

picture of signals obtained from muons in the neutrino shield, with a

480 um thick silicon diode. The corresponding energy spectrum is shown

in fig. 1.9(b), and it has the characteristic form of the Landau

distribution, apart from the low energy tail.

1.4.2 Current sensitive preamplifier

Traditionally, low noise nuclear amplifiers use a JFET as input

element. The noise sources in the JFET are the thermal noise in the

channel, equivalent to a series voltage generator

"7 8kT if ,-,•,-,. v s = — — (1.13)

^m

and the fluctuations in the conductive channel under the metallic gate,

equivalent to a parallel current generator

f 2C 2

i 2 = 4kT Af (1.14) P Tt2g m

where Af is the frequency interval and f the frequency. The

transconductance g must be as large as possible to reduce the noise, =m but a large drain current gives a high power dissipation. At high

frequency the noise term (1.14) becomes predominant because of the factor

f 2. Therefore, in this case the JFET is not very advantageous, and the

characteristics of bipolar microwave transistors were studied for

application in high rate amplifiers [1.23]. Recent types of microwave

transistors feature high current gain & and high transition frequency

f„, with a low base spreading resistance r D T J, (10 to 30Œ) , which is T. DO

accomplished by a very thin base region, requiring a precisely controlled

diffusion process. The noise sources in a bipolar transistor are the base

spreading resistance r. o l and the shot noise in the collector current,

which can be represented as a series voltage generator

vl = 4kT(r B B, + 2^-)Af (1.15) ym

and the fluctuations in the base current I , equivalent to a parallel

current generator

i 2 = 2ql 0 Af (1.16) p B

The transconductance g m = q I E / kT , where I £ i s the emit ter c u r r e n t .

- 21 -

For high frequency operation the bipolar microwave transistors

present comparable or better noise performance at currents which are ten

times lower. Therefore, bipolar transistors instead of JFET can be used

as input transistors in preamplifiers which are designed for high particle

rate detection in high energy physics [1.17]. The current sensitive

configuration with a 10 kQ feedback resistor R f was built, as shown in

fig. 1.10, using a bipolar transistor (NEC578) with f = 5 GHz and a

current gain 100. The output voltage is directly related to the input

current

Vout = - ^ f t 1- 1 7'

MICROSTRIP DETECTOR PREAMPLIFIER

lOnF i ±100nF

_,,_ 1 1 1 a

ÏT1 = NE578,NE021,NE734

VBIAS T 2 . B F T 2 5

T3 =BFS17

Fig . 1.10 C i r c u i t diagram of transimpedance ("current s e n s i t i v e " ) preampl i f ie r .

In t h i s conf igura t ion , the equivalent noise sources for the se r i e s generator can be wr i t t en (using (1.15) and (1.16))

ENC* = | { C^n 4kT(r , + j~)) J? ( w ( t ) ) 2 d t (1.18)

and for the p a r a l l e l generator

ENC* = i { 2 q l + 1*T} J œ w ' ( t ) d t ( 1 . 1 9 ) p z a K o

where w and w' are the "weighting functions", which describe the signal

filtering by the electronic processor [1.24]. The feedback resistor gives

an additional contribution to the parallel noise.

The noise optimization is possible if the weighting function is

chosen such that the resulting noise contains equal contributions of the

parallel and series generators. With the actual values of the components,

the equivalent noise resistance R. = 7 kft and the series noise

resistance R v = R B Bi + kT/QIe = 75 P.. An optimum filtering time

T C can be determined [1.24]

- 22 -

C . VR...R in x v

which for an i n p u t c a p a c i t a n c e of 15 pF i s then 10 n s .

(1.20)

Measurements of the n o i s e as a f u n c t i o n of i n p u t c a p a c i t a n c e a r e r e p o r t e d in f i g . 1 . 1 1 , for t h r e e d i f f e r e n t b i p o l a r i n p u t t r a n s i s t o r . Up t o 20 pF a n o i s e of ^ 1400 e l e c t r o n s r . m . s . i s measured , c o r r e s p o n d i n g to 12 keV e q u i v a l e n t in S i . These measurements were made wi th a 40 ns g a t i n g on a c h a r g e s e n s i t i v e ADC. I n an e x p e r i m e n t a l s i t u a t i o n , measur ing the t o t a l a b s o r p t i o n peak of the gamma (122 keV) of S 7 C o , a FWHM of 16 keV i s found.

NOISE

" rms (electron )

3 0 0 0 -

2000 U

1000

0 10 50 100 INPUT CAPACITANCE ( pF )

150

Fig. 1.11 Noise characteristics as function of input capacitance for three different bipolar transistors, BFT 25, NEC 578 and NEC 021. Data are normalized for the effective weighting function.

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2. MINIMUM IONIZING PARTICLES IN SEMICONDUCTOR DETECTORS

The penetration of charged particles in matter has been studied in

detail by many people [2.1]. The aim of this chapter is to review some

features which are relevant to particle detection in high energy physics,

with thin silicon detectors, and to present results of energy loss

measurements in silicon. Some special properties of high energy muons in

a thick shield will be reserved for the next chapter, where also are

discussed the complications which arise in the actual muon flux

measurement, due to the secondary radiation which accompanies these muons.

2.1 Energy loss of high energy particles

Energetic particles penetrating in matter, loose gradually energy via

several types of interaction, until they come to rest or end in a decay or

a nuclear interaction. The interaction of charged particles with the

atomic electrons, via their electromagnetic field, results in excitation

and ionization of the electrons and the generated free charge provides in

semiconductor detectors the means to detect these charged particles.

Neutral particles interact with nuclei and can cause atomic displacements,

thereby degrading the structure of materials. Only occasionally can

neutral particles be detected via charged reaction products.

2.1.1 Excitation and ionization

In a semiclassical description energy transfers from the incident

particle to the electrons are treated as a function of an impact parameter

b, e.g. [2.2], In the limit of high incident velocity, the electrons can

be considered as a free electron gas, and for an energy transfer e

between 0 < e < e_ the collision probability w(e) per unit energy IïlâX

per g cm"2 can be written as (e.g. ref. [2.3])

w(e) = -- (1 - e 2 ~ ) (2.1) 2 E

c max with a parameter k

k = 2 , z2 a/ V „ .15354 g 2 Z [ M e V c n . g - i ] ( 2 . 2 )

m e c2 e 2 A e 2 A

- z, M are the charge and the mass of the incident particle,

- v = gc is the velocity of the incident particle,

- c is the light velocity in vacuum,

- m is the rest mass of the electron, e

- q is the unit electric charge,

- N. is 6.025 x 1 0 2 3 , Avogadro's number,

- Z and A are the atomic number and atomic weight of the target material, - q2/m c 2 = r is the classical electron radius. ^ ' e e

- 24 -

For Si, in the limit 6 + 1 , the value of k is .0766 MeV cm 2g _ 1. The closer the collision in terms of b, the more energy is transferred. The maximum transferable energy e

r b and for relativistic particles min c

corresponds to a minimum impact

2m c 2B 2 2m 1 +

M /l • ^ ) *

M (2.3)

For a muon with momentum 200 GeV/c this e is 190 GeV. Electrons max

produced by collisions are designated as "delta-electrons" or "knock-on" electrons (sect. 2.1.2).

On the other hand, because the energy is quantized, the energy transfers cannot be arbitrarily small, and there is a b which

max corresponds to a minimum quantum of transfer. For such large impact parameters, however, one has to take into account the collective behaviour of the electrons. The energy transfer gives rise to longitudinal oscillations of the electron gas called plasmons [2.4] (sect. 2.1.3).

For practical calculations the mean energy loss dE in a path length dx is given by the formula of Bethe-Bloch

.2 a 2 _ dE d E P m e c 2

£ = k SLn £-x L i 2 d

max 2B 2 - 2 £ - 6 ;)

(2.4;

where k is given by (2.2) , £ by (2.3) . I is in principle the mean max

excitation potential per atomic electron. In practice, it absorbs a number of shortcomings of the formula and is adapted to the measured energy loss values. It amounts to 172 eV for Si and 273 eV for Fe [2.5]. The term 2 C/z represents the shell correction, i.e. the reduction in energy loss on inner shell electrons for slow incident particles. In the case of high energy particles, the complications presented by the binding energy of the electrons are of less importance. On the contrary, the density effect correction 6 plays a preponderant role for ultra-relativistic particles in solids. It arises from the polarization of the medium by the field of the incident particle (also called "screening"), and results in a reduction of the loss in distant collisions. Sternheimer and Peierls [2.6] have worked out the general expression, using the variable X which is related to the particle momentum p

X = i0^h 'log — 6 /l -

4.606 X + C - a(Xi - X) for Xo < X < Xi

4.606 X + C 0

for X > Xi for X < Xo (2.5)

They were able to determine the parameters such, that the accuracy is better than 2% for all materials. For silicon the values are C = -4.425,

- 25 -

Xo = .2, Xi = 3.0, a = .1596 and m is always 3. Crispin and Fowler [2.7] made a comparison with experimental data. In fig. 2.1 curves are shown of dE/dx in Si with and without density effect.

1.0

0,9

0,8

0,7

~l I I I M i l l ~1 I I ! I I I I I I I I I I I I M ~ l 1 I I I I I I

( M

eV/m

m

0,6

con

0,5

Sil

c 0,4

LU T3

X -a

0,3

0,2

0,1

0

d)

+ 486 /xrn thick

• 980 /xm thick

iL _i I i i i m l j i i 11 m l j i i i i m i i i 11111

10 100 1000

/ 3 / V H 3 2 of MUON

10000 100000

F ig . 2.1 The mean energy loss of muons or pions in s i l i c o n , as a function of Che muon momentum p/Mc = 8 / / l _ 8 2 in MeV mm"' or keV urn"'. The d i f fe ren t curves ind ica te (a) The energy los s ca l cu la t ed with formula (2 .4) without 6. (b) Includes the densi ty ef fec t <5. (c) Is the r e s t r i c t e d energy l o s s , with a maximum energy t r a n s f e r of .5 MeV, using formula

( 2 . 7 ) . The measurements indicated on curve (c) refer to the averaged value of the Landau energy d i s t r i b u t i o n , for a 980 um thick s i l i c o n d e t e c t o r , inc luding m u l t i p l i c i t y co r rec t ions ( s e c t . 2 . 3 . 1 ) .

(d) Is the most probable energy loss in 1 mm of S i , ca l cu la t ed with formula ( 2 . 1 3 ) . Measurements with pions and muons in a 980 pm and a 486 vim thick de tec to r are shown. The discuss ion i s given in 2 . 3 . 1 .

Formula (2.4) describes co r r ec t ly the macroscopic average energy l o s s , but the s t a t i s t i c a l nature of the ionizat ion process has to be taken in to account if one i s in te res ted in the case of a s ingle p a r t i c l e . Moreover, i t a lso may turn out t ha t the co l l i s i on cross sect ion (2.1) i s too crude an approximation; in other words, not a l l impact parameters b have equal p r o b a b i l i t y . This i s p a r t i c u l a r l y obvious in the case of p a r t i c l e channeling in a c r y s t a l . The electron dens i ty i s much lower in the open c r y s t a l d i r e c t i o n s , which r e s u l t s in a lower energy loss for

- 26 -

particles which channel along such directions [2.83. The small acceptance angle for channeling of high energy particles causes the fraction of channeling muons to be negligible in the case of muon flux measurement.

2.1.2 Delta electrons Delta-electrons or "delta-rays" are generated as part of the

electronic excitation process. For high energy particles the maximum transferable energy e is nearly as high as their total energy. However, the probability for such a transfer is low, as can be seen from max

the collision cross section (2.1) If £ + e m =„ then w(e) -• 0 for e ^ 1.

For spin 1/2 particles an additional term was introduced in formula (2.1), leading to the "Bhabha formula" (see e.g. Rossi, chapter 2 [2.9])

w(E, e)de 1 - s2 ~£-max

+ i (- ) E + Mc"

de (2.6)

In order to evaluate the fraction of the energy loss due to collisions involving an energy transfer above some value n, the integral

{ max e w(E,E)de

has to be calculated as fraction of the total ionization loss. Alternatively, one can determine how much energy is lost in transfers below rw This corresponds approximately to the energy deposited in the thin detector by a high energy particle, and will be considerably lower than the value calculated with (2.4). It is customary to use a modified formula, which excludes transfers above the energy n, and call this the "restricted energy loss" [2.10].

dE dx restr

Jin 2m ec zB n (1 B 2)n i 2 U - ;) 2m c 2

e (2.7)

Obviously, the energy loss of the particle itself is not physically restricted, but only its manifestation in the detector. In table 2.1 some results are listed for muons in Si. For 20 GeV muons and n = 10 MeV, which pertains to electrons which certainly will escape from a 1 mm thick silicon detector, a fraction of 21% is found. The effects of delta rays depend not only on the type of detector, but also on the surrounding material. For silicon detectors it is found that the energy deposit per unit of path length is a function of the total detector thickness (sect. 2.3) . Although energetic delta electrons are escaping, they may significantly contribute to the signal before they have left the detector. Therefore, the "restricted energy loss" formulation is not quite appropriate. This problem was treated in detail by Laulainen and Bichsel [2.11] for protons up to 50 MeV, by summing the energy loss of the electrons along their pathlength up to the detector boundary.

- 27 -TABLE 2.1

Fraction of energy loss via large energy transfers for muons in silicon

Muon energy

dE dx max n dE

dx restr % of dE .„, — with dx

GeV keV . mm" ' GeV MeV keV . mm" ' transfers > n 2 444 .339 .1 316 29 X

.5 345 11 1.0 357 20 10.0 398 10

100. 435 2 20 521 13.0 .1 328 37 %

.5 357 31 1.0 370 29 10.0 411 21

100. 452 13 1000. 492 5

200 570 190. .1 330 42 % .5 358 37 1.0 371 35 10.0 412 28

100. 453 20 1000. 494 13

2000 611 1989. .1 330 46 1 .5 358 41

1.0 371 39 1000. 494 19

2.1.3 Plasmon generation

From characteristic energy loss experiments in Electron Loss Spectroscopy (ELS) [2.12], it is known that the four valence electrons of silicon, which are relatively loosely bound, constitute an electron plasma, with density n = 2 x 1 0 2 3 c m " 3 . The energy of the corresponding volume plasmon (there also exist surface plasmons), with plasma frequency LO , is given by P

Hu)_ W m 16.7 eV (2.8)

This form is strictly valid only for a free plasma. The theoretical value shows a remarkable agreement with the energy loss observed on low energy electrons, passing through a 100 nm thick Si film (16.6 eV) [2.13].

Although it was speculated, that the ionization of electron-hole pairs in semiconductor detectors proceeds exclusively via intermediate plasmons [2.14], which led Klein [2.15] to propose an alternative approach for the calculation of the Fano factor (see 1.1), this only covers part of the reality. In fact, if the momentum transfer exceeds the value Xk , where the critical wave vector k corresponds to the screening length in the Thomas-Fermi approximation (about .1 nm) , the interaction should be treated as a single electron excitation of the now unscreened electron.

- 28 -

In addition, it should be remarked that high energy particles interact also with the bound electrons from the inner shells, which do not participate in the plasma oscillations. But the plasmon generation is certainly the intermediate process, responsible for the ionization in distant collisions, for impact parameters > .1 nm.

2.1.4 Radiative energy loss If the particle velocity is greater than the phase velocity of light

in the medium, there is energy loss via Cerenkov radiation, which can be observed also in ELS, together with the plasmon generation [2.16]. The Cerenkov radiation gives only a small contribution to the electronic excitation, and is essentially included in the energy loss formula (2.4). This in contrast with the radiative energy losses caused by the interaction of a relativistic particle with the nuclear field (bremsstrahlung). The cross section for this process increases linearly with energy. Because the particle mass enters inversely in these interactions, the radiative losses are generally only noticeable for fast electrons. But above 100 GeV they also become important for muons.

The critical energy for electrons in a material can be defined as the energy at which the electronic collision loss is equal to the radiation loss [2.17]. A different definition (chapter 15.4 in Jackson [2.2]) relates the critical energy to the lower limit of the electron energy E for complete screening above which the radiation cross section becomes nearly constant as a function of the emitted photon frequency

Z l / 3 m (2.9a)

In this region the radiative loss is proportional to the energy of the particle. For electrons (M = m ) in Si the latter critical energy is 40.7 MeV and for muons (M = 105.6 MeV/c2) it is 1700 GeV. When using the first definition mentioned above, these "critical energies" are 48 MeV respectively ^ 500 GeV.

For radiative energy loss one length Xo

can introduce [2.2] the radiation

X5 i^NZ2 Si 3 fie

2 2

Mc 2

In 192 M 1 /3 Z m

(2.9b)

A complete list for all elements has been given by Tsai [2.34], who uses a more precise formula. For electrons in Si the radiation length is 21.8 gem"2 (9.36 cm). In analogy, one can define the muon radiation length for muons in the TeV region, which has a value of 2.5 x 10 5 g cm"2 (320 m) in iron.

- 29 -

The probability of radiative energy loss in a thin (< 1 mm) silicon detector is even for electrons rather small. Radiative energy loss in the surroundings of a detector may be important, because the photons or shower particles could reach the detector. In the next chapter the various radiative processes and their effects will be discussed for muons in iron

2.2 Fluctuations of energy loss in a thin silicon layer The mean energy loss formula (2.4) gives the infinitesimal value

dE -s— for an idealized, infinite absorber. The stochastic nature of the energy loss process causes fluctuations in the actual energy loss of individual particles integrated over the same thickness. For a rather thick layer the energy loss probability distribution is Gaussian around the mean value, but for a thin layer the straggling of the values around the average is non-symmetric, with a long tail of rare events towards the high energy side. The most probable energy transfer, represented by the mode of the probability distribution is therefore lower than the mean value. Using the previous definitions of k (formula (2.2)) and E

max (2.3), a layer will be called "thin" in the following discussion if for a thickness of s g cm"2

(k/c m a x) s « 1 (2.10) For a 1 GeV muon this condition is achieved (< .01) for s < 13.1 g cm"2, i.e. for a thickness x < 56 mm.

The theory for the energy loss in the case of a thin detector was first developed by Landau [2.18], hence one speaks generally of the Landau energy distribution. A generalization of the Landau theory for thicker layers and lower energies was worked out by Vavilov [2.19] . A comparison of further improvements in the calculations was made by Bichsel and Saxon [2.3] . The practical situation for silicon detectors in high energy experiments is well described by the thin layer approximation of Landau.

2.2.1 The Landau distribution The Landau probability distribution function f(s, A)dA describes the

probability that a particle in traversing a thickness s undergoes an energy loss between A and A + dA. This function is the solution of the transport equation for the particle, taking into account the collision cross section w(e) (2.1). Landau introduces the parameters X and Ç = k.s

A A. = —

2m v 2 C in Ë + i

I2(l - B2) -* - lnJL emax (2.11)

- 30 -

4200 l 1 1 1

Spectrum 3

-

3360 280 GeV -

-' 1 r Peak CH 290 ^ 2 5 2 0 - , -3 O / h

1680 • j \ t

—

840

/ /

\ s

1 i " " " ' • — ' • ' i

250 450 650 Channel number

850 1050

Fig. 2.2 A measured pulse-height spectrum of 280 GeV muons is compared with a calculated Landau distribution. Both the position and the height of the calculated distribution are adapted to the measurements, but no correction is made for noise in the measurement. The hump around channel 550 is ascribed to accompanying delta electrons .

400

E =L

• v . > to CO o

_1

en o>

300

200

-

1 1 1 1 1

0

1

-

o o +

- o -

/ + + + this experiment

o calculation, Paul _ x

X

1 1 1

x electrons

1 1 1

0 Silicon Thickness (mm)

Fig. 2.3 Mode of the energy deposition as a function of silicon detector thickness. The calculated value for 200 GeV muons is indicated by the curve. Calculated values by Paul [2.23] are also shown (0). The mean energy loss at the minimum ionization is 387 eV/um. The calculated restricted energy loss with n = .5 MeV is 358 eV/um. The measured points were obtained with pions or muons for various energies between 60 and 340 GeV, except the points (x) at .1 mm, which were obtained with 8.2 GeV electrons (upper point) and 51.1 GeV electrons (lower point) by Ogle et al. [2.33].

- 31 -

dE C is Euler's constant, 0.577, A is the average -^ , k and the other symbols

were explained with eqs (2.1)-(2.4). With these parameters a Laplace

transformation to a variable u can be used to write

f (s,A) = | *(X> = | 2^ T T ~ e U l n u + X udu (2.12) ^ ^ a-i<=°

The integral in (2.12) was tabulated by Borsch-Supan [2.20] and the

maximum value 4>(X) = .18066 occurs for X = -.225, which corresponds

in the probability distribution to the mode or most probable value for the

energy loss in the thickness s. In fig. 2.2 a Landau energy loss

distribution is shown, which was calculated for 280 GeV/c muons passing

through a 400 um thick Si detector, using the computer routine DISLAN,

installed in the CERN computer Library [2.21]. It was pointed out by

Maccabee and Papworth [2.22] that using the Bôrsch-Supan evaluation the

mode A _, of the energy loss distribution has the value mp 3

A = C mp *

2m c 26 2C I in - B 2 + .198 - 6 (2.13) . I 2d - 62) J

which is somewhat different from Landau's value, and includes also the

density effect correction 6. The most probable energy loss decreases

faster than linearly with the absorber thickness, and in fig. 2.3 the

calculated values for silicon are compared with measurements. Calculated

values, which were published by Paul [2.23] are slightly too high, because

he used Landau's value of X = -.05 instead of X = -.225 for the

maximum. The variation with momentum of the most probable energy loss in

1 mm of Si is also shown in fig. 2.1, curve (d).

Recently, Kblbig and Schorr [2.35] presented asymptotic expansions

for the integral ()>(X} as X + ± œ. They mention that <J>(X) has its maximum at

X = -0.222783 with <t>(X) = 0.180656.

A discussion of the validity of the Landau and Vavilov approximations

was qiven by Jarmie et al. [2.24]. Apart from the condition ks << e 3 max

(2.10) also k s >> electron binding energy (2.14)

is required. This condition breaks down in the limit of very thin absorbers

and at low momentum. For a 1 GeV muon in Si, k = .0773 MeV cm 2, and this

condition becomes important for a thickness below "v- 30 pm (1 mg cm"2) of

Si. Jarmie et al. give a number of graphs which delineate the limits of

the validity regions, where the accuracy is better than 25%. The

influence of shell effects in the case of very thin foils was described by

Knop et al. [2.25]. The energy loss shows then clear quantization, as in

the case of plasmon generation, which was not taken into account in the

theory by Landau, and also would lead to deviations from the predicted

behaviour at very low thickness.

- 32 -

2:2.2 Escaping energy Landau considered the distribution of energy losses of the particles

traversing a thin layer, but not the energy deposited within this layer, and some fraction of the lost energy may be escaping from the layer, in the form of delta-rays or photons. This could in particular occur for events in the tail, which result from a large energy transfer. Badhwar [2.26] modifies the collision cross section w(e) by introducing a limiting energy n» in analogy to the derivation of the restricted energy loss, and then proceeds to find the probability distribution, which tends to be more Gaussian and has less events in the high energy tail. The mean value of the energy deposited is much reduced by the phenomenon of escaping electrons, as is clear from the curve (c) in fig. 2.1, but the most probable value only slightly. As was mentioned already in the context of delta-rays (2.1.2), the model used to obtain the restricted energy loss is not quite appropriate for the case of silicon detectors. In fact, it was developed to describe the energy loss measurements by grain counting in nuclear emulsions, and only grains belonging to the primary track normally are included. Electrons beyond the cut-off energy n (in the restricted energy loss formula (2.7)) always travel some distance through the silicon and deposit energy, before escaping. An extension of the work by Laulainen and Bichsel [2.11] to higher energy might be of interest, but it will be shown in sect. (2.3) that by adjusting the r\ for the restricted energy loss already a quite satisfactory agreement between data and calculation is obtained. For example, Esbensen et al. [2.8] argue that n = 500 keV for the case of .9 mm Si is accurate to 30% and their calculated values for the restricted mean energy loss then correspond exactly to their measurements.

Photons from bremsstrahlung energy transfers also influence the energy loss distribution for high energy electrons in thin absorbers. Matthews et al., [2.27] fold these losses with the distribution for ionization losses, considering the radiation length of the absorber, and thus taking into account the escaping photons.

2.3 Measurements of energy deposition in silicon detectors 2 ."3.1 Measurements at CERN with muons, pions and protons The energy deposition of muons, pions and protons of 60-300 GeV in

silicon detectors of different thicknesses was measured in the H2 hadron beam and the M2 muon beam in the North Experimental Area of the CERN SPS. Two detectors were exposed perpendicular to the beam direction, in air, mounted in one box, as shown already in fig. 1.8, adjacent to their preamplifiers. The detectors were fully depleted by applying a bias voltage in excess of what is needed for total depletion, so that the thickness of the silicon wafer can be used to calculate the specific

- 33 -

energy deposition. All the wafers were mechanically measured after the exposure, with 1 ym precision, and were found to have thickness uniformity better than 3 um or 1%.

The signals from the voltage sensitive preamplifiers (described in 1.4.1) arrived via a 15 m long coaxial cable in either a timing filter amplifier TFA (ORTEC 474) with a 200 ns time constant or a spectroscopy main amplifier MA (CANBERRA 1413 or NUCLEAR ENTERPRISE 4657). The unipolar analog signal from the spectroscopy amplifier was analyzed in a multichannel pulse-height analyzer (SILENA BS 27/N) which could be gated with a logic signal from the coincidence of discriminators on both the bipolar signal from the same spectroscopy amplifier MA and the signal from the timing filter amplifier TFA. The coincidence signal from the TFA is generated by the smaller of the two detectors, mounted in the same box, as is shown in fig. 2.4. In this way one can exclude the signals generated in the edge region of the main detector, that may suffer from incomplete charge collection, and therefore constitute a low energy tail in the pulse height distribution.

TINY DETECTOR TELESCOPE /

' Room for ( hybrid) • preamplifier

• ' H i • • • - - < • ' . - • . . • ' • • . • ' • • - •••• ••• M - i a - » - " - • '

/ ' Room for ( hybrid) • preamplifier

1 1 / ' Room for ( hybrid) • preamplifier ———/ . ' Room for ( hybrid) • preamplifier ' Room for ( hybrid) • preamplifier MS 5 . A I centering rings

' Room for ( hybrid) • preamplifier

H M=> <-C— Pb plug . 1 PCB

» 1.6mm PCS

F i g . 2.4 Diagram of Che t e l e scop ic arrangement of two s i l i c o n d e t e c t o r s , each on a p r in t ed c i r c u i t board (PCB) which a l so contains the p reampl i f i e r . Epoxy and aluminium r ings provide a pe r fec t alignment of the smaller and the bigger d e t e c t o r . In between the de tec to r s i s shown a lead plug. which can be removed. The entrance and e x i t windows in the aluminium box are thinned to .2 mm.

In order to record severa l spectra a t the same time, a number of boxes were aligned in a s t ack . In the pion beam H2, the spect ra of de t ec to r s which have some mater ia l in f ront , show one or more peaks in addi t ion to the normal peak in the Landau d i s t r i b u t i o n . These add i t iona l peaks occur a t mult iple values of the most probable energy loss of the s ingle d i s t r i b u t i o n . As the counting r a t e was very low, 10 to 500 events per second, and the i n t e n s i t y of the addi t ional peaks increases for the downstream d e t e c t o r s , these peaks are supposed to be due to i n t e g r a l numbers of p a r t i c l e s , coming from in te rac t ions of beam p a r t i c l e s in the de tec to r s and box ma te r i a l , a l together a few mm of Si and Al.

- 34 -

The mode (most probable value) A for the energy deposition can easily be determined from the position of the main peak, but to calculate the average energy deposition from the data, an integration over all channels must be made. However, the excess contribution from events with a multiplicity higher than one has to be subtracted. On the other hand, events with a large energy deposit, exceeding the range of the multichannel analyzer, could not be registered, and their contribution to the average has to be estimated and added. It was noted that there are too many events in the high energy tail. In one spectrum, which had the most probable peak in channel 90, some counts were registered even beyond channel 900. Many of these high energy deposits may have been caused by interactions, either in the silicon or in the material around. Unfortunately, it was not possible, in the context of these measurements, to place the detectors in vacuum, nor could an interaction veto be provided. Pulse height distributions, which were obtained recently with silicon microstrip detectors, and where more stringent conditions on the selection of traversing particles could be imposed by using high precision wire chambers, have a much smaller proportion of high energy events in the tail. It could even be shown, that these high energy events often give rise to energy deposition in more than one strip, and therefore have a lateral extension beyond the strip width of 50 pm.

A pion spectrum, which in particular shows the effect of multiplicity peaks, is given in fig. 2.5(a). With muons the effect of higher energy deposition in downstream detectors is much lower. Here only electrons, escaping from upstream material may give rise to multiplicity effects. An energy spectrum, obtained with a 486 urn thick detector in the 280 GeV muon beam was already shown in fig. 2.2. The theoretical Landau distribution does not predict the experimentally observed width, as shown in fig. 2.2. In order to correct for the undesired multiplicity, an artificially widened distribution is applied to perform the multiplicity correction on the data. Folding in of noise is not sufficient; tentatively the widening was attributed to electron escape from the silicon volume [2.8], as it is more pronounced for thinner detectors. A good fit of the tail on the high energy side could be obtained by increasing the parameter X (2.11) while keeping A and Â fixed. Although this procedure is not justified theoretically, it means physically that one chooses a distribution which belongs to a smaller value of Ç, i.e. a smaller thickness than the real one. The drawn curve in fig. 2.5(a) is such an "improper" Landau distribution, in which \ is multiplied by a factor 1.25. When this fitted distribution is subtracted from the experimental data, the result is another Landau distribution (fig. 2.5(b)).

- 35 -

The multiplicity correction and the high tail correction generally are 10%-20% of the measured energy deposition and although they are subject to large errors, the net effect on the uncertainty of the resulting mean energy deposition is of the order of 5% only.

600-

480-

360-

240

3600 1 ! 1 J

0)

2880 _

c

U 2160 "o

| 1440 _ . 3 Z i \

780

n ^ * " ' i . ' wit.aWMcw

Fig- 2.5

(a) The measured Landau energy distribution shows multiplicity in the 486 um thick detector, with 120 GeV n~. An "improper" Landau curve is fitted with X multiplied by a factor 1.25.

(b) The difference between the smoothed original distribution and the "improper" curve has the typical form of a second Landau distribution. The peak (514) lies at a value slightly smaller than twice that of the original peak (261).

450 650 Chonnel number

1050

For each chain of detector, amplifier and multichannel analyzer the energy calibration was performed several times. The correspondence between channel number and energy was determined by recording the spectra of the radioactive sources 5 7 C o (122 keV), l 3 3 B a (80 keV, 300 keV, 356 keV) and 2 0 5 H g (279 keV) . The zero energy point and the energy per channel were calculated, by fitting a straight line to the mode values of the total absorption photon peaks of the spectra, as a function of their known energy. At the same time, the noise of detector and electronics can be determined from the peak width, and typically a 20 keV FWHM was found. Long term stability was not better than 2% to 6% for different chains, and seemed to be related to temperature changes which influence the input FET of the preamplifier.

- 36 -

The results of these measurements are shown in fig. 2.1 as a function of 8//1 - S 2. The absolute energy in MeV mm"1 or kev urn'1 is compared with the calculated values. The curve (d) is the mode of the energy loss distribution in 1 mm of Si, and agrees quite well with the data for the detector of 980 pm thick. The most probable energy deposit in the 486 pm thick detector is situated below the theoretical curve, as expected. In fig. 2.3 are the results plotted as a function of the detector thickness. Each point is the average of the values for the various beam energies above 10 GeV. The value for the 3.2 mm thick detector has a large uncertainty, because of a relatively thick insensitive back contact (this was a Si-(Li) detector). For the thin detectors, the values are systematically below the theoretical value, which may be explained as the difference between the theoretical energy loss and the experimental energy deposition. The difference is about 10%, which is somewhat more than the experimental error of 5%, estimated from the mentioned instability.

The mean energy deposition, after the corrections, is plotted also in fig. 2.1 and can be compared with the restricted energy loss curve (c) , which was calculated for energy transfers below .5 MeV. This value was suggested by Esbensen et al. [2.8], for a silicon thickness of .5 to 1 mm, and regardless of the microscopic phenomena, it leads to a prediction which is compatible with the present data, within the admittedly still large errors.

2.3.2 Comparison with other data for silicon The most recent data, and virtually the only available as yet for

hadrons above 1 GeV, are from Esbensen et al. [2.8], who measured at 2, 6 and 15 GeV/c, at the CERN PS. They publish absolute energy deposition values for one thickness of silicon only (.9 mm), but they differentiate between n , TT , K , K , p and p. Within the quoted ±5% overall uncertainty, no difference is noticeable between the particles, under the non-channelling conditions. For positive channelling particles, the energy deposition may be half as low as the "random" value, for negative channelling particles it may be 30% higher. Also, there are marked differences between the various particles in the case of channeling. Table 2.2 summarizes the "random incidence" data of Esbensen et al., and gives the highest and lowest values observed at each momentum. Our data obtained at 3.5, 60, 120 and 340 GeV/c are also shown in table 2.2, and both series agree well, although it is disappointing that the accuracy is not better than ± 8%.

For lower particle momenta a few more experimental studies are published. Julliot and Cantin [2.28] discuss the response of silicon detectors of various thicknesses between .5 and 5 mm for pions of 470 MeV. Maccabee et al. [2.29] show several results, a.o. for 370 MeV

- 37 -

pions and 730 MeV protons but only approximate values can be read from their graphs. Finally, data for electrons, pions and protons were published by Aitken et al. [2.30]. Only the electron data and one of the pion measurements (200 MeV) approach the region of minimum ionization.

TABLE 2.2

Comparison of energy deposition data in silicon in keV mm"'

For .9 mm Si 1 For -v. .5 mm Si Momentum Particle Energy de

Mode | Mean position Mode Mean

.20 it 330 (2.16 mm) [2.30]

.47 H 293 280 [2.28]

.73 P 370 340 [2.29] 2 it , k, p 315

288 360 340

' [2.8]

3.5 297 357 286 356 This expt. 6 295 346

15 279 295 285

334 353 343

[2.8]

60 307 365 288 364 301 356 283 357

120 302 355 281 368 This expt. 340 T 3 k , p 304 360 282 350 ! 280 u 303

In conclusion it can be said that the more recent measurements (Esbensen et al., Julliot et al. and this experiment) agree quite well on a most probable energy deposition of .29 MeV mm"1 for a 1 mm thick detector, and .28 MeV mm"1 for a . 5 mm detector. The value of the mean energy deposition is .35 MeV mm"1 and is roughly the same in all cases. Note that this value is \. 10% below the calculated mean energy loss for

-1 the minimum ionization in an infinite piece of silicon (.387 MeV mm ).

2.3.3 Measurements with electrons Because electrons are at the minimum of ionization at a relatively

low energy (" 1.2 MeV in silicon) many more data on electron energy loss are available. A number of papers discuss the true energy loss in thin metal foils, measured by analysis of the incident and transmitted electrons. A recent review of this type of measurements was given by Matthews et al. [2.27]. Original data were publishsed by Buskirk et al. [2.31] for Be, Sn and Gd, and they review their earlier data for Al, Cn, Pb. Another paper, by O'Brien et al. [2.32] presents data for 50 and 100 MeV electrons in C, Al and Cu, of .1 to 1 mm thick. In table 2.3 the values obtained for Al are reported, because these should be very similar to those for Si.

- 38 -

TABLE 2 . 3

Comparison of ene rgy l o s s and e n e r g y d e p o s i t i o n by e l e c t r o n s

Energy MeV

Absorbe r T h i c k n e s s

mm

Ener Mode

keV mm" '

gy l o s s Mode

keV g" ' cm2

R e f e r e n c e

Aluminium ene rgy l o s s

50 . 2 2 6 3 0 0 . 4 1111 50 .452 330 .7 1225

100 . 6 6 3 3 3 5 . 0 1239 [ 2 . 3 2 ] 50 . 997 3 3 4 . 7 1239

100 1.33 3 4 0 . 8 1266

5 3 . 6 2 . 7 0 3 8 1 . 1411 7 4 . 6 2 . 7 0 3 8 5 . 1425 9 6 . 9 2 . 7 0 4 3 3 . 1602 [ 2 . 3 1 ] 5 3 . 6 5 . 3 4 4 0 4 . 1499 7 4 . 6 5 .34 4 0 8 . 1513 9 6 . 9 5 .34 4 0 4 . 1499

S i l i c o n ene rgy d e p o s i t i o n

4 5 8 2 . 1 6 3 1 8 . 1365 [ 2 . 3 0 ] 3 3

. 5 1.15

284 . 3 0 8 . 7

1219 1325

[ 2 . 2 8 ]

1.5 . 1 0 1 246 . 1057 8 2 0 0 . . 1 0 1 254 . 1091 [ 2 . 3 3 ]

15300. . 1 0 1 239 . 1027 [ 2 . 3 3 ]

5 1 1 0 0 . . 1 0 1 2 4 1 . 1035

Energy deposit ion measurements in s i l i con were made by Aitken e t a l . [2.30] with e lec t rons between 150 and 800 MeV, and by J u l l i o t e t a l . [2.28] with 3 MeV e l e c t r o n s . Ogle e t a l . [2.33] used the e l ec t rons in the tagged photon beam at Fermilab to measure the energy deposi t ion in a 101 ym thick s i l i con detector a t energies from 8.2 to 51.1 GeV. Their data are given in table 2.3 and two points are shown also in f i g . 2 . 3 . They suggest tha t the lower values a t high energy can be explained by a r e l a t i v i s t i c e f f ec t , the thickness of the de tec tor being smaller than the maximum coherence length for the energy t r a n s f e r s .

- 39 -

MUON ENERGY LOSS AND FLUX MEASUREMENT IN A SHIELD 3.1 Introduction and review of shielding calculations

Muons do not participate in strong interactions, and unlike electrons, loose relatively little energy by radiative processes, because of their much bigger mass. Therefore, high energy muons are, apart from neutrinos, the most penetrating of the known particles. Considerable amounts of shielding are needed to stop the muons in present and future high intensity beams around accelerators. Extensive studies are conducted for the design of such muon shields, because of the cost and the space involved, the environmental safety and the requirements of the experiments themselves.

Muon shielding problems were discussed, in relation to the electron beams of the Stanford Linear Accelerator (SLAC), by Nelson, Kase and Svensson [3.1]. With scintillators, nuclear emulsions and dosimeters they measured the muon flux and the absorbed dose in the shield behind a dump target, on which an electron beam of 14 GeV or 18 GeV was dumped. They also calculated the flux at the measurement locations and found agreement to better than 10% at small angles. Beyond an angle of 40 mrad the measurements give considerably higher values than the calculations.

Calculations for the shielding of 500 GeV muons in the FNAL neutrino beam were published by Alsmiller et al. [3.2], Theriot [3.3] and Roe [3.4]. Roe calculates a shielding thickness which should have a leakage of not more than 1 in 1 0 1 2 . Ladu et al., [3.5] present results of Monte-Carlo calculations for 100, 200 and 500 GeV muons in earth, in the form of graphs of survival probability as function of shielding thickness. Hartmann and Leutz [3.6] proposed a magnetized iron shield with a central current conductor, such that the muons of one sign are spiralling towards the centre, and the wrong sign muons are swept out of the shield. In theory, the amount of iron shielding needed can thus be reduced, but the wrong-sign muons, which originally present only a few %, will dominate at the end of the shield, especially because those of highest energy are only little deflected.

For the shielding of TeV muons the treatment of the radiative processes becomes much more critical, and also the choice of shielding material becomes more important. Recently, Mokhov et al. [3.7] presented some considerations for the design of a 3 TeV muon shield. For the calculation of the muon flux in the shield of the CERN neutrino beams, several Monte-Carlo programs are available, of which NUBEAM by Visser is the best documented [3.8] .

Calculations for the shielding behind the CERN muon beam were made by G. Stevenson with the computer program TOMCAT (see also sect. 3.3.2).

- 40 -

In sect. 2 of this chapter a review will be given of the theoretical description of the energy loss of high energy muons. An important feature of this energy loss process at high energy is the generation of secondary electrons and photons ("spray")» In a global shielding calculation the presence of these cascade products, mostly of low energy, can be ignored. But a measurement of muon flux in a shield is strongly influenced by the effect of these cascade products on the measuring device. Therefore, in sect. 3.2.4 an attempt will be made to calculate the density of secondary electrons inside the absorber.

In sects 3 and 4 several measurements will be discussed, which provide information on the composition of the ionizing radiation in a shield. The importance of secondary electrons is put into evidence, via tests using the absorber box. A comparison between the measurement results and the previously mentioned calculations shows at least qualitative agreement. Also measurements in an electron beam, and coincidence counting are discussed, in view of the additional information provided on the behaviour of electrons in a shield.

The study of the composition of the flux is of course closely related to the absolute calibration of the muon detectors. In the latter case, one is interested in the muon component only, but it is not trivial to distinguish in the response of a detecting system between muons and the others. A further discussion of the absolute calibration will be given in chapter 9.

3.2 Energy loss of high energy muons The energy loss of charged particles by excitation and ionization has

been discussed in the previous chapter. The radiative processes require some more study, since these are responsible for a significant energy loss of muons above 100 GeV. The stochastic occurrence of large energy transfers in delta-electron production ("knock-on electrons") and in radiative interactions, causes an important straggling in the range of high energy muons. Since long this subject has been of interest in cosmic ray studies, and an extensive discussion is presented by Adair and Kasha [3.9]. Muons which do not suffer any "catastrophic" interaction, but only loose energy by the "continuous" ionization, will travel much farther in the shield. Range measurements in the SPS neutrino beam shielding for various muon energies will be discussed in chapter 11.

- 41 -

3.2.1 Bremsstrahlung, pair production and nuclear interaction The differential probability for a rauon with energy E to transfer an

energy £ via the bremsstrahlung process is 4>h(E, v) , with v = e/E. The formula given by Petrukhin and Shestakov [3.10] is used:

<|>b(E, v)dv = D . C(E, v) . F(v) dv (3.1)

A- N / e \2 Z(Z + 1) q" 137 Al m I , 2,2 1 u/ (ni cM e

C (E,v) = in 2 1 8 9 \ z-2/3 3 m

e 189/2.718 "Y __v z" 1/ 3

2 * m E * 1 - v * e F(v) = (i - A v + v2)/v

The energy loss by radiation is then found by numerical integration v

b, (E)E = E j m a X v<ME,v)dv (3.2) D o

The value of v indicates the value for zero cross section max

v = 1 - 3^7718 ^ zl/3 ( 3 . 3 , max 4 E

For e +e~ pair production an analytical expression has been worked out by Kokoulin and Petrukhin [3.11 and 3.12] which is valid above 5 Gev and has a maximum deviation of 2.3% to the rigorous calculations by Kel'ner and Kotov [3.13]. In a comprehensive review article, Wright [3.14] discusses the various theoretical treatments and the shortcomings of approximate formulae. Especially the formula of Kobayakawa [3.15], which is often used, a.o. by Serre in the tabulation of muon energy loss [3.16] yields a result which is up to 40% lower than the rigorous calculation. The analytical expression by Kokoulin and Petrukhin is regarded as the best approximation available. It has been ascertained by Fortney et al. [3.17] that the predictions of these quantum electrodynamic calculations correspond to the experimentally observed cross section and momentum partition, in the case of 200 GeV pions in a neon-filled bubble chamber. A more recent measurement of pair production by cosmic ray muons was published by Chaudhuri [3.32], and it confirms the correctness of the calculations of Kokoulin and Petrukhin.

- 42 -

According to Kokoulin and Petrukhin the differential probability 3p(E,v,p) is then written as follows

,2 2 N 1 Z(Z + 1) q" 1 - v

>p(E,v,P)dvdp = 3 - ^ — 7 ,„ 2, 2

4 - 7 -137' (m ec<)

rae ?o + —

e m 2

u

dvdp (3.4)

$ e = | [(2 + p 2 ) ( 1 + M + Ç ( 3 + p 2)]Jln [1 + j) + 1

P

+ ^ — - (3 + p 2 ;

o y = <j[(i + p 2 ) ( i + | s i ) - -| (i + 2 i i ) ( i - p 2 ) ] m i l + ç) +

+ U1 - P -JJ. + (1 + 2i)(l - P

2) U 1 + Ç j y

,-V3. 189 z ~ ' J y u + T n i + Yj L = In

2m / 2 . 7 1 8 189 Z ' (1 + Ç ) ( 1 + Y J T7T

1 + E v ( l - p 2 )

_M 7 - l / 3

L, = J6n

189 -*• Z m

e. V (1 + - ) ( ! + Y u )

T73-

1 + 2m / 2 . 7 1 8 189 Z ' ( 1 + Ç) (1 + Y )

e ^ u_ E v ( l - p 2 }

Y e = 5 - p 2 + 4fc (1 + p 2 )

2 ( 1 + 3Ji.)«,n(3 + j) - p 2 - 2 £ ( 2 - p 2 )

4 + p* • + 3fc ( 1 + p 2 )

(1 + p 2 ) ( | + 2Jt)S,n(3 + Ç) + 1 - | p 2

I = v 7 ( 2 - 2v)

Again v = e/E and p = ( e + - e_ ) / e i s t he asymmetry c o e f f i c i e n t of the

ene rgy d i s t r i b u t i o n of t h e e l e c t r o n p a i r . The l e a s t momentum t r a n s f e r r e d

to t h e nuc l eus i s

2m 2 (1 + U

E v ( l - p ' ) t-r-, where Ç m v 2m L e . 1 - v

The other variables have the usual meaning, as defined in chapter 2

The limits of p are

0 < Ipl < ft max f 4m e_

E v 1 6m2

IL E z (1 - v) J

(3.5)

As an example, the total cross section was calculated by numerical

integration over v and; p, for muons of 200 GeV. In fig. 3.1 the

differential cross sections for ionization (i) , bremsstrahlung (b) and

pair production (p) *açe clotted as a function of the energy transfer e.

For transfers above a few hundred Mev the pair production process

- 43 -

dominates, until -v 10 GeV, where the bremsstrahlung takes over. In fig. 3.2 this is made even more clear, by plotting the percentual contributions for each value of z, again for incident 200 GeV muons. For the ionization cross section the Bhabha formula (2.6) as given by Rossi [2.9] has been used.

Fig- 3.1 probability per gem" per GeV of Che muon in iron for 200 GeV muons, as function of Che energy transfer e [GeV], for the ionization (curve (i), formula (2.6)), Che bremsstrahlung (b), formula (3.1), and the eleccron pair production (p). The curve (s) is the sum of these three conCribuCions.

>

o

I n t e r a c t i o n —, CD < CD O CE CL

10" r

10

10"

-3 _

,-5 = -10

I0 ' D r

,-7 =-10

I0" 8 r

10

10"

-9 _

! i i - r r ru i • T 1 M l l | I I i 111MI i i i T ! M tj 1 i i i i r rd

r n

k ( P ) ^ 1

r (

V M s ) =

r ( b ) \

1

r

r

1

r i . . 1 i i I i i l l i i • , M , 1

i 1 1 , i M

10" 10"^ I 0 _ l I

ENERGY TRANSFER (GeV)

10 10'

F i g . 3.2 Percentual con t r ibu t ion of the ionizacion ( i ) , bremsscrahlung (b) and p a i r produccion (p) Co Che CoCal inCeracCion probabiliCy for energy c rans fe r e (GeV), for 200 GeV muons in i r o n .

ENERGV TRANSFER i GeV

- 44 -

Finally, the nuclear interaction of high energy muons can be described as a photoproduction interaction of a virtual photon of the electromagnetic field of the muon with a nucléon, or with the nucleus as a whole. The measured cross sections of real photons can then be used, to approximate the muon cross section for this process. For example, Constandt et al. [3.18] propose for the energy loss parameter b (E)

bn(E) .61 + 1.86 x 10" 3 in — 10 6 [cm2 g'1] (3.6)

The total energy loss dE in a thickness dx of matter is now described by

dE dx

dE dx + b, (E)E + b (E)E + b (E)E (3.7)

ion where b b, b and b n are given by integration of the cross section over all energies, similar to formula (3.2). The mean energy loss in iron as a function of the 6//1 - B 2 of the muon is plotted in fig. 3.3. The contributions of the various processes are shown as part of the total loss. in table 3.1 the percentual losses are tabulated for a few energies, for aluminium, iron and lead absorbers. A material with high atomic number and high density has an increased radiation yield, but the lower ionization stopping power annihilates this advantage, except for Tev energies.

TABLE 3.1

Mean energy loss of a muon in 1 cm of several metals

Energy GeV Absorber Total

MeV/cm Ionization

% Bremsstr.

% Pair

X Nuclear

% 2. Al 4.96 100.

Fe Pb

13.39 14.88

100. 100.

20. Al 5.92 98.1 .9 1.0 -Fe 16.36 96.5 1.6 1.9 -Pb 19.87 89.7 4.4 5.9 -

100. Al 7.1 87.4 4.8 6.7 1.1 Fe 21.4 79.0 8.2 11.7 1.1 Pb 34.5 55.6 17.5 26. .9

200. Al 8.3 76.2 8.9 12.9 2.0 Fe 27.3 63.5 14.1 20.7 1.7 Pb 53.6 36.7 24.9 37.3 1.1

500. Al 12.0 54.5 17.1 28.0 3.4 Fe 45.3 39.4 23.5 34.6 2.5 Pb 113.5 17.9 32.4 48.4 1.3

800. Al 15.7 42.3 21.7 31.9 4.1 Fe 63.6 28.5 27.8 40.8 2.9 Pb 175. 11.8 35. 51.9 1.3

- 45 -

18

i 1 1 M III] I i i i i n i | i 1 1 1 1 1111 1 - i M i i n j i T i l l III] 1 1 1 1 1 m

PJ E u

16

14 -

1 1 —

en • / /

( MeV

12 h ~

LOS

S 10 - /(b) -

ENER

GY 8

6

4

2

-

k / A

To -r\ i 1 1 M i l l ! 1 i > l l m l i ' 1 1 i I i n u l i i i i 11 m

To - 1 i io i o 2 io 3 i o 4 io 5

/8 -v / i - / 3 2 OF MUON

F i g . 3.3 Mean energy l o s s of muons in iron [MeV g" ' cm 2 ] . The con t r ibu t ions of i on i za t i on ( i ) , nuclear i n t e r a c t i o n (n) , bremsstrahlung (b) and pair production (p) are cumulated, so that the upper curve (p) ind ica tes the t o t a l mean energy l o s s .

3.2.2 Production of secondary e lec t rons and photons Via the energy loss processes the muons produce e l e c t r o n s , posi t rons

and photons of high energy, which i n i t i a t e electromagnetic cascades in the absorber ma te r i a l . Using the in te rac t ion p r o b a b i l i t i e s , described above, the number of secondaries i n i t i a l l y produced in 1 cm of shielding was ca l cu la t ed for several muon energ ies . Resul ts are given in table 3.2 for 10' incident muons per cm , which is representative for the actual number of muons in the experimental situation in the SPS muon flux measurement. In the columns "e" the sum of "knock-on" electrons and electron-positron pair production events is given, not taking into account that in fact two particles are produced in the latter case, but with an individual energy which may be less than the indicated minimum energy

- 46 -

transfer. The total number of interactions above 10 MeV increases slowly with energy for aluminium and somewhat faster for iron and lead. But interactions with large energy transfers (> 1 Gev) increase four to five-fold for iron and lead going from 20 GeV to 200 GeV muons.

TABLE 3.2

Number of secondary electrons and photons, produced by 10 6 muons in 1 cm of absorber

Muon energy M inimum energy transfer in the interaction > 10 MeV > 100 MeV > 1 GeV > 10 G eV > 100 GeV

GeV e Y e Y e y e Y e Y 1 cm of Al

20 20218 39 2021 24 149 10 2 2 - -100 21500 53 2780 38 266 23 15 9 - -2 00 21950 59 3390 44 417 29 22 14 0 2 500 22080 66 4450 51 815 36 52 20 2 7 800 22150 67 5110 55 1127 40 91 24 2 10

1 cm of Fe

20 58800 199 6050 121 427 51 5 6 - -100 68670 Ilk 9890 194 960 117 43 44 - -200 69140 301 13000 224 1722 145 76 69 1 10 500 72900 337 18500 261 3770 183 227 105 4 34 800 74900 350 22000 279 5370 201 431 122 8 48

1 cm of Pb

20 81300 684 9300 420 571 177 7 18 - -100 107000 936 22600 668 ,2194 403 69 155 - -200 122000 1033 33340 768 4800 501 167 241 3 35 500 144000 1150 52500 892 11900 625 690 360 10 115 800 156000 1204 64450 952 17400 685 1395 419 24 165

The secondaries initially produced by the muons, generate more electrons and photons. The analytical description of this shower process is complicated, if not impossible (Rossi [2.9]). Therefore one generally has recourse to a Monte-Carlo simulation program to find the shower particle distributions as a function of depth in the absorber. Messel and Crawford [3.19] published the results which they obtained for lead, copper and air, for electrons and photons from 1 MeV to 10 GeV. A versatile Electron-Gamma-Shower program (EGS) was developed by Ford and Nelson [3.20] and can be used to simulate rather complicated situations with several absorbers. A different approach was used by Adler et al. [3.21] who published tables for 10 and 100 GeV electrons and photons, calculated by numerical integration of the cascade equations.

- 47 -

In 3.2.4 a very much simplified method will be used to estimate the effective number of electrons from the showers, initiated by the muon energy transfers in the iron muon shielding. It is based on measured or calculated average electron ranges and electron shower developments, for different electron energies.

3.2.3 Absorption of electrons and photons The transport of electrons and photons in materials and the charge

deposition distribution has been the subject of many experimental and theoretical studies. Tabata et al. [3.22] and [3.23] have measured the charge deposited by 4 to 24 MeV electrons and the range (fig. 3.4) in several absorbers, by means of a charge collector inserted between the absorber sheets. Very similar results were obtained by the author, using silicon detectors in "current" mode, where a thin detector measured the intensity of a burst of electrons from a 19 MeV betatron, installed near BEBC, and another detector measured the deposited charge behind some thickness of absorber (fig. 3.5). The charge deposited represents the number of surviving electrons. The relation between this number of electrons and the thickness of absorber is called a "transition curve". In this case no absolute values were obtained because the silicon detectors could not be calibrated. The reference measurement by the upstream detector has to be used with caution, because it is influenced by back-scattering of electrons from the following absorber. Depending on the energy and the atomic number of absorber, this backscattering can be several tenths of the incoming intensity [3.24]. One should remember that in an electromagnetic shower always some fraction of the electrons and photons move backward (see e.g. the Messel and Crawford tables [3.19], where this fraction is made explicit).

Silicon detectors, measuring the integrated charge inside an absorber, are sensitive to all ionizing electrons, down to the lowest energy. As the range of a 100 keV electron in iron is only ^ 10 urn, however, the contribution of electrons with energy below this value will be very small. The response of silicon detectors in counting mode will be illustrated in 3.3.1.

For the photon absorption in the silicon detector, on the contrary, the efficiency is high below ^ 100 keV and drops very rapidly for higher photon energies.

48

< ce

o rr t-o

10" 10 I0 2

ELECTRON ENERGY

10= I0 b

MeV ;

F i g . 3.4 The e l e c t r o n range in iron as function of the e lec t ron energy. Curve: ( a ) R e p r e s e n t s the t o t a l pathlength, as ca lcu la ted by Berger and Se l t ze r [ 2 . 1 7 ] .

I s based on Monte-Carlo simulation by Wit t ig [3.2 7] . Are measurements of Tabata e t a l . [ 3 . 2 3 ] .

(b) (c) (d) Is the distance for probability N e = 1 in the shower development, for cut-off

energy E £ 1 MeV and 10 MeV. (e) Is the distance of the shower maximum t m a x (see fig. 3.6) also for E c = 1 MeV and

10 MeV. (d)and (e) are based on Monte-Carlo results from Messel and Crawford for copper [3.19] and on numerical integration by Adler et al. [3.21] for iron. The point just below curve (c) is the result of the measurement made with silicon detectors, shown in fig. 3.5.

4.0 6.0 8 0

Thickness of absorber ( g cm_ï!

Fig. 3.5 Transition curves for 19 MeV electrons, measured with thin silicon detectors inserted between the absorber sheets. The mean range found for iron (54 gem"2 or .68 cm) is reported in fig. 3.4 and is in reasonable agreement with the other data.

- 49 -

3.2.4 Calculation of the electron density in an absorber In view of the complicated energy spectrum of the muon-generated

electrons and photons and therefore the long calculation time for Monte-Carlo simulation of the electromagnetic cascade down to the energy of < 1 MeV, an attempt is made here to obtain results in a simpler way.

For a given energy of incident muons, the mean value of the number of generated electrons and photons in each energy bin is determined for a certain absorber thickness, using the formulae discussed earlier. It is justified to use mean values since the number of muons ("v 10 6 cm"2) is large. All possible energy transfers are well represented in each beam pulse.

For electrons in a certain energy bin it is decided over which distance they can reach the "measurement plane", which can be a hypothetical plane inside the shield, or the transition from iron shield to air gap. This mean electron range is tabulated for electron energies from .1 MeV to 1000 GeV. In fig. 3.4 the electron range is shown as a function of energy. Up to -v. 30 MeV the experimentally determined projected range (mean range) is used [3.22,3.23]. Above % 500 MeV the shower tables from Messel and Crawford and from Adler et al. are used to find the distance over which the number of electrons is larger than one (curves (d) ) , and to find the distance of the shower maximum t , where

max the number of electrons in the cascade is highest (curves (e)). Between 30 MeV and 500 MeV an interpolation is made. The total electron pathlength as calculated by Berger and Seltzer [2.17] is also shown for comparison in fig. 3.4. Below ^ 300 MeV this length is up to four times longer than the mean projected range.

Above "v 100 MeV it does not really make sense to talk about the range of an electron, because of the cascade effects. In a second paper of Adler et al. [3.25] it was pointed out that the transition curves which describe the cascade development for various absorbers and at various incident energies, have the same longitudinal scaling if distances are expressed in units of the distance of shower maximum t • In fig. 3.6 the curves calculated by Adler et al. for 10 GeV and 100 GeV electrons in iron are plotted. Beyond ^ 3.5 times t the probable number of r -1 max r

electrons drops below one, and this distance could be regarded as the electron range.

In fig. 3.6 is indicated how the cascade is approximated by a simplified parametrization, in order to carry out the computation. If t is the distance of the point where an electron is created by the muon, to the "measurement plane", then the number of electrons N in this plane can be represented by

- 50

5 o

o ce

LU —I LLJ

U_

o or LU CD 2: ID

" ' m a x

F i g . 3.6 The shower cascade development for 10 GeV and 100 GeV e l e c t r o n s in iron [3 .21 ] , The d is tance t i s expressed in u n i t s t m a x ( t m a x = 8 r ad i a t i on lengths = 13.9 cm for 100 GeV and t m a x = 5.4 r ad . lengths = 9.4 cm for 10 GeV, both for 1 MeV cut-off e n e r g y ) . The broken l i n e s ind ica te the paramet r iza t ion used to c a l c u l a t e the number of e l e c t r o n s in the shower, which reach the measurement p lane .

N = (N - l ) - r -e max t

+ 1 0 < t < t max

N e

- N max

N e = e • b t + 3 . 5b

N e = 0

t < t < 1.25 t max - - max

1.25 t < t < 3 . 5 t max - max

t > 3.5 t max

(3 .8 )

where N i s the maximum number of e l e c t r o n s a t t for t h e i n c i d e n t max max

e l e c t r o n e n e r g y , b = {In N__ 1/2.25

The r e s u l t s of t h e computa t ion a r e shown in t a b l e 3 . 3 . The numbers of e l e c t r o n s a r e g i v e n for v a r i o u s muon e n e r g i e s , as f r a c t i o n per i n c i d e n t muon (column d ) . For compar i son , t h e r e s u l t s o b t a i n e d w i t h a Monte-Carlo

- 51 -

program by Bassompierre [3.26] are also given in column e. He quotes an uncertainty of 20 to 30%, mainly due to the small number statistics. For the present numbers one cannot claim a better accuracy, however, there is no influence of statistics in this case, as mean values are used. The approximations used in the calculations introduce the uncertainty on the result. Some experimental results will be discussed in the next section.

TABLE 3.3 Number of e l ec t rons >, 1 MeV accompanying a high energy muon in the

equi l ib r ium in an absorber of " i n f i n i t e " dimensions

(a) (b) (c) (d) (e) (f) (g)

Muon energy (GeV)

From ionization and pair production

From bremsstr.

Total electrons

Elee trons Bassompierre

[3.26]

Increment of energy loss

compared to loss of 2 GeV muons (MeV/rad.length)

Number of electrons (d) divided by the increment (f)

Iron absorber Rad. length

1.76 cm 2 .08 .0008 .08 - -20 .26 .017 .29 .13 ± .04 5.23 .056 100 .53 .16 .69 .28 ± .06 14.1 .049 200 .85 .35 1.19 .41 ± .06 24.5 .049 250 1.02 .45 1.46 - 29.6 .049 300 1.17 .54 1.71 .61 ± .11 34.8 .049 500 1.80 .85 2.64 .91 ± .31 56.1 .047 800 2.74 1.21 3.95 - 88.3 .045

Lead absorber Rad. length

.56 cm 2 .05 .0005 .05 -20 .11 .019 .13 2.79 .046 100 .29 .25 .54 11.0 .048 200 .56 .62 1.18 21.7 .054 250 .71 .79 1.50 27.2 .055 300 .86 .94 1.81 32.7 .055 500 1.52 1.45 2.98 55.2 .054 800 2.63 1.99 4.62 89.6 .052

Probably the most important deviat ion from the r ea l s i t ua t i on i s the approximation t h a t a l l muon energy t rans fe rs give r i s e to forward e lec t rons or photons. By taking into account the emission angle, the ef fect ive d is tance should decrease and r e su l t in a lower number of accompanying e l e c t r o n s . This e f f ec t should be more pronounced for the low energy t r a n s f e r s . However, a l so in the Monte-Carlo simulation of Bassompierre the angular d i s t r i b u t i o n s are neglected for the knock-on e l ec t rons , the pai r production and the bremsstrahlung.

By choosing a cut-off energy of 1 MeV a number of low energy e lec t rons i s d is regarded. The effect i s est imated by repeating some

- 52 -

calculations for a cut-off of 10 keV. In the case of a 2 GeV muon the number of electrons is then .12 instead of .08. Although the energy deposited in a detector by the numerous low energy electrons is an important contribution to the signal, they cannot be accounted for in the calculation because of the great uncertainty on their behaviour. Their behaviour is probably more determined by the geometry close to the detector than by the muon shielding. The effective range of a 1 MeV electron in iron is only .24 mm.

All energy transfers of the muon are treated in the same way, no difference is made between the shower development of initial photons, single electrons or electron pairs. Recent values for the cross section for these processes are used, which are somewhat higher than those employed by Bassompierre (+ 20% for pair production).

One may have a check on the internal consistency of this calculation by comparing its result to the evolution of the total energy loss of the muon as a function of energy (table 3.1). In the last two columns of table 3.3 is shown the increment of average energy loss in one radiation length (i.e the energy loss minus the energy loss at 2 GeV) and the ratio of the calculated number of electrons divided by the excess energy loss. The limit of 2 GeV has been chosen, because below that energy muons do not give rise to penetrating cascades. It appears that this ratio is around .049 for iron and around .053 for lead, largely independent of the muon energy. According to the global model of cascade development, the number of electrons of low energy should be independent of the primary energy transfer. Our approximation does not lead to a violation of this basic feature. Each electron ^ 1 MeV corresponds to a muon energy loss of ^ 20 MeV in a radiation length. Besides, an easy rule is found in this way to calculate the number of accompanying electrons at other energies.

The absolute values predicted by the model are two or three times as high as those of Bassompierre. The difference may only partly be explained by the difference in pair production cross sections. Obviously, the difference in results is largely due to the difference in the approximation method. As there seem to be no experimental data on this subject, some tests were designed to measure the number of accompanying electrons. These are discussed in the following sections.

3.3 Measurements to determine the influence of the secondary radiation The calculation of the accompanying electrons and photons in a high

energy muon flux in a homogeneous absorber still can be made rather precisely, at least in principle, but the response of a detector is already less foreseeable. The situation becomes much more complicated if the detector is not closely imbedded in the shielding, but mounted in a relatively large air gap, and surrounded by frames and other material.

- 53 -

A straightforward method to measure the true muon flux uses coincidence counting with a muon hodoscope. Several examples will be given in this section. The hodoscope should have at least two planes which are well separated so that electron-induced cascades do not produce coincidences. The electron content of the muon flux is then manifest in the signal heights, but not in the coincident count rate. This method does not give precise results if the count rate is too high because of dead-time errors, as will become clear from the tests described in 3.3.3.

A second method to measure the true muon flux and which can be used regardless of the time structure of the flux, consists in integrating all charge, generated in the detector and determining independently the number of muons responsible for the generated charge. Usually, this calibration is performed by counting muon tracks in nuclear emulsions, as will be described in detail in chapter 9. It is supposed that one is able to distinguish the muon tracks from the tracks of high energy cascade electrons. However, the calibration is time consuming, and limited to a flux below 10 6 cm" 2, above which it becomes impossible to follow separate tracks in the emulsion. Also, the emulsion calibration does not give direct information on the relative contributions from muons and secondaries in the detector signal.

In sect. 3.4 measurements will be described in which the integrated charge is modified by the insertion of pieces of absorber. In this way, information can be obtained on these relative contributions and especially how these relative contributions change as a function of muon energy, and of position in the shield

3.3.1 Cascades by high energy electrons in a tiny counter telescope When only a small space in the muon shield is available for

measurement, it may be impossible to place detectors sufficiently far apart to suppress coincidences due to electron or photon initiated cascades. And when the flux is high, so that one has to use small detectors to obtain a reasonable counting rate, the distance is also limited by the required acceptance. In this case, a lead converter of a few mm could be employed between the detectors and a cascade development identifies the incident particle as an electron. To study the feasibility of this approach, some measurements were made with the tiny telescope [1.22], described in sect. 2.3.1 (figs 1.8 and 2.4). It was used in the muon shielding behind M2 (sect. 3.3.2) and in the muon shielding of the neutrino beam (3.3.3). In this section measurements will be reported which were made in a test beam at SLAC [3.28], with 4 and 15 GeV electrons and pions. The insertion of 3 mm of lead has nearly no influence on the pion energy deposition spectrum in the downstream detector, but it produces multiplicity for the electrons for 4 GeV (fig. 3.7) .

- 54 -

T i 1 r

0.05

G.04 ^

C.03

0.02 —

O.Oi

0.00

C.025

r 0.020

0.015

0.010

0.005

0.000

EX305 BEAM = PI-P= 4.0 GEV/C PB= 3.0 MM TOTAL CTS.= 6877_J

' " ' i ' " fii/il tu y , . ' , ' ",'-''' = " ' ••"'w.'-.r-'tf'.t../.,...,,,!,.,.,,.. .,-j. .

50 00 150 200 25 CHANNEL N0.

i i i 1

EX306 3EAM=E-

1 ' L D= 4.0 GEV/C °B= 3.0 MM TOTAL CTS.= 22571

là '"

I I I I I l _

50 100 150 CHANNEL N0.

200 250

Experimental pulse height spectra in a silicon detector for IT and e f momentum (4 GeV/c) and the same converter thickness (3.0 mm of lead).

55 -

EXPERIMENT SIMULATION

C T ;!

w\ \ .

EX306 8EAM-E-P- 4.0 GEV/C PE- 3.C r*M T0TAL CTS.- 22571 ClACE * lfSOi

• h * . 530 750 1000

ENERGY CKEVl

; H

WW,

TH3C6 BEAM-E-P- 4.0 GEV/G PB- 3.0 MM T0TAL CTS.- 22525 G-CH.SM03THING

0 250 500 75C 1000 1250 1500 ENERGY (KEV)

EX307 8EAM-E-P- 4.0 GEV/C PB- 5.0 MM T0TAL CTS.- 18B55

250 5CG 750 1000- 1250 1500 ENERGY (KEV)

V ' \

TOO? BEAM-E-P- 4.0 GEV/C PB- 5.0 MM T0TAL CTS.- 19304 Û-CH.SM00THING

Hm ftwaste.

5CC 750 1000 1250 1500 ENERGY ;KEVI

L , . . , , , , , . 1 . , , , 1 . . , • I ' ' ' • r ' ' \ EX3I0 BEAM-E-

- P- 15.0 GEV/C _ PB- 5.0 MM ]

k 1 Î 0TAI CTS.- 43346:

\

- ! yv \ :

"./, ':.., . i . . . . i . . . 250 500 750 1000 1250 1500

ENERGY 1K£V)

, " TH310A BEAM-E-

0.0020 1 : 1 P- 15.0 GEV/C _ PB- 5.0 MM T0TAL CTS.- 41241

0.0015 - 1 I O-CH.SM00THING

0.0010

0.00G5

L i ! I '

I , , , , I , I, ,1 250 500 750 10O0 1250 1500

ENERGY [KEV]

y 1 ? ' 3 ' . § Comparison of experimental spectra ( l e f t ) with the corresponding s imulat ions ( r i g h t ) . For each couple the sca le s are chosen the same. The lowest energy peak r ep re sen t s s ingle e l e c t r o n s , the second peak two e l e c t r o n s , e t c . The p r o b a b i l i t y to de tec t three e lec t rons i s always higher than for two e l e c t r o n s because of pa i r product ion . For 3 mm of lead the agreement of experiment and s imulat ion i s qu i t e good, but for th icker lead plugs there are d i s c r epanc i e s , e spec i a l l y on the high energy side. ' This i s discussed in the text.

A simulation was made with EGS [3.20] to p red ic t the pulse height spectrum and the p a r t i c l e m u l t i p l i c i t y for var ious inc ident e l ec t ron energies and d i f f e r en t converter th icknesses . In f i g . 3.8 a few experimentally obtained spectra are compared with these s imula t ions . There i s fa i r agreement, except t ha t m u l t i p l i c i t y one occurs l e s s often experimentally than predic ted . Also there i s an excess in the ca l cu l a t i on of very high m u l t i p l i c i t i e s (above 1000 keV). The measured peaks are somewhat broader. In the simulation absorption of photons in the de tec tor was not taken in to account, which may explain some of the d i sc repanc ies .

- 56 -

In table 3.4 a summary is given of the experimental conditions and the classification of events according to signal height. For an absorber of 7 mm of lead about 80% of the high energy electrons cause a signal in excess of that for 2 minimum ionizing particles. On the other hand, 7 mm of lead in the telescope eliminates electrons below % 30 MeV. In fig. 3.9 the multiplicities obtained by simulation for energies from .1 to 10 GeV are shown as function of the converter thickness. Clearly, there is a region, between 30 MeV and 800 MeV, where this telescope is not effective for distinguishing electrons from muons. Muons or pions do not give rise to multiplicity greater than one, and can be represented by a horizontal line in fig. 3.9. However, if additionally one could observe the multiple scattering caused by the absorber, the device could become more powerful. With the introduction of a silicon microstrip detector as analyzing detector, this would become possible. At the same time, a higher intensity could be handled, when the elements of this microstrip detector are made smaller than ^ .1 mm 2.

TABLE 3.4 Summary of experimental cond i t ions in SLAC t e s t and the c l a s s i f i c a t i o n of events

according to the s ignal h e i g h t , for var ious energies and converter thicknesses

(a) (b) (c) (d) (e) (f)

Run Particle Momentum GeV/c

Lead converter thickness (mm)

Percentage of Events with

signal (*) > 2 x mode ;

Apparent average

multiplicity

Percentage of Events with

signal (*) > 2 x mode ;

EX302 Ti 4 0. 14. 1.3 EX305 n 4 3. 18. 1.5 EX304 TT 4 7. 23. 1.7 EX309 It 15 5. 20. 1.6

EX301 e 4 0. 15. 1.4 EX306 e 4 3. 58. 2.5 EX307 e 4 5. 77. 3.4 EX303 e 4 7. 84. 3.9 EX310 e 15 5. 88. 4.3

(*) mode: Most probable value of s i n g l e m u l t i p l i c i t y peak.

With t h i s t e s t i t i s shown, in a well defined geometry, tha t the s i l i con de tec to r s are s e n s i t i v e to the e l ec t rons and photons in an electromagnet ic cascade.

- 57 -

M/No

Fig. 3.9 Results of the EGS simulation of particle multiplicity N in the silicon detector as a function of the lead thickness At, for different energies of the incident electrons N n.

i 1 1 1 1 1 1 1 r

CHARGED PARTICLE MULTIPLICITY N/N„

3.3.2 Muon fluxes in the earth shield behind M2 In the North Experimental Area of the CERN SPS there exists the

possibility to measure the muon flux from the muon beam M2, which operates usually at an intensity of "v- 2 x 10' at a well defined momentum between 100 and 300 GeV/c. Two measurement pits are excavated at 292 m from the beginning of the earth shield, one on axis, the other 3.94 m from this axis. The vertical intensity profiles were measured with scintillators, with nuclear emulsions and with silicon detector telescopes, and the measurements are compared with the predictions of the muon transport code TOMCAT. A preliminary account of this work was given by Nelson et al. [3.29]. A profile measurement, obtained with a silicon telescope, is shown in fig. 3.10. Only those counts are accepted, which produce a coincidence in the two silicon detectors of the telescope (without lead converter in this case). By integration of the profile, a total number of passing particles of 3 x 10 7 is found, which has to be compared to an incoming flux of 2 x 10 7 muons of 180 GeV/c. After 292 m of soil (p = 2.0) the average muon energy is ^ 100 GeV and according to table 3.3 there is ^ .6 electron > 1 MeV per muon expected in iron and lead, and presumably the number in soil should not be much different. In spite of the rather big uncertainties both in measurement and calculation, the conclusion must be that the electron contribution is really important.

1 1 '"I 1 ' I " i

Beam center-4.55m

8 0 0 I

• 1 •

6 0 0 - i

• 4 0 0

• ^ 1

-•

200 •

• 1 » ? 1 1 1 1

0.5

0.4'

0.3.

0.2!

0 I 2 3 4 5 6 " Depth m pit

Fig. 3.10 Vertical profile measurement in the earth shield of the M2 muon beam, obtained with a coincidence measurement in a silicon telescope.

8m

- 58 -

To obtain more p r e c i s e information, an extended s i l i con detec tor t e l e scope , shown in f i g . 3 .11 , was used and the flux measured with the s i l i c o n de tec to rs i s compared to r e s u l t s of two s c i n t i l l a t i o n counters CI and C2 of dimension 5 x 10 cm 2, upstream respec t ive ly downstream of the s i l i c o n te lescope . In table 3.5 the r e s u l t s are shown for three measurements. The f i r s t and the l a s t with de tec tor 41 upstream, and the second one with the te lescope reversed, so tha t detector 62 is then facing the beam. The e l e c t r o n i c chain was already described in sec t . 2 . 3 . 1 .

F i g . 3.11 Extended s i l i c o n d e t e c t o r t e l e s cope, used for muon flux measurement in the s o i l s h i e l d . Two t iny t e l e s copes are separa ted by a 12 cm long lead absorber . The s c i n t i l l a t o r CI i s placed at the l e f t of t h i s te lescope (upstream) and the s c i n t i l l a t o r C2 at the r igh t .

TABLE 3.5

Extended counter te lescope in ea r t h sh ie ld

Detector Type Nominal area cm2

Evaluated area cm2

Normal position counts

Reversed position counts

Normal position counts

CI Scintillator 5 x 20 id 1000 1000 1000 C2 Scintillator 5 x 20 id 1018 1027 1008 Cl, C2 Coincidence 802 798 831 41 Silicon .26 .35 1270 1011 1173 42 Silicon .63 .77 1250 1136 1180 61 Silicon 2.0 id 1112 1160 999 62 Silicon .65 .79 1135 1195 1040 41, 42 Coincidence .35 1020 80% 929 92% 935 80% 61, 62 Coincidence .79 1045 92% 1095 92% 960 93% 41, 42, 61 Coincidence .35 940 74% ( 65 6%) 855 73%

The f i r s t d i f f i c u l t y i s the effect ive surface area of the s i l i c o n d e t e c t o r s . Using the nominal a rea , as speci f ied by the manufacturer, near ly twice as much flux i s found by the s i l i con as by the s c i n t i l l a t o r s . The numbers in table 3.5 are obtained by using the evalua t ion of the s e n s i t i v e area already out l ined in s ec t . 1.2.2, i . e . the

I ^ i' M

- 59 -

effective radius of the sensitive area consists of the nominal (visible) radius plus a distance equal to the depleted thickness. This should take into account the lateral extension of the detector depletion region. Although in this way good agreement is found between scintillator flux and silicon flux, it must be clear that the silicon data cannot be used for absolute measurement without more precise calibration. One may use however the relative values and the coincidence rates.

All data are normalized to 1000 counts in the upstream scintillator CI. The detectors in front of the lead indicate ^ 15% more flux than those behind. The coincidence rate between 41 and 42 is 80% of the counts in 41 in the normal position and 92% in the inversed position when both are behind the 12 cm lead block. It looks, as if at least 12% of the counts in 41 are due to low energy electrons, which are not separately recorded by the upstream scintillator CI. Detectors 61 and 62 have in both positions 92% coincidences. The coincidence rate for 41, 42 and 61 is 75% in the normal position. From this, it can be concluded that in the position of measurement, on axis, there are not more than 25% of the counts in 41 due to an electron, not accompanied by its parent muon. Hence, the electron contribution to the counting in detector 41 can be situated between 12% and 25%. In addition, the pulse height spectra of the silicon detector signals show 5-10% of events with multiplicity 2.

From these measurements it becomes clear that the surface area definition of silicon detectors has to be improved considerably to make them useful for flux counting of particles which cannot be stopped by a collimator. To obtain the electron contribution with high precision, more sophisticated apparatus is needed.

3.3.3 Coincidence counting in the neutrino shielding The measurements in the high energy electron beam and in the long

spill muon beams, described in the two previous sections, were meant to evaluate the possibility of using the tiny silicon telescope in the muon shielding of the SPS neutrino beam. In spite of the conclusion, that the telescope in this form will not yield a more precise muon flux measurement, it was used for some time in the Nl wide band neutrino beam (sect. 6.2.1) which has a spill time of •v 2 ms. The count rates and the coincidences of two detectors in the telescope (without absorber) were compared with the signals of charge-integrating detectors in the reference box, in the muon flux measurement pit 5, where the flux is relatively low. The intensity was varied by changing the position of the box, lateral to the beam. In fig. 3.12 are plotted the relations between the count rates for the small ( 6 mm 2) and the big ( 35 mm 2) detector on the one hand and the integrating detectors on the other hand.

- 60 16000

14000

12000

o 10000 t -o UJ

C3 8000 -

O 6000

4000

2000 -

8000

7000

1000 2000 3000 4000 COUNTS IN SMALL DETECTOR

Fig. 3.12 Relations between several counting detectors and integrating detectors in gap 5. Ta) Counts in big counter vs. counts in small counter. Each point represents the average

over 5 or 10 SPS pulses. (b) True count rate in big counter, calculated for 90 ns dead time correction. (c) Integrated charge in reference detector 4 (scale on right axis) vs. counts in small

counter. The arrow gives the magnitude of the calculated counting loss. (d) Integrated charge in reference detector 4 (scale on right axis) vs. charge in

reference detector 343 (scale on x-axis, the unit is pC instead of count).

Curve (a) represents the counts in the big detector corresponding to those in the small detector. Beyond ^ 3500 counts per pulse, the big detector has a considerable loss, observed count rate n, by

The true count rate N. is related to the

n, Nb = T^rTT nb(l + nbt) (3.9)

- 61 -

where T represents an effective dead time. It is supposed that the counter is of the non-paralyzable type [3.30]. Then a dead time T - 90 ns is found for the big detector (curve (b)) which is appreciably longer than the signal duration of ^ 40 ns. The dead time is in reality probably shorter, and the larger counting loss is due to the non-uniform distribution of the particles over the 2 ms spill time. This explains also the observed spread in the points (c) (fig. 3.12), which shows the charge in one of the "reference" detectors, integrated over the whole spill-time, versus the counts in the small detector. The time distribution of the number of particles in the spill has a Gaussian envelope (see sect. 6.2, fig. 6.3) but there is a fine structure, and also the overall form and duration may change as a function of the operating conditions of the accelerator. The integrated charge is not affected by changes in this time distribution, but the count loss is. The count loss of the small detector for a 50 ns dead time is in (c) indicated with an arrow, and is compatible with the spread of data points. The relation between counting and integration (c) is still linear over the measured flux range. Curve (d) represents the relation between two of the integrating "reference" detectors for data points corresponding to those of curve (c) , extended with points taken at higher intensity in pit 4. This relation is a perfect straight line.

Due to the non-uniform time distribution of the particles the usual formulae, like (3.9), are not strictly valid, because these are derived under the assumption of randomly distributed particles (Poisson statistics). However, by lack of a more appropriate description, the dead time corrections are applied, using the effective T = 90 ns.

Coincidences between the signals of the small and the big detector were determined using the NIM logic signals from discriminators. The duration T of these signals was T = 20 ns. If n and n, are the

3 s b observed counting rates, and N ^ the true coincidence rate, then the random coincidence rate is given by

n , = (n, - N . ) (n - N , ) 2 T (3.10) random b s,b s s,b

Using the measured coincidence rate n , instead of the true coincidence rate N„ . the rate of random coincidences was calculated. The measured s , b values of the random coincidence rate were ^ 1.3 times higher than the calculated values, again due to the unequal distribution of particles.

The true coincidence rate N . can be calculated from the observed coincidence rate by multiplication with the correction factors for the count rates N b / n b and N s / n g . Using (3.9) one finds

Ns,b = ( ns,b " nrandom ) (1 - n.r) (1 - n rf ( 3 - 1 1 )

- 62 -

The dead time x is here the effective counter dead time T = 90 ns. Applying the corrections, a true coincidence rate of 80 ± 5% is found for all intensities. It should be noted, however, that the correction factor becomes even more than 2. for the high flux in the centre of the pit. Again it is found, that the results also show a dependence on the spill conditions, via the effective dead time.

The results obtained with the silicon counter telescope can be compared with the track count in a nuclear emulsion, which was exposed simultaneously for the purpose of absolute calibration of the integrating detectors. In this test, the telescope detectors registered 15130 resp. 53549 counts during 4 pulses, corresponding to a corrected flux of 304 000 resp. 385 000 cm"2 for the small resp. the big detector. The corrected coincidence rate at this time was 92% of 304 000, which would indicate a flux of 280 000 particles . cm" 2. The result obtained from the nuclear emulsion is 330 000 tracks cm" 2, from which 60 000 cm"2 (18%) are identified as electrons (chapter 9). There is fair agreement between these two results, in view of the large corrections for the counters. Although the use of smaller (e.g. microstrip) silicon detectors could improve the dead time losses, a much better result would be obtained if the beam were operated in a slow extraction mode. However, this interferes with normal data taking of the neutrino experiments, and was not possible until 1982. Some tests were made during the beam dump runs and the NNB operation with special calibration boxes equipped with scintillators, using a slow extraction mode. Preliminary results will be mentioned in sect. 9.4.1.

The use of coincidence counting for the measurement of the angular distribution of the muons in the shield will be discussed in chapter 9. A conclusion will be, that coincidences between two small detectors, placed at some distance, are mainly due to muons. Some fraction of the non-coincident counts is due to secondary electrons and photons. To identify these, a large analyzing detector, preferably shielded by an electron absorber, has to be placed a few cm downstream of the counting detector. Several tests were performed with the absorber box (sect. 7.2.3), but the counting inefficiencies only permit the statement, that secondaries can be seen. In table 3.6 the count rates of a detector just in front of a 3.2 cm long absorber (iron or lead) and one just behind, are normalized for their area and compared to the count rate of another silicon counter, which was placed 34 cm upstream, directly on the iron shield. The increase in the middle detector is attributed to backscattering, the decrease in the downstream detector to absorption of low energy electrons in the absorber.

- 63 -TABLE 3.6

Count rates with and without absorber in wide band neutrino beam

Relative count rate Normal ized count rate in air

Upstream detector 100. 100. 100. 100. Middle detector 66.6 100. 103.1 104.4 3.2 cm absorber - Air Iron Lead Downstream detector 2742. 100. 98.6 95.4

The measurements, described in this section, showed that there is a contribution of secondary particles to the counted flux, but it also became obvious that the short spill time, aggravated by the non-uniform time distribution of the particles, complicates considerably the use of coincidence counting. Results agree with emulsion counting, however, and useful information on the muon angular distribution could be obtained.

3.4 Secondary radiation in integrated charge measurements using absorbers Measurements with small pieces of absorber material, sandwiched

between silicon detectors, performed in 1974 in the 24 GeV PS neutrino beam, showed already that the integrated charge signal depends strongly on the material around the detector. In the SPS neutrino beams it was noticed [3.31] that the moving lift, when it is positioned in front of fixed detectors, causes a change of the signals of these downstream, fixed detectors. An increase is seen in the centre of the beam, a reduction of the signal height is caused for detectors off-axis. This "calbox-effeet" will be discussed further in sect. 9.1.2.

A more systematic study of the influence of material in front of integrating detectors could be performed, using the "absorber-box", of which the technical details are given in sect. 7.2.3 (figs 7.10 and 7.11). A well defined thickness of material, either iron or lead (or air), can be sandwiched between the middle and the downstream detectors. An upstream detector, at a fixed distance from the downstream one is needed as reference, because the middle detector, which is pressed against the absorber, is affected by backscattering, and may have a signal increase of several per cent. The ratios between the downstream and upstream detector signals, averaged over 20-100 pulses and corrected for a possible offset, are determined for various absorber thicknesses, in various positions in the shield, and for various beam energies. A flux normalization is made in air, comparing the signals of the absorber box with those of the fixed detectors. These are calibrated, as described in chapter 9. The reproducibility of the ratios is generally better than 1%, provided the operating bias voltage of the detectors is not changed.

- 64 -

3.4.1 Influence of secondary radiation on the muon flux profiles The absorber box was first of all used to measure the profiles of the

flux with and without absorber. In fig. 3.13 the profiles in gap 1 are shown for the 200 GeV parent momentum, narrow band neutrino beam. Behind 12 cm Fe absorber the beam has a FWHM of 9 cm, but in air this is 11 cm. The absorber really absorbs nearly half of the flux at 60 cm from the centre, but it creates 40% more signal on the beam axis. As will be shown later, a thickness of 12 cm of iron practically restores the equilibrium between muons and secondary radiation, and therefore the profiles measured with the maximum absorber length of 12 cm represent the true muon flux profiles.

The same phenomenon is seen in fig. 3.14 for gap 2 (v 200 GeV and v 300 GeV) except that the magnitude of the changes is much reduced. Here are plotted the signals (normalized to 10 1 3 protons on target) for the upstream detector (U), the middle detector (M) and the downstream detector (D) in air and the signal of D behind 12 cm of iron in a few positions. Somewhere between 20 and 30 cm from the beam axis the absorber has no visible effect on the detector signals, and it is concluded that in this region the proportion between secondaries and muons is the same as inside the iron. In this location the relative detector sensitivities for U, M and S were determined by comparison with the fixed, calibrated detectors. Using these "true" sensitivities, the radiation profile in the gap, parallel to the beam, can be mapped. In fig. 3.15 the result is shown for gap 2 at 200 GeV and at 300 GeV. The curves in fig. 3.15 illustrate that the proportion of secondary ionizing radiation depends on both radial and

10

GAP 1 200 GeV

9EHIND p e FWHM 9 cm

i\lN AIR FWHM II cm

" Fit! 3.13 Profile of muon flux gap 1 along Che horizontal axis for 200 GeV narrow band neutrino beam, measured by a charge integrating detector in the absorber box, with (Fe) and without (AIR) an iron absorber plug of 12 cm thick, in front of the detector. The width of the beam at half of the maximum (FWHM) is indicated, and it is smaller behind the iron than in the air. The beam profile is asymmetric, and the peak at the right is also visible in the following gap 2 (fig. 3.14).

-SO -40 -20 20 40 RADIAL DISTANCE (cm )

- 65 -

longitudinal position in the gap, and there is no equilibrium situation at the plane of the fixed detectors. The effect increases with higher energy. The measurement structures are built against the downstream walls of the gaps, and because the widths of the gaps are not everywhere the same (see Appendix A for exact dimensions) , the distance from upstream iron wall to detectors varies. It is smallest in gap 2, largest ("v 2 m) in gap 1.

x

-80 -60

RADIAL D'STANCE ( cm

Fig. 3.14 Profiles of muon flux in gap 2 along the horizontal axis: (a) For 200 GeV narrow band antineutrino beam. The curve U is measured at 17 cm from the

upstream wall of the gap, the curve M at 28 cm and the curve D at 38 cm, all in air. The points A are measured with 12 cm of iron absorber in front of D.

(b) For 300 GeV neutrino beam, measured with detector D with (Fe) and without (air) absorber.

F ig. 3.15 Longitudinal profile along beam axis of the density of ionizing radiation in gap 2, as measured with a charge integrating silicon detector. The ordinate is expressed as the charge (pC) generated in a detector of .1 cm thick, 2.4 cm2 of area, and is normalized to a proton density of 10 1 ' on the target.

10 20 30 to

Oistsnce from shield ( cm)

- 66 -

3.4.2 Secondary radiation as a function of muon energy As a part of tests with various parent particle momenta in the SPS

narrow band antineutrino beam (see also chapter 11) the amount of secondary radiation emerging from a 12.4 cm thick iron absorber has been measured as a function of beam momentum. The momentum was increased from 60 GeV to 200 GeV in steps of 10 GeV and additional measurements were made at 220, 230, 260, 275 and 300 GeV. At the highest momenta the flux became very low, so that the precision of these data is limited by fluctuations and noise.

The absorber box was during these tests positioned in the centre of gap 1. The average muon momentum in that position is 10-30% lower than the nominal beam momentum. The detector signals were normalized to the flux in the upstream detector, when there was only air between upstream and downstream detector. The presence of the middle detector caused an increase of 5% of the signal of the downstream detector, at 200 GeV.

In fig. 3.16 the signal increase of the downstream detector caused by the 12.4 cm iron absorber is plotted as a function of the mean energy loss in iron (table 3.1) of muons with the nominal momentum. Over the momentum range under consideration the increase in mean energy loss is nearly linear with momentum, and so is the generated secondary radiation. Extrapolation from the data points shows that no secondary radiation is generated below an energy loss of 1.8 Mev per gem"2 which corresponds to ^ 4 GeV muons. At this energy, all energy loss is caused by ionization only, and it is near to the minimum ionization loss (1.47 MeV per g cm"2) .

18 g

1.7 |

16 , |

1.5 |

u I 13 £

O

1.2 |

CE

11

10

Tota l energy loss MeV g _ 1 c m 2

Fig. 3.16 The signal increase in a charge integrating detector in the centre of gap 1, caused by the insertion of 12.4 cm of iron just in front, for various beam momenta between 60 GeV/c and 260 GeV/c. Each point is a ratio with/without iron, plotted against the total energy loss of muons of the nominal beam momentum. Se^ the text for the curves (c) and (r) (right vertical scale for (r)).

18

I

/ i n

1.7 - / • -

16 -// *

-

15 ~ U - / ' -

1.3 - /••/ -

1.2 - I d / y -

11 - -10 -< ! 1

- 67 -

The measurements reported in fig. 3.16 suggest a correlation with the radiative part of the muon energy loss. Curve (r) shows the increase of this radiative contribution with increasing energy. It is the ratio of the total energy loss and the pure ionization loss. The agreement between the data and the curve (r) would become even better if they were plotted as a function of the true average muon momentum in the centre of gap 1. Curve (c) represents the same data points, taking the mean momentum as • 80% of the nominal beam momentum.

3.4.3 Secondary radiation as a function of absorber thickness Measurements could be made with the absorber box, varying the

thickness of absorber in a given position in the flux. Most interesting are the results, obtained in the centre of the gaps, for 200 and 300 GeV beams. These results can be compared with the calculations, performed in sect. 3.2.4. In table 3.7 the measured increase of signal for the maximum absorber thickness of 12.4 cm is compared with calculated values for infinite absorber thickness. The calculation was made for an average muon energy in the centre of the gap. The "maximum" muon energy, which is found by subtracting only energy loss through ionization is indicated for comparison. In the calculation it is found that for high energy muons (> 50 GeV) the electron contribution builds up rather slowly with thickness and the saturation occurs only after 30-50 cm of iron, corresponding to 18-30 radiation lengths. This is due to the penetration of showers, initiated by energy transfers > 1 GeV, which do not occur frequently, but contribute a large fraction of the mean energy loss of high energy muons as has been shown already in tables 3.1 and 3.2. The calculated signal increase N (p)/N , mentioned in table 3.7, represents the number of electrons N (u) , present in the iron in the equilibrium situation after > 100 cm, for N incoming muons of the average assumed x u energy. These calculated values are considerably higher than the measured signal increase, but it will be shown that the presence of electrons, which originate from the cascade in the nearby iron, could explain this difference.

Whereas in table 3.7 only the values for the maximum possible absorber thickness are compared, the evolution of the measured values as a function of thickness is shown in fig. 3.17. These measurements do not show the slow increase of signal height with thickness, which is produced in the calculation, but the signal increase is immediately there, for thin layers of absorber, indicating a residual electron/photon component in the flux which is incident on the absorber.

It was found possible to explain in a first order approximation the measured behaviour by assuming in addition to the muons a residual electron component in the air gap, at the position of the detectors. The inferred number of residual electrons N0(e) per number of muons N

8 10 12 i 1 i 1 1 r

GAP 1

Thickness of iron (cm)

2 4 6 8 10 12 T 1 1 1 1 r

GAP 2

i 1 1 r 10 12 14 16 18

i 1 1 r

0.80

0.70

0.60

0.50

040

0.30

0.20

0.10

0

040

0.30

0.20

0.10

-220 GeV

/

/

-160 GeV

GAP 3

-100 GeV

Fe -• •

/

/

Fe

-140 GeV -90 GeV -40 GeV

Fe

j i i _ i r

J I L

- Fe Pb

_L J L 0 20 40 60 80 100 120 20 40 60 80

Thickness (g cm"2) 20 40 60 80 100 120 140

Fig. 3.17 The increase of the charge integrated signal of a detector on the beam axis, as a function o the thickness of absorber in front of it. The lower part of the figure shows the results in gaps 1, 2 an 3 for the 200 GeV beam, the upper part relates to the 300 GeV beam momentum. The average muon energy i the centre of the gap is shown in each case (table 3.7). The measurement points are connected by curves t guide the eye.

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is given also in table 3.7. These electrons, 30 to 100 cm in air, downstream from the iron shield, must have an energy spectrum distorted towards the higher energies, because of the larger scattering angles for low energy electrons, and the limited detector acceptance. The proportion of low energy electrons must be smaller, the wider the air gap. This explains why in gap 1 fv 100 cm of air) the number of residual electrons is only .35 whereas in gap 2 (30 cm of air) it is .4 at 200 GeV and .6 at 300 GeV. Also the beam profile influences this proportion. In a homogeneous flux the spectrum distortion would obviously be much smaller, because large angle, low energy electrons can then reach the central measurement position from the sides. This explains the tendency from gaps 1 to 3, of the effect to become smaller and smaller, the wider the muon beam.

TABLE 3.7 Comparison of ca l cu l a t ed and measured inc rease of charge i n t eg ra t ed s igna l

per incoming muon, for maximum absorber thickness (124 mm)

Beam momentum 200 GeV 300 GeV Gap 1 2 3 1 2 3

Average muon energy (GeV) 140 90 40 220 160 100

Maximum muon energy 185 150 115 285 250 215

Iron absorber Calculation N (u) .90 .64 .40 1.31 .99 .69 Measurement .37 (.40) .16 .08 .58 (.70) .26 .16 Residual electrons N0(e) .35 .40 .30 .36 .58 .46

Lead absorber Calculation N (y) .80 .48 .22 1.31 .92 .54 Measurement - .22 .09 82 43 -

The inferred number of res idual e lec t rons N 0(e) can be determined supposing that the number of e lec t rons N e ( e ) , o r ig ina t ing from the r e s idua l e lect rons No(e), becomes neg l ig ib l e for a thick layer of i ron . The measured e f fec t can be represented by

s ignal in absorber _ u e N + N (u) + N (e)

N^ + No (e) = 1 + measured difference signal in air

The calculated signal increase does not include the residual electrons

signal in absorber = Nu + N e ( u j _ = 1 + c a l c u l a t e d difference signal in air N„

By taking N (e) = 0 one can find the No (e) from the measured signal lifference, in the limit of thick absorber.

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Using this number of residual incident electrons, the calculated signal increase can be adapted to the measured signal increase, in the limit of a thick absorber. Assuming for these electrons an energy spectrum proportional to 1/E2, but modified at lower energies for the effect of the scattering, the calculated signal increase follows closely the measured behaviour, up to a thickness of ^ 1 cm of iron. In the region of intermediate thickness of absorber the calculation gives results with more or less pronounced oscillatory behaviour due to the simplifications in this first order approximation. The developments of the cascades produced by muons on the one hand and by electrons on the other hand should have a gradual overlap. Photon induced cascades are neglected altogether.

In principle, it is also possible to simulate precisely the measurements in a Monte-Carlo calculation. However, in the framework of this report it was not possible to do more than just indicate the factors which affect the flux measurement, and to guide the way for a more detailed approach, if required in the future.

3 .5 Conclusions A considerable fraction of the energy loss of high energy muons in a

shield results in a "spray" of energetic electrons and photons. Precise equations were formulated for the generation of these secondary products. To calculate the actual contribution of this secondary ionizing radiation to a flux-measurement by charge integration, an approximation method was described, which makes use of empirical data for the mean electron range and of Monte-Carlo results for the cascade parametrization. Although the trend of the calculated values is confirmed by the measurements with the absorber box, there remain quite large discrepancies which could originate from inadequacies in the model. Several complications in the measurements are difficult to take into account. Attempts to improve the model yielded results which were only slightly (i> 20%) higher or lower.

The information obtained from the measurements is not decisive, because the muon flux was "contaminated" with residual electrons before entering the absorbers. In the neutrino beams the muon momentum distribution complicates the interpretation of the measurements.

Altogether, the impression is that the calculated number of electrons is too high to be true. However, the measurements show that the Monte-Carlo results of Bassompierre [3.26] are too low.

Compared to charge integrating detection, the coincidence counting method would be much more straightforward for the measurement of a muon flux, although also in this method the influence of the secondaries has to be carefully taken into account. It was shown in 3.3.3, however, that counting cannot be used in the usual extraction modes for the neutrino beams.

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NON-LINEAR RESPONSE OF SILICON DETECTORS The muon flux in the shield ranges from a few muons per cm2 to 10 8

per cm2 and these may all arrive together, within a short time interval (10 ns to 3 ms) . The signal current density correspondingly varies over many orders of magnitude, from a few nA cm"2 for a single minimum ionizing particle to .1 A cm"2 for the 10 ns long bunches of 10 7 muons cm"2

in the PS neutrino beam. It is not obvious that linearity of the detector response is maintained over a range of 10 8. Therefore a special study has been devoted to this question and comparisons are made with operating conditions of silicon detectors in nuclear physics, where a broad experience has been accumulated.

In sect. 4.1 the signal formation will be described. In sect. 4.2 several causes for charge collection inefficiencies are given. In 4.3 an opposite effect is discussed, namely an anomalous increase of the signal of the detector, which was observed in the PS neutrino beam. This phenomenon, explained as charge injection from the contact has been studied in detail. First, because it was the most unexpected non-linearity observed in the PS-muon flux measurement. Second, because it provided new insight in a still not completely understood non-linearity in heavy-ion spectroscopy, generally referred to as "charge multiplication".

It should be mentioned, however, that for most types of detectors a complete charge collection guarantees linearity up to a signal current density of "v. .05 A cm"2. Non-linear behaviour occurs only in some rather extreme cases, like 10 1 3 particles s"1 cm"2, or in detectors with incomplete charge collection ("trapping").

1 Signal formation and charge collection time The signal of a semiconductor detector consists of the collected

charge, which is proportional to the energy deposited by the particle in the depleted region of the detector, as has been discussed in chapter 1. The cloud of electron-hole pairs, generated around the particle track, is initially neutral, and if the ionization is very dense, recombination of electrons and holes may be significant. This is one of the causes of a non-linear behaviour, called "pulse height defect", which is often encountered with heavy ions.

The passage time for a minimum ionizing particle in a 1 mm thick detector is only 3 x 10"' 2 s and the stopping time for a non-relativistic particle is of the same order of magnitude. Under the influence of the electrical field in the detector the electrons and holes are separated and drift to the electrodes, which constitutes the signal current. According to Cavalleri et al. [4.1] this signal current i can be described by an extension, of a theorem by Ramo [4.2] including a fixed space charge,

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dQ v 1 = dt = * d t 4- 1'

and the induced charge AQ in the external circuit AQ = q ^ (4.2)

where q is the unit charge, v its velocity due to the electrical field, Ax the displacement of the charge perpendicular to the electrode and d the sensitive thickness of the detector. The correctness of this description has been verified by Williams and Lawson [4.3], who observed a flat top in the current pulse, at saturated carrier drift velocity, in a thick overdepleted Ge detector.

The drift velocity v has been given in formula (1.5) and is a function of temperature and electrical field. Measured values can be found in ref. [1.6] in sect. 1.3 by Alberigi Quaranta et al. and also in [4.4]. At room temperature the saturated value for electrons in silicon is ^ 7 x 10 s cm s"1 and for holes ^ 5 x 106 cm s" ' , both at electrical fields in excess of 10 kV cm"1. This leads to a total charge collection time of ^ 14 ns for electrons and ^ 20 ns for holes in a 1 mm thick Si detector. It may be useful to remind that the mean thermal velocity of carriers in Si at absolute temperature T is v., - 7 x 10 s x /T cm s"l.

The separation of electrons and holes starts immediately after their generation, so that the current risetime can be extremely short. However, the shape of the current or voltage signal in reality depends mostly on the detector equivalent circuit parameters and the shaping properties of the signal amplifier. For example, in the case of a partially depleted detector, the resistance of the field free region increases the signal risetime.

The "plasma-effect" can give rise to a delay in the detector response, because in a dense cloud of ionized electron-hole pairs screening is thought to prevent field penetration until diffusion or erosion of the plasma has taken place. Such "plasma delay times" are inversely proportional to the applied field and range from 5 to 40 ns [4.5 and 4.6]. Seibt et al. [4.5] propose the expression for the plasma time T P

, 0 (ni E ) 1 / 3 -T = 1.32 x 10" 1 0 L ' [10"9 s] (4.3) p t

where n, is the linear carrier density [cm"1] along the track, E is the energy in MeV deposited by the particle in the detector, and F is the electrical field in Vcm"l at the position of the track. For a 80 MeV fission fragment, with a range of 25 ym we find n, = 8.8 x 109 cm" 1. If the field in the detector is 2000 V cm" 1, the plasma time T = .6 ns. Applied to the ionization by minimum ionizing particles (m.i.p.) it would result

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in a plasma time x ^ 10" 1 1 s. However,the notion of plasma time ir

i s a l toge ther i r r e l e v a n t for m . i . p . , because the charge density i s too low to c r ea t e a screened plasma. There i s complete f i e ld penetrat ion in t h a t case .

In f i g . 4.1 are shown the current pulses induced in a 500 urn thick diffused Si detector by the muon flux from the PS neutrino beam. The r i se t ime i s ^ 20 ns , and at 270 V bias the pulse duration i s ^ 70 ns , so t h a t the current has dropped to the zero leve l before the next bunch of p a r t i c l e s a r r i v e s . The PS bunches have a duration of 10 ns and an i n t e r v a l of 100 ns between bunches. The dependence of the charge co l l ec t i on time on the applied bias vol tage is c l ea r ly i l l u s t r a t e d . No further amplif icat ion was used in t h i s case , and the s ignal cur ren ts measured over a 50 S termination are .1 to .2 A for the 2 cm2 big d e t e c t o r .

Fig. 4 . 1 Current pu l s e s , induced by the muon flux in a 500 um thick d e t e c t o r , measured over a 50 fl termination as a function of time. The current in ind iv idua l pulses shows v a r i a t i o n s , r e l a t ed to the v a r i a t i o n of the number of acce le ra ted pro tons . The dura t ion of each proton bunch i s 10 n s . (a) A complete PS e x t r a c t i o n ; 18 pulses were

d i r ec t ed to the neu t r ino a r ea . Horizontal d i v i s i o n s 500 ns .

(b) At 70 V bias there i s no t o t a l dep le t ion ; not a l l charge i s c o l l e c t e d , as can be seen from the s t i l l decreas ing cu r r en t , when the next pulse beg ins . Horizontal d i v i s i o n 100 n s .

(c) At 270 V bias a l l charge i s co l l ec ted a f t e r 70 ns (50 ns per d i v i s i o n ) .

In fig. 4.2 similar pulses are shown obtained with a 100 um thick surface barrier detector, for 15 V and 25 V bias voltage.

duration of the bunch,

(b) Bias 25 V, all charge collected within 70 ns, risetime 14 ns.

4.2 Charge collection efficiency For a number of reasons the charge collected at the external contacts

of the detector may be less than the original number of excess electron-hole pairs generated by the energy transfers of the incident particle. Only if the collection efficiency changes as a function of the number of charge carriers involved, this inefficiency is seen as a pulse height non linearity in particle spectrometry (pulse mode). A constant fraction inefficiency may pass unnoticed through the usual energy calibration procedure.

A constant fraction inefficiency is often caused by temporary trapping of carriers, and becomes therefore visible as a non-linear behaviour if the time intervals for signal charge integration are varied. If the integration time is shorter than the (de) trapping time not all charge is collected. If one wishes to measure the deposited energy via the absolute value of collected charge any inefficiency causes an error in the measurement.

4.2.1 Insensitive layers In a silicon detector the signal comes mainly from the collection of

the excess charge, generated in the region which is depleted of free carriers (space charge region, sect. 1.1). The excess electrons and holes generated in the field-free undepleted region recombine with the available free carriers and thermal equilibrium is restored, with a mean free carrier lifetime T, which is a strong function of the purity of the silicon crystal. During the lifetime T the carriers diffuse over a mean length L = JËrî, where D is the diffusion coefficient (see table 1.1) .

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T h i s d i f f u s e d charge g i v e s on ly a smal l e x t r a c o n t r i b u t i o n to t he s i g n a l i n a p a r t i a l l y d e p l e t e d d e t e c t o r , i f t he p a r t i c l e s have t r a v e r s e d the u n d e p l e t e d r e g i o n . The undep le t ed r eg ion can t h e r e f o r e be r ega rded as an i n s e n s i t i v e l a y e r of t h e d e t e c t o r .

Apa r t from the undep l e t ed r e g i o n , a l s o the c o n t a c t l a y e r s a r e i n s e n s i t i v e . The energy l o s s in the m e t a l l i c c o n t a c t s themse lves does not g i v e r i s e to f ree c h a r g e . But a l s o some p a r t of t he u n d e r l y i n g s i l i c o n r e p r e s e n t s a dead l a y e r . In d i f fu sed or i o n - i m p l a n t e d d e t e c t o r s t he t h i c k n e s s of a l l i n s e n s i t i v e m a t e r i a l in f r o n t of t he s e n s i t i v e r eg ion ( t h e s o - c a l l e d "window") can be ^ 100 nm but in s u r f a c e b a r r i e r d e t e c t o r s i t can be t h i n n e r . Elad e t a l . [ 4 .7 ] r e p o r t a minimum dead l a y e r of 27 nm ( s i l i c o n e q u i v a l e n t t h i c k n e s s ) on a g o l d - S i n - t y p e d e t e c t o r , which in t h i s s p e c i a l case could be a t t r i b u t e d t o the gold l aye r (15 ug cm" 2 ) on ly . The e f f e c t i v e window t h i c k n e s s can be de te rmined by measur ing the energy l o s s of monoene rge t i c , s t opp ing p a r t i c l e s , whi le v a r y i n g the angle of i n c i d e n c e . Windows a r e o b v i o u s l y no t impor t an t for m . i . p .

The r e d u c t i o n in c o l l e c t e d charge for a p a r t i a l l y d e p l e t e d d e t e c t o r i s i l l u s t r a t e d in f i g . 4 .3 by comparing t h e o u t p u t of d e t e c t o r PDB13 for v a r i o u s b i a s v o l t a g e s V to t he ou tpu t of a r e f e r e n c e d e t e c t o r which i s exposed to the same muon f l u x . The r e l a t i o n s h i p between t h e two d e t e c t o r s i s p e r f e c t l y l i n e a r over the dynamic range for each of t h e a p p l i e d b i a s v o l t a g e s , even for 0 V. One may p l o t the c o l l e c t e d charge f r a c t i o n v e r s u s t h e d e p l e t e d t h i c k n e s s , i t s e l f p r o p o r t i o n a l t o /V ( formula ( 1 . 3 ) ) . The r e s u l t i s shown in f i g . 4 . 4 , t o g e t h e r w i t h the cha rge c o l l e c t i o n e f f i c i e n c y (energy peak p o s i t i o n ) for 5.486 MeV a lpha p a r t i c l e s ( range in S i ^ 28 um) i n c i d e n t on

F i g . 4 .3 Comparison of de t ec to r output s ignals for v a r i o u s de tec tor b i a s v o l t a g e s , corresponding to i nc reas ing deple t ion th ickness in detector PBD 1 3 . Above 210 V bias the output does not change any more, and the detector i s t o t a l l y dep le ted . The s igna l s a r e compared to the output of re fe rence de tec tor 44. A s i g n a l of I V in reference d e t e c t o r 44 corresponds to 1280 muons per cm 2. For the b i a s vol tages >, 180 V the da ta po in t s are not shown, and only the l inea r l eas t squares f i t s through these points a re g iven .

the d iode s ide or t h e r ea r s i d e of t he same

a > < z o en

0-

3 O

O

O LU

1 2 3 4 5 6 7 8 9 10 n F T F T T O R n i l T P I I T ^ i r . M f l l I \ / \ n c r n o n 1 1

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detector (PBD4) function of

The increase of the muon s igna l i s a nearly l i nea r The slower V and therefore of the depleted th ickness .

increase near and above the t o t a l deplet ion vol taqe V, , is re la ted to 3 depl

a lateral extension of the depleted region (sect. 1.3.3). The detector is insensitive for alpha particles impinging on the back contact until the depletion region extends more than ^ 800 ym through the volume. This illustrates again, that an undepleted region is nearly insensitive but also it shows that diffusion of charge contributes to the signal (the a range is much shorter than the undepleted thickness). In sect. 4.3.3 an example will be given of enhanced collection (funneling) from the undepleted region.

DETECTOR PBO 4 mn thick

; F i g . 4 .4 Charge co l l ec t ion as a function of applied bias vo l tage

-J V B>

25 100 225

OPERATION VOLTAGE

_1 . 400

f o i p a r t i c l e s ( s 'Am source) inc iden t on diode side and on ohmic back con tac t , and for the completely t ravers ing minimum ion i z ing muon f lux . All values a re normalized to 100% for s igna ls a t l a rge opera t ion vol tage (300 V). In abso lu t e value the s ignal he igh t for a on the rea r i s smaller than on the front (see a lso f ig . 1 .4) .

4.2.2 Carr ie r trapping and detrapping Deep energy l eve l s E. near the middle of the energy band gap

(E - E ) in the semiconductor capture free c a r r i e r s (e .g . e lec t rons from the conduction band) and may r e l ea se them again to the same band with a detrapping time constant T . The f rac t ion of charge trapped i s propor t ional to the densi ty N of trapping cen t r e s and the i r trapping c ross sect ion o . The following discussion i s r e s t r i c t e d to trapping in the semiconductor volume, but t rapping can a l s o occur a t the surface or in oxide l a y e r s . A comprehensive account of t rapping phenomena i s given by Mayer in chapter 5 of ref . [ 1 . 6 ] .

If the energy level subsequently captures a second ca r r i e r of opposite s i g n , the level a c t s as a recombination cen t re and both the

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electron and the hole disappear. This is the inverse of the generation process. In the case of detrapping, the carrier is not lost, but reemitted to the same band and the time constant T can be written as

E c - E t 1 kT

T° • \^r°-t ' ( 4 - 4 >

The temperature dependence of the effective density of states N 3/2 1/2 c

( T ' ) the thermal velocity v (% T ' ) and the capture cross section o can in first approximation be neglected compared to the exponential factor. For a typical a = 10" 1 S cm2 a time constant of t_ = 10"" s is found for E

c ~ E t = * 4 e V a n d k T = - 0 2 6 e V (room temperature). If more precise values for the energy level E. are required, the (exponential) temperature dependence of the capture cross section a has also to be taken into account. At liquid nitrogen temperature (kT = .006 eV) T n = 1 0 l 7 s, and no detrapping will occur for this energy level. In fact, the value of the energy level can be found by measuring the temperature at which the reemission takes place. This so-called "Thermally Stimulated Current" (TSC) method will be discussed further in chapter 5 because it enables the study of (e.g. radiation induced) defects in silicon.

The number of trapped carriers is proportional to the capture cross section a. and the density of unfilled trapping centres N.. A density of N. ^ 101"* cm"3 in a 1 mm thick detector would give rise to a few per cent of trapped carriers. As long as not too many traps become filled, the detector response remains linear, because for each pulse a constant fraction of the carriers is taken away. As already was remarked, such a constant fraction inefficiency can only be noticed, if a comparison is made of the detector response at different temperatures or for different time constants.

For a certain type of diffused junction detectors it was found that the signals obtained by charge integration (current mode) in the PS muon flux did not correspond to the charge expected from the sensitive silicon volume. Moreover, the ratio of actual charge over expected charge varied considerably from one detector to the other (see also Appendix C) . When these peculiar detectors were used later in the SPS monitoring system, this ratio of actual charge over expected charge turned out to depend on the total integration time and it was higher in the long ( 5 ms) extraction ("spill") than in the fast ("v- .5 ms integration) spill. The signal measurement in the PS system occurred already after .3 ys. In fig. 4.5 some signals are shown at the output of the charge integrating circuit in the NFM system, for the SPS fast extraction. The spill itself is 23 ys (see sect. 6.2.1) and the hold of the integrated signal occurs ^ .5 ms after the beam passage. For a normal detector the integration

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of the charge, generated during beam passage, i s followed by a constant l eve l . The pecu l ia r de t ec to r s , however, show a continued increase of s ignal during severa l ms after beam passage. This i s a c lear sign of detrapping, and apparently up to 40% of the charge i s i n i t i a l l y trapped in some of these d e t e c t o r s , while in others trapping i s l e s s or nothing a t a l l . This f r ac t ion of trapped charge i s so big, tha t i t seems d i f f i c u l t to explain by volume t rapping. The t r ap densi ty in surface (oxide) l aye r s may be much higher and could be used as explanation.

Obviously, the measured signal depends in th i s case very much on the timing of the measurement. The varying degree of t rapping makes the detector response p r a c t i c a l l y unpredic table . In add i t ion , the strong temperature dependence of the detrapping time constant may cause shor t term v a r i a t i o n s in the s e n s i t i v i t y of de tec tors which suffer from t rapping .

(a) Upper t r a c e : de t rapp ing ; Lower t r a c e : normal d e t e c t o r . Each (hardly v i s i b l e ) d iv i s ion

presents 50 u s .

(b) Upper t r a c e : normal de tec to r (QDA8) , Middle t r a c e : de t rapping (SDA.15); Lowest t r a c e : no beam. 500 ys per d i v i s i o n .

F ig . 4.5 Comparison of de t ec to r s which do and do not show t rapp ing /de t rapp ing of charge c a r r i e r s . The h o r i z o n t a l sca le of these osc i l loscope p ic tu res i s t ime. The beam passage l a s t s in a l l cases 23 ys , the s igna l a t the output of the charge i n t eg ra t i on c i r c u i t i s displayed on the v e r t i c a l a x i s , in 500 mV r e s p . 100 mV per d i v i s i o n . The small v e r t i c a l displacement v i s i b l e in (b) and ( c ) , i s caused by a t r a n s i e n t of the hold mode, but does no t influence the rea l measured v a l u e .

(c) Two consecut ive s i g n a l s , showing detrapping are compared with no beam (de tec to r SDB1). 500 us per d i v i s i o n . The de tec to r leakage cur ren t i s s l i g h t l y overcompensated, and t h i s causes a slow decrease of the in teg ra ted s ignals with time ( s e c t . 8 . 2 . 3 ) .

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4.2.3 Pulse height defect for heavy ions The Pulse Height Defect (PHD) , which is observed in silicon detectors

for heavy ions, like fission fragments, [4.8] is defined by the difference between the energy of the incident heavy ion and the energy of a light ion (e.g. an alpha particle) which produces the same pulse height in the detector. The response of a detector for a specific ion is nearly linear as function of the real energy of the ion, but for different ions the signals are not proportional. Schmitt et al. [4.9] studied the absolute calibration of silicon detectors, using 20 to 120 MeV bromine resp. iodine ions. They found a PHD of 1.3 MeV resp. 5.5 MeV.

An almost trivial contribution to the PHD comes from the detector entrance dead layer ("window"), because heavier particles have a higher specific energy loss. As mentioned in sect. 4.2.1, this contribution can be determined by studying the angular dependence of the signal height.

A second contribution to the PHD comes from the increased energy loss in nuclear collisions, which do not result in the generation of electron-hole pairs. Light particles therefore generate more free charge for the same total energy loss. Using the theory of nuclear stopping developed by Lindhard et al. [4.10] calculations of the energy loss in nuclear stopping were published by Pinch for ^ 100 MeV fission fragments [4.11] and by Potter and Campbell [4.12] for low energy ions. Also for this contribution one can make a correction.

A residual PHD remains after having accounted for the previous two contributions, as shown a.o. by Finch and Rodgers [4.13,4.11]. This residual part of the signal defect is attributed to recombination of electrons and holes inside the dense ionized plasma, during the time that the electrical field cannot penetrate this plasma. The density of the plasma is higher for heavier ions. The plasma time T has been given already (formula 4.3). Finch, following Miller and Gibson [4.14], expresses the fractional carrier loss by

where n is the original number of charge pairs, on the number which is subsequently lost by recombination and T is the recombination lifetime. This recombination lifetime is mainly determined by the concentration of recombination centres and has about the same value as the minority carrier lifetime To (10-1000 ys) if the generated carrier density n is much less than the equilibrium concentration of carriers in the conduction band (5 x 10 l 3 cm"3 for 100 ftcm n-type Si). But for a higher generated carrier density ( 1 0 1 7 cm"3 for fission fragments) the recombination lifetime drops to much lower values (T /to ^ 10" 3, i.e. T - 10 ns) and becomes

- 80 -

comparable to the plasmatime. Finch then calculates a pulse height defect as function of the minority carrier lifetimes TO and suggests that discrepancies found between results of different authors may be related to differences in the x 0 of their silicon detector material.

For heavy ions the existence of the PHD is now well established, because the true energy of the ion can be determined in a calibration experiment, e.g. by time of flight. In the case of a pulsed muon flux, comparable generated carrier densities are obtained, as will be shown in more detail in sect. 4.3.4. However, there are no obvious methods to show that there is also loss of signal by recombination. In analogy to the situation for heavy ions, a loss of the order of 10% is quite conceivable.

4.3 Anomalous charge injection On various occasions experimentalists have found anomalously large

signals from silicon detectors, generally when the irradiation caused a high ionization density, like in the case of fission fragments. Although the effect is still not completely understood, it has become evident that it arises from injection of charge out of the metallic contact, and it can be largely suppressed by special surface treatments in the case of silicon surface barrier detectors.

Also in the muon flux measurement in the PS neutrino beam an abrupt increase of signal has been observed above a threshold ionization density, for nearly all types of silicon detectors. The injection of excess charge could be studied in more detail because the ionization density is homogeneous in the whole detector volume, contrary to the situation for a fission fragment, which causes dense ionization in a small volume near the surface only, for which the size is not easy to determine.

Recently, the introduction of microstrips on the detector surface has provided again more information, as it was found that collection on one strip can be accompanied by injection on the adjacent strip.

The anomalous charge injection has to be distinguished from the avalanche charge multiplication as used in avalanche diodes. In a high field region ( 10 s V cm"1) the originally generated carriers obtain such a high drift velocity, that they produce secondary free carriers by impact ionization, which leads to a cascade. The breakdown field for Si at room temperature is 3 x 10 5 V cm"1, but for diode structures it may be higher, depending on geometry and doping (see e.g. ref. [1.8]). In the case of anomalous charge injection, the electrical fields are much lower (> 103 V cm"1 ) .

- 81 -

4.3.1 Signal multiplication for fission fragments Britt and Wegner [4.8 and 4.15] were the first to study the

multiplication phenomenon systematically with 2 5 2 C f (californium) fission fragments. The normal spectrum has two partially overlapping peaks, as shown in fig. 4.6, with the low energy peak at "V/ 80 MeV corresponding to the heavy fragments and the high energy peak at "\< 105 MeV from the light fragments. The fragments range in silicon is ^ 25 urn. This spectrum can be described by a number of parameters (table 4.1), as proposed by Schmitt et al. [4.16], which enable an easy evaluation of the quality of a detector for fission fragment spectroscopy. The multiplication is sometimes seen as an increased high-energy tail, or at higher fields a third peak may develop. Also, the complete spectrum may move to higher energy values, but in this case Schmitt's parameters do not reveal the multiplication.

Britt and Wegner observed that multiplication increases with electric field and decreases with the angle of incidence. The maximum multiplication occurs if the fragments enter perpendicularly. Lowering the fragment's energy by an absorbing foil also reduced the effect. Although Britt and Wegner originally concluded that there is a threshold penetration depth, this is not the real explanation. From the absence of multiplication with a particles it follows that multiplication is related to the ionization density. Walter [4.17] explains the multiplication as a charge injection phenomenon, related to the surface condition at the metallic contact. He suggests a tunneling injection through the thin oxide interface layer as the multiplication mechanism. A sodium dichromate treatment, which increases the thickness of this oxide layer, could prevent the injection.

TABLE 4.1

Evaluation of ion-implanted detector for fission-fragments

Parameter (fig. 4.6)

Schmitt's criteria ref. [4.16] Observed value

V Nv > 2.85 2.7

N H / N V ^ 2.2 2.1

V NH -\< 1.30 1.28

AL/(L-H) < 0.38 0.34

AH/(L-H) < 0.45 0.43

(H-HS)/(L-H) < 0.70 0.705

(LS-L)/(L-H) < 0.49 0.495

(LS-HS)/(L-H) < 2.18 2.21

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F i g . 4.6 A typ i ca l pulse he ight spectrum for 2 5 2 C f f i s s i o n fragments. Schmi t t ' s spectrum parameters are i nd i ca t ed . L r e f e r s to the high energy l i g h t fragment, H to the low energy heavy fragment. NL and NH are the r e l a t i v e numbers of counts in the peaks L and H, N v i s the number of counts in the v a l l e y . (LS - L)/(L - H) i s a s e n s i t i v e ind ica t ion for m u l t i p l i c a t i o n , i f t h i s i s charac te r ized by a high energy t a i l . If m u l t i p l i c a t i o n s h i f t s the complete spectrum to higher energy, Schmi t t ' s parameters cannot d i s t i ngu i sh m u l t i p l i c a t i o n .

CHANNEL NUMBER

Further s tud ie s of the mul t ip l i ca t ion were undertaken by Belcarz, Heijne, Muller and S i f f e r t [ 4 .18 ] , using not only surface b a r r i e r s but a lso ion-implanted d e t e c t o r s , which enabled to vary several parameters. Their f indings w i l l be repor ted in the r e s t of t h i s sec t ion .

In f i g . 4.7 the influence of the dose of implanted boron ions , both for l i g h t and heavy fragments of 2 S 2 C f i s shown. For a low dose implantat ion (5 x 1 0 1 2 cm"2) one observes a stronger increase of the peak pos i t ion than for a higher dose. On the cont ra ry , no e f fec t i s seen from varying the ion energy from 15 keV to 60 keV, which corresponds r e spec t ive ly to ^ 50 nm and ^ 200 nm [1.13] penet ra t ion depth of the B +

ions ( f i g . 4 . 8 ) . For a fixed energy and dose the mul t ip l i ca t ion was seen to diminish for higher post- implantat ion annealing temperatures ( t ab le 4 . 2 ) , which produce a higher proportion of ac t iva ted doping atoms and reduce the r a d i a t i o n damage defect dens i ty . These r e s u l t s a l l i nd i ca t e that the mul t ip l i ca t ion i s an inverse function of the concentra t ion of e l e c t r i c a l l y act ive boron atoms (acceptors) in the doped i n t e r f ace layer , but i s not r e la ted to the width of the doped l a y e r .

In add i t ion , i t has been found that the mul t ip l i ca t ion increases for lower detector opera t ing temperatures, as i l l u s t r a t e d in f i g . 4 .9 . This behaviour suggests t h a t the mul t ip l i ca t ion can be re la ted to the c a r r i e r mobi l i ty and/or to t rapping phenomena. Walter [4.17] observed a r e l a t i o n between the m u l t i p l i c a t i o n and the thickness of the gold contact in

2000

1500

o o

U_ 1000-

LLJ

3 500

1000 2000 3000

Fig. 4.7 The peak positions of the 2 S 2 C f spectrum as a function of detector bias, for different doses of implanted boron. For low dose the peaks increase more in the "plateau-region", which indicates stronger anomalous injection. The energy of the implanted boron ions was 15 keV.

Fig. 4.8 The peak position, corresponding to the most — 30» -probable pulse amplitude for *z the light fragments of a = 2 5 2 C f source, as function J of detector bias voltage. 2 There is a slight increase -f of signal for the lower 3 implantation energy, which can be explained by the o thinner insensitive layer on ^ the surface of the detector, Ij because the junction occurs ^ 28 nearer to the surface. The <

dose was 5 x 10 1 3 cm" 2 u for all implantation " energies. j?

27X

- 84 -

TABLE 4.2 Mul t ip l i ca t ion in ion-implanted d e t e c t o r s , expressed as

(Apulse/Abias)/fragment energy (normalized s igna l increase with b i a s vol tage)

Heavy fragments Light fragments Dose 5 x 10 l 2 1.01 1.17

10 1 3 0.80 0.97 5 x 10 1 3 0.48 0.55

Annealing temperature 400 °C 0.72 1.03 500 °C 0.66 0.70 600 °C 0.48 0.55 700 °C 0.37 0.50

ISO 190

TEMPERATURE ( K )

Fig. 4.9 The multiplication, expressed as the area of the anomalous tail in the 2 5 2Cf spectrum, at various detector temperatures.

- 85 -

surface barriers. In our case all surface barriers and most implanted detectors had a 30 nm thick gold contact layer on the junction side. It was found that an implanted detector without gold contact does not show multiplication, but that multiplication appears only after the gold is deposited (fig. 4.10). Also the inverse is true, the injection disappears if the gold contact is etched away. Of course, this test is impossible to do with a surface barrier detector, but also with ion-implanted diodes it was not always possible to obtain a good contact without the gold layer.

20

17.5

1 r

B implanted diode

without gold contact 16-5-1977 with gold contact 23 -5 -1977

100 200 300 4 0 0

BIAS VOLTAGE

Fig- 4.10 multiplication, function of the voltage, appears after deposition of gold contact layer.

The as

bias only the

The quality of the ion-implanted detectors used in this study, was evaluated by applying the criteria of Schmitt [4.16], as was done also, for example, by Klema et al. [4.19] in a recent study of energy resolution of heavy-ion detectors. In table 4.1 the values for a typical ion implanted detector are compared to Schmitt's parameters, and they compare quite well. This means that energy resolution, uniformity, edge effects, etc. are acceptable, but it does not mean that there is no multiplication. The degree of multiplication can be expressed as the relative signal increase "A pulse" for a difference in bias voltage "A bias", which is a measure of the slopes of the saturated parts of the curves in e.g. fig. 4.7, etc:

(Apulse/Abias)/fragment energy

The values obtained in this way, are reported in table 4.2, illustrating the tendencies already mentioned.

All diodes discussed so far were made from n-type resistivity silicon. Belcarz [4.20] recently studied phosphorus implanted p-type diodes, and found also in this case charge injection for fission fragments, but at higher fields. On n-type diodes the increase of the injection with the thickness of the gold layer was again verified, but in the case of a palladium contact the injection is constant beyond a contact thickness of 10 nm.

- 86 -

From these measurements it is clear that the generated carrier density in the volume, or rather that at the interface with the contact, plays a critical role in the charge injection phenomenon. However, only approximate values for these densities can be calculated, depending on assumptions for the initial volume taken by the electron-hole plasma, the plasma time, the expansion rate and the penetration of the field. Walter [4.17] suggests an initial diameter of 3 ym for the plasma column, of 25 ym length. This gives an initial ionization density of ^ 1.5 x 10 1 cm"3 electrons and holes. After 5 ns full separation has occurred, and the hole cloud which approaches the interface, has a diameter of 400 ym. The hole density is then ^ 8 x 10 1 2 cm" 3. These numbers are shown in table 4.3, and have to be compared to an ionized donor density of 1 0 1 3 cm"3 in the high resistivity Si.

TABLE K.3 S i - D E T E C T O R S in PULSED BEAMS

C E R N P S L O S A L A M O S S L A C C a n a l i «t * i T o v e •> ,1 f i ss ion f r a g m e n t

4.25 4.26 4.27 4.28

P A R T I C L E S . E N E R G Y /u*/u" 1 - 1 5 GeV P* 8 3 0 keV e' 1 9 - 2 2 GeV e" 3 0 keV foton 6143 A 100 M e V

P U L S E D U R A T I O N 1 0 n s 1/us-1ms 1.6/us 150 ps 1 ns 5 ns

P E N E T R A T I O N D E P T H a l l 1 5 /Um a l l 7 /um 2/urn 2S/um

S P O T S I Z E homogeneous 5 0 m m ' 4 m m ' 3 m m ' 7.1 m m ' 7.10'Sn.m! 1 .3 ,10 'mm !

N U M B E R OF P A R T I C L E S < 2 . 5 , 1 0 6 3 .6 .10 s . 1.3,10'° 2 . 5 . 1 0 3 . 2 . 5 . 1 0 ' < 3 . 3 . 1 0 s < 2 . 1 . 1 0 ' j e * 1 NORMAL 1 I £ 0 CM 2

E N E R G Y D E P O S I T E D < 1 0 " 3 , 1 0 " . 1 . 1 , 1 0 * < 9 . 1 0 " . I O " 1 . 3 . 1 0 " 1.4.10' 5 7 . 7 . 1 0 ' ° • v PER PULSE CM 2 10 10O.UPI in IS *im m IOO . i i in 7 Mm in 2 * m m 2S Aim

I O N I Z A T I O N D E N S I T Y PEP PULSE

c h P . I R S CM 3 « 2 . 8 . 1 0 " S . S . 1 0 " 2 , 1 0 " < 2 . 5 . 1 0 " < 3 . 9 . 1 0 " < 1 . 8 , 1 0 " I . 5 . I O " 8 . 5 . 1 0 "

r h P . I R S CM 3 SEC ' < 2 . 8 . 1 0 ' ° < 2 . 1 0 " < 1 . 6 . 1 0 " 2 . 6 . 1 0 " 1 . 8 , 1 0 " 3. . 1 0 " 1 . 7 . 1 0 "

p h p » I R S C M ' S E C ' 2 . 8 . 1 0 " 3 . 1 0 " 1 . 6 . 1 0 " 1 . 8 , 1 0 " 3 . 6 . 1 0 * ° 4 . 2 . 1 0 " ( S I G N A L C U R R E N T D E N S I T Y I . p i . . . out

4.3.2 Charge injection in microstrip detectors In a silicon microstrip detector [1.17, 1.19] the response of a

single strip may present a trivial non-linear behaviour because some of the charge may be collected on an adjacent electrode. The sum of the charges collected on several adjacent strips should however be proportional to the total deposited energy.

In a recent study of Kraner et al. [4.21] there are some indications that also in this type of silicon detector a less trivial anomaly, in the form of charge injection may occur. This was seen as a negative signal on a strip, adjacent to the one with the normal, positive signal. The strips were in this experiment aluminium deposits of 100 ym or 50 ym wide (200 ym or 100 ym pitch) and constitute the ohmic back contact of the

- 87 -

diode. The negative signals were observed under irradiation of this rear side with 2 5 2Cf fission fragments, but they also became apparent with 2.5 MeV and 1.2 MeV protons from a microbeam of 20 ym cross section, if the beam had been positioned for some time on the same spot. Irradiation at the junction side did not produce this effect. In the case of the protons the anomaly seems related to radiation damage, which in a microbeam accumulates very rapidly on a small area. The holes generated by the incident proton, which should be collected towards the junction, may be trapped for some time near the rear metallic contact, and while the electrons are rapidly collected, this "fixed" positive charge causes electron injection from nearby neutral metal electrodes.

It has not yet been possible to make detailed measurements of collected charge, however it has been clearly shown that there exists under certain conditions injection from a metallic contact, which normally remains non-injecting.

4.3.3 Charge funneling in partially depleted structures The influence of ionizing particles, especially natural alpha-

particles of ^ 5 MeV, on the operation conditions of microelectronic silicon devices has become of interest because of the ever smaller dimensions of the device elements. The amount of charge involved, e.g. in the switching of a memory element, may become comparable to the charge generated by ionizing particles, which may then give rise to undesirable, so-called "soft failures" of the device.

A renewed effort is given to the better understanding of charge collection in silicon, e.g. in [4.22]. But recent measurements, a.o. by Campbell and Knudson [4.23], have shown that an unexpected phenomenon causes an enhanced charge collection. This is called "charge funneling" [4.24] and is caused by the modification of the depletion layer and the electric field in the structure by the generated free charge itself. A drift field is created along the column of charge in the previously field-free region of the undepleted silicon, and causes a prompt collection of much of the charge. Without this funneling, only little charge should have reached the contacts by slow diffusion, much later in time and distributed over a larger area, as shown in fig. 4.11, taken from [4.24].

Although the charge funneling causes anomalously high signals from highly ionizing particles, the model which is developed to explain the phenomenon does not have to include anomalous injection from the contacts. Instead, it is assumed that charge which in the undisturbed field configuration would have been lost, is funneled back towards the point of particle entry. The undepleted region apparently cannot always be regarded as an insensitive layer (sect. 4.2.1).

- 88 -

40 coil

CHARGE COLLECTION BY ' FUNNELING AND DIFFUSION

Fig. 4.11 Schematic representation of the influence of charge tunneling on the profile of the

charge, after of an

in a partially

c o l l e c t e d pene t r a t i on a - p a r t i c l e deple ted s t r u c t u r e .

CHARGE COLLECTION BY DIFFUSION ALONE

s i l i c o n device The d e p l e t i o n

reg ions under the c i r c u i t nodes in the undis turbed s i t u a t i o n are indica ted by the broken l i n e s . The expected charge c o l l e c t i o n ( Q c o l l ) p ro f i l e i s given by the fu l l l i n e , for the case of d i f fus ion only. Funneling c o n s i s t s in a temporary ex tens ion of the d e p l e t i o n along the a - p a r t i c l e t r a c k , such tha t the c o l l e c t e d charge d r i f t s back to the c i r c u i t node h i t by the p a r t i c l e ( a f t e r McLean and Oldham [4.24]) .

The resemblance between the funneling and the anomalous charge in jec t ion for f i s s ion fragments l i e s in the necessary condit ion tha t the densi ty of generated e lec t ron-hole p a i r s exceeds some threshold , which i s r e l a t ed to the s t a t i c charge dens i ty in the device, e i t he r in the volume or a t the in te r face l aye r .

ARRAY OF CIRCUIT NODES

4.3.4 Charge in jec t ion in a pulsed flux of muons

The high ion iza t ion densi ty which l o c a l l y occurs around a heavy ion track in a s i l i c o n de tec to r , can a lso be achieved through the whole detector volume, using a pulsed high energy a c c e l e r a t o r , or another type of pulsed source of intense r a d i a t i o n . Hemmendinger e t a l . [4.25] used a 830 keV proton beam to study the behaviour of s i l i c o n de tec to r s in pulses of 1 us to 1 ms, for beam cur ren t s of .29 uA to 10 pA. They r e p o r t an onset of anomalously high s igna l cur ren t a t 10 uA beam c u r r e n t . Taking into account the beam s ize and penet ra t ion depth , as mentioned in table 4 . 3 , they achieve an ion iza t ion densi ty of 2 x 1 0 2 1 cm"3 s " 1

e lec t ron- hole pa i r s (e-h) and a s ignal cu r ren t densi ty of 3 x 10 cm"2 s " 1 c a r r i e r s . These values are comparable to those obtained with f i s s ion fragments on a microscopic a rea .

An abrupt increase of the s i g n a l s of some s i l i c o n de tec to rs above some threshold in the pulsed muon f lux of the PS neutr ino beam has been observed. This occurred f i r s t for r e l a t i v e l y thick de t ec to r s (> 500 urn) in the c a l i b r a t i o n box when they were moved from a low flux region in the sh ie ld in to the h ighes t flux near the cent re of gaps, but only af ter the s t a r t of the PS-booster r ing operat ion in 1974. This had ra ised the maximum muon flux by a factor ^ 3 . In a subsequent t e s t program, the e f fec t was seen in severa l experimental d e t e c t o r s , and i t could be re la ted to a threshold i on i za t i on , beyond which the s igna l increases very rapid ly with f lux.

- 89 -

In fig. 4.12 a typical example of the abrupt signal increase is shown. The ouput of detector 6 is compared to that of reference detector 292 while both are pushed from a low flux region in the muon shield to a high flux region. They are mounted in a moveable calibration box, both well aligned, perpendicular to the passing muons, and should therefore register the same flux. Using the calibration value of the reference, the threshold for injection can be determined (1.2 x 10 7 muons cm"2 per pulse).

Fig. 4.12 - Signal of detector 6 as function of the muon flux, measured by a reference detector. Charge injection starts at a threshold of 12 x 106 muons cm"2. Bias voltage on detector 6 is 60 V. The electronics saturates at 10 V output, which causes the accumulation of points along the upper horizontal edge.

There is ample evidence that one observes really injection in detector 6, rather than saturation in detector 292. The reference detector could be simultaneously compared to one or two other detectors in the calibration box, and also it could be compared to several fixed detectors. All these relations remain linear, while detector 6 (or different other injecting detectors) shows the signal increase. Apart from this, the variation of the flux intensity is obtained by moving the detectors from low to high flux or vice versa. The flux profile is well known and changes gradually, certainly not representing discontinuities as observed in fig. 4.12.

Bunch test - The total detector signal in the PS was the integrated charge from 18 bunches of particles, each bunch lasting for 10 ns, as has been shown in fig. 4.1(a). It is possible to increase the ionization density for a given amount of integrated flux, by reducing the number of bunches. In fig. 4.13 the result of this test is shown for the same pair of detectors. For the highest density (3 bunches) the charge injection already occurs at 3 x 10 6 muons cm"2. This proves that the injection is related to the ionization density in a single bunch. Moreover, the current pulses as illustrated in fig. 4.1, were recorded for some of the

• y

o

- f

9

/

8 2 12.3 16,4 20.5 24,6 28.7 32,8 36! 41.0

MUON FLUX x I 0 6 c m " 2 i REFERENCE DET 292

- 90 -

detectors under injecting and non-injecting condition (in a high and a low flux region) . The shape of the pulses was not visibly affected by the injection, and therefore the injection is a fast process (at most a few ns) which is contemporary with the normal charge collection. In table 4.4 the threshold flux for injection and the corresponding ionization density during one bunch are reported for 3 detectors. Detector 6 is a surface barrier, detectors 17 and 19 are ion-implanted detectors and have a somewhat higher threshold. The threshold expressed in ionization density per bunch is nearly constant for each detector. The slightly lower value for many bunches could be explained by the influence of a few low intensity bunches, which do not cause injection. With only three bunches, the occurrence of 1 or 2 low intensity bunches brings the integral below the injection threshold, and therefore the value found for three bunches is the most precise. It is ^ 10 1 2 carriers cm" 3, at first sight amazingly low when compared to the initial ionization density for heavy ions (-v 10 1 7 cm" 3). However, if one supposes that the injection takes place at the interface with the contact, the initial ionization'

10 ~! r~

- 6

? *\

o sit

/*t 13 i8 bunches JL_ I : L 12 15 18 21 24 27 30

Fig. 4.13 The threshold flux for charge injection in detector 6 becomes lower as the number of bunches is reduced from 18 to 13.8 and 3. The electronics saturates at 10 V signal.

MUON FLUX x I 0 b ( R E F E R E N C E DETECTOR 2 9 2 )

Threshold flux and ionization density for injection

18 bunches 13 bunches 8 bunches 3 bunches

Detector Total flux Ionization per bunch

Total flux loni zat ion per bunch

Total flux loniza t i on per bunch

Total flux Ionization per bunch

muons cm"2 carriers cm 3 muons cm 2 carriers ci 3 muons cm"2 carriers cm'3 muons cm"2 carriers cm"]

6 11.7 x 10' 7.2 x 10 1' 8.0 x 10' 6.8 x 10'' 6.1 x 10' 8.4 x 10" 2.7 x 10' 1.0 x 10"

1? 21. x 10' 1.3 x 10" 16.5 x 10« 1.4 x 1 0 " 11.4 x 10' 1.6 x 10" 5.9 x V0' 2.2 x 1 0 "

19 17.7 x 10' 1.1 x 1 0 " 13.2 x 10' 1.1 x 1 0 " 10. x 10' 1.4 x 1 0 " 5.2 x 10' 1.9 x 1 0 "

- 91 -

density has to be translated into the rate of arrival of the carriers at the interface. The important parameter is then the signal current density, i.e. the number of electrons or holes cm"2 s"1. This number is given on the bottom line of table 4.3. For the fission fragments the carriers are much spread out during the plasma time and the subsequent drift towards the contact, so that the effective signal current density becomes comparable for all situations in which injection is observed (2-4 x 10 1 9 cm - 2 s" 1).

In fig. 4.14 the signal of detector 13 is plotted as a function of flux. First, the detector showed injection above 1.5 x 10 7 muons cm"2. Afterwards, once the gold contact metallization had been removed, and a new contact was made with the wire directly glued on the implanted layer, no injection was observed up to 2 x 10 7 muons cm"2. This shows that also here the injection is related to the metal layer on the surface.

1 0 -

o

Q

o RUN 204 BIAS 100 V

del 13 RUN 702 BIAS 120 V no Au contact

20 25 FLUX MUONS x 10

F ig . 4.14 Signals of de t ec to r 13 as function of muon flux (measured by a reference de tec to r ) with and without a gold contact l aye r . Removing the gold e l imina te s i n j e c t i o n .

The dependence of the in jec t ion threshold on the e l e c t r i c f ie ld i s p lo t t ed for some de tec to r s in f i g . 4 .15 . With increas ing f ie ld the c r i t i c a l flux becomes lower, but seems to s t a b i l i z e beyond % 2500 V cm" 1 . For de tec tor 400 the threshold i s p a r t i c u l a r l y low. This i s a surface ba r r i e r detector with a f a i r l y large area , but there i s no obvious reason for i t s except ional ly low threshold .

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o

x „ 6

=> 10

T~:

""29° det 16 b

<

o

det 296

det 400

1000 2000 3000 4000 5000 6000

ELECTRIC FIELD V cm"

Fig. 4.15 The critical flux for anomalous charge injection, as function of the average electric field in some detectors. The detector numbers correspond to the numbers in table 4.5.

A list of all tested detectors is made in table 4.5 {the numbers used here do not correspond to the detector numbers used in the other chapters of this report). Generally, the detectors could be operated at bias voltages well above the total depletion voltage V . The surface barrier detectors show on the average a lower flux threshold than the ion-implanted detectors. Several surface treatments were applied, both to the surface barrier detectors and the ion-implanted detectors. Within the available statistics, no influence on the flux threshold for injection could be found. All these detectors were successively mounted, two or three at a time, in the same calibration box, together with one or two reference detectors. Each time the same amplifiers have been used. Therefore, the signal increase cannot be explained as an amplifier effect.

A further discussion follows at the end of this chapter.

4.3.5 Charge injection in a pulsed flux of electrons From 1975 onwards, the PS neutrino beam did not exist any more.

Studies of the charge injection were continued for some time at the Medium energy Electron Accelerator (MEA) in Amsterdam, while this machine was still under construction. A fairly homogeneous beam of 10-20 cm2 area, of 35-80 MeV energy at various intensities, and pulse durations of .5 to 15 us could be delivered to a stack of detectors, which were placed in air, immediately behind the beam exit window.

93 -

TABLE 4 . 5

D e t e c t o r c h a r a c t e r i s t i c s and t h r e s h o l d f l ux for i n j e c t i o n

Total Detector Type Treatment Area Thickness depletion

VD Capacity Resistivity Actual

voltage Threshold

flux

mm 2 urn Volt pF kflcm Volt x 10 6 ctrr'

2 SB H15 28.2 243 5 18.9 47. 100 9. 3 SB H15 28.2 222 20 10.6 14.5 70 8.7 4 SB - 28.2 232 15 13.4 16. 50 10.1 5 SB - 28.2 270 50 13.6 4.7 60 8.1 6 SB - 28.2 226 30 15.6 6.3 60 11.7 7 SB H5 28.2 199 50 18.1 9.5 50 8.7 8 SB H5 28.2 341 50 10.2 7.7 100 6. 9 SB H5 28.2 255 30 10.6 7.6 50 12.9 10 SB B10 28.2 288 50 13.2 4.2 100 6. 11 SB BIO 28.2 364 50 12.2 2.7 90 8.8 12b SB BIO 28.2 301 30 12.3 4.2 75 > 9

13 115 - 28.2 281 50 6.4 4.7 100 13. 14 115 - 28.2 299 30 7.2 8.5 60 20.2 15 115 H15 28.2 284 50 6.8 5.8 80 13. 16b 115 H15 28.2 277 50 5.7 7.6 100 14. 17 115 B 28.2 281 20 5.4 100. 60 20. 18 115 B 28.2 207 20 9.0 10. 80 16. 19 160 - 28.2 379 70 5.0 7.7 100 12.2 20 160 - 28.2 410 70 4.5 18. 120 15. 21 160 H15 28.2 268 30 7.1 7.3 50 > 19 22 160 H15 28.2 257 30 7.3 32. 150 12.4 23 160 BIO 28.2 274 20 7.2 27. 75 16. 24b 160 BIO 28.2 238 50 7.0 6.6 100 16.4

290 SBP - 50 98 10 46. 2.5 25 13. 299 SBP - 50 97 8 42. 2.5 60 12. 300 SBP - 25 100 12 40. 2.5 10 > 35 4 00 SBO - 327 219 90 165. 2.7 60 1.5

293 DS28 - 50 300 60 10. 296 DS35 - 50 300 120 5.6

Type

SB Sur face b a r r i e r 115 I o n - i m p l a n t e d 15 keV B + i o n s 160 I o n - i m p l a n t e d 60 keV B + i o n s DS Dif fused j u n c t i o n

Treatment

H5, H15 b o i l i n g HN03 , 5 r e s p . 15 min. B10 b i c h r o m a t e t r e a t m e n t 10 min.

R e s i s t i v i t y i s deduced from C-V measurement

No special calibration of the flux was made, but the known muon-flux calibration factors were used to estimate the number of electrons hitting the detectors. Values of 10 8-10 9 electrons cm"2 were found, well above the intensity in the muon flux.

Except at the lowest possible beam, all detectors used were extremely non-linear and apparently suffered from both injection and saturation. Pulse shapes observed over a 50 ft termination indicated a completely vanishing collection after ^ 1 ys, as sketched in fig. 4.16. Under such circumstances no meaningful measurements of the charge injection could be expected. An upper limit of silicon detector application seems to be established, around the ionization density of 10' 9 e-h pairs cm"3 s"l, corresponding to 1 A cm"2 of signal current.

Fig. 4.1.6 The shape of a current pulse generated in Che detector by the electron beam pulse can be observed directly on an oscilloscope, over a 50 Q termination. The detector is connected as shown in the upper part of the figure. The pulse shapes seen at low beam intensity and at high intensity are sketched. The ordinates are not to scale, and represent currents of ^ .1-10 A. At high intensity the detector is "blocked" after 1 1 US.

A few similar experiments have been reported in the littérature. Lakin [4.26] at SLAC used a surface barrier detector in a 19-22 GeV electron beam, up to 2.5 x 10 7 particles cm"2, in a beam spill of 1.6 us. Lakin reports a proportional behaviour, and no injection effects were observed. The ionization density in the detector remained a factor 10 below the injection threshold found in the muon flux at the PS. The calculated values are also reported in table 4.3.

Canali et al. [4.27] have irradiated silicon detectors with 150 ps long bursts of 30 keV electrons, to show the existence of space charge limited currents, if the injected carrier density is higher than the density of ionized donors. They also achieved ionization densities in the region of interest for anomalous injection. Vice versa, their results show that injection coincides generally with a space charge limited current density. This implies that the charge collection is slowed down, and that a reservoir of generated charge is maintained for a short time.

In a similar experiment, Tove and Andersson [4.28] illuminated silicon surface barrier detectors with laser light pulses, and studied the transient space charge limited current under high irradiation levels. The ionization density is ^ 10 l s carriers cm" 3, which is well above the injection threshold. They did not observe injection, but mention an increase of the space charge limited current at high irradiation intensity, ascribed to the expansion of the temporary charge reservoir. The laser light (614.3 nm wavelength) is stopped within ^ 2 um so that the drift length for the holes is extremely short.

detector

94 -

oscilloscoop

50 fi

300 V

Low beam intensity

I 10 time lus)

XT

High beam intensity

10

- 95 -

4.3.6 Discussion

From the experimer .il results it can be concluded that the signal multiplication, observed with heavy ions or high intensity pulses of electrons or muons, is caused by transient injection of carriers from the contacts. Two conditions must be fulfilled.

(a) A critical rate of carrier generation in the detector must be exceeded. For most detectors the threshold is ^ 1 0 1 2 cm" 3 in 10 ns. Although this value is comparable to the density of ionized donors, there is no clear correlation with the silicon resistivity. However, a correlation with the interface charge is evident.

(b) A sufficient charge-reservoir must be available near the interface, in the form of a metal contact layer. A gold electrode with a thickness of 40 ug cm" 2 contains ^ 1 0 1 8 free electrons cm" 2, but the ion-implanted layer alone has % 10 1 1* cm" 2. Also the thickness of gold is of importance.

The anomalous charge injection seems a fairly general phenomenon, once the amount of charge collected near the interface becomes comparable to the fixed charge. By increasing the fixed charge with a higher implantation dose or with a thicker oxide layer the injection is suppressed, but it is likely to appear again at a higher ionization density. The thickness itself of the interface is less important, contrary to a conclusion of Walter.

Not only the density, but also the time plays a role, as can be deduced from the results with the microstrip detectors. If holes remain trapped for some time near the surface, there is injection, contrary to the case of instantaneous collection. With a high ionization density, the carrier collection occurs in fact under space charge limited conditions, which also causes some delay.

In fig. 4.17 a simple diagram illustrates the situation in a n-type ion-implanted detector, just at the penetration of a particle (A), and a few ns later, when the electrons and holes are separated (B) and the holes approach the interface layer. The space charge distribution is locally (in the case of isolated heavy ions) or generally (in the short muon pulse) disturbed. If the collected holes represent a charge, comparable to the fixed charge in the implanted layer, temporarily there will exist a strong electric field over a short distance. For example, in detector 19, which is 379 urn thick, the ionized donors in the volume represent a fixed positive charge (at full depletion) of only 2.6 x 1 0 1 0 cm" 2, which under equilibrium conditions is balanced by equal fixed negative charge in the junction. The thickness of this negatively charged layer in the junction can be estimated to be less than 1 nm. The collected charge at the injection threshold of 1.9 x 1 0 1 2 cm" 3 (table 4.4) is 7.2 x 1 0 1 0 holes, which is far more than the fixed charge.

- 96 -implanted

3 Soie layer SU bsf r a t e

3 Soie P-'ype | n - t y p e

Ê 9 © ]® Pi i

© ® ©

r ® Q ; + - • _ • _ .

: ® 9 ®

© ©

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z ®«î®. * © © * ©

z : s ; ® © ® * ~ ©

z e a ; ® * © * ©

300A IOOOA interface

SPACE CHARGE DISTRIBUTION

© B I A S

- ®

®

Fig. 4.17 Diagram of Che charge distribution in a B implanted diode on n-type silicon, in equilibrium^^ when the particle enters, and a few ns later (B) when the generated electrons and holes are separated, and the holes approach the interface.

The transient situation, created by the cloud of holes near the

interface, resembles that of strong inversion in a MOS structure. Green

and Shewchun [4.29] described current multiplication by tunneling of

electrons through a thin oxide ( 3 nm) when minority carriers (holes)

are provided to enhance the inversion layer. In a subsequent paper [4.30]

they studied the frequency response of such a system and found that gain

only occurs at relatively low frequencies (< 10 kHz). However, the

tunneling injection of electrons from the metal, caused by a high density

of collected holes, is expected to occur only in n-type silicon devices,

whereas the anomalous charge injection has been observed also in p-type

detectors. Also, a straightforward tunneling seems excluded in view of

the results obtained with the implanted detectors. The distance between

gold contact and interface layer is i- 100 nm, which is far too thick for

a tunneling process as described by Green et al.

So far, three explanations have been qualitatively proposed for the

anomalous injection discussed here:

(a) Electron tunneling through the oxide layer (Walter).

(b) Emission of secondary electrons from the metal into the silicon, due

to the impact of the particles, and

(c) Avalanche multiplication.

From the experiments described here, it may be concluded that

tunneling cannot be the common explanation for injection in surface

barrier and ion-implanted and diffused diodes. Given the necessity to

find a different explanation for the ion-implanted and diffused detectors,

the explanation (a) becomes questionable also for the case of surface

barrier detectors. The emission of secondary electrons from the metal can

- 97 -

be excluded, because there should have been a correlation with the front-layer thickness for the various ion-implantation energies (fig. 4.8). Also, the injection in the muon flux was the same for both orientations of the metal contact, facing the beam or downstream.

Finally, the classical avalanche multiplication was excluded because the required field strength is > 10 s V cm"1 (see for measurements e.g. [4.31]). However, it was noted that the collection of a large number of charge carriers gives rise to a strong field, which exists temporarily near the interface. Some of the carriers might be accelerated sufficiently to cause a transient avalanche multiplication. Still, this explanation does not account for the role of the metal contact.

In conclusion, a number of measurements have provided new data concerning the anomalous injection phenomenon. These seem to rule out the tunneling model, but also the alternative transient avalanche model is not completely satisfactory. The muon beam measurements enabled a precise determination of the required ionization density for this injection. Measurements in a medium energy electron beam showed that at somewhat higher ionization density the detector response becomes quite unpredictable.

4.4 Conclusion A non-linear response of silicon detectors can occur in the case of a

high ionization density (> 1 0 1 8 cm"3 s"1) which prevents field penetration or which provokes anomalous injection. Or it is related to the temporary loss of charge at trapping centres, in the volume or at the surface.

The first mentioned situation is common in heavy ion spectroscopy, but was also encountered in the PS muon flux measurement, due to the short spill time. In the SPS muon flux measurement the spill times are 100 to 10000 times longer for an approximately equal intensity, and therefore the SPS measurements do not suffer from this problem.

Charge trapping has been observed in many of the detectors which were used in the PS measurement, but the calibration procedure could practically eliminate the influence on the final result. In the SPS flux measurement most of the new detectors did not show trapping, except sometimes after prolonged irradiation (see chapter 5). A few old PS detectors which continued to be used at the SPS were even more non-linear than before due to the changes on signal integration times.

- 98 -

As far as the good detectors are concerned, there is "v< 100% charge collection efficiency and therefore a linear response, from single particle detection up to ^ 10 1 2 cm" 2 s"1 minimum ionizing particles. This is not true for detectors exhibiting trapping, which may be linear for some part only of this extended range.

In addition to the response of the detector, the electronics channel may present a non-linear characteristic. Especially, low signals and high signals may be distorted due to noise or offset, respectively saturation. The linearity of the electronics used in the SPS muon flux measurement will be discussed in sect. 8.1.2.

- 99 -

5. RADIATION DAMAGE IN SILICON DETECTORS The energy loss of ionizing particles results mostly in excitation of

electrons, as discussed in the chapters 2 and 3. A small fraction of the energy transfers concerns atomic or nuclear interactions, and may lead to the displacement or désintégration of atoms from the silicon crystal, causing damage to the detector material. These collisions may be carried out by secondary electrons as well as by the primary particle itself. The average energy, required to displace a Si atom from its lattice position is "v- 25 eV, but in some directions the escape is easier than in others. To displace an atom in a head-on collision, an electron needs an energy in excess of ^ 150 keV (displacement threshold). Although subthreshold damage effects exist (e.g. related to ionization enhanced mobility of atoms), they are far less important than the "head-on" displacements. Neutrons, with their much larger mass are very effective in causing displacements. Because neutrons do not loose energy by ionization, all their energy is lost in atomic collisions, and because the recoil atoms also may receive considerable energy, a cascade develops resulting in a large number of displacements close together (local damage cluster).

A displaced atom leaves a vacancy behind, which disturbs the electronic band structure of the crystalline silicon. The vacancies are stable only at low temperature (< 55 K for n-type) , and recombine with other vacancies or with impurity atoms (oxygen, phosphorus, ...) to form new defect structures which are stable at room temperature. Defects present localized energy levels in the forbidden band gap, and can trap electrons or holes, or both successively (recombination). The fate of the ejected, interstitial silicon atom is not well known. Sometimes it is seen to replace an impurity atom in the lattice (Watkins1 replacement mechanism [5.1]) e.g. a substitutional Al atom becomes interstitial, the ejected Si atom taking its place. Or the Si may migrate to dislocations in the crystal, or to the crystal boundary. The trapping of charge carriers in deep energy levels was already mentioned in 4.2.2, in the context of incomplete charge collection.

In the course of irradiation, the number of radiation induced defects steadily increases, and this radiation damage influences progressively the macroscopic electronic properties of the silicon device. When the defect density has become comparable to the built-in charge density in the device, serious degradation of the functioning can be expected.

An extensive treatment of radiation effects in silicon has been given in a recent textbook by Van Lint et al. [5.2]. Originally, the main objective of the study of radiation damage has been the production of semiconductor devices capable to operate or survive under irradiation. Presently the accent is more on the use of radiation induced defects for

- 100 -

the study of the electronic properties of the semiconductor. Generally, the work has been confined to "reasonable" densities of defects and impurities, in the region lO'^-lO16 cm" 3. Comparatively little study has been devoted to the very high purity silicon, used in radiation detectors.

The work described in this chapter has been performed at CERN in collaboration with several outside groups specialized in this field. The aim was, to study the defects created in silicon by high energy muons, in order to understand the performance of different types of detectors as function of irradiation dose.

In the first part 5.1 the emphasis is on the production and identification of the microscropic point defects in several types of silicon crystals. Identification of the microscopic defect structures may help to understand the macroscopic device behaviour. In the second part 5.2 these macroscopic aspects of the degradation of manufactured silicon diode detectors are discussed, comparing several types of detectors with respect to diode reverse current, capacitance, minority carrier lifetime and energy resolution. This is the more traditional approach for the study of radiation damage in detectors, with the aim of establishing an upper limit for the useful lifetime of a certain type of detector in a given type of radiation, see e.g. the overview in the ORTEC detector instruction manual [5.3].

5.1 Defects in silicon introduced by irradiation with muons 5.1.1 Introduction Different techniques are available to study radiation induced defects

in semiconductors, among which optical absorption measurements, Electron Paramagnetic Resonance (EPR) and electrical measurements (capacitance, current), as explained in ref. [5.2]. They aim at measuring the energy difference between the defect associated energy level and the bandgap, and identifying the microscopic structure in the crystal which gives rise to the electrical properties. This may be possible by studying defect orientation in a magnetic field and under uniaxial pressure, as a function of temperature. Often one requires many samples with different doping densities.

The older techniques were comprehensively discussed by Corbett [5.4], in his book on electron radiation damage (1966) . The most powerful remain the EPR and ENDOR (Electron Nuclear DOuble Resonance) techniques, which enable a microscopic characterization of the defect and its electronic structure in the host lattice. The defect concentration has to be at least ÎO^-IO 1" cm"3 for most of these techniques.

- 101 -

The Thermally Stimulated Current (TSC) and Deep Level Transient Spectroscopy (DLTS) techniques have been refined in recent years to provide also quite specific information on radiation induced defects. The TSC is very sensitive to even small concentrations of defects, provided a good quality diode can be made on the silicon sample. The DLTS [5.5] uses the transient capacitance variations related to the emptying of traps in the edge of the depletion region, after a temporary change of bias voltage. Reviews of the current and capacitance transient techniques were given by Sah [5.6] and [5.7]. The advantage of the capacitance measurements is to distinguish between minority and majority carrier traps, which is not possible with TSC. However, when using Schottky barrier diodes only majority carrier traps can be detected, because there is no minority carrier injection at forward bias voltage.

A second advantage of capacitance methods is a better noise immunity, because the capacitance bridge employs a high frequency, phase sensitive detector. Also this advantage vanishes in the particular case of high resistivity silicon. It was found that for a measurement frequency of 1 MHz, usual in DLTS, the capacitance of the irradiated diodes is practically constant, approaching the geometrical capacitance. This phenomenon will be further discussed in 5.2.

Defects with shallow energy levels (< .1 eV) are of less practical interest, because they do not trap the charge carriers for a long time. Most of the study is directed to the deep defect energy levels. A thorough review of deep energy levels has been given by Milnes [5.8] .

It is of interest to study the special case of GeV muons in high resistivity silicon, because it might be that different or unknown defect structures are produced due to the large energy transfers, or that production rates are much different from what has been observed with the usual 1.5-15 MeV electron irradiations.

As mentioned earlier, data on microscopic defects in high resistivity silicon are scarce anyhow. One reason is that there is only little economical interest in this material, another reason is that some of the usual methods for defect study are difficult to apply to this type of silicon. This is especially true for the capacitive methods, as will be explained in some detail in sect. 5.2.3.

On a number of muon irradiated samples three different methods were applied, namely TSC, DLTS and EPR. The first two techniques require a diode structure on the sample, and for EPR small silicon bars are used. The experimental procedures will be further described in sect. 5.1.2. As it turned out that neither DLTS nor EPR gave usable results, the following

- 102 -

sects 5.1.3 and 5.1.4 are concerned only with TSC. The last section 5.1.5 will summarize the results and conclusions.

5.1.2 Experimental procedure Silicon slices of various resistivities, both p-type and n-type, were

at first exposed in different locations in the iron shield of the CERN PS neutrino beam, as shown in fig. 5.1, during several weeks. They received an integrated dose, depending on their position varying from 10 9 to 2.5 x 10 1 3 cm"2 of muons with average energy x 2 GeV. Later also exposures were made in the SPS iron shield, and these samples received a dose of 1 0 1 2 to nearly 101"* cm"2 of 20 GeV muons.

After the irradiation, the slices were cleaned and etched, and surface barriers were formed by evaporating gold and aluminium contacts. The diode structures were made only after the irradiation, to exclude effects from the radiation on the lightly oxidized surfaces. Then Thermally Stimulated Current (TSC) measurements were made to determine the concentration and the energy level of radiation induced defects.

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Fig . 5.1 Schematical drawing of the CERN PS neut r ino beam, during 1974-1975. P o s i t i o n s of the i r r a d i a t e d s i l i c o n samples ins ide the measuring gaps 1, 2 , 3 and 5 are i n d i c a t e d . The muon flux i s h ighes t in gap 1, in the c e n t r e .

To enable comparison of the TSC data from the muon-irradiat ion with defects introduced by other types of r ad ia t ion , a l so some samples were exposed in a neutron flux from a research reac tor and in a coba l t

- 103 -

irradiation facility. At the same time, also some prototype detectors were irradiated. The irradiation conditions will be described in more detail in sect. 5.1.4.

Some slices were cleaned after the TSC and cut into bars, so that they could be inserted in a resonant cavity of an Electron Paramagnetic Resonance (EPR) spectrometer. No signals could be detected in this measurement, most likely because the concentration of defects was below the detection threshold. It is also possible that the defects happened to be in a charge state, which is not paramagnetic, so that it is undetectable with EPR. Shining light on the samples did not lead to any change. Therefore, the identification of the defects via their very specific EPR spectra proved impossible. Some of the samples used in the EPR measurement had been exposed in the SPS muon shield for nearly one year and accumulated a muon dose of "v. 1011* cm"2. Probably a much higher dose is required to enable EPR.

The DLTS method is using a high frequency ( 1 MHz) capacitance measurement, and the capacitance variations resulting from the filling and emptying of trapping centres under varying bias voltage constitute the signal to be detected. It was found that in the high resistivity silicon the high frequency capacitance was independent of the applied bias voltage (see sect. 5.2.3). Hence no signal was detected in the samples.

5.1.3 Thermally Stimulated Current (TSC) measurements In TSC measurements, the sample in the form of a diode is cooled to

liquid nitrogen or helium temperature, then the energy levels associated with the defects are charged by carrier injection (forward bias voltage or illumination) , a constant reverse bias is then applied and the diode is heated in a controlled way, while the reverse current is measured. If a sufficient concentration (> 10 1 0 cm"3) of a well defined energy level is present, the detrapping of carriers from this level will be stimulated at a certain temperature, which causes a peak in the measured current. A quantitative formulation will be presented further in this section.

The description of the apparatus and an overview of the methods of analysis of TSC are given by Muller [5.9]. He applied the method to study damage induced by boron implantation [1.14], which is localized near the surface, whereas the muon induced damage is uniformly distributed through the volume. An account of the following results has been given already [5.10] .

- 104 -

In f ig . 5.2 the TSC curves measured for 5 muon (^ 2 GeV) i r r ad i a t ed samples of 1000 iîcm n-type Si revea l a t l e a s t one de fec t , which causes a cur ren t peak at a temperature of 163 K, for a heat ing r a t e 6 of .75 K s " 1 . The peak height i s p lo t t ed as a function of the muon i r r a d i a t i o n dose in f i g . 5 .3 , and shows a l inear increase as expected. The in tegrated muon flux was measured with a s i l i c o n d e t e c t o r , using the ca l i b r a t i on procedure with nuclear emulsions, described in chapter 9.

100 110 120 130 140 150 160 170 180

T E M P E R A T U R E ', K )

F i g . 5 .2 TSC c u r v e s for 5 s a m p l e s of 1000 ftcm n - t y p e S i , i r r a d i a t e d i n t he PS s h i e l d i n g w i t h •v 2 GeV m u o n s , w i t h d o s e s as i n d i c a t e d . The e x c i t a t i o n was by + 1 V forward b i a s , t h e c u r r e n t measurement a - 1 0 V r e v e r s e b i a s , h e a t i n g r a t e B = . 7 5 K s " 1 .

0 1 2 3 x I 0 ' J IRRADIATION

4

DOSE

5 6 7 ( MUONS / c m 2 )

F i g . 5 . 3 E v o l u t i o n of TSC peak h e i g h t a s f u n c t i o n of muon i r r a d i a t i o n d o s e . The peak h e i g h t i s t h e o b s e r v e d TSC c u r r e n t a f t e r e x c i t a t i o n by forward b i a s , c o r r e c t e d for t h e c u r r e n t i n t h e a b s e n c e of e x c i t a t i o n . The dose r e c e i v e d depends on t h e p o s i t i o n of t h e sample i n t h e s h i e l d . The e x p o s u r e t i m e was for a l l samples t h e same .

The temperature T for which the TSC peak occurs , enables the determination of the energy l eve l associated with the defec t . If the defect i s an e lec t ron t rap the energy difference (E - E.) r e l a t e s to the edge of the conduction band E r e l a t e s to the valence band edge

in :t <B<- - E v ) .

the case of a hole trap it In the following an

- 105 -

electron trap will be assumed, indicated by subscript n (p for hole). Instead of writing E - E. , which is a positive energy, E will be taken zero. E. then must be given a negative value.

The value of T depends on the heating rate, because the number of released carriers is related to both temperature and time. At the temperature T the derivative of the current is zero m

^4P = 0 (5.1) The current can be approximated by the thermally stimulated emission of trapped carriers only

I (T) = | q Ax en(T) nfc(T) (5.2)

where q is the unit charge, A is the area of the diode junction and x is the depletion thickness, which varies between filling and measurement, if the space charge is modified, giving rise to a displacement current, which is just half of the generation current [5.11]. The heating starts at a temperature To and proceeds with a rate 6, so that at time t the temperature T is

T = To + Bt (5.3)

The emis s ion r a t e of e l e c t r o n s e (T) i s t h e r e c i p r o c a l of t he d e t r a p p i n g t ime T , a l r e a d y mentioned in chap t e r 4 , formula (4 .4)

(E c - E fc) f t e n ( T ) = i - = N c v t h o t e" kT = N c v t h o f c e k T (5.4)

For a c o n c e n t r a t i o n of d e f e c t s N. t h e number of f i l l e d t r a p s n . ( T ) depends on the Fermi energy l e v e l E„ t r

N t n. (T) = 1 (5.5) t ft fF

1 + e kT The change in concentration of filled traps with temperature (time) is related to the capture rate c by empty traps (concentration p.) and the emission rate e„

n dn t

dt" = cn pt - en nt ( 5 ' 6 )

of which the term c p. can be neglected if there is no retrapping at the temperature of measurement.

- 106 -

The calculation of the derivative (5.1), taking To + St = T , results then in

N v , a kT2

Et = k T m l n S(2kTm + E t ) m <5'7>

which can be simplified, by neglecting the temperature dependence of N and v., th

N v t u a t kT2

E t - k T m l n C 3 E t ^ <5'8>

In practice, however, it is not possible to neglect the temperature dependence of N and v , completely, and additional measurements are needed to eliminate their influence. One approach, developed by Muller [5.9] is that of the "delayed heating", which requires a series of TSC measurements, starting each time with completely filled traps from an initial temperature T.. By varying the delay t. at this temperature, one allows a partial deexcitation of the traps, proportional to exp (-t. /tj,) • In fig. 5.4 the TSC curves are shown for different delay times at T. = 144.3 K. The TSC peak is plotted in the insert as function of this delay time. The mean time T n of detrapping is graphically determined to be 310 s at this T.. In this way one has the additional equation

- E^ TD = N v' a" e k T i <5'9>

c th t

Using eqs (5.8) and (5.9) the values E. = -0.40 (± .02) eV a = 10" 1 S cm2

were found for the most prominent defect, and for the smaller peak at T = 129 K the values are m

E f c = -0.29 (± .02) eV n = 10" ^ cm2

In f i g . 5.5 these values are i l l u s t r a t e d in a bandgap scheme, which also shows the most important vacancy re la ted poin t defects in s i l i c o n . Both the divacancy (V-V) and the phosphorus-vacancy (P-V) have an energy level which corresponds approximately to the value of E. found for the predominant de fec t .

- 107 -

Fig. 5.4 TSC curves for one sample, allowing increasing de-excitation times ( 1 to 7 minutes) at Tj_ = 144.3 K. The graphical determination of in = 310 s is shown in the insert (b).

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100 MO 120 130 140 150 160 170 180

TEMPERATURE ( K )

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(G8) IG2)

101)

Fig . 5.5 Deep energy leve ls in the forbidden band gap of s i l i c o n . At the l e f t , the two leve ls found in n- type , high r e s i s t i v i t y s i l i c o n a f t e r muon i r r a d i a t i o n . At the r i g h t , the energy l eve l s and cheir charge s t a t e s are given for a few iden t i f i ed d e f e c t s , r e l a t e d to the s i l i c o n vacancy. The charge s t a t e s which can be observed by Electron Paramagnetic (Spin) Resonance (EPR or ESR) a r e indicated by a c i r c l e and the corresponding EPR spectrum i d e n t i f i c a t i o n i s wr i t t en below (G7) e t c . [ 5 . 1 ] .

The phosphorus concentra t ion in the high r e s i s t i v i t y s i l i con i s lower than the observed defect concentra t ion , which favours the i d e n t i f i c a t i o n of the defect as the divacancy. Watkins and Corbett [5.12] have assoc ia ted the EPR spect ra G6 and G7 to the V-V+ and V-V~ charge s t a t e s r e s p e c t i v e l y .

- 108 -

G7 is observed in high resistivity n-type Si, if the Fermi level is situated below (Ec - 0.4)ev. Raima and Corelli [5.13] studied two energy levels, (Ec - 0.39 eV) and (Ec - 0.54 eV), using photoconductivity as function of the wavelength of polarized light. They compared stress-induced reorientation and annealing behaviour for photoconductivity, infrared absorption and EPR, and their conclusion was that the energy levels represent indeed two different charge states of the divacancy.

Tne annealing behaviour observed for the muon irradiated samples also supports the interpretation of the defect as divacancy. The peak height was unchanged six months after irradiation and first measurement, whereas it disappeared gradually upon annealing between 150°C and 350°C. The phosphorus-vacancy defect shows some annealing, even at room temperature, and disappears at 150°C.

The peak height shows a /v B dependence, which indicates a homogeneous distribution of defects through the crystal, as expected with high energy particles. The area below the TSC peak is proportional to the number of traps, and supposing that all defects become charged at low temperature, an approximate calculation of the defect concentration is possible. The silicon volume can be determined from a C-V measurement. The introduction rate of .2 defects per cm 3, normalized to incident ^ 2 GeV muons per cm 2, was determined for the principal defect. For the second defect the introduction rate was found to be different for different batches of silicon, between .02 and .1 cm" 1. The introduction rate for the principal defect was found to be a factor 40 higher in 400 iicm and 8000 ficm n-type Si, irradiated later in the SPS muon shield with -\- 20 GeV muons.

5.1.4 Comparison with electron and neutron irradiation In the muon shield of the PS-neutrino beam not only muons were

present, but also secondary electrons {delta-rays, as discussed in chapter 3) and an important neutron background. Measurements of the neutron flux and the neutron energy spectrum were made by Hôfert and Coninckx [5.14]. In the first measurement gap, after 2.55 m of iron shielding, the total neutron flux is nearly equal to the muon flux, but the profile is flatter. The ratio neutron/muon is ^ .5 on the beam axis and ^ 1.5 at a radius of .45 m. In the second gap, after 3.65 m the ratio n/p is ^ .03 and in the third gap, after 5.55 m of iron it has dropped to .002 (on axis).

Measurements of the neutron flux in the SPS muon shield showed a negligible neutron component, in comparison to the muon flux. The reason is, that the neutrons originate from the proton-beam target, which was

- 109 -

quite close to the muon shield in the PS (see fig. 5.1) but far away in the SPS (fig. 6.1).

The neutrons cannot be responsible for the point defects observed after the muon irradiation. First, because after the SPS irradiation the same defects were found, with a higher introduction rate ("v- 8 cm' 1). And also, because the concentration of defects is linear with the muon dose (fig. 5.3), but does not correspond to the neutron dose in the different irradiation positions. Finally, by irradiating identical silicon samples in a nuclear reactor neutron irradiation facility, the effect of thermal neutrons was studied separately. Thermal neutrons form the largest part of the neutron flux in the PS shield [5.14]. The irradiation took place in the Austrian research reactor ASTRA. Some details about this facility can be found in the compilation of radiation damage test data by Beynel, Maier, Schonbacher and Stolarz-Izycka [5.15]. The neutron flux in the standard neutron irradiation facility (SNIF) is 4.8 x 10 9 n cm" 2 s" 1, and in three irradiations, different samples received 4 x 10 1 2, 2 x 10 1 3

or 1011* n cm" 2. The TSC curve in fig. 5.6 for n-type Si after a dose of

'CO 120 140 160 i80 100 120 140 160 8 0 200 100 120 140 160 180 2 0 0 '

Tempcro'ure ! K i

Fig. 5.6 TSC curves obtained with different n-type and p-type silicon samples, after irradiation with GeV muons 6°Co gamma rays or reactor neutrons. These curves show that a variety of defects can result, depending on the properties of the silicon itself and the type of radiation. In some cases is not only shown the current after forward bias excitation which fills the deep traps at low temperature,but also the current increase without this excitation.

- 110 -

10 l " cm"2 shows a weak peak at 180 K, and for p-type Si there is only a general increase of current, without any peak. At room temperature the diodes have a high reverse current, apparently due to the radiation damage, but which is not related to point defects. In sect. 5.2 it will become apparent that the neutron damage is the most severe in the detectors. However, the TSC method does not help to identify this type of damage. This comes from the fact that the neutron damage clusters present such a disorder, that no clear energy levels are created.

The effects of low energy electron and gamma irradiation can be studied using a 6 0Co irradiation cell. The gamma rays of 1.17 MeV and 1.33 MeV convert in electrons, which cause the major part of the radiation damage in the silicon. In fig. 5.6 the TSC curves are given for several irradiation doses (3 x 10 6 rad to 6 x 108 rad) . In p-type Si a single defect level is visible, with T = 128 K, which appears also in the muon- irradiated p-type sample. In the n-type Si the TSC peaks after 6 °Co irradiation are completely different from those after muon irradiation, and they are even different for two different n-type Si batches. The 1000 s2cm sample shows a peak with T = 147 K which is

m absent in the 4600 i2cm samples.

The comparison of the TSC curves of muon-irradiated samples on the one hand (figs 5.2 and 5.6) and 6 °Co and neutron irradiated samples (fig. 5.6) on the other hand, shows at least one typical muon-related defect level, with T = 163 K.

m

5.1.5 Discussion From the three methods employed, only the TSC measurements have

permitted to monitor the introduction by the muon irradiation of at least one point defect, with an energy level E - E. = .4 eV. Because the initial phosphorus concentration in the n-type Si was lower than the final defect concentration, it is likely to be the divacancy, rather than the phosphorus-vacancy defect. Also the annealing characteristics support this identification. The introduction rate of this defect is higher than what is usually found in 1.5 MeV electron irradiations, especially at the SPS muon energy.

It should be remarked, that the defect concentration is after all very low compared to the possible concentration of unintentional, electrically inactive impurity atoms in the silicon. These may give rise, with or without a related vacancy, to a multitude of deep energy levels (see e.g. Milnes [5.8]), and could cause a significantly different behaviour for silicon of different origin.

- Ill -

Unfortunately, the effect of the most deleterious component in the radiation, namely the low energy neutrons, could not be studied by the TSC method as no clear peak in the current curve is generated, but rather an overall increase of current.

As the divacancy can be annealed by heating to 350°C this might present a possibility of recuperating damaged silicon slices, provided the mounting stands this temperature. No fast diffusing metal, like gold, should be on the slice. Aluminium is tolerable, as long as one does not approach the eutectic temperature of Al and Si (577°C).

5.2 Degradation of silicon detectors The microscopic defects created by the energy transfers to atoms in

the silicon lattice act as generation-recombination trapping centres for the charge carriers. They cause an increase in the reverse current of a diode and a shortening of the minority carrier lifetime. For a transistor it results in a decrease in current amplification, and e.g. at CERN Backenstoss et al. [5.16] used this effect to study the radiation damage caused by 590 MeV protons. For a silicon particle detector the main deleterious effect is the noise, which accompanies the increased leakage current. The current also may prevent full polarization of the diode, which then causes charge loss in the undepleted region. Finally, if the carrier lifetime becomes very short, charge loss may occur via recombination at trapping centres (sect. 4.2.2).

A different kind of device degradation occurs, if there is a change of the effective resistivity of the silicon, due to the compensating effect of the deep energy levels. Ultimately, this may lead to a change of the conductivity type, e.g. from n-type to p-type.

A study of the effects from various types of radiation was made, using different prototype detectors, in order to select the detectors for the NFM system (see also 7.1.2). Afterwards, the behaviour of the selected detectors in the muon flux could be continuously monitored via their leakage current compensation (8.2.3) and via comparison with calibration detectors which were most of the time kept outside the radiation. At regular intervals also the capacitance of the detectors was measured, as a function of the bias voltage. Finally, minority carrier lifetime measurements were made for the junction detectors, using the reverse recovery method. In the following sections the results of this work will be described.

- 112 -

A pictorial representation of the radiation history of most of the

NFM detectors is given [5.17] in fig. 5.7 and shows above all that the

degradation, measured by current and detector noise, is worse for high

resistivity, n-type silicon. This can be understood if the defects have

primarily an acceptor-type behaviour, and thus cause compensation of the

available donors.

••• normal operation

— degraded operation

o epoxy failure

* total failure

(a)

-i i i i 11 J_ i i i

•I 1

- J 1 I I — 1 1 , 1

10* 10" 10" 10"

Fluence muons cnv 2

10'' 10'

Fig. 5.7 The total muon dose (number of muons per cm2 ) until 1981 received by the silicon detectors employed in the NFM system. The operational status of each detector is indicated. For some detectors complete failure (*) occurred, others had a recoverable epoxy failure (o), still others could continue operation for some time, while degraded ( ) ; due to high noise, high leakage current or incomplete charge collection. The class (a) detectors are n-type surface barrier detectors, from top to bottom in order of decreasing resistivity ( 25 kflcm to ^ 1 kftcm); (b) are ion implanted detectors of ^ 1 kucm Si; (c) are p-type Si diffused detectors of ^ 2 kûcm.

- 113 -

5.2.1 Radiation test of prototype detectors From 5 commercial manufacturers 20 prototype detectors were

purchased. Initial measurements were made of diode current and capacitance as a function of applied reverse bias voltage (I-V and C-V characteristics) and energy resolution for a-particle detection. Some detectors were then stepwise irradiated with y~radiation in a 6°Co cell and after each step the characteristics were measured. The other "dummy" detectors were not irradiated, but kept for comparison. After this test the same detectors were irradiated in the neutron flux of the ASTRA reactor (see 5.1.3). Finally, the sensitivity of the detectors in the muon flux was determined and could be compared to that of the unirradiated dummy.

5.2.2 Diode reverse current and minority carrier lifetime The diode reverse current is the sum of several components. There is

an electron-hole current i caused by thermal generation in the space charge ("depletion") region. It is described by the Sah-Noyce-Shockley model, assuming that there is only a single deep energy level for generation-recombination (see e.g. Milnes [5.8], chapter 8).

n. i = q A ) L -^- (5.10) g ^ D 2TO

This generation current i is proportional to the depleted volume, and thus to x D (formula (1.3)), to the intrinsic carrier density n^ = /np (formula (1.1)) and inversely proportional to the mean carrier lifetime T 0. The introduction of radiation defects reduces the carrier lifetime and therefore increases i . The current increases exponentially with the temperature, because of n. (formula (1.1)).

A second component to the reverse current is the generation of minority carriers in the undepleted region, within a diffusion length of the space charge region. This so-called saturation current is obviously not present in a totally depleted diode. If there are undepleted regions on both sides of a p-n junction the current may be expressed by the following formula

Vi-Vi p ' n (5.11)

D and D are the diffusion coefficients (table 1.1) , p and n are the minority electron and hole concentrations in the corresponding regions of the junction. Also the saturation current is inversely proportional to the carrier lifetime, and therefore sensitive to radiation damage.

- 114 -

A third contribution to the current, in the case of surface barrier detectors, comes from the electron emission over the potential barrier V,,

i = A T' e e

q(V E - AV) kT (5.12)

A is the Richardson constant for thermionic emission. The correction term AV represents the barrier lowering by the image force effect (see Sze [1.8] , chapter 8) .

The final, and often the most important current component comes from generation at surface states or conduction through surface channels at the edge of the device. Although this surface leakage current is primarily related to the manufacturing procedures, it is known that surface oxides, used for passivation, are extremely sensitive to radiation damage. An inventory of the understanding of charge effects at the Si-Si02 interface has been compiled by Deal [5.18].

The minority carrier lifetime To can be measured in a diode via the reverse recovery lifetime t , which is visible if the diode is

s switched from forward to reverse bias. The diode remains conducting in the reverse direction, as long as there are still stored minority carriers to sustain this reverse current V After time the reverse current, which initially can be adjusted to be equal to the forward current i f, recovers to a much smaller steady-state value, with a time t , as shown in fig. 5.8. The behaviour is described by the error function erf

erf > T° 1 + xr

(5.13)

if

TIME—^

0.1 ir / ^

\ ( _ t s t r —

Fig. 5.8 Diode current transient observed after switching a diode from forward bias (current if) to reverse bias. During a time

which t s a reverse current i r flows, t,. to 0.1 limited

afterwards decays in a time The reverse current i r is external resistance and the forward adjusted to make if = i r.

by bias

an is

- 115 -

2 6 - 10 - 1976 1 - 8 - 1 9 7 8 2 8 - 2 - 1 9 8 3

Fig. 5_.£ Oscilloscope pictures of the reverse recovery transient for diffused detector QDA 10 (number 323). The minority carrier lifetime degraded from ^ 20 us in 1976 (left), via 11 ys in 1978 (middle) to 4 ps in 1983 (right). The accumulated muon fluence was approximately 2 x 1 0 1 2 cm"2 in 1978 and 8 x 1 0 1 2 cm"2 in 1983.

In a forward-biased surface barrier diode there is no minority carrier injection, and quite often there is no forward current at all, indicating that also the rear "ohmic" contact has become rectifying. Therefore, the reverse recovery cannot be used to determine the carrier lifetime in surface barrier detectors.

In all detectors an increase of leakage current with irradiation dose has been seen, although for some detectors preceded by a decrease in the early stages. The reverse current as a function of /v~ is plotted in fig. 5.10 for a n-type Si surface barrier detector (PBB1) , before and after irradiation with 2.4 x 10 7 rad 6 0Co gamma rays. The current at the total depletion voltage (38 V) has increased from .2 yA to 2.3 \ih, and develops mostly over the range 0-7 V. The C-V characteristic for this detector after irradiation indicates total depletion at ^ 7 V, instead of 38 V. Apparently, the deep energy levels, responsible for the current, have caused a resistivity increase, or even a type reversal. They seem to be responsible for the current, which up to 7 V increases linearly with /vl. The current becomes gradually lower with time, which might indicate a room-temperature anneal of the defects. The neutron irradiation (2 x 10 1 3 cm"2) which took place afterwards, destroyed practically the detector, as can be seen from its dramatic current increase at very low bias. Total depletion has shifted to 165 V, and the current is "v 70 \ih at that voltage.

116 -

3 -

<

after 2*10 3n cm"2

Sep 76

after 60Co Feb. 76

March 76

April 76

initial Jan. 76

10

bias voltage /v

Fig . 5.10 Leakage cur ren t as funct ion of b i a s voltage (/V) for de tec to r PBB1, before i r r a d i a t i o n and a f t e r 2.4 x 10 7 rad of 6 0 Co gamma rays . In following months there i s a gradual recovery. After a fu r ther i r r a d i a t i o n with 2 x 1 0 ' 3 neutrons cm"2 the current becomes very high (i- 70 pA). The r e s i s t i v i t y of the n-type Si was o r i g i n a l l y •N. 9000 iîcm.

A second example of leakage current degradation i s shown in f i g . 5 . 11 , for detector QDA51, which i s a diffused detector on p-type S i . The 6 0 Co i r r a d i a t i o n , which coloured a Pyrex t ray dark brown, hardly influenced the diode reverse cu r r en t , contrary to the tenfold increase observed in the n-type Si de tec tor ( f ig . 5.10). However, a lso in t h i s case the neutron i r r a d i a t i o n was very des t ruc t ive and caused a "v- 25 uA reverse c u r r e n t . Also the apparent r e s i s t i v i t y decreased from ^ 5000 ftcm to •v 400 ftcm and t o t a l depletion sh i f ted from 23 V to 70 V, ind ica t ing an add i t i ona l "doping" with acceptor- type r ad i a t i on de fec t s .

An overview of the prototype t e s t s i s given in table 5.1 and for a l l de tec to rs the reverse c u r r e n t s before and after i r r a d i a t i o n s are shown. I t can be seen tha t the c u r r e n t , normalized for the detector volume, i s general ly la rger for the small d e t e c t o r s , which ind ica te s the importance of surface and edge c u r r e n t s . The detec tors which were not i r r a d i a t e d kept a f a i r l y s t ab le reverse c u r r e n t . The gamma i r r a d i a t i o n caused a s i g n i f i c a n t increase for 3 de tec to r s (the bigger ones, with higher r e s i s t i v i t y ) and no e f fec t or even a s l i gh t decrease in current for 6 o t h e r s . The neutron i r r a d i a t i o n caused a ca tas t rophic current increase in a l l t e s t ed de tec to rs (factor 100-500). The de tec to rs QDA52 and QDB1 were used in gap 1 respec t ive ly gap 2 for a 10-day run in the PS muonflux, where they received nominally ^ 1 0 ' 2 respect ively % 1 0 ' x muons cm" 2 . But in addi t ion they a lso received the neutron f lux, as described in 5 . 1 . 3 ,

TABLE 5.1

Test of prototype detectors

INITIAL CHARACTERISTICS AFTER IRRADIATION

Detector Type Dimensions 2

mm x Mm

Si Type

licon Resist. De m

Bias vol Full

Depletion

tage (V) Operation

Initial Leakage current

pA pA/cm3

t 0Co y dose rad (Si)

Leakage Current after y pA/cm3

Energy resolution FWHM '"Am

keV

Neutron dose n/cm2

Leakage Current after n pA/cm3

LIA 1 Implanted 30.2x100. n 1700 11 50 .48 159. 20 LIA 2 Implanted 30.2x105. n 2800 9 50 .56 177. 2.4x10' 95. 17 10'» 14800. OIA 1 Implanted 25. xl01.4 n 720 23 60 .04 16. 2.4xl07 24. 18 10'» 6490. OIA 2 Implanted 25. x 93.2 n 1080 15 70 .06 26. 16 OKA 1 k. surf. bar. 30. x 90. n 720 19 50 .305 113. 2.2x10' 83. 16 10 , s 10500. OKA 2 k. surf. bar. 30. x 93.2 n 780 22 50 .24 86. 14 QDA 51 Diffused 30. xl07. P 5600 23 50 .071 22. 2.4x10' 25. 30 l O " 6822. QDA 52 Di ffused 30. xl07. P 4100 22 50 .076 24. 1 run PS 460. 40 250.

LBB 11 surf. bar. 110.4x305. n 3300 65 70 .117 3.5 2.4x10' 10. 19 4x10" 327. LBB 12 surf. bar. 108.5x295. n 1800 110 150 .114 3.6 18 NKB 1 k. surf. bar. 100. x320. n 2400 65 120 1.4 56. 2.4x10' 38. Rear side

better 23 NKB 2 k. surf. bar. 100. x312. n 4700 58 120 .43 14. 18 PBB'l surf. bar. 105. x288. n 6700 43 150 .29 10. 2.4x10' 76. Rear side

only i> 26 2x10" 1950.

QDB 1 Diffused 100. x310. P 8200 72 100 .83 27. 1 run PS 64. 36 QDB 2 Diffused 100. x310. P 8800 60 100 .52 17. 30

NKC 1 k. surf. bar. 200 x510. n 4900 120 230 .3 29. 2.4x10' 176. Rear side only *** 30

NKC 2 k. surf. bar. 200. x515. n 4400 145 250 1.7 16. 27 OKC 1 k. surf. bar. 450. x219 n 5900 40 85 1.96 20. 1.8 PBC 11 surf. bar. 219. x513. n 5900 151 200 .46 4. 3. 19 PBC 12 surf. bar. 222. x505. n 8900 113 200 .62 5. 2.35x10' 83. Rear side

only 'x» 40

Detector type: Ion implanted detector (see sect. 1.2.3) Diffused detector (see sect. 1.2.1) Surface barrier detector (see sect. 1.2.2) "Key hole" surface barrier detector - detector with guard ring structure

- 118 -

which caused a reverse current degradation of 20 x for QDA52 and 2x for QDB1. Several detectors similar to these QDA52 and QDB1 were exposed later in the SPS muon shield to a much higher integrated muon flux without any degradation (fig. 5.12). This enables the conclusion that it were indeed the low energy neutrons which caused the degradation in the PS.

0.15

< 3.

•r 010

0 . 0 5 -

r - T : i ; i i i i | i i

(a)

/ _ _ _ _ _ _ - ( d ) -

-~ ~~ / / (b) — - " / y / —

^ ^*S^~^^"~

S —- *** ^^"^r^^ (w ~~ / --"**' J^-^^^^'*^ / ^ ** ^ ^\^^^

/ ^ ** ^^***^\^^*/^ — / <•"'* ^ * * * * * * * ^ _ ^ — ^ ^ " " " \ , ^ ^ ^ — / ^^-^^--~~~ \ s ^

/ - ^ ^ ^ ^^*^ j ^ > - - ^ ^ ^

i i i i i i i i i 1 i i i

10

bias voltage /v

30

- 25

20

« <

- 10

Fig. 5.11 (a) 24-1-1976 before irradiation; (b) 24-4-1976 after 2.2 x 107 rad of "Co; (c) 1-5-1976; (d) 7-11-1976 a f t e r lO 1 " neutrons cm"2 ( r igh t -hand s c a l e ) ; (e) 20-9-1977. Leakage current as function of bias voltage (/Vg) for •v 5000 ftcm p-type resistivity. After 2.2 x 107 rad of changed much, but after 10'* neutrons the current is i 25 pA afterwards.

detector QDA 51, which has 6"Co the current has not

It recovers gradually

COMPARISON OF SAME TYPE DIFFUSED DETECTORS

1.5 OET 152 PS EXPOSURE MUON SHIELD

% JAN. 76

. MAY 76

-1.0 • FEB. 78

• SEP. 77

0.5

DET 117 SPS MUON SHIELD

Fig. 5.12 The exposure of two similar diffused, p-type detectors led to a low reverse current for detector QDA17 (det. 117) in the SPS muon flux and to a high current for detector QDA 52 (det. 152) in the PS muon flux, to which a nearly equal but undetected neutron flux is added, and which originates directly from the nearby target region.

Fluence muons cm" z «10'

- 119 -

5.2.3 Change of diode capacitance The diode capacitance is inversely proportional to the square root of

the applied reverse bias voltage and the silicon resistivity, according to formula (1.4). The radiation induced defects can act as additional doping, either compensating the existing doping impurities or adding to it. The energy levels of the radiation defects generally are deep in the bandgap (.2 - .5 eV) , whereas the normal donor or acceptor levels are shallow (phosphorus E .044 eV, boron E + .045 eV) Therefore, the Fermi level in the semiconductor moves towards the middle of the bandgap ("intrinsic silicon") if the concentration of radiation defects becomes important.

The C-V characteristic and also the total depletion voltage may change, as mentioned already in the previous section. A complication arises from the fact that the change of charge-state of deep levels has a relatively long time constant. Therefore, the measured capacitance of a diode becomes frequency dependent if deep levels are present. At low frequency the deep levels can adapt their charge state with the variation of the space charge region, imposed by the measurement signal. But at high measurement frequency, e.g. at 1 MHz, the deep levels remain in the same charge state. The result is that the effective free carrier density is very low at high frequency and the measured diode capacitance approaches that of the totally depleted diode, regardless of the applied bias voltage.

An analysis of the frequency dependence of an n p diode with deep energy levels was made by Schibli and Milnes [5.19]. An extension to the particular case of a large density of a deep level acceptor impurity N. together with some shallow donors N,, was given by Schibli [5.20]. An overview of the theory can be found in Milnes ' book [5.8] in chapter 8. The equivalent circuit for the frequency dependent capacitance of the p-n junction can be represented by a series connection (fig. 5.13) of two capacitances C, and C f. C, is the regular junction capacitance (formula (1.4)) measured at low enough frequency. Cf is the frequency dependent part which accounts for the penetration of the variable space charge into the undepleted region.

I 1 Cdc C f I

Fig. 5.13 Equivalent circuit for the frequency dependent capacitance of a diode, containing deep energy levels as well as shallow impurity levels. C(jc is the regular, bias dependent part and Cf is the frequency dependent part. I

-diode J_+_L Cdc C f

- 120 -

V2a2e W l

k T Nt ^T <5-15> a) is the capacitance measuring frequency, N is the deep level concentration and uu describes the capture frequency at the deep level, which is the inverse of the trapping/detrapping time constant. In the case, treated by Schibli, ui = c'm where m is the free carrier density when the Fermi level coincides with the deep level E?, and c is the capture coefficient of the neutral deep level for a free electron. At high frequency to the value of C f may become small and is then determining for the measured diode capacitance.

This behaviour is illustrated in fig. 5.14, where the capacitance of detector PBC12 is plotted as a function of measuring frequency, for several doses of gamma irradiation. For increasing dose the frequency dependence becomes more pronounced (curves a, b, c). At the same time, the compensating effect of the deep levels is visible in the overall lowering of the diode capacitance. Under continued irradiation the trend reverses and the capacitance increases again (curves d, e). This might indicate a reversal from n-type to p-type silicon, but it also has to be assumed that the aluminium rear contact has become the rectifying barrier.

The effective C f were calculated for the measurements (e) taking the 1.59 kHz capacitance value as C, . There are not enough data to fit

-1/2 the a) ' dependence of formula (5.15) , but the influence of the deep energy levels is clearly seen.

5.2.4 Energy resolution The energy resolution of the prototype detectors was measured with

2l>1hm a particles incident on the front or the back electrode, and resulted in spectra like the ones already shown in fig. 1.4. The width of the main peak at half of the peak height (FWHM) was for most detectors between 15 and 20 keV, generally 1 or 2 keV better on the front than on the back. After the gamma irradiation the energy resolution was the same for those detectors which did not change in leakage current. The others, mainly those with higher resistivity, showed a degradation especially for the front incidence. On the back of the detectors the oc-spectrum became visible at much lower bias voltage, and was of much better quality than that obtained on the front electrode, although worse than the spectra obtained initially. These results are also summarized in table 5.1 and once more suggest a type reversal from n-type to p-type, with the aluminium contact becoming the rectifying contact. The holes, which in a surface barrier detector are collected on the gold contact apparently traverse the damaged detector thickness more easily than the electrons, whereas in the undamaged detector the electron collection is often better. This indicates that the radiation defects are acceptor type, which reinforces the previous conclusions of type reversal.

- 121

Q .

OJ i-i c no G m a.

T3 o

I i 1 i i i i i | i i I I 1 "1 1—I | | I L

DETECTOR PBC 12 ' I

m - * - ^ \ .

10 ^ ^ ^ \ " V ~~= a) - - (ei _ ^^ \ : (b)

(c) —i^i^^^llf^l^-^jV

1

(b)

(c) ^ N.

1 i I I 1 1 1 1 1 1 1 1 1 1 1 1 ! 1 1 1 1 1 1 1 i-

10 100

Measuring frequency (kHz)

1000

Fig. 5.14 Measured capacitance of detector PBC12 as function of measuring frequency at 2 V reverse bias. (a) Initially, the capacitance is the same for all frequencies; (b) After 1.1 x 10 s rad of 6 0 C o . The compensation of the original doping is visible

as an overall decrease of capacitance; (c) The capacitance has even more decreased after 2.2 x 10 6 rad, but also the frequency

dependence is more apparent; ( d) After 1.15 x 10' rad the capacitance is increasing again, but the frequency

dependence remains; (e) After 2.35 x 10' rad the capacitance at 2 V is 530 pF for 1.59 kHz and 150 pF at

1 MHz. The total depletion value is 60 pF at 1 MHz. The total depletion value is 60 pF respectively 40 pF;

(f) Calculated Cf corresponding to (e). The arrow indicates a -1/2 dependence.

The damage from the neutron irradiation was such, that no reasonable a-spectra were measured any more. Signals were only visible at very high reverse bias voltage but resulted in peaks of several hundred keV width.

The charge collection in the prototype detectors in the muon flux test showed severe degradation after the neutron irradiation. Even at a much increased bias voltage a charge loss was observed for 4 detectors out of 7. All were more noisy. In table 5.2 a comparison is made of the S (formula 1.7) of detectors before and after neutron irradiation.

D

The charge loss is related to carrier trapping at the radiation induced defects and depends on the integration time and the detrapping time (see chapter 4).

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TABLE 5.2

Collection efficiency before and after neutron irradiation

Detector Before irradiation Minimum voltage D

Neutron dose

After irradiation Minimum ,. S„ Charge loss voltage D °

H A 2 OIA 1 OKA 1 QDA 51 LBB 11 PBB 1 QDB 12

15 30 15 50 60 45 50

1330 1600 1800 1650 119 124 193

10'" 10'" 10'" 10'"

4 x 10'3

2 x 10'3

2 x 10'3

110 75 60 100 70 165 100

3000 2150 2300 1650 120 185 195

56% 25% 22%

-v- 0% -v. 0% 33%

t 0%

5.2.5 Long term muon irradiation in the SPS The degradation of all detectors installed in the SPS neutrino filter

can be continuously monitored via the leakage current measurement, as will be described in 8.2.3. In addition to the average value of the leakage current, also an indication of the current noise is available via the offset measurements, which are performed every 15 min. The r.m.s. value of a large number of these offset measurements is shown for example in tables 7.2 for several runs (column "offset"), or for several detectors in one run in table 7.4 (column "noise"). Also the measured leakage currents are shown in these tables. It may be noticed that an increase of leakage current generally is stopped by diminishing the bias voltage (table 7.2(b)). Obviously, this influences the charge collection efficiency, and makes a recalibration of the detector necessary. The calibration procedure will be described in chapter 9. After recalibration the detector still can be used, although one might regard this a "degraded" status. In fig. 5.7 such degraded operation is indicated by a full line. Complete failure of a detector is indicated by an asterisk.

Examples of leakage current degradation will be shown in figs 7.1 and 7.2, and it will become clear that also other causes of degradation may exist, besides the irradiation. Especially humidity should be mentioned, but also some detectors are found to be sensitive to the oxygen of the air and have to be kept in an inert nitrogen or argon atmosphere.

The absence of an important neutron background in the SPS muon shield alleviated considerably the problem of radiation damage, in comparison with the situation in the PS muon shield, where a rapid detector degradation had been observed.

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5.3 Conclusion The macroscopically observed degradation of silicon detectors can be

understood in terms of microscopic atomic displacements, and some indications on the properties of these microscopic defects were obtained.

Comparisons between several types of radiation once again showed that low energy neutrons are the particles which cause most damage. Because ionization detectors do not detect these neutrons, they are easily overlooked in an experimental situation, and this may complicate the estimation of radiation damage effects to be expected. From the 6 °Co gamma irradiation was learned that the effects are very much depending on the type of the silicon material and the way it is processed.

Detector degradation by radiation defects is unavoidable, but one can reduce its incidence on the measurement by designing a system with possibilities of current compensation, recalibration and redundancy. Also one has to keep in mind, that electronic parts often are much more sensitive to the radiation than the detectors themselves.

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PART II - MUON FLUX MEASUREMENT FOR THE SPS NEUTRINO BEAMS

In this part the technical aspects of the muon flux measurement in the SPS neutrino beams are described. First a fairly detailed description of the neutrino experimental area is given in chapter 6, followed by the description of the muon detector system NFM in chapter 7. The electronics for the NFM system is treated in chapter 8, and in chapter 9 the detector calibration procedures are discussed. Chapter 10 is a presentation of the results of muon flux measurements under various beam conditions. A range measurement was performed while varying the parent beam momentum, and this is the subject of chapter 11.

6. THE SPS NEUTRINO BEAMS 6.1 The West Area Neutrino Facility WANF

The Super Proton Synchrotron Accelerator can provide proton beams up to 450 GeV to experimental zones in the North Area and in the West Area. The neutrino beams are directed to the West Area where a number of massive neutrino detectors is aligned along the beam axis. A scale drawing of the whole WANF is shown in fig. 6.1. First comes the Big European Bubble Chamber (BEBC), with its related external detection systems. Second in line is the counter building with the 1400 t electronic detector of the CDHS (CERN-Dortmund-Heidelberg-Saclay) Collaboration, (experiment WA1) which can be equipped with a steel target or a H2 or D 2 target of 35 m 3. In the same building follows the "fine grain" electronic detector of the CHARM (CERN-Hamburg- Amsterdam-Rome-Moscow) Collaboration, WA18. The next building housed the heavy liquid bubble chamber Gargamelle (GGM) until its breakdown during 1978. In the last building the Bologna-CERN Collaboration WA44 has constructed an apparatus to search for quarks in high energy neutrino interactions.

A "Wide-band Neutrino Beam" Ni (WNB) and a dichromatic "Narrow-band Neutrino Beam" N3 (NNB) are installed, and can be used alternatively with the respective proton targets T9 or Til and their related secondary beam elements. These are situated 35 m underground in the neutrino cave, at 400 m distance from the extraction in the main SPS ring. Access to the neutrino cave is via the auxiliary building BA7, where also the power supplies and controls for the installations in the cave are housed, including the Neutrino Beam Control (NBC) NORD-10 computer. A list of exact distances in the WANF layout can be found in Appendix A. We use the Neutrino Beam Coordinate System, the x-direction along the WNB axis with the origin 2 m upstream of the projection of the construction coordinate "R" on this x-axis, i.e. 1.45 m before the present target centre point "T9".

IRON PLUG LATER MODIFICATIONS (1979)

cT~~ibm PIT e

BLDG 274

PIT

BLDG 274

SHIELDING MAGNET 11979) n

BEAM DUMP CAVE (1982)

[PfJ:

J » n U -

BE8C | | w>lh I—AIR"- EARTH SHIELD

EPI EMI IRON SHIELD wilh

MEASURING PITS - — NEUTRINO CAVE — I

I T 9 . T "

EXTRACTEO PROTON BEAM

Fig. 6.1 Scale drawing of the West Area Neutrino Facility WANF. The region of the iron shield with flux measurement pits is also shown enlarged. Later modifications are indicated: in 1979 a magnet just after pit 5 and additional shielding behind building 274 were installed. In 1982 a cave for the proton beam dump was built.

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Both positive and negative secondary particles are produced in the interactions of the protons in the target, mainly pions (90%) and kaons (9%) . They decay predominantly into a muon and a neutrino, and are therefore designated as "parent" particles. The parent particles are focused to form the desired beam, which points upwards with an angle of ^ 42.5 mrad. They enter a 290 m long evacuated tunnel, where some fraction decay. At the end of the decay tunnel all undecayed parents and other hadronic particles are stopped in the first few metres of a long iron shield. The rest of the shield is needed to stop the muons, so that in the end only the neutrinos are left in the beam.

The number of muons and their range are related to the number of neutrinos and their energy via the kinematics of their decay from common parents. Therefore muon flux measurement at several depths in the shield is performed by the so-called Neutrino Flux Monitoring system (NFM). This system uses another NORD-10 computer, which together with the controls for the BEBC EMI (External Muon Identifier) is situated next to the BEBC control room in Building 192. Both on-line information on the neutrino beam quality and off-line data on neutrino flux and neutrino energy spectrum are available from the NFM to all the earlier mentioned neutrino beam users. A view of the NFM access huts to the pits VI, etc., is given in fig. 6.2.

Fig. 6.2 View of the NFM measurement zone, above the muon shielding. The huts VI, etc., give access to the vertical shafts, leading to the gaps in the shield.

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6.2 The production of neutrinos and muons 6.2.1 The proton beam One revolution of the SPS internal proton beam along the 6.9 km

circumference takes 23 us. The circulating beam has a gap of 2.1 us [6.1] which allows the rise and fall of field in the injection and extraction kicker magnets, the latter deflecting the accelerated beam towards the West Area switch-yard. The extracted beam may have the SPS RF structure at 200 MHz, with 2 ns bunches (this was especially the case for the extractions during acceleration at 330 and 350 GeV/c) or it may be debunched, depending on the timing of the extraction. For the neutrino target an intensity of around 10 1 3 protons per pulse is generally obtained.

Three extraction modes are available for neutrino operation. In the fast extraction, FE, a rapid deflection of the circulating proton beam with fast kicker magnets during whole or part of the revolution time, brings the deflected beam into the field of the septum magnet, the first element of the extraction channel. The pulse duration is at most 23 us, and therefore this mode can be used only for the NNB, where the event rate in the counter experiments is rather low.

Using a half-integer resonant method the fast-resonant extraction (FRE) is obtained with a spill duration of 1-3 ms. This mode is generally used for the WNB because it presents an optimal compromise between the conflicting timing requirements for bubble chambers and counter experiments. The intensity distribution may still have the 200 MHz and 43 kHz modulations, within some Gaussian shaped envelope. The spill form can be measured via the resulting muon flux and fig. 6.3 gives examples of the different extraction modes, as seen by a muon detector.

The third mode is the coherent fast-resonant extraction, in which the protons leave the machine every other revolution during three or more revolutions. In this mode all protons are extracted, so it can be used only as the last extraction process in the cycle.

6.2.2 Proton Intensity Monitors The extracted proton beam (EPB) is brought onto target T9 or target

Til via three switching magnets. Both targets consist of beryllium or aluminium rods of 10 cm length, diameter 2, 3 or 10 mm, which are supported between thin (2 mm) beryllium plates. The overall length of T9 is 200 cm (up to 11 rods of 10 cm, 110 cm of Be). Between the rods there are 9 cm spaces. The length of Til is 50 cm of Be consisting of 5 pieces of 10 cm. Both T9 and Til target boxes provide for 3 targets and one empty passage. The target is cooled with a flow of He gas [6.2]. After the targets collimators are placed. In the NNB it reduces the wide band background and its acceptance is 2-3 mrad.

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In the wide band beam the collimator has a variable 5-8 mrad acceptance and an axial absorber ("beam stopper") can be used in the antineutrino WB to reduce the wrong sign background.

(1)

(2)

(3)

Fig. 6.3 Proton beam extraction modes, as seen by a muon detector in the shield. The detector can be used to show the instantaneous flux (a) or the integrated flux (b). (1) Fast Resonant Extraction FRE ("fast-slow") 500 ys/div; the lower curve shows the

current in a scintillator. (2) Fast Extraction FE ("fast") 10 ys/div. of a single SPS turn. (3) Coherent FRE ("multiple fast"), 6 or 7 turns of the SPS.

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For the steering of the proton beam onto the target a number of monitors are placed in the TBIU box (Target Beam Instrumentation Upstream) and in the downstream TBID. These monitors use the signal which is obtained from the secondary emission of electrons when a thin metallic foil is bombarded with the protons of the Extracted Proton Beam (EPB) [6.3]. In addition to the total intensity measurement differential signals from "split-foils" are also available for position information. The target multiplicity is obtained from the comparison of upstream and downstream integral SEM (Secondary Emission Monitors). Both TBIU and TBID are displaced simultaneously with the targets on a common girder. A description of the performance of T9 and its monitors is given by Kalbreier et al. [6.4].

The correctness of the beam steering onto the target is checked regularly via the proton intensity monitors and can also be verified via the symmetry of the muon flux (see chapter 10) . Parameters which can be adjusted by the SPS operators are the beam size (1.2 mm horizontally, 2.7 mm vertically [6.4]), the beam or target position and the beam or target angle.

The SEM's can be used in all extractions, and are standard for all SPS targets, but in the beams for the WANF proton beam current transformers (BCT) are also installed to measure the proton intensity. Actually these were used to calibrate the SEM, directly, or indirectly via activation measurements [6.5].

The proton beam represents a current of ^ 80 mA in the FE, or v .8 uA in the FRE, and acts as the "primary" winding of a transformer in which, by induction, a current is generated in the "secondary" coil. A BCT can be very precisely calibrated with a one-turn winding, carrying a calibrator current pulse from a high precision current source, and it gives, in principle, an absolute, precise and time-independent measurement of the proton beam intensity. The above mentioned SEM foils are not absolute, and their characteristic emission changes slowly with increasing irradiation [6.5].

Since 1979 a BCT was available for the FRE. It has two 180 turn coils which are capacitively coupled to obtain a differential signal, which is amplified, integrated and converted [6.6]. Because of the very low signal level, special high gain amplifiers are required, which inherently exhibit drift. During the 1979 WNB operation one of the line amplifiers slowly degraded, thus necessitating frequent adjustment of the calibration factor. A special sequence of offset readings (two times 8 ms integration time) is triggered before and after the beam pulse and the calibrator pulse, each also integrated over 8 ms. With these precautions, the intensity was measured and found to be exactly proportional to the intensity measured with the BCT installed in the main SPS ring, which are

- 131 -

more reliable because of the higher current. But after total extraction (in FRE), always 2-4% less protons are found by the BCT than there were in the ring. This difference could be explained by a true loss during the transfer or by protons which are not extracted, but left in the SPS main ring.

In the NNB and in the WNB from 1980 onwards, the most reliable number for the proton intensity comes from the BCT. A pulse by pulse comparison between this proton signal and the muon signal from the central detector in pit V2 is shown in fig. 6.4(a) for the WNB and in 6.4(b) for the NNB. Observed variations are the result of variations in targeting or focusing. The comparison between SEM signals and BCT on the one hand and muon flux signals from a Si-detector and a whole-beam ionization chamber on the other hand confirms this.

WNB RUN 105

Fig. 6.4

(a) Comparison of BCT proton signal with V2 muon signal (WNB). The points below the normal line are due to bad targeting or focusing.

7

6 -

5

(.

3

2

5 6 7 8

BCT Proton intensity

10 11 12

(b) Comparison of BCT proton signal with V2 muon signals. The Si detector signal is indicated with * (left scale) and the signal from the beam ionization chamber with o (right scale). A clear correlation between the two muon signals is observed, and it shows that the targeting and focusing vary from pulse t pulse. These points are not representative for the real fluctuations because they are selected from ^ 5 hours running.

7 r » r

I 3

NNB RUN 90

3 4 5 6 7

BCT Proton intensity

,10°

Data acquisition on all these devices is under control of the SPS WEXTR (West Area Extraction) computer, and via a CAMAC data-link a "calibrated" proton intensity is communicated to the NBC computer in building BA7. There the proton data are combined with other data from the neutrino cave and sent to the NFM computer (NBC-NFM data-link) and distributed to users. Due to this rather complex transmission and because of additional high priority tasks of the WEXTR computer, the calibrated proton data arrive irregularly and most of the time only after the next extraction. For about 3% of the pulses the register is already updated

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before it is read, so the data for those pulses are lost. Signals in analog form, which do not pass via all these links but are transmitted directly from the device to NFM, are available for the upstream integral SEM and for the BCT, a few ms after extraction. One should use therefore the uncalibrated signal and determine afterwards the correlation with the calibrated values.

The proton intensity is used to normalize the muon fluxes from pulse to pulse, which indicates the targeting efficiency. The Monte-Carlo beam simulation programs are also normalized to the proton intensity.

6.2.3 Neutrino Parents The important decay modes for high energy muon-neutrino generation

are given in the case of positive parents (reactions for negative parents are similar)

TT+ •* u + + v 100%, with lifetime 2.60 x 10~ 8 s (6.1)

K + •»• y + + v 63.5%, with lifetime 1.24 x 10" 8 s (6.2)

K + + IT 0 + ir + 21% ( 6 . 3 ) I + L-+ y. + v

y

E l e c t r o n n e u t r i n o s a r e a l s o g e n e r a t e d , b u t i n much s m a l l e r q u a n t i t i e s , v i a K + + T r ° + e + + v 5% ( 6 . 4 )

u + •* e + + v + v 100%, w i t h l i f e t i m e 2 . 2 x 1 0 " 6 s ( 6 . 5 ) e y

There is also the decay

39%, with lifetime 5.18 x 10" 8 s (6.6)

is not focussed by the beam elements.

Most of the neutrinos and muons come from decays (6.1) and (6.2). In the beam dump experiments, where one searches for "abnormal" neutrino sources, other less common, but well known decays have to be considered [6.7] .

In order to determine the neutrino spectrum, the yield of pions and kaons from proton interactions in the target should be known. Comprehensive measurements are being performed, as approved experiment NA20, in the North Area H2 beam, with 400 GeV/c protons on a Be target. Some results of the K, ir yield measurements are now available [6.8]. For technical reasons it is not possible to use an identical target, and therefore a re-absorption correction must be applied. In the actual

0 ± + KT° •* IT + e + v L e

b u t t h e u n c h a r g e d p a r e n t K '

- 133 -

neutrino targets the reabsorption of pions and kaons is minimized by having a small diameter (02 or 03 mm) rod and several separated pieces (sect. 6.2.1). The total length has been generally 50 cm for Til, i.e. ^ 1.5 interaction length, and 110 cm for T9, i.e. *» 3 interaction lengths. The effectiveness of targeting and proton usage can be monitored with the target multiplicity, measured by the SEM's but also with the muon yield, normalized to the number of protons.

6.2.4 Decay kinematics Charged parent particles are selected and focused, either in a narrow

(sect. 6.3) or a wide (sect. 6.4) momentum range and transported to the decay region, which starts 124 m after the targets. Up to that point a 200 GeV/c pion has had a 1.1% probability to decay already. The decay tunnel is evacuated at A< 1 Torr to reduce interaction of the parent particles, and for this reason also the entrance window is thin (2 mm titanium, see Appendix A). The decay angles 9, which are a function of the energy transfer to the neutrino, may then be calculated relative to the nominal beam direction (fig. 6.5). This is mainly important for the narrow band beams. The decay tunnel is 290 m long, and for a momentum of 200 GeV/c 2.60% of the entering pions and 17.7% of the kaons decay. The decay path can be shortened with a 6 m long iron hadron stopper, which is movable inside the vacuum tunnel. In this way, in the NNB, the energy definition of the neutrino by its radial distance can be improved, at the expense of intensity. However, this facility has never been used so far.

3ni„-neutrino decay angle 5 V

I TT

wm

TARGET DECAY REGION

Fig . 6.5 A pion decays in to a neut r ino and a tnuon, with decay angles 9 V and 6^ ( in the l abora to ry system), which are r e l a t e d to the neutrino and muon momentum. Therefore , the neu t r ino momentum can be inferred from the posi t ion of an i n t e r a c t i o n in the d e t e c t o r . However, the uncer ta in ty on the decay point gives a spread to the decay ang le , from 6min t o emax> w n i - c n introduces an unce r t a in ty on the energy determinat ion.

The two-body decay of the pion or kaon i s i so t rop ic in the centre of mass system ( c . m . s . ) , which i s the parent r e s t frame. The Lorentz t ransformations of momentum and angle in the c .m.s . (primed var iables) give the energy E , and angle 9 of the decay muon in the labora tory frame (normal v a r i a b l e s ) .

SHIELD DETECTOR

- 134 -

For a parent pion with mass the transformation gives

momentum

E_E' + p' p„cos6 TT U U IT

and energy E

•1 < c o s 9 ' < 1 (6.7)

tg9 M sin6'

E cose' + E' —r-TT U p'

0 < 9' < TT (6.8)

This results in a flat momentum spectrum of the muons with a range of momenta from .57 to nearly 1. times the pion momentum for muons from pions, and from .05 to nearly 1. times the kaon momentum for muons from kaons. The neutrino momentum is approximately the complement of the muon momentum. Schematically this is shown in fig. 6.6. The emission angle of the neutrino is related to its energy, but an uncertainty AE is introduced by the uncertainty of the decay point. As shown also in fig. 6.5 this uncertainty is limited by the angles 9 . and 9

min max' corresponding to a decay at the beginning or the end of the decay region

AE Y 2 [B2 - 9 2 . ] ' max min E 2

i + x_ o 2 - e 2 . ) 2 max m i n '

Y = 1//1 - B" is related to the energy of the parent particle via the velocity £ = v/c. The muon has a maximum emission angle in the laboratory frame which decreases with increasing parent momentum, as shown in fig. 6.7. The angles for the pions are of the same order as the beam divergence ( .2 mrad) but for the kaons they are much larger. These two components can be recognized in the muon flux profiles, shown in figs 3.13 and 3.14.

>

I

200 "WS=; —_i I i I i I ' I i I i i i i i i \ - ^ ^ 200 GeV parents • - —

180 \ \ ^~"-\^

y V K -y

y 160 y

/ 140

^^-~--^ v 120

_ 100 —

/ \ / \

—

an _ / \

/ \ ^—--'" v* -- / \

60 / ^ y

\ \

40 _. /

/ ^ y / ^ y ^

20 -

_i__a y

-—"""I

y

I i i , i i i i i 1 l 1 , ! I

200 190

36 54 72 90 108 126

Muon decay angle in cms (degrees)

144

114

86

162 180

Fig. 6.6 The relation between c.m.s. muon decay angle and neutrino and muon momentum for 200 GeV/c pions and kaons. The maximum neutrino momentum corresponds to the minimum muon momentum, and is obtained when the muon goes backward (180°) in the c.m.s. system. Maximum neutrino momentum and minimum muon momentum is indicated on the right vertical axis. The index IT or K indicates the muon/neutrino parent particle.

- 135 -

Muon decay angle in c m s (degrees) Parent momentum (GeV)

Fig . 6.7

(a) the transformation of the c.m.s. angles 6' into the laboratory angles 6 for 200 GeV parents for neutrinos and muons. The muon angles are restricted to some maximum value. The index TT or K indicates the muon/neutrino parent particle.

(b) Evolution of the maximum muon angle with parent momentum for muons from pions (lower curve) and for muons from kaons (upper curve), which have a much wider distribution.

6.3 The Narrow-band Neutrino Beam (NNB) [6.9] The high intensity of the SPS together with the higher number of

forward neutrinos at high energy, as can be seen from the decay kinematics, permitted the construction of a narrow-band neutrino beam, which selects a narrow momentum bite of parent v and K particles, and rejects mainly lower momentum parents. The neutrino event rate then becomes much lower, though still acceptable, with the gain of a less complicated experimental situation. The advantage of the NNB comes mainly from the neutrino energy information which is contained in the location of the vertex in the neutrino detector, given that the parents are monoenergetic. Also the rejection of wrong sign parents is more effective than in the WNB.

6.3.1 Sign and Momentum Selection of parents A detailed drawing of the installations in the cave is given in

Appendix B, both for NNB and WNB. All elements are indicated by a code, which will also be used here. The WNB elements (horn and reflector) are placed on the main axis. The NNB magnets are placed beside them, and the change from one beam to the other does not require more than one week. The proton beam direction in the case of the NNB makes an angle of 15.6 mrad with the main WNB axis, which is the direction from target to experiments. This is designed to reduce contamination of the beam from pion and kaon decays which take place before the momentum selection. The NNB axis converges with the WNB axis, and both meet near the neutrino detectors, "v» 700 m downstream.

- 136 -

Seven quadrupole magnets (QVS 111006-QVS 111024), which can be coupled in various ways (Ql -Q3) focus the parents and protons leaving the target. The particles are bent with the magnets MVC 111028-MVB111067 for the momentum selection, which is accomplished with two slits (XCHV 111069-71) which have 1 m long iron jaws, adjustable either horizontally or vertically. The momentum bite which is normally accepted is -^ = 5% r.m.s. r.m.s. Anything other than the focused particles with selected momentum, hits a large iron proton dump (TDX 111055) . Final focusing and bending directs the beam towards the WANF detectors but it makes an angle of .6 mrad with the main WNB axis. The beam divergence depends on the focusing configuration and ranges from .1 to .4 mrad. The whole beam line is pulsed with a pulse duration of ^ 50 ms and .30 Hz as maximum repetition rate. Measurements of the performance of the magnets are reported by J. Maugain [6.10].

A number of focusing configurations has been worked out for several momenta [6.1] and the beam has been used for extended periods at 275 GeV/c, at 200 and 300 GeV/c. During the long shut-down in 1980-81 the beam elements were rearranged, so as to enable running at higher energies [6.11]. The lateral distance between NNB and WNB has been slightly reduced.

6.3.2 Monitoring of the Parents Beam A number of monitors for the NNB is installed in the cave, as

indicated in fig. 6.8. On platform XCET 111102, besides an ion grid chamber and a SEM grid monitor, a Cerenkov counter is measuring the time structure of the extraction.

NEUTRINO CAVE NNB MONITORS SHIELD

SEM split foils

T8IU TBID 690739 690743

segmented ION chamber

total beam ION chamber-

SEM grid profiles

ION grid profiles

ION chamber intensity

Fig. 6.8 A schematic drawing (not to scale) of the narrow band beam monitors in neutrino cave and shielding. SEM are secondary emission foil monitors, ION are ionization chambers, BCT are beam current transformers.

- 137 -

The total intensity of the parent hadron beam is measured with two BCTs for hadron intensity measurement, similar to the one described for the proton intensity measurement, but with much higher sensitivity.

The BCT signals have to be corrected for the effect of background particles from several sources, like delta electrons, which increase the signal in a negative beam and reduce it in a positive beam. The difference between positive and negative values is around 10% [6.12].

The beam composition can be determined with a differential Cerenkov counter with variable gas pressure placed on platform XCVD 111121. This device does not discriminate individual particles, because the rate is too high, but gives an integral response, and only the relative content for each type of particle is given. In the 200 GeV negative beam pions and kaons are the main constituents, in a ratio of about 20 to 1. In the 200 GeV positive beam many protons are also present. The K/TT ratio in this case is .14 [6.13]. In both cases muons are "trapped" in the beam line. These represent a negligible fraction of the BCT signal, but later on in the shield they become a sizeable part of the muon flux.

Just after the decay tunnel, another set of monitors is installed which measure the undecayed hadron flux and are used for beam steering. These monitors are not reliable for intensity measurement because of secondary radiation generated in various places around the monitors. Up/down or left/right asymmetries are however very precisely detected.

6.3.3 Monitoring in the Muon Shield The muon flux monitoring in the iron shield can be performed with the

silicon detectors of the NFM system, as will be described in chapter 7. The plates which support a regular array of small detectors can be aligned with the narrow band beam axis, such that the detectors are in symmetrical positions and can be used for beam steering and profile measurements. Apart from the NFM monitoring system, two more muon monitors are placed in the shield specially for the NNB. The first one is a segmented ion chamber, in pit VI, which permits monitoring of the beam shape and position. The second device is a whole beam ionization chamber of 95 x 95 cm 2 in pit V2, placed against the upstream iron disc.

Results obtained with the NFM system in NNB running will be given in chapter 10.

6.4 The Wide-band Neutrino BEAM (WNB) 6.4.1 Focusing with a magnetic horn The focusing of parent particles over a wide range of momenta can be

achieved with a system of coaxial magnetic lenses, originally conceived by

- 138 -

van der Meer [6.14]. The first element is usually called a "horn" and further ones "reflectors". Their design was continuously improved during the operation of the PS neutrino beam. The first horn for the SPS was optimized for maximum flux over all neutrino energies. A second one which was used during the 1979 and 1980 WNB periods was designed to optimise flux at high momenta at the expense of a reduction at lower momentum. Design studies are facilitated by the interactive computer program RHINO [6.15] and the Monte-Carlo program NUBEAM [3.8].

In a magnetic horn, charged particles are deflected by the radial magnetic field between two coaxial conductors carrying equal and opposite currents. The field B [Tesla] at a radius r is proportional to the current i [kA] and inversely proportional to the radius r [mm]

B = . 2 - (6.10)

For a focusing lens, the bending force on a particle JB dJ- must be proportional to its radius, so the length of the field region traversed must in general vary quadratically with radius. An empirical formula derived from the thin magnetic lens approximation can be used as a first order approach to the profile of the inner conductor.

E d.9' 60 i + k E

(using r = d.9) (6.11)

which gives the length I [m] of the field region at radius r to focus parallel particles of energy E [GeV/c] and emission angle 6 [mradians] from a point source at distance d [m] from the principal plane of the lens. The factor k represents the deviation from a thin lens approximation and can take values from 1 to -1 depending on whether the effective deflection point is before or after the principal plane (k = 0.6 for the SPS horns).

To obtain the required field strength, given the large diameter of the horn, a current of ^ 100 kA is needed. The current pulse has to provide the maximum field in coincidence with the beam passage. The electrical circuit of horn and capacitor bank is a damped LCR oscillator, such that -~ > R z . The equation describing the horn current i as a function of time t has the form (fig. 6.9)

i = B e a t s i n b t (6 .12)

Current

Time ( ms 1

Fig . 6.9 The shape of the cu r r en t pulse in the horn and r e f l e c t o r .

139

The maximum current value is i M 100 kA for the horn, 120 kA for the reflector, and is determined by technical limitations. The maximum current occurs at t = x , so that the amplitude 6 can be written as

ax M e w sin b XJJJ (6.13)

The parameters a and b are related to the LCR circuit parameters r>2

a = " 2L a n d 1_ LC 41/

a and b determine the peak time x„ and the cross-over time x. The condition 1 = 0 gives b = —, _~ 0 gives b = —. For the peak time -rr = 0 and one finds

b tg b x M

The values of x and t., are 8.5 ms and 3 ms for the horn and 7.5 ms M

and 3.1 ms for the reflector. The focusing of parent particles of one sign implies defocusing of

the opposite sign. Particles of higher momentum are less affected by the field and above 200 GeV/c hardly any enrichment (or rejection) can be achieved with the present installation.

6.4.2 Timing of the extraction In fig. 6.10 the complete timing sequence for the WNB is presented,

including also the muon detector timing which will be discussed in more detail later. Originally, the timing sequence was initiated by "event 63" in the SPS acceleration cycle, which is generated 150 ms before beam extraction. From 1981 onwards, instead of "event 63" one uses "event 38", which precedes the extraction by exactly 1000 ms.

SPS

TIMING

BEAM

Event 36 J

-1000 ms

63 Extraction _±_J

-150 ms 0

l / ' N i , i i 5 ms

Fig. 6.10 Timing sequence in the WNB. The scale in the upper diagram is different from the others.

HORN

NFM

BEBC

0 5 ms CONTROL DATA BOX INTEGRATE HOLD READ

' ' 11 i i I i i "I i i • I - 3nn ms -600 -//- 0 5

F l a s h I I. i

10 ms

0 5 ms

- 140 -

6.5 The Muon Shield The main part of the muon shield consists of cast iron discs of about

14 000 kg each, with a measured density of 7.25 g cm" 3. They are 40 cm thick and circular with 250 cm diameter, except for a flat part on the bottom and the two "ears" for lifting them up. They were introduced in the 270 cm wide excavated tunnel via pit V7, which is equipped with a suitable crane, and moved on rails to their final position. Then they were blocked by injecting soft concrete all around. Seven gaps were left open, for installation of the muon monitoring arrays. They have varying widths, minimum .9 m, depending on the smoothness of the discs and the precision of stacking. Exact dimensions are given in Appendix A. The first three discs have a hole of .95 m diameter on the NNB axis, which is filled with the .50 x .50 x 1.50 m iron calorimeter block, surrounded by isolating glass wool. The mass which is lacking aroung the calorimeter is substituted by the "anticalorimeter" in gap 1, just after the first 9.25 m of shield. After June 1981 the calorimeter (and anticalorimeter) were removed to install the beam dump (6.5.2).

Until 1979 the shield had 425 discs, with a total mass of 123 kg.cm"2 to which about 44 kg.cm"2 of earth shield must be added (total 167 kg.cm" 2). Later modifications are discussed in sects 6.5.1 and 6.5.2.

6.5.1 The Shielding Magnet The shield was found to be inadequate for muons which come at an

angle of > 3 mr from the target region, at 400 GeV/c operation for WNB. These muons have a high probability of being scattered in the surrounding earth, and travel along the shield towards the neutrino detectors. Temporarily the maximum proton momentum was restricted to 330 GeV/c for antineutrino and 350 GeV/c for the neutrino WNB operation. Two solutions were proposed to reinforce the shielding [6.16] of which the magnet solution was chosen. Therefore early in 1979 40 discs were taken out of the shield, just after gap 5, and were replaced by a 10 m long toroidal magnet, with an overall diameter of 6 m and field free iron core of 2 m diameter. Fig. 6.11 is a picture taken during the installation of this magnet. The total maximum bending power at 5 kA is 12 Tm. The purpose of the magnet is, to bend muons which are outside the original shield either further outwards or back into an iron plug which provides sufficient shielding for these muons. This iron consists of 4700 t, which were placed just behind building 274 as a "plug" of 39.2 m long, by 4 x 4.5 m, slightly irregular because of the West Hall foundations. The total shielding since then is up to 180 kg.cm"2, namely 145 kg.cm"2 of iron and 35 kg.cm"2 of earth.

- 141 -

The effect of the shielding magnet can be adequately measured by comparing the flux profiles in front of the magnet and 122 m downstream just before the iron "plug". Vertically moving detector boxes were installed therefore in pit V5 and V8. They have a vertical range of -1 m to +10 m. Measurement results for different focusing conditions in 400 GeV/c WNB are shown in fig. 6.12. It should be noted that the results in V8 are obtained with a Si-detector in "pulse mode" by counting single particles, using the voltage sensitive amplifier discussed in 1.4.1. The flux here is too low for "current mode" integration.

n from bear axis

Fig. 6.12 Vertical flux profiles measured in pit V8 in the 400 GeV/c WNB with the shielding magnet at 3200 A (++++) and at -3200 A ( ) , which is in this case the "wrong" polarity. The iron "plug" extends to ± 3 m from the beam axis.

- 142 -

The muon background in the experiments is now satisfactory when the magnet is powered with 3 kA. There is some reserve left to inflect higher energy muons in a 450 GeV/c WNB with the maximum current of 5 kA.

6.5.2 The proton beam dump During the 1980-81 shutdown the shield underwent a further

modification with the installation of a proton beam dump in front of the shield. Twelve discs were taken out, including the calorimeter (and the anticalorimeter) to make place for this dump. The shielding therefore was reduced to 176.5 kg cm"2. New windows were mounted on the entrance and exit of the decay tunnel, and also the decay tunnel vacuum was improved to .1 Torr [6.17].

Three different dumps of 3.025 m length are available. They are made out of copper, and are cooled by forced air. The apparent density is different for each dump: it is one for the solid block, one half and one third for the dumps where the mass is reduced by making slits. The dump is put into place by a remotely controlled crane. The upstream end is then at .772 m from the decay tunnel exit window. In between there is a beam monitor (BSG 222420). Immediately (.025 m) behind the dump is a 3.025 m long "catcher".

- 143 -

7. THE NEUTRINO FLUX MONITORING SYSTEM (NFM) The neutrino flux, both in NNB and WNB can be indirectly measured via

the muons, which originate from common parent particles. Muon counters in the form of silicon diode detectors are distributed in the muon shield and transmit their signals to the NFM computer. The real time program OPCOM uses this information to monitor the beam quality and writes all measured data on magnetic tape. Off-line treatment yields in the first place calibration factors for the muon detectors and secondly the muon flux distribution.

This chapter treats the muon detectors, their layout in the shield and the mechanical parts which were designed to enable detector calibration. Some details were already described in [7.1] and [1.12].

7.1 The muon detectors For the early muon flux measurement at CERN one used sealed

ionization chambers [1.2]. These are still used elsewhere [7.2], but from 1969 onwards silicon diode detectors were introduced progressively in the CERN monitoring system [7.3], because they are easy to handle, they are smaller and therefore serve better as a flux probe, and they have a better long term stability. Also for the NFM system it was decided to use these detectors, and about 120 were purchased from commercial manufacturers, who fabricated them according to the specifications outlined in the next section. Some prototype detectors were tested before the final purchase was made, as described in sect. 7.1.2.

7.1.1 Detector specifications The detectors should respond linearly to fluxes from 10 2 cm"2 to

10 8 cm"2 per extraction. Normalized per second, and taking various forms of extraction into account, the limits are 10 s to 10 1 2 cm"2 s"J. As was discussed in chapter 4, problems may arise at a flux above 10 1 3 cm"2 s"1. Fluxes below 10 5 cm"2 s"1 are difficult to measure by charge integration, as is explained in sect. 8.1.4, due to the uncertainties from noise and leakage current compensation.

The detectors should have a stable and well defined sensitive volume, expressed as the reciprocal sensitivity S_ (defined in sect. 1.3.3). Four different sizes (table 7.1) were chosen to cover the full dynamic range. Individual specification of the detector dimensions was required, and these should not differ by more than 10% from the nominal value. The detector thickness is measured mechanically to better than 1 um. The sensitive area is nearly circular and calculated by taking the average of three measured diameters. These specified values and corresponding nominal sensitivities are tabulated in Appendix C.

- 144 -TABLE 7.1

Detector size definition

Type Thickness ym

Area mm 2

Nominal sensitivity

muons cm"2 pC" ' ( formula (1.7)

Lower flux limit

muons cm"2

Higher flux limit

muons cm"2

A 100 30 2440 15 000 7 x 10" B 300 100 234 1 500 7 x 10 7

C 500 200 69 400 2 x 10' D 1000 200 31 200 107

The detectors should be totally depleted at a bias voltage below 250 V. This requirement puts a lower limit on the resistivity of the silicon to be used, which especially for the 1 mm thick detectors can be difficult to meet, because 20 kficm n-type silicon with very good characteristics is not easily available.

The diode reverse current of detectors at operating bias at 21°C should initially be less than .3 uA cm"2. The range of the leakage current compensation is .01 to 10 yA, and because of progressive degradation of the detectors, a low initial value is desirable. This specification had to be relaxed however for type D detectors, which were accepted if the current was below 1 uA. Detectors with high initial current invariably are noisy and have a limited useful lifetime.

The noise of the detectors should be about 20 keV FWHM for 5 MeV a particles (sect. 1.3, especially fig. 1.4). Although this requirement seems irrelevant for operation in current integration mode, it is a conventional indication of the detector quality. More relevant was the requirement that the detectors should have less than 10 pA r.m.s. noise in the bandwidth .1 Hz to 1 kHz, but none of the manufacturers agreed to specify on this point.

Finally, a manufacturer data sheet should accompany each individual detector.

7.1.2 Prototype testing To evaluate the properties of different detector types from various

manufacturers, a prototype testing was performed on 20 detectors from 5 suppliers. Two detectors of a certain size and type were purchased, one detector was submitted to the irradiation tests, the other served as comparison. Before and after the irradiation the current voltage (I-V) and capacitance voltage (C-V) characteristics were measured. The C-V

- 145 -

measurements were done at 1.59 kHz with a Wayne-Kerr B642 bridge and at 1 MHz with a Boonton 72BD capacitance meter. Total depletion voltage and doping density were derived from a plot of 1/C2 vs V as was shown in fig. 1.1. In general, the results found at CERN before irradiation were conform to the supplier's data. The results of the irradiation tests were described in chapter 5. Three tests were performed: a high dose 6 0Co gamma irradiation, a neutron irradiation in a research reactor and a 12 hour test in the muon flux in the back stop of the g-2 experiment production target. In fact, all detectors went through this last test, in which the new detectors were compared with three old detectors which were carefully calibrated during the previous years. In this way, their sensitivity S n was determined, corresponding to the true sensitive volume. The flux in this g-2 back stop was around 101* par tides.cm"2 .

A spectrum of 2 U A m a particles was recorded both for the front ana the rear side of each detector. The charge collection as a function of bias voltage was examined, both for the a spectra and in the muon flux. A comparison was already made in fig. 4.4, where the a peak-heights for front and back irradiation and the flux charge collection were plotted as a function of the square root of the bias voltage V. The collected charge in the muon flux is roughly proportional to the depleted thickness. The charge collection for the a particles on the contrary depends on the local electric field strength.

The first conclusion of the prototype test was that none of the detectors fulfilled all requirements at the same time. Therefore it seemed most appropriate to purchase a mixture of detectors from several suppliers, so that the risk could be reduced.

All finally delivered detectors were checked on their diode characteristics (I-V and C-V) and on their sensitivity as a function of bias voltage. In fact this initial testing in the g-2 back stop was the only occasion that all detectors were in an identical measurement situation, and therefore the data obtained, which are given in Appendix C, constitute a good basis of comparison.

Acceptance test reports were written to each of the suppliers, in which all measurement results were summarized. A number of detectors was not accepted, these were either repaired or replaced.

7.1.3 Detector performance and detector degradation After the initial testing in 1976 most of the detectors were

installed in the muon shield, and used nearly continuously, i.e. the bias voltage remains on during a 10 day period, then it is reduced to a few

- 146

LEAKAGE C U R R E N T EVOLUTION DET 13 ( I O k X 2 c m ) - - , , ,

0 .8-

0.6}

< oo

" 0.4 k

1977- -|-!978-|-l979-|-l980-i—-|

0.2 • • • /

I 2 3

Fluence muons cm-2 , 10"

F ig . 7.1 Evolut ion of de tec tor leakage current as a function of r ad i a t ion dose. The years of opera t ion are a l so i n d i c a t e d .

LEAKAGE CURRENT EVOLUTION DET 114

Fluence muons c m " 2

Fig . 7.2 Anomalous increase of the c u r r e n t which was caused by epoxy k i t f a i l u r e . Curing r e s t o r e s the cur ren t to a value which r e f l e c t s the true degradat ion by i r r a d i a t i o n ( the curing temperature i s too low for r a d i a t i o n defect anneal ing) .

- 147 -

volts during the 3 day stop, and put on again for the following 10 days, etc. When the beam is changed from NNB to WNB a somewhat longer stop is needed and detectors have to be rearranged to suit the different flux profile. On the occasion of long shutdowns the detector diode characteristics were checked. In addition, all detector leakage currents are measured on-line every 15 min. The fluctuations on these measurements, taken into account the slow temperature dependent variations, are an indication of the detector noise. These data also enable a good monitoring of the detector performance. A typical evolution of a detector with radiation dose and time is shown in fig. 7.1. Although the current has more than doubled, this detector is still perfectly useable after four years of operation. A periodically occurring problem is experienced with detectors of type QDA, as shown in fig. 7.2. These all show a sudden increase in leakage current, around the same time, each 1-2 years, which is independent of dose. As already mentioned in sect. 1.2.1 this problem is related to the curing of the epoxy kit and can be solved by an overnight bake-out.

Detector histories are recorded for all detectors and two examples are given in table 7.2. For each physics run the calibrated sensitivity, the operating bias, leakage current, noise and integrated flux are recorded. From table 7.2(b) it can be seen that changes in the bias may influence the sensitivity S n. A reduction in bias is necessary when the leakage current exceeds the range of compensation (10 yA) or when detector operation becomes very noisy. From that time on the detector data still can be used but are less reliable. In chapter 5, fig. 5.7, a summary was given of the lifetime in terms of radiation of most of the detectors. Further observations concerning radiation damage were also made in chapter 5.

7.2 Layout of the monitoring system 7.2.1 Detector positions The. detectors are mounted inside water-tight isolating boxes, made of

cast epoxy, charged with ground dolomite rock. High precision concentric machining enables a detector positioning of better than 0.1 mm. The detector box and mounting are illustrated in fig. 7.3. These boxes can be placed on octogonal support plates in the gaps in the shield. The respective shielding thickness for each of these gaps is given in Appendix A. A standard geometry of detector positions, as shown in fig. 7.4 allows full detector circles at radii of 150 mm, 300 mm and 450 mm. Incomplete circles can be equipped at 600 and 750 mm and horizontally diametral positions at 900 mm are available. The positions are numbered "nm", where n is the angle modulo ir/4 and m is the radius modulo 150 mm. A photograph of a fully equipped plate is shown in fig. 7.5.

TABLE 7 . 2 ( a )

D e t e c t o r 308 Type 01B 6 D e p l . b i a s 3 3 . 0 V 132 0 G-2 S D I n i t i a l c u r r e n t 1.40 pA

P o s i t i o n 1 r . m . s .

Run P i t Angle ( ° )

R a d i a l D i s t a n c e

( cm)

Amp1i f i e r number Ga ins Channel

number 1 . : B i a s V

N o i s e (V) SD Emuls ion c a l i b r a t i o n Muon f l ux

cm* 2 pe r run

Leakage c u r r e n t

uA

53 59

P3 P3

180 , 180,

0 0 45 1- 32 81 4 7 . 9 . 268 101.0

100 .5 108.5 NB 200 GeV

.653 1 0 ' ° . 133 65 P3 180 , 0 45 32-128 81 2 6 . 7 .043 101.0 .607 1 0 s .090 66 P3 180, 0 45 16-256 81 4 7 . 8 .009 101.0 113 . NNB 275 GeV .674 1 0 5 .114 67 P3 180 , 0 45 2 - 32 81 4 6 . 8 .069 9 8 . 8 .342 1 0 " . 122 68 P3 180, 0 45 4 81 4 7 . 8 .008 9 9 . 0 .190 10 ' > .129

168 P3 180 , 0 45 4 81 4 7 . 6 . 023 9 9 . 0 .713 1 0 1 0 .134 69 P3 180, 0 45 2 - 4 81 4 7 . 8 .047 100.0 .223 10 ' ' .147 70 P3 180, 0 45 4 - 8 81 4 7 . 8 .004 100.0 .147 1 0 " . 143 71 P3 180, 0 45 2 81 4 7 . 3 .025 9 9 . 3 .423 1 0 ' ' .157 72 P3 180, 0 45 1- 2 81 4 7 . 3 .001 100.0 .483 1 0 " .166 73 P3 180, 0 45 1- 4 81 4 7 . 3 .003 9 5 . 5 9 6 . 6 WNB 300 GeV .298 10 1 ' . 155 74 P3 180, 0 45 2-128 81 4 7 . 3 .014 104.6 .247 10" .159 75 P3 180, 0 45 8 81 4 7 . 4 .002 9 9 . 5 .435 1 0 ' ° .142 76 P3 180, 0 45 4 - 16 81 4 7 . 4 .009 100.0 9 6 . 4 WAB 330 GeV . 1 4 1

176 P3 180, 0 45 4 - 8 81 4 7 . 3 .002 100.0 .110 1 0 " .144 77 P3 180, 0 45 2 81 4 7 . 4 .016 105.0 .613 1 0 " . 158 78 P3 180, 0 45 2 81 4 7 . 4 .002 100.0 .177 1 0 ' ' .164 79 P3 180, 0 45 2 81 4 7 . 4 .010 105.0 .537 1 0 " . 168 80 P3 180, 0 45 2-128 81 4 7 . 4 .003 105 .5 .173 10 ' ' .182 81 P3 180, 0 45 4 81 4 7 . 4 .002 100.5 9 6 . WAB 330 GeV .138 1 0 ' ' . 1 9 1 82 P3 180, 0 45 4 - 8 81 4 7 . 4 .004 105 .0 .159 1 0 " .229 83 P3 180, 0 45 2 81 . . 003 105.0 9 3 . 2 WAB 350 GeV .398 1 0 " . 2 1 8 84 P7 0 , 0 115 3 2 - 64 245 53 .8 .132 100 .5 .355 10* .267 85 P7 0 , 0 115 64 245 4 7 . 0 .029 104.3 .639 10 9 . 285 86 P7 0 , 0 115 64-128 245 4 6 . 9 .020 104 .3 .625 1 0 8 . 3 1 1 87 P7 0 , 0 115 1- 64 245 4 6 . 9 . 0 1 5 101.0 102. NAB 200 GeV .476 10 9 . 343 88 P3 0 , 0 115 64 245 4 6 . 8 .007 104 .1 .594 1 0 9 .300 89 P2 0 , 0 115 64 245 4 6 . 8 .005 104 .1 .996 10 9 . 335

90 P2 0 , 0 115 8 - 64 245 4 6 . 7 .005 104 .1 9 7 . 4 8 3 . 7

NAB 200 GeV .144 10 "> . 3 8 8 91 PI 0 , 0 115 128 245 4 6 . 7 .087 104.2

9 7 . 4 8 3 . 7 .110 1 0 9 1.312

94 P2 0 , 0 115 2 245 4 6 . 6 . 003 9 7 . 8 .413 1 0 " 1.046 95 P5 0 , 0 115 2 245 4 6 . 7 .052 9 7 . 7 .339 1 0 ' ° 2 . 1 1 1 96 P4 0 , 0 115 8 245 4 6 . 6 . 0 1 1 9 8 . 0 .812 1 0 ' - . 988

105 P4 o, 0 115 16 245 4 6 . 6 .059 9 3 . 6 .166 1 0 ' - 3 .464 106 P4 0 , 0 115 16 245 4 6 . 6 .068 9 4 . 3 .547 10 ' ° 3 .674 107 P5 o, 0 115 16 245 4 6 . 6 .147 102.0 .549 1 0 9 3 .618 108 P3 o, 0 115 16 245 4 6 . 6 .052 9 6 . 5 .283 10 ' ° 2 .855 109 P2 o, 0 115 2 - 16 245 4 6 . 6 .077 9 7 . 5 .108 10 ' ' 2 . 396 no P2 o, 0 115 2 245 4 6 . 6 .008 9 6 . 5 .395 1 0 " 2 .854 111 P2 o, 0 115 2 245 4 6 . 5 .050 9 6 . 5 .382 1 0 ' ' 2 . 313

TABLE 7.2(b)

Detector 35 Type LBD9 Depl. bias 200. V 17.5 G-2 S n Initial current .730 yA Position r.m.s.

Run P i t Angle ( ° )

R a d i a l D i s t a n c e

(cm)

A m p l i f i e r number Ga ins Channel

number B ias V Noise

(V) SD Emulsion c a l i b r a t i o n Muon f l ux cm" 2 p e r run

Leakage c u r r e n t

uA

59 P3 180, 75 50 2 - 8 86 1 7 1 . 1 . 0 9 1 16 .0 .149 10 l 0 . 212 65 P3 180, 75 50 128 86 177.4 .125 17 .0 .809 10 7 . 4 1 1 66 P3 180, 75 50 8-128 86 174.0 .040 17.0 .110 10" . 3 8 5 67 P3 180, 75 50 1- 2 86 174 .1 . 012 15 .6 . 868 1 0 1 0 . 3 9 5 68 P3 180, 75 50 2 - 4 86 177.8 .017 15 .3 .487 1 0 " .442

168 P3 180, 75 50 2 - 4 86 179.5 .045 15 .3 .174 1 0 1 0 . 479 69 P3 180, 75 50 2 - 4 86 161.6 .074 15 .8 .565 1 0 " . 4 5 1 70 P3 180, 75 50 2 - 8 86 177.7 . 0 1 0 1 5 . 8 .374 1 0 1 0 . 517 71 P3 180, 75 50 1 86 175.0 .020 15 .4 . 9 3 1 1 0 " .554 72 P3 180 , 75 50 1- 2 86 175.5 .004 15 .4 .129 1 0 " . 6 1 8 73 P3 180, 75 50 1- 4 86 176.4 .010 14.7 13 .5 WNB 350 GeV . 7 3 1 1 0 " . 638 74 P3 180 , 75 50 2 -128 86 176.4 . 428 16 .0 . 2 5 1 10" . 6 8 3 75 P3 180, 90 51 8 87 182.2 .034 15 .4 .676 1 0 9 . 9 2 8 76 P3 180 , 90 51 8- 16 87 182 .1 . 038 15 .5 .955

176 P3 180, 90 51 8 87 1 8 2 . 1 .036 15 .5 .160 1 0 ' ° . 959 77 P3 180, 90 51 2 87 182 .1 . 019 1 5 . 5 .797 1 0 " 1.009 78 P3 180, 90 51 2 87 182 .1 . 0 1 1 15 .5 .244 1 0 " 1.062 79 P3 180, 90 51 2 87 182 .1 .015 15 .5 .699 1 0 " 1.116 80 P3 180, 90 51 2-128 87 182.2 .067 16 .0 .249 1 0 " 1.165 8 1 P3 180, 90 51 8 87 182 .2 .039 1 6 . 0 . 2 0 1 1 0 " 1.196 82 P3 180, 90 51 8 87 182 .3 .044 14 .5 .205 1 0 " 1.276 83 P3 180, 90 51 4 87 • . 038 15 .5 1 4 . 1 WNB 350 GeV .587 1 0 " 1.275 84 P3 180, 45 48 64-128 84 142.4 .486 1 6 . 0 .192 1 0 9 . 860 85 P3 180, 45 48 8 - 16 84 112.5 .002 16 .4 . 103 1 0 " .277 86 P3 180, 45 48 2-128 84 159.3 . 058 16 .4 .359 10» .892 87 P3 180, 45 48 32-128 84 149 .9 .049 16 .0 .979 10 9 . 625 88 P3 180, 45 48 32-128 84 149.9 .014 16 .3 .360 1 0 9 . 632 89 P3 180, 45 48 8-128 84 149.8 .017 1 6 . 3 . 898 10 9 . 6 1 8 90 P3 180, 45 48 1-128 84 149 .8 .013 16 .3 . 883 1 0 9 . 6 2 1 9 1 P2 4 5 , 30 17 128 56 146.6 .145 16 .3 .116 10* .532 94 P3 180, 90 63 4 87 176.6 .042 14 .5 .445 1 0 " 2 .039 95 P3 180, 90 63 1- 2 87 176.7 .017 14 .7 .116 1 0 " 2 .145 96 P3 180, 90 63 2 87 176.6 . 0 2 1 14.4 .678 1 0 " 2 .216

105 P3 180, 90 35 4 - 16 87 5 7 . 3 . 0 0 1 1 8 . 1 . 428 1 0 " .180 106 P3 180, 90 35 2 - 4 87 57 .2 . 0 0 1 17.7 .119 1 0 " . 2 2 5 107 P3 180, 90 35 2 - 8 87 5 6 . 6 .147 18 .0 .868 1 0 " .387 108 P3 180, 90 35 2 87 57 .2 . 0 0 1 18.0 .588 1 0 " .242 109 P3 180 , 90 35 4 - 16 87 5 7 . 3 .002 18 .0 .432 1 0 " .234 110 P3 180 , 90 35 2 - 16 87 5 7 . 2 . 003 1 8 . 0 . 566 1 0 " .247 111 P3 180, 90 35 4 -128 87 57 .2 .019 18 .0 .389 1 0 " . 2 5 3

- 150 -

ZiM^—Ul T h e disassembled detector mounting together with the tight box. The detector shown here, has an active area of 2 cm 2.

Fig- 7-4 Geometry of the detector support plate. The numbers are the position indicators.

- 151 -

Fig. 7.5 Photography of fully equipped support plate.

Because the WNB and the NNB are not on the same axis it was necessary to make the complete support plate moveable. The range of movement is 360 mm in the horizontal y direction and 326 mm in the vertical z direction, with a precision of .2 mm, which corresponds to one step of the absolute encoder. The plate position is measured from the encoder in each SPS cycle, but to avoid errors, the power supply of the motors which drive the plate, usually is cut during an experiment. The geometrical survey has provided the WB position expressed in encoder counts. The WB axis is the theoretical x axis in the neutrino beam coordinate system, and is therefore used as the point (0, 0) for each pit-coordinate system. In table 7.3 the relative coordinates for the old NNB and the upgraded NNB (since 1982) are given. An example is shown in fig. 7.6 (gap 2).

TABLE 7.3 Relative positions of the wide band and narrow band beams in the shielding

Gap number VI V2 V3 V4 V5 V6

Wide band position X 1234 1360 1358 1232 1360 1369 Relative to plate "garage" y 1158 1395 1394 1213 1390 1305 (Encoder counts in .2 ram)

Idem in mm X 246.8 272.0 271.6 246.4 272.0 273.8 y 231.6 279.0 278.8 242.6 278.0 261.0

Narrow band position X 203. 194. 184. 173. 163. 150. Relative to wide band Y 170. 162. 154. 145. 136. 126. 1976-1980 (in mm)

Position of upgraded X 176. 171. 161. 152. 144. Narrow band beam y 142. 131. 122.8 112. 101. From 1982 (in mm)

- 152 -

[CONCRETE

' IRON

GAP

NN3

Upgraded NNB

WNB

^

Fig . 7.6 The pos i t ions of WNB, old NNB and upgraded NNB in gap 2. Also the zero point of the encoder of the p l a t e i s shown ( 0 , 0 ) . The ind ica ted dimensions are in mm. Usually, p o s i t i o n s in the plane do not re fe r to the encoder zero p o i n t s , but are indica ted in the Neutrino Beam Coordinate System, taking the t h e o r e t i c a l WNB axis as the o r i g i n .

The plate support as a whole is sliding on rails so that i t can be serviced outside the gap, which is too narrow for this . The plate is perpendicular to the beam axis, and thus inclined by ^ 2° to the vertical. A picture of the

situation of gap, plate support and cables is shown in fig. 7.7.

Generally, the left hand horizontal radius of each pit is fully equipped with detectors (positions 40 to 46) as well as one

*V ~~ fA H H I.| K^IIIIEl or two circles. Which circles are &* ^^HL Irfî SBUIU1I chosen depends on the flux profile

to be measured, because the highest sensitivity to beam displacement is obtained by placing detectors where the gradient is maximal. Apart from this basic outfit, more detectors can be placed anywhere else, and also an additional

detector box can be mounted on top of another. These "second layer" aetectors are put in place for example if i t is probable that a detector will break down soon, as judged from an increase of leakage current and noise. During a run one has no access to the detector pi ts , and therefore some redundancy is necessary.

Lia-/

7.7 Pic ture of a p l a t e s u p p o r t , complete with cabl ing in a gap. In fac t , t h i s is not a t r u e underground i n s t a l l a t i o n , but a 1:1 model.

There are conventions for the attribution of read-out channels to detector positions. In table 7.4 an example is shown of the detector layout (for pit V3) which is printed for each neutrino run. Altogether

TABLE 7.4

NFM detector layout for pit V3 during run 105 -400 GeV WB starting on 800306150000 a n n e l t e l .

P o s . Disc

R a d i a l cm

Det Num.

ec t o r Name

T o t . d e p l . ( V o l t )

B ias Real

Leak (uA)

Noise ( V o l t )

Ampl i . Number

S e n s i t . (u /pC cm 2 )

s D . o n - l i n e

C o r r e c t i o n f a c t o r

Comment I n t e g r a t e d

F l u x ( p / c m 2 )

81 040 0 302 LBB 2 2 5 . 30 . .684 .002 45 113 .0 115 .0 .983 A .510 1 0 " 82 041 15 301 LBB 1 2 5 . 2 3 . . 191 .001 46 9 9 . 0 9 9 . 0 1.000 A .450 1 0 " 83 042 30 90 QDB 11 7 2 . 6 9 . .720 .019 157 152.0 151.0 1.007 P r o t o t y p e .346 1 0 " 84 043 45 21 OIB 4 4 5 . 5 4 . .259 .002 48 9 2 . 5 9 1 . 4 1.012 .226 1 0 " 85 044 60 33 QDC 2 250 . 144. 1.008 .002 49 4 7 . 0 4 6 . 0 1.022 .134 1 0 " 86 045 75 44 QDC 3 280 . 1 9 1 . .356 .002 50 4 3 . 5 4 5 . 0 .967 .788 1 0 " 87 046 90 35 LBD 9 200 . 57 . .180 .001 35 18 .1 15.4 1.175 .428 1 0 1 0

88 002 30 17 QDB 1 6 0 . 9 4 . 1.183 .033 52 165 .0 165.0 1.000 .372 1 0 " 89 012 30 19 QDB 2 8 0 . 152. 1.034 .013 53 163.0 165.0 .988 .362 1 0 " 90 022 30 20 QDB 3 8 0 . 116. . 923 .017 60 165 .0 160 .0 1.031 .352 1 0 " 91 032 30 22 QDB 4 4 0 0 . 5 9 . 2 .135 .055 55 143.0 142.0 1.007 150 V .335 1 0 " 92 052 30 310 QDB 9 150. 5 5 . .467 .002 155 152 .0 148.0 1.027 Box 201 .354 1 0 " 93 062 30 87 QDB 7 130. 8 7 . 1.971 .034 96 160.0 157.0 1.019 .368 1 0 " 94 072 30 88 QDB 8 140. 8 9 . .940 .006 72 150 .0 149.0 1.007 .379 1 0 " 95 003 45 7 LBB 5 3 5 . 4 7 . .210 .001 59 104.0 105.0 .990 .252 1 0 " 96 013 45 18 OIB 3 4 8 . 154. .245 .001 106 104 .0 9 8 . 0 1.061 .247 1 0 " 97 023 45 55 SDB 9 160. 9 4 . .714 .010 63 106.0 103.0 1.029 .230 1 0 ' ' 98 033 45 85 QDB 5 140. 8 8 . .765 .004 105 144 .0 142.0 1.014 .227 1 0 " 99 053 45 69 OIB 12 240 . 192. .429 .002 H I 79 .0 8 1 . 0 .975 Rep. .230 1 0 ' '

100 063 45 31 LBB 8 3 0 . 2 2 . .219 .004 64 130.0 134.0 .970 .243 1 0 " 101 073 45 86 QDB 6 140. 8 9 . 1.118 .006 99 147.0 146.0 1.007 60 V-100 V .251 1 0 " 102 140 0 10 LBB 6 0 . 4 3 . . 083 .002 107 157 .0 152.0 1.033 Abs . box mi .507 1 0 " 103 142 30 30 LBB 7 100. 79 . .123 .001 67 118.0 120.0 .983 Abs. box up .339 1 0 ' ' 104 001 15 347 OIB 7 3 5 . 3 5 . 2 .076 .002 54 103 .0 101.0 1.020 .477 1 0 " 105 004 60 27 LBC 2 8 0 . 3 0 . .139 .001 42 4 3 . 5 38 .2 1.139 .154 1 0 " 106 107 108 070 0 32 LBC 5 8 0 . 185 . .472 .004 134 3 6 . 0 39 .4 .914 .510 1 0 " 109 110 111 112 113 114 115 116 CALBOX 341 QDA 1 3 8 . 4 6 . . 303 .002 31 1327.0 1330.0 .998 CAL S5 .912 10'° 117 CALBOX 384 SDA 50 100. 7 3 . 1.302 .007 62 187.0 195.0 .959 CAL S I , 1 . 961 10'° 118 CALBOX 381 OIB 5 4 4 . 4 4 . .488 .011 14 9 6 . 9 9 8 . 4 .985 CAL S4 .419 1 0 1 0

1 19 CALBOX 345 OBD 3 280 . 192. .643 .007 97 16.2 16.0 1.013 CAL S3 .439 1 0 " 120 CAL BOX 390 OBD 7 200 . 177. .565 .010 119 15 .8 16.2 .975 CAL S2 .412 10 ' °

- 154 -

there are 256 read-out channels for the muon detectors, and 40 are attributed to each pit, i.e. 1-40 to VI, 41 to 80 to V2, etc. The channels 241 to 256 are kept for special detectors. Inside each block of 40 channels the first 7 represent the left horizontal axis, the second 7 the innermost circle which is fully equipped, and the next 7 represent eventually an (not necessarily fully) equipped second circle. Further channels, from 22 to 35, can be used for special detectors. From run 94 (summer 1979) one detector (conventionally in position "70", channel 28) has been added directly on the upstream iron wall on axis in each pit (except V2). This detector is not influenced by displacements of material within the gap, and can therefore be used as a reference. The channels 36 to 40 in each block of 40 channels are attributed to detectors in the calibration box of that pit.

The detector positions are regularly written onto the magnetic tape, as data block 210. This contains 256 words of 16 bits. The highest order bit indicates if the channel corresponds to an operating detector. Further the pit number, layer and position are encoded. One bit is used to signal errors in the amplifier gain setting, another is set after off-line analysis to indicate noise-free operation which justifies the use of offset compensation. All data formats for NFM tapes are described in detail by Cavallari [7.4,7.5].

7.2.2 The calibration boxes The detector support plate described in the previous section is

placed near the downstream wall of the gap. In front of it a remotely controlled girder ("lift") can be positioned on any point in the plane y = -985 to y = +1235 mm, z = -618 to z = +751 mm, with the origin y = 0, z = 0 defined again at the gap centre, the WNB axis. The range of the movements of both lift and support plate is indicated in fig. 7.8. As a matter of fact, the position encoders have a zero value in their respective garage positions, and are incremented positively only. These numbers are read and written on tape, in each NFM data block 200. The positions in mm with respect to the y = 0, z = 0 centre point then can be found using the relative coordinates of this survey point which are given in data block 202, together with the encoder steps given in block 223. The encoders for the calibration lift are reset to zero when it starts from the garage position, while passing a magnetic pick-up. The encoder step in this case is .3 mm and for the absolute precision one relies on the uniformity of the horizontal and vertical cog-bars on which a pinion is driving the encoder shafts. Temperature variations are not exceeding a few degrees around 13°C, due to the depth of the pits. Moreover, during the experiment one cuts the forced air ventilation, which would cause warming up (summer) or cooling down (winter). As a result, mechanical

- 155 -

dilatation is less than .5 mm over 2 m, and it is less for a relative position. A picture of a support plate with calibration box lift under construction is shown in fig. 7.9.

0 » , 9P;T .'-.

36C ^ 1 N8 ' ~

\0ETECTQR SUPPORT PLATE

;ÎM, Fig. 7.8 Movement range of de tec tor support p la te and c a l i b r a t i o n box. The p l a t e i s shown in the WNB p o s i t i o n and in the garage pos i t ion . The area covered by the c a l i b r a t i o n box i s indicated by . . _. _ . 0 are the garage p o s i t i o n s .

Fig. 7.9 View of the l i f t , which can move in two d i r e c t i o n s in front of the d e t e c t o r support p l a t e .

Each l i f t i s always equipped with a normal c a l i b r a t i o n box, and can at the same time receive a second box of the same type, for example the reference box or a box containing single p a r t i c l e counte rs . in

- 156 -

SITUATION OF CALIBRATION BOX (IN CENTER POSITION)

CONCRETE

FIXED DETECTORS IN EPOXY BOXES

INTERGAP NORMAL REFERENCE CALIBRATION BOX BOX (OPTIONAL) Q l

enu. 3 0 0 •

// IRON >j

/, <f> = 2500 /)

1 I—i Inn Urn

MOVABLE L'FT

ALUMINIUM PLATE

| 42.5 mrod

Fig . 7.10(a) Scale drawing of the l i f t equipped with normal c a l i b r a t i o n box and reference box. Also shown are the upstream d e t e c t o r and some de tec tors on the support p l a t e .

b ) SITUATION OF ABSORBER BOX

CONCRETE

ABSORBER BOX WITH MAXIMAL

ABSORBER THICKNESS 126 mm

FIXED DETECTORS IN EPOXY BOXES

302-ABSORBERS IN OR OUT

(IRON OR LEAD)

V-— 126—1—92—• COMPRESSING

MIDDLE DETECTOR

I - • •

— 150-

DOWNSTREAM DETECTOR

ROCK

PLATE

| | SUPPORT 21 |fl STRUCTURE

env 300 -

ALUMINIUM PLATE

10 cm J_42.5 m,

Fig . 7.10(b) The l i f t equipped with the absorber box.

- 157 -

fig. 7.10(a) a scale drawing shows the situation of the lift in the gap. It should be noted that the lift is moving upstream of the fixed detectors on the plate, and therefore influences them, as will be discussed in sect. 9.1.2 (calbox effect). The upstream detector however is not influenced, because backscattering is only important for very low energy particles.

Inside the calibration boxes and reference box a set of five detectors of different sizes is stacked, with the bigger one generally upstream, the smaller ones downstream. These detectors can be positioned to better than .6 mm in front of a fixed detector and provide then simultaneous flux data for the relative calibration. If the reference box is installed, it is possible to compare continuously detectors in the calibration box with those in the reference box, because they remain aligned all the time.

The lift installation not only serves the intercalibration of detectors, but was also designed to facilitate scans around circles or along cross sections. An advantage is that in this case the same detectors measure at different positions, eliminating intercalibration uncertainties. Also, profiles can be obtained with much better spatial resolution. The smallest displacement is .3 mm, but it has to be realized that a smearing effect occurs if the gradient is very steep, due to the relatively large diameter of the detectors (6 to 16 mm). This is presently only important in the NNB in pits VI and V2. The possibility to move in small steps turned out to be very useful for the measurements of the muon angular distribution by coincidence counting technique (sect. 9.4.1) .

7.2.3 The absorber box To investigate the radiation field inside a gap, in an experimental

way, both as a function of position and surrounding material, a distance controlled device was designed, called "absorber box" (fig. 7.11). It can be mounted on the lift, but it excludes the use of a normal calibration box, as is shown in fig. 7.10(b). The box has three detectors, which are normally identical so as to give nearly equal response. The first ("UP") and last ("DOWN") are mounted on the walls of the box, 210 mm apart. The middle one is on a holder which can be used to compress a stack of circular absorber blocks against the DOWN detector. The stack can be built up of maximally 8 pieces, resp. .5, 1, 2, 4, 8, 16, 32 and 64 mm thick, of either iron (Fe) or lead (Pb) of 99.9% purity. Also one may use no absorber at all, and position the MIDDLE detector anywhere between UP and DOWN, which provides the basic coefficient of comparison between the three detectors. In table 9.1 is indicated when and where this absorber box has been used. The results obtained with this box were already discussed in sect. 3.4.

- 158 -

F i g . 7.LI P ic ture of the absorber box.

Some t e s t s , in which an addi t iona l box i s placed on the l i f t , with s ing le p a r t i c l e counters , e i t he r s i l i con de tec to r s or s c i n t i l l a t o r s , w i l l be discussed in d e t a i l in 9.4 (see also sec t . 3 . 3 . 3 ) .

- 159 -

8. DATA ACQUISITION AND ELECTRONICS

The operation of the NFM system is controlled by the real time

FORTRAN program OPCOM, which is running in a NORD-10 computer in the beam

control room, under the operating system SINTRAN III. The main NFM

functions are fully automatized, using specially designed electronics,

conform to CAMAC specifications [8.1]. The main part of the electronics

consists of the amplifier control and detector signal processing, followed

by the data transmission via a serial CAMAC link. A second part of the

electronics system concerns the communication of data to several users via

data links. The third part is the control of movements of detector

support plates and calibration boxes.

The CAMAC configuration is sketched in fig. 8.1 and consists of 7

crates, numbered 1, 2, 11, 12, 13, 14 and 28. The various links are also

indicated.

8.1 Signal processing

The particles generate free charge in the detector, which is

collected and measured as an integrated current signal by a charge-

sensitive preamplifier. Because of difficult access and possible

radiation damage to the MOS elements of the amplifier, it was preferred to

install all electronics outside the radiation zone in the electronics

barrack B294. Schematically the signal processing is summarized in

fig. 8.2. The 256 analog channels are fed into a multiplexed ADC which is

read via a CAMAC interface.

8.1.1 The direct coupling

The conventional capacitive AC-coupling of detector and charge-

sensitive preamplifier, cannot be used for measurement of very long

current signals because it is differentiating the signal. The duration of

the muon flux detector signal may be ^ 3 ms, or exceptionally even

^ 100 ms, and with AC coupling a varying part of the signal would be

lost. A second effect from the rather long signal integration time, in

the case of capacitive coupling, with a capacitor CL , would be the

signal loss via the reload current with the time constant

R D C T = 106 xlO' 9 = 10"3 s

B Li

In the usual configuration with capacitive coupling the current through

the bias resistor R B contributes to the noise, in parallel with the

signal source at the input of the amplifier. This contribution to the

noise can be reduced by increasing the value of R to *o 10 9 Ù, but

an unacceptable bias voltage drop occurs for a typical leakage current of

1 pA. In a direct coupling, on the contrary, the resistance is not

parallel to the input of the amplifier and the value of R^ does not play

a role.

NFM Control Room

STATUS-»-BCT - » — SEM u

CRATE 2

SOR c c & a DATA BOX *~ S T A T U S > = =

CAMAC LINKS

NBC 4 — WA

=H CRATE

1

1 4 1 WA 18 4 -

EHt<

*

NORO 10

NFM PERIPHERALS

MAGNETIC TAPE

G-DISC &

T4010 o PRINTER/,

PLOTTER o-TV

STATUS

Electronics Barrack B 294

SERIAL CAMAC LINK

Pit V8

SERIAL

REPEATER

CRATE

11

amplifier control

BP SCC

V////Z.

RM3tO

AMPLIFIER CRATES

detector data •SS/S/////SJ

CRATE 12

f? SCC

42:

x 6

7T ' 512 analog lines

512 MUX 14 BIT ADC

scalers, digiswitch

CRATE

13

w-BP SCC

n m i / i i i

CRATE

It

• spill measurement, timing

uiiiiiimiiirr,

BP

Vl/JJ/l

SCC

zz

O DECODER

m/i/iiiiT7TTr £> COUNTER DECODER

CRATE 28

fi= BP SCC

scalers

movement

Measurement Pits V1-V6

bias line

signal line

X40

* ( K I

detector box

current/temperature measurement shielding magnet

- • M

6 x support plate movement

slow servomotor

calbox lift 0 x movement

- / A C ) absoluti y-J encoder

4 fast servomotor

ic ) incremental encoder

1 CAMAC configuration of NFM system. A Serial Drive Receiver (SDR) in crate 2 is connected to the crates via s rs. In each crate a Serial Crate Controller (SCC) is connected via a ByPass (BP).

- 161 -

Signal current

'in

Output 256 analog to digital signal multiplex digital transmission 0-10V conversion

S/H -//-256

ADC -//- SERIAL CAHAC

NORD 10

COMPUTER

t 256 Charge sensitive Sample/ silicon integrating hold DATEL defectors amplifier SYSTEK 256

multiplexing ADC

Fig. 8.2 The signal processing

Finally, with AC coupling an important noise is due to the necessity of polarizing the very long cable (80 m to 200 m) with the detector bias voltage.

A direct coupling of the detector therefore was chosen, in the configuration which is shown in fig. 8.3. The signals are transmitted via a bipolar, non-twisted cable, with independent shield, using one wire for the signal and another for the signal ground. With the direct coupling not only the particle-generated charge signal is integrated but also the charge resulting from the diode leakage current and from non-averaged low frequency noise sources. The charges induced by movement and irradiation of the cable fall into this last category, and therefore a special low-loss, low-noise cable construction (fig. 8.4) had to be adopted. A polyethylene insulation is surrounded by a semiconducting graphite containing layer, which provides the flow path for the balancing of induced charges. The noise from the cable is reduced in this way by a factor ^ 100, and corresponds now to a r.m.s. signal < 2 mV.

The leakage current component in the signal has to be subtracted via a leakage current compensation but in this way one has at the same time a detector current monitoring (sect. 8.2.3).

To minimize the noise from pick-up, there is only one ground point, located in the electronics barrack. Only there the signal ground wire is connected to the cable shield, as is shown in fig. 8.3. The detector shield, the aluminium encapsulation shown in fig. 7.4, is connected to the cable shield only. The bias voltage is carried by a different cable, standard type, and its screen is not connected in the detector box, to avoid a loop between the cable screens. This second cable can also be used to drive a light-emitting diode which is mounted in front of the detector, to simulate the beam.

CHARGE AMPLIFIER MODULE r TIMING I O J-LFROM CONTROLLER UNIT

Q COMP

MUON-FLUX TEST

1 L =80 to 150m

RESET CDE

iïdrd

fOETECTOR BOX IN NEUTRINO PIT e

V h TO ANALOG DATA " ACQUISITION SYSTEM

CABLE

n —/Ë—rVbi-t ^ s / | LADDER 1 _ / / L-t-O-f . / NETWORK ~ |

LOW NOISE | * PREAMPLIFIER |';;:|

+Vref

r<^ 1

-300V 0

4 o—^s^ 1/30 HIGH VOLTAGE

AMPLIFIER

-Vref i

m -

UJ >_

ui u ï

g œ

O

'wœk

1

4-12

w <y 2 ul ir o o; o

_S

J

R 9

MUX

ST.

LOGIC W -ARI „ AR2 „

LOGIC LOGIC

•••••: :rvw;;:v.::-.::v.,f

LOGIC

••••.•••.:•.!-.•. . ^ • . • . • • • • - . • • : . - : - : : ' .

ST. - . ADDRESS HANDSHAKE—»

LOGIC

V„ TO ANALOG DATA ACQUISITION SYSTEM

m «qui >

Fig. 8.3 The connection of detector and charge amplifier control registers RI, R2 and R3

- 163 -

LOW-NOISE C A B L E C O N S T R U C T I O N

Fig . 8.4 Low noise cable used for d i r e c t coupling

PVC JACKET

METAL BRAID SCREEN

CONDUCTOR

CABLE GUIDE

- POLYETHYLENE INSULATION

SEMICONDUCTING LAYER

No elements of the amplifier are inside the detector box, except the resistors and capacitor of the low frequency filter, which eliminates noise from the bias voltage source.

8.1.2 Charge sensitive preamplifier with programmable gain The charge sensitive preamplifier is a conventional integrator

consisting of a wide band JFET input operational amplifier (LF356, National Semiconductor) with a negative feedback loop, which however is modified to obtain a programmable gain. A binary attenuator resistor ladder network (AD1807 KD) has high precision thin film resistors (ratio inaccuracy better than ± .2%) and excellent long term stability (less than .01% drift per year). It is used as a divisor and the bridge ratio

Rz K = Ri + R2

is set digitally by the register R3 to a value 2 (n = 0, 1,...,8). A virtual change of the feedback capacitance C_ is obtained, which gives a

1/K on the output signal V 0 for a detector current relative gain G i

1 V° = C^K f ls d t (8.1)

The basic gain is preset to lO"1* V.pC"1 with a variable gain amplifier which follows the hybrid sample/hold circuit. The overall gain instability is less than .2% for gains up to 128.

The charge integration is started by opening the amplifier gate, about 200 us before the arrival of the beam. The gate lasts as long as needed, depending on the beam conditions, and afterwards the sample/hold is triggered to hold, 50 us before the reset of the amplifier. This reset is performed by a MOSFET transistor, which causes a spurious charge

- 164 -

on transition, due to the gate-source capacity. To avoid a feedthrough

effect, an adjustable capacitor injects an equal and opposite amount of

charge at the amplifier input ( Q C 0 M p ) - This adjustment is made during

the initial trimming of the amplifier. The timing of the amplifier

operation is shown in fig. 8.5.

PREPULSE BEAtf

E J E C T I O N / / \

5 0 ^ 2 - 3 m s

ScESET A M P L

G A T E

I IN

1

| S

TEGPATE

5 rr,s

ScESET

I IN

1

| S

TEGPATE

5 rr,s

O U - P J T CHARGE

A M P ^

I IN

1

| S I

- M P L E

I IN

1

| S I

- M P L E

_ ^ G C S A M P L E /

I IN

1

| S I

- M P L E

H O L D

4 ms

H O L D

S A V P . E / HOL D

STùFT

1 ! _ ST STùFT

1 ! _ ST OP

J A ' A

A C Q u l S I T . C N

STùFT

1 ! C G I -

9 B . . E - A S "

A u A C Q J SiT

~ 3 0 0 m s

C H A W B E P

on 1 OP

BEBC

- P I C G E P

IS F

C G I -

9 B . . E - A S "

A u A C Q J SiT

~ 3 0 0 m s

C H A W B E P

on

F ig. 8.5 Timing sequence of amplifier

The calibration of the charge amplifiers is performed from time to

time in a separate computerized set-up. Signals simulating the detector

input are generated by a 16 bit DAC and fed to the input via a calibrated

capacitance. Slope and offset of the output are calculated by the least

squares fit of a straight line. The variable gain amplifier is adjusted

to obtain the basic gain to .1%. The higher gain settings are verified.

It can be shown that the gain G is not exactly a power of 2, but (for

G = 1, 2, 4, 256)

( l + f ) (8.2)

where A is the open loop gain of the operational amplifier. For the LF356

the open loop gain is 2 x 10 5 and this results in an inaccuracy of .13%

for the relative gain 256.

More important is the inaccuracy on the resistor network, which is

±.4%. The maximum error for gain 256 is then .5%, but it is

considerably lower for the lower relative gains.

The amplifiers cannot be calibrated in situ and therefore an

uncertainty has to be taken into account, which comes from the cable

capacity C- at the amplifier input

G V» = -

v i + t + c-;; • ! o D

(8.3)

- 165 -

In the worst case the cable length is 200 m with C- = 16 nF, C„ = 10 nF, 1 r

G = 256, A = 2 x 10 5. The uncertainty is then Ci G _ 16 256 _ C„ A ~ 10 2 x 10^ ~ "** (B-4> F

Finally, the instability of the feedback capacitor, with porcelain dielectric (Vitramon) , is ^ .1% per year and the temperature coefficient is -v 25 ppm/°C.

Altogether, the inaccuracy and non-linearity in the amplification are at most ±.5% for the gain 256 and ±.3% for lower gains. Fluctuations in the amplifier are on the .01% level. It will be shown later, that the fluctuations on the signal are completely dominated by the detector noise and the detector leakage current.

A picture of the complete amplifier module is shown in fig. 8.6.

Fig. 8.6 Picture of the amplifier module. The charge amplifier is located on the small plug-on board in the upper left corner.

8.1.3 ADC and data acquisition The negative output signal is sampled during the integration time and

held beyond the reset of the integrator. This hold status lasts for more than 300 ms, the time needed for data acquisition. The signals are presented on 256 parallel input channels to the multiplexed analog to digital converter (DATEL, system 256) . The conversion is done sequentially, using high speed successive approximation, starting from the address to which the DATEL is set. The digital signal contains 13 bits plus a sign bit, and these are left adjusted to the 16 bit words of the computer such that 9.9976 V corresponds to 32764. A conversion factor of .00030514 has to be applied to the digital data, to obtain the signal in volts. After each conversion a signal is exchanged and the address is incremented. The processing time per address in "^50 ys.

- 166 -

A CAMAC interface for the DATEL was built.

8.1.4 Special treatment for very low signal The lowest flux which can be detected with a type D detector

(S D = 30 muons cm"2 pC"1) at the highest amplifier gain is ^ 200 muons cm"2 in the 2 ms extraction. The collected charge is 3 x 10 7 e-h pairs in 2 ms corresponding to a signal current of ^ 1 nA. This signal current is integrated together with a (compensated) detector leakage current of ^ 1 pA (see 8.2.3). Obviously such a small signal will be drowned in the noise if the leakage current is net stable.

For a special calibration run at low flux a somewhat different electronic layout was adopted. Selected low current detectors were cooled with a Peltier element to reduce the leakage current further. The amplifier circuit was mounted adjacent to the detector and the timing circuit of the integration was modified to allow jitter-free gating, precisely timed with the calibrating scintillation counters (see 9.4.1).

8.2 Amplifier control In order to minimize operator intervention the amplifier control is

done by computer, which enables at the same time to keep record of the various parameters. As shown in fig. 8.1 there are 6 sets of modules corresponding to the six measurement pits, each controlled by the CAMAC control module RM 3.40, which is connected via an interface to CAMAC crate 11. The RM 3.40 and his three slaves control the read and write cycles on the registers of 40 amplifiers, addressed from 0 to 39. The address is hardware defined by the position of the amplifier in a crate, and corresponds at the same time to a well-defined detector position, as explained in 7.2.1.

There are three 16-bit registers, Rl for the bias voltage generator, R2 for the leakage current compensation and R3 for the relative gain setting (see fig. 8.3). A block diagram of the control unit is shown in fig. 8.7. Details of the control functions will be given in the next sections.

An extension to the DATEL of 256 multiplexed channels is used in relation with the amplifier control module, to convert the analog values of the applied bias voltages for the corresponding 256 detector/ amplifier channels.

- 167 -

8.2.1 Setting of bias voltage and gain The CAMAC control module RM 3.40 performs the addressing and data

transmission for a maximum of 40 amplifiers in the following way. The first operation is the addressing mode, in which an amplifier address (0 to 39) and a working register (Rl, R2, R3) are selected. This operation precedes each data transmission, which can take place either randomly or sequentially. Random write or read proceeds in the selected register of the selected amplifier. In the sequential mode the data are written or read in the selected register of all amplifiers starting from the selected address.

CAMAC REGISTER RM 3.40 CONTROL UNIT < • " %

<% DATES 16 bits

CAMAC FUNCTION

V

D W DATA REGISTER

16 bits t>

_K ADDRESS . y COUNTER

6 bits ^>

J\ ADDRESS y REGISTER

DATA WRITE

x16

ADDRESS AMPLIFIER

«6

AODRESS REGISTER

«3

MANUAL/ ~~\ COMPUTER ~S WRITE SETTING

OCTAL

MANUAL/ A COMPUTER """ADDRESS DISPLAY

BCD

WRITE

DATA

v-<r>

_N MANUAL/ _ > COMPUTER

REGISTER DISPLAY R1 R2 R3

3 S \ READ

WRITE

LOAD PULSE

_ ^ MANUAL / _» . COMPUTER

READ/WRITE

MANUAL/COMPUTER-LOAD

HANDSHAKE

CLEAR

c < >

16 » BUFFER

DATA READ

x16 c RECEIVER/ DRIVER

DATA DISPLAY OCTAL

c READ fl: DATA

X >

Fig . 8.7 Block diagram of the RM 3.40 con t ro l u n i t , which c o n s i s t s of two modules. At the l e f t the CAMAC r e g i s t e r module, and at the r i gh t the control u n i t for 40 ampl i f i e r s .

The b ias vol tage generator uses a high voltage amplifier (LM 310 National Semiconductor) which is regulated by a 12-bi t DAC, such tha t a programmed f rac t ion of the t o t a l high voltage i s appl ied . Or ig ina l ly , a maximum vol tage of 300 V was foreseen, but in p rac t ice not more than 220 V can be applied because of l imi t a t ions of the LM 310. For each individual amplifier the bias voltage is further l imited by the s e t t i ng of a front-panel d ig i swi tch . This s e t t i n g i s done manually and should correspond to the maximum admissible bias voltage for the corresponding de t ec to r . This so-ca l led "overbias s e t t i ng" i s compared to the ac tua l ly applied vo l t age , and if the "overbias" i s reached, no further increments are appl ied.

- 168 -

The bias setting is controlled by a special program which is activated in the beginning of a run or after a voltage drop has occurred. It proceeds in steps so that the bias voltage is applied only gradually to the detectors. The output of the high voltage amplifier is connected to the detector bias cable. A monitoring of this real applied bias voltage is obtained by measuring the analog value (via a voltage divider) on 256 auxiliary channels of the DATEL ADC. These data are available in block 204 on the magnetic tape. The value in volts is obtained by multiplying with 0.009156.

The programmable gain, as described in 8.1.2, is obtained using a resistor network. The active resistance is determined by the register R3, in which only one out of eleven significant bits should be set. Which gain is applied, is written in block 203, where for each channel the exponent "EXP" is given. The gain G can be found with the formula

G = 2 E X P 10"" (8.5) It may occur that another bit becomes set, due to a transient signal. As a precaution, galvanic isolation was applied for all signals to avoid this feedthrough in the RM 3.40 from the CAMAC. But to be aware of an error on the gain, the register R3 is first read before a new setting is applied. If this actual setting does not agree with the setting previous programmed, an error flag is set for this channel in block 203, bit 15 of the corresponding word. Afterwards, the correct gain bit is rewritten in R3.

The gain setting which is required, is computed automatically using a tabulated beam profile, taking into account the sensitivities of the detectors and the actual proton intensity. Normalized beam profiles for positive and negative WNB and NNB are stored on the disc of the NORD-10. Only the gains in channels 1 to 21 in each pit are automatically computed, such that their output is between 1 V and 5 V. The gains for the additional detectors, the calibration and reference detectors have to be set by the operator, in the beginning of a physics run, and these normally should not be changed.

8.2.2 The offset of the amplifiers The main cause for the existence of an offset voltage on the output

signal of the amplifier is the direct coupling of the detector at the input. A digital current compensation system is therefore incorporated which corrects for the detector leakage current. But its conception is such that it suppresses all other offsets at the same time, like zero drifts and sampling errors.

One source of offset voltage, the charge injected by the transition of the MOSFET reset transistor, is compensated separately by an injected charge Q r n p , as already discussed. If the trimming of the compensating

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capacitor was not perfect, the resulting offset will also be corrected by the leakage current compensation.

8.2.3 Leakage current compensation Any uncompensated detector leakage current will give rise to a signal

at the output of the integrator. It is by measuring this signal that a perfect current compensation can be obtained. Every 15 min, or more often if the currents are unstable, e.g. during system start-up, a compensation sequence is initiated automatically by OPCOM. It starts by measuring the signals when there is no beam, but using exactly the same timing sequence. To cope with possible fluctuations, this is repeated 20 times within 1 min, and the averaged offset voltages are determined. These are also written onto magnetic tape, in block 206, as mV normalized per ms of gating time. On the basis of these measured offset voltages, the new compensation current I is calculated with the following algorithm

xc = Zc + " 7 A I c ( 8 - 6 )

AI is the uncompensated current which gives rise to the non-zero s i 9 n a l VOFF

G . AI . T r

VOFF " CI ( 8 - 7 )

r with sampling time Tn, gain G and feedback capacitance C , (20 nF) . Now the compensation current is adapted to I and finally the offset after compensation is measured once and written on tape, block 201 (same format as the normal block 200). After some iterations, the offset will become close to zero, as is clearly shown in fig. 8.8. It can be seen that the signal in the beginning (left) when there is no compensation, is saturating at nearly 14 V. When there is correct compensation (right), the resulting offset is a few mV and the beam signal ( 150 mV) can be measured.

U t F ig. 8.8 Leakage current compensation. LEFT: The vertical scale on this oscilloscope picture is 5 V per (harldy visible) division, the time scale is 1 ms/div. stepwise the compensation is established. RIGHT: Vertical scale .1 V/div., time scale .5 ms/div. Nearly correct compensation, upper track with beam, lower track without beam. Only a "* 50 mV signal from uncompensated leakage current is left. The beam signal represents ^ 160 mV, and would have been completely invisible on the left picture, where the leakage current causes a signal > 10 V.

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The current compensation is obtained by a DAC controlled voltage to current converter. To have a very high impedance at the output, a MOSFET transistor is used. The 12 bit DAC is set by the register R2, which also selects the range of compensation: 100 nA, 1 \ih or 10 pA maximum current can be selected by a simple commutation of the feedback resistors. The resolution decreases for the higher ranges, because only 12 bits are available. The least significant bit represents respectively .024, .244 and 2.44 nA. This leads to a residual offset, depending on gain and sampling time, which cannot be corrected. The content of the R2 registers is written on tape at the end of the compensation sequence.

Bits 0 to 11 indicate the setting and bits 13, 14 or 15 indicate the scale factor (.1, 1. and 10. \iA respectively). Thus I = scale-factor x setting x 0.0002442. Although this compensation corrects for other causes of offset, the main part reflects the detector leakage current, and therefore the compensation current is a monitor for the detector current. It follows the temperature variations in the pit, but if these are very important, the feedback of 15 min is too long and some systematic offset may appear. The same happens, if there is a continuous increase in detector current due to radiation damage. In these circumstances the offset measured after the compensation is a true offset and has to be subtracted from the flux signals during the following 15 min.

Sometimes, fluctuating offsets are measured but these are not related to a continuous change of current. These instantaneous current changes are due to low frequency noise, and in this case the measured offset should not be subtracted from the signal. In the off-line treatment of the detector data, the bit 12 (block 210) is set if the offset subtraction is enabled (see also 7.2.1).

In the detector history tables (table 7.2) the averaged compensation current for each run is displayed. The r.m.s. value of the offsets after compensation is used as a measure for the detector noise. Generally, an increase in current is accompanied by an increased noise. Beyond 10 iiA detector current no compensation is possible, and the detector has to be replaced, or it has to be used at a lower bias voltage, which implies a loss of stability if it is not fully depleted any more. From fig. 4.3 it became apparent, however, that the short-term linearity may still be acceptable. In figs. 7.1 and 7.2 the in-situ measured leakage current was used to illustrate the behaviour of the detector as a function of radiation and as a function of time.

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8.3 Movements Control As described in 7.2 there are two independently moving parts in each

gap, the support plate and the lift. Both can be displaced in the y and z-direction by commands which are controlled by CAMAC modules in crate 13 (support) and in crate 14 (lift) . The support is driven by a slow electromotor. The rotation of the shaft causes the displacement via a gear box, and is also coupled to the absolute encoder. When the encoded position becomes equal to the required position the motor is stopped. The encoders are read every SPS cycle, the results are BCD numbers which are written on tape in bits 2-14 of the corresponding data words in block 200. The maximum range is 1799 steps of .2 mm. When a support is being displaced, the bit 0 of this word is up. Other status information is given in the following word [7.4].

The lift is driven by a fast electromotor, which can in principle move the lift with a load of ^ 10 kg from one detector position to another within the 10 s between two SPS ejections. An incremental encoder with .3 mm step is driven by a pinion on a precision cog-bar. When the true position is less than 10 steps from the required position, the motor is switched to low speed, to avoid overshoot. The lift positions are also written on magnetic tape, but in binary code.

Several computer programs are available which perform automatically a sequence of movements, e.g. for a circular scan at several radial distances, or for a calibration of all fixed detectors on the support plate. Sometimes, the lift remains oscillating around the required position, and the bit which indicates movement stays up. The command is then aborted after 1 min, and an error message is sent to the operator. The system then waits simply for the next movement command. All the automatic movements begin with a "garage" command, which positions all lifts outside the gap. The incremental encoders are all reset to zero on departure from the garage. When they finally return, after the complete sequence of movements foreseen in the program, one may check if any error has occurred. Bit 15 in the lift position word will be zero in case of an error, and the complement of the indicated position represents the number of incorrectly added increments.

8.4 Data links to users Data to and from users can be transmitted via serial CAMAC links

(fig. 8.1). Data concerning the proton intensity and the beam elements in the neutrino cave are received from the NBC computer in BA7 via the NBC link. Beam and flux data are transmitted to the EMI computer, to the WA1 computer and to the WA18 computer. Previously there also was a link to the GGM computer.

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Some data do not arrive via a link. The BEBC databox, which is also used for the NFM time indication, the GGM databox, and the WA1 and WA18 status and run number information is directly sent to data acquisition modules in CAMAC crate 1. Several other data, like direct signals of the proton SEM and the proton BCT, arrive in crate 2, also in the beam control room. Various data which are generated in the electronics barrack 294, are read in crate 12. These comprise the spill length measurement, several scalers which can be used for counting-detectors, a digiswitch module which is used in connection with the absorber box, and the current and temperature measurement of the shielding magnet. They are written in tape block 200, as described in ref. [7.4].

8.4.1 NBC data link The Neutrino Beam Control (NBC) computer has the control of the beam

elements in the neutrino cave, either the NNB magnets or the horn and reflector for the WNB. It also does the data acquisition of the special NNB monitors, which are installed in the cave and in pits V x and V2 (sects 6.3.2 and 6.3.3), and it transmits data from the SPS and WEXTR computers to the NFM and also directly to WA1.

The NBC computer, which is situated in BA7, can be accessed via a number of terminals, one of them is in the NFM beam control room. It enables operator intervention if the audio-visual alarm indicates malfunctioning of one of the magnets. Such an alarm can be checked immediately against the muon flux.

Although the NBC computer keeps record on its own magnetic tape unit, all relevant data are assembled in blocks and transmitted via the NBC-NFM data link. They appear on the NFM magnetic tape as blocks 240 to 245. Block 240 (from June 1977 this becomes block 243) is the narrow band beam standard block, which is sent every SPS cycle. It contains the proton beam data, the hadron BCT values, the beam profiles, the spill time structure and the Cerenkov data block. A detailed description is given by Sigurdsson in ref. [8.2] and also by Cavallari, [7.4]. Block 241 (from June 1977 this is block 244) contains mainly the data from the hadron calorimeter, which was taken out of the shield in 1981, to make place for the new beam dump. The calorimeter block is sent every 20 min. In block 242 one finds the narrow band reference data, which are the collimator positions, the beam magnet settings and detector positions. This reference block is transmitted every 100 SPS cycles, or when a change has occurred.

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8.4.2 Off-line data For the off-line data analysis the program OPCOM writes all original

measurement results on magnetic tape. Every SPS cycle the NFM standard block 200 and the NBC standard block 243 (earlier 240) for NNB or 245 for WNB are written. Other data blocks were already indicated, and the exact data formats are described by Cavallari [7.4]. The NBC block is always written, unless the NFM or the NBC or the link is down. The NFM block is written only if some threshold flux is exceeded. However, from 1981 onwards, it also is written whenever one of the counter experiments is taking data, even at low flux.

The original data tapes are changed every day, and copied together on CDC closed-shop master tapes. These can be used to determine the detector calibration factors, as will be described in chapter 9. Using the updated calibration factors, the muon flux for any desired selection of SPS cycles can be obtained from the master tapes. To enable such selection, run numbers, status information and film/photo numbers are given in each block. H. Klein and E. de Wolf designed a program [8.3], which compresses the information to 100 pulse averages in a HYDRA data structure, and which enables the user to work with much shorter data tapes (see also [8.4]).

Some users also write the on-line distributed flux data directly on their data tapes. Clearly, this facilitates the correlation of events with the flux. However, the on-line data contain implicitly already calibration factors, which are not too precisely determined. Therefore, a correction factor has to be applied afterwards, if good precision is required. Such correction factors are shown in table 7.4. They are the ratio between the off-line and on-line sensitivities.

8.4.3 Bubble chambers and counter experiments The CAMAC links to the EMI (BEBC) and the counter experiments WA1 and

WA18 are one way only, and each SPS cycle a data block (identification number 221) of 400 words is transmitted. It contains the muon flux data, expressed as muons.cm"2, in two words. Each time, the first word contains the mantissa and the second word the exponent of the power of 10. First the fluxes along the left hand radius of each of the six gaps are given, then the fluxes in the eight positions of the symmetry circles, each time preceded by the gap number and the radius of the circle. The muon flux data are followed by proton beam data and the horn and reflector currents.

Apart from the data transmission via serial CAMAC links, there is also some exchange of data otherwise. The flash trigger for BEBC and GGM is derived from a discriminator module, which is hardware connected, normally to the charge amplifier of the central detector in gap 2. The discrimination level is set manually. Until 1981 this trigger was also

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used to determine if muon flux data had to be written onto magnetic tape, or not. The BEBC data box information is read through an interface in CAMAC crate 1. The same is the case for the status information on the counter experiments.

8 .5 Organization of OPCOM At 600 ms before the proton extraction an interrupt in the NFM

computer stops the current activity, which might be for example a leakage current compensation sequence. At 1 ms before extraction the BEBC data box is read, which gives the time indication in the following NFM data block. At 6 ms after extraction the muon detector signals are read via the serial CAMAC link to the barrack B294, which may take 200 ms. During this time other link interrupts are not effective, but afterwards these have highest priority, except when data are written onto magnetic tape.

Afterwards, lower priority programs continue execution, like movements control for scanning or calibration, composition of the display, etc. Some programs are initiated by the time scheduler, like the current compensation of the detectors (every 15 min.) or the shielding magnet current measurement (every 20 min.).

Of course, programs can also be activated by the operator, who has the OPCOM control on the terminal in the beam control room (fig. 8.9) . The different possibilities are described by Cavallari in the NFM users guide [8.5] .

Fig. 8.9 View of the NFM control room, with the NORD 10 computer at the left.

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Some programs, especially the circular and cross sectional scans, produce their output not only as lists of fluxes but also in graphics form. This can be viewed on the screen or plotted on a Versatec hardcopy unit.

The possibilities for on-line beam monitoring will be further described in chapter 10.

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TABLE 9.1

Calibrations by Che reference box and by emulsions

Run number Date Beam type Energy

(GeV) Commmen t Position of Refbox Absbox

Emulsion exposure

49 1976 Dec. NB + 200 2 (4) 50 1977 Jan. NB + 200 2 (4) 51 Jan. NB + 200 2-3 52 Jan./ NB + 200 2-3 NFM system 53 Feb. NB -200 2-3 incomplete 54 Feb. NB -200 2-3 incomplete 18 Feb. gaps 2,3 55 Feb. NB 2-3 operating gaps 56 57

March March

NB NB -200

2-3 2-3, 5 are indicated 21 March, gaps 2,3,5

58 March NB 200 2-3, 5 59 April WB 400 test 2-3, 5 60 April NB 200 2-3 (4) 5 61 April NB 200 2-3 (4) 5 62 April/ NB 2-3 (4) 5 63 June NB 1-2-3 (4) 5 i 64 June NB 200 1 65 July NB + 275 1-2-3-5 1 66 July NB + 275 All 1 11 July, gaps 2-5 67 July/ WB + 350 All 4 68 Aug. WB + 350 3 69 Sept. WB +400 70 Sept. WB + 350 6 71 Oct. WB + 350 5 72 Nov. WB + 350 6 73 Nov. WB +350 2 28 Nov., gaps 2-5 74 Dec. Proton dump 400 1 16 Dec. gaps 1,2 75 1978 March WB -350 3 76 March WB -330 3 15 March, gaps 2-5 77 April WB + 350 3 78 April WB + 350 1 79 April/ WB + 350 5 80 May WB + 330 4 81 May WB -330 4/5 30 May, gaps 2-5 82 June WB -330 3 83 July WB + 350 3 5 July, gaps 2-4 84 Aug. NB -200 1 85 Sept. NB + 200 2 1 86 Sept. NB + 200 4 3 87 Oct. NB -200 5 4 20 Oct., gaps 2-5 88 Oct./ NB -200 3 4 89 Nov./ NB -200 (Also +200) 2 3 90 Dec. NB -200 (also +200) 2/3 2 14 Dec. gaps 2-4 91 1979 April/ Proton dump 400 1 92 May Proton dump 400 1 94 June WB -400 2 3 95 July WB + 400 5 5 4 July, gaps 2-5 96 July WB -400 4 5 18 July, gaps 2-5 97 Sept. NB -200 Protons 450 3 2 10 Oct., gaps 2-5 98 Oct. NB -200 450/400 3 1 99 Nov. NB -200/+300 3 1 100 Nov./ NB + 300 2 1 29 Nov., gaps 2,3 101 Dec. NB +300, tests 2 1 102 1980 Jan. NB + 300 4 2 103 Feb. NB +300, test at +. 200 5 3 7 Feb., gaps 2-5 104 Feb. NB Tests 5 3 105 March WB -400 4 106 March/ WB +400 4 25 March, gaps 107 April WB -400 5

j 108 April/ WB -400 3 [ 109 May WB +400 2 ; 110 May/ WB + 400 (also -400) 2 27 May, gaps 2,3

111 June WB -400 2 112 1981 Aug. WB -400 2 113 Aug./ WB -400 114 Sept./ WB -400 29 Sept., exp. stack 115 Oct. WB +400 116 Oct./ WB -400 27 Oct., exp. s tack 117 1982 Feb./ New proton dump 118 March New proton dump 119 March/ New proton dump 120 April NB + 100, 200 121 Apri 1 NB + 200 122 May NB + 200 123 June NB ±100 124 July New proton dump 125 Aug. New proton dump 126 Aug./Sep. New proton dump

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DETECTOR CALIBRATION To relate the detector signal to the muon flux, which is expressed as

the number of muons per cm 2 per burst, it is necessary to calibrate the detectors. In principle, the calibration proceeds in three steps. First, a few detectors in the "reference box" are exposed in the beam, together with a device which measures precisely the number of muons which pass through these reference detectors. This is the "absolute" calibration and the device is generally a nuclear emulsion. But some attempts were made to use electronic counters, silicon detectors or scintillators, which recognize muons via a coincidence method (sect. 9.4).

The second step is the comparison of the detectors in the normal calibration boxes ("calbox") with those in the reference box ("refbox"). All these are traversed by the same muon flux, because they are precisely aligned on the lift. The reference box is moved from one pit to another between runs, because generally there is no access during a run. In table 9.1 is indicated in which pit the reference box was exposed, and when an emulsion exposure has been made.

The third step then is the calibration of all fixed detectors via the comparison of their signals with those of the calibration box, when the lift is positioned exactly in front of a fixed detector. In reality, the calibration proceeds just in the inverse order and this third step, the "relative calibration" is the first to yield results. A renormalization on the basis of the time consuming emulsion analysis can be applied any time afterwards.

9.1 The relative calibration 11 is not taken for granted that the calibration factor of a detector

is constant over some period of time. The procedure of repeated relative calibration has been designed to determine this factor over and again, at least for each run independently. The calibration sequence is executed automatically every 8 hours, and in addition also the data from scans can be used for calibration purposes. In both cases, the calbox, eventually with refbox in front travels to a given position, which is aligned in front of a fixed detector, and once it has come to rest, it stays there until five acceptable bursts are written onto magnetic tape. Then it continues to the next position. In this way, between 100 and 500 calibration data are available for each detector in a run.

All these relative calibration data were extracted from the master tapes for each run and for the separate pits written onto temporary CDC 7600 computer disc files, in the following form. A record contains the month, day and time indication, the detector and channel number if the

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m m

O

S^ I 1 1 I I I I L _ ! 1 5 4 6

DET 395 (V) 10

>

4 6 DET 370 (V)

Fig. 9.1 Two examples of relative calibration of one detector against another. The signal heights for both detectors (in volts V) are correlated, if both detectors are hit by the same muon flux. The recursive fitting eliminates a number of confusing data points, and yields a correct result. The "noise" in the upper plot is due to incomplete current compensation. The correlated wrong points in the lower plot are caused by incorrect positioning of the calibration box, somewhat besides the fixed detector 42. The lines drawn through the data points represent the results of the successive iterations.

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calbox was in front of a fixed detector, the signal of this fixed

detector, and the signals of the five calbox or refbox detectors. The

signals V-0 are the output voltages V of the amplifiers, corrected with

the most recent measured offset voltages 0, if the offset correction is

enabled.

9.1.1 Calibration boxes

In a first operation, the calbox data are used in the linear least

squares fitting program "RELCAL", which calculates the slope and the

intercept of the linear relation between signals of each couple of

detectors in the calbox (or refbox). Quite frequently there are corrupted

data, generally because of insufficient leakage current compensation or

because of detector noise, related to a large leakage current. Therefore,

the fitting program has three recursions, excluding each time some of the

original data. In the first pass all signals are accepted and a straight

line fit is calculated. In the second pass, all data which are more than

.1 V from the line are excluded, and a new fit is made. In the third

pass, all original data are used again, but now those which are more than

.05 V from the previously fitted line are excluded. In fig. 9.1 examples

are given of such a fitted line, where a certain number of confusing

points were correctly eliminated.

For one beam pulse the output signals of two detectors x and y

represent the same muon flux N and therefore, using formula (1.6)

V - 0 V - 0 M = S

X -- - = S Y -X 2 (9 1) N u bD G bD G i y - i )

The offset-corrected signals V - 0 and V - o are plotted x x Y y

in the graphs of fig. 9.1 and can be fitted with the straight line V - 0 = C (V - O ) + O (9.2) y y y ,x x x y ,x

The coef f ic ien t C i s the s lope of the s t r a i g h t l ine and 0 i s the y r x y i x

in te rcep t with the y - ax i s . Under normal circumstances the i n t e r cep t s are found to be close to zero, and they are fur ther ignored. The r e l a t i o n between the rec ip roca l s e n s i t i v i t i e s i s then

SY = ^X i_ s

x

Û D G C D ( 9 . 3 ) x y , x

The line fittings are done for each value of the gain G or G

separately. It is found that the change of gain does not introduce a

noticeable non- linearity. In table 9.8 the coefficients for a few

detectors (364 vs. 308, 343 and 381) are given for relative calibrations,

spread over four years.

% CHANGE OF SIGNAL DUE TO CALBOX % CHANGE OF SIGNAL DUE TO CALBOX NARROW BAND 200 GeV WIDE BAND 4 0 0 GeV

rmol col ibrol ion box only o normol box. togelher wi lh relerence box .normal calibration bo» only onormol box , logclher with reference box

Note change of scales Note change of scales

Fig- 9.2 The percentual change of the fixed detector signal in various positions, due to the presence of the calbox ("calbox effect"). The left column shows the effect along the horizontal radius of the gap, the second column shows the effect along a circle, whose radius is indicated by the arrow in the left column. If a detector is "shielded" by another, superimposed "second layer" detector, the calbox effect is reduced. If a detector is shielded with 4 cm of lead, the effect nearly disappears. On the other hand, it does hardly make any difference if two or only one box is on the lift. It is mainly the lift structure itself which is responsible for the effect.

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Once the relations between the calbox detectors are established, a comparison is made of the fixed detectors with each of the calbox detectors, using again the data on the computer disc file with the same program RELCAL. The result is then a complete matrix of coefficients, relating the fixed detectors with the calbox and refbox detectors. From a preliminary sensitivity, assigned to the central detector 343 in the refbox, one calculates the relative calibration factors for all other detectors.

An alternative approach has been used in the program NFMPLY [8.3]. The reciprocal sensitivity S of the fixed detectors is determined directly by comparison with the muon flux, calculated by averaging the measurements of all calbox detectors. Although this approach is simpler than the one used here, it offers less possibilities to detect and correct anomalies in detector response.

9.1.2 "Calbox effect" on fixed detectors Unfortunately, the calibration box and the lift on which it is

mounted, modify the radiation which reaches the fixed detector behind. This effect can be easily studied by comparing the signals of the fixed detector to a non-disturbed intensity measurement, e.g. to the upstream detector signal, or directly to the proton intensity signal (however, the target efficiency may change with time, as well as the muon energy spectrum, which influences the muon to proton ratio). The calbox effect is then expressed as the ratio of the disturbed signals to the normal signals, both normalized to the same intensity reference. Only when both are perfectly aligned the effect on the fixed detector by the calbox is of direct importance for the calibration. Other detectors in the vicinity may be more disturbed than the detector actually calibrated [9.1], but this distorts the flux measurement only for a short time, and has no influence on the calibration.

In fig. 9.2 the calbox effect is plotted as a function of radial and azimuthal position, for the 400 GeV wide band beam and for the 200 GeV narrow band beam. The discussion given in chapter 3 on the influence of secondary radiation in the shield, provides the basic understanding of the calbox effect. The equilibrium which exists inside the iron between the muons and their electromagnetic cascade products is gradually lost in the air gap, the equilibrium in air being quite different. Secondary electrons redistribute in the air gap and diffuse outwards, contributing more to the signals of detectors at large radii. When the calbox is brought in position, it restores to some extent the situation existing inside the shield, generating secondaries in the centre and absorbing them at large radius.

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The relative calibration factors for the fixed detectors are thus obtained in a situation which is different from their normal flux measurement conditions. A correction factor for the calbox effect can be applied to the relative calibration factors, and these corrected sensitivities can be used in formula (1.6) to find the muon flux in the normal situation, without calbox in front:

N = (relative calibration x calbox factor) x V - O

= SD X V - 0 [cm"2] (9.4)

The calbox correction factors are listed in table 9.2. For example, in the centre of gap 2 the signal of the fixed detector is enhanced by the calibration box. The S D determined in the calibration is therefore too low, because without the calbox more muons are required to produce the same signal in the detector. The correct sensitivity S , and the correct muon flux (in the absence of the calibration box) is found by multiplying the relati' (narrow band, 200 GeV) . multiplying the relative values of both the S and the flux with 1.08

TABLE 9.2 Correct ion factors for the calbox e f f e c t

Position 40 41 42 43 44 45 46 Radial distance (cm) 0 15 30 45 60 75 90

350 GeV WNB, 400 GeV W NB

Gap 1 1.07 1.07 1.04 1.03 1.01 .90 2 1.02 1.015 1.01 1. 1. .98 .96 3 1.02 1.02 1.015 1. .99 .98 .97 4 1.01 1.01 1.01 1.01 .99 .99 .98 5 1.01 1.01 1.01 1.01 1. .99 .99 6 No cor rectior \

200 GeV NNB

1 1.11 1.08 .90 .82 .85 .88 2 1.08 1.03 .93 .85 .85 .85 .85 2 ref. [6.13] 1.06 1.04 .94 .89 .89 3 1.05 1.03 1. .96 .90 .85 4 1.02 1.02 1.01 1. .98 .97 5 1.01 1.01

300 GeV NNB

1 top layer 1.23 1.06 1.02 . .96 .94 1.05 1 second layer 1.09 .99 .98 2 top layer 1.10 .88 .88 2 second layer 1.04 3 1.03 1.06 .97 .86 .85 4 1.03 1.03 1.00 .97 .93 .91 5 1.01 1.01 1.01

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Once all the coefficients between fixed detectors, calbox detectors and refbox detectors are determined, a complete and coherent set of sensitivities can be calculated by "linking" all detectors to one basic sensitivity of a detector in the reference box. The sets of sensitivities given in the listing in Appendix C are such coherent sets. The stability of these relative coefficients is generally better than 2% over long periods of time. Some examples will be reported in table 9.8.

The calibration factors contain implicitly the local contribution of the secondary radiation. Therefore, these calibration factors are strictly valid only for the detectors in the calibrated position. In general, however, the change of the calibration factor when a detector is relocated in a different position, is below 10%. A notable exception is the "upstream" position, directly on the iron shield, in gaps 1, 2 and 3. In table 9.3 is shown that the S of a detector placed in gap 1 upstream is 18% lower than when it is placed on the plate (in WNB) .

TABLE 9.3 Gap 1, 400 GeV WNB

Detector S in posi t ion

upstream S_ in pos i t ion

on p l a t e Difference

115

117

1164

1190

1413

1460

18%

18.5%

9.2 Absolute calibration with nuclear emulsions 9.2.1 The exposure of emulsions The exposure of emulsions in the gaps interrupts the normal beam

operation for "v» 2 hours, and is performed when stable machine conditions can be expected. The extraction has to be stopped to give access to the underground pits for mounting the emulsions flat on the calibration boxes, i.e. roughly perpendicular to the beam. The emulsions are generally 200 um thick, of the type Ilford G.5, and backed by a 1.25 mm thick glass plate of 25.4 x 76.2 mm and individually wrapped in black paper covers. With adhesive tape they are simply fixed on the calbox; The resulting precision of alignment is ^ 1 mm. The box is then moved to the selected position in the gap, which in general is the central position. The disturbing effect of the emulsions on the calbox signals can be neglected, as was concluded from tests with an absorber box containing Al sheets instead of iron sheets in front of the detector (chapter 3).

Once the emulsions are in place', data for each SPS cycle, including "no beam" cycles, are carefully written onto magnetic tape so as to avoid missing a spurious pulse of muons. However, normally a well defined number of pulses can be extracted: one, two or more, as needed to obtain a predefined integrated flux in the positions of exposure.

- 184 -

Emulsions in gaps 2 and 3 may already contain 5 x 10s tracks . cm"2

after one pulse. Then these have to be taken out, before additional pulses are given to obtain similar numbers of tracks in emulsions, placed in gaps 4 or 5. By experience it was found that the optimal counting efficiency is achieved between 3-6 x 10 s cm"2. Then, also the errors due to background subtraction and area determination are minimal. If the flux on the beam axis exceeds 6 x 10 5 cm" 2, the exposure has to be done at some distance from the centre. In this case, there is a gradient in the flux in the emulsion, and to minimize the error related to this, the emulsion is fixed always with the long side tangent to the equi-flux lines. Also the scanning has to be done along this direction.

After the exposure, the emulsions are developed as soon as possible to avoid fading of the latent image and to stop further accumulation of background. This background can be determined from an emulsion of the same batch, which accompanies the other emulsions everywhere, except that it is not exposed in the gap. Generally, the background is only a few per cent of the number of counted tracks, and very few background tracks are perpendicular to the emulsion. On two occasions, a fairly large number of muon-like tracks were seen in the background plates ( 25 000 cm"2) , and these batches presumably were hit by particles from some beam on the site.

A muon background in the earth, originating from other beam lines in the West Area and occurring during the exposure period would of course not be seen in the background plates. Although the West Area beams normally continue operation during the emulsion exposures, there is no danger for such background in view of the large lateral distance.

9.2.2 Counting of tracks in the emulsion The developed silver grains (average diameter .6 urn) are

stochastically distributed along the trajectory of the particle in the emulsion, with interspaces of 4-10 ym. Due to the small depth in the focal plane of the microscope, no more than one or two grains of the perpendicular track can be seen at a time. But the human eye reconstructs the tracks, from the grains which follow one after another on a local spot when the microscope objective is moved up and down several times. The numerous "meaningless" grains, which are distributed everywhere through the emulsion are in this way eliminated. To recognize tracks in this way requires some training, and it has been found that observers may be "blind" to some special types of tracks, or on the contrary, may be too imaginative in linking some "meaningless" grains together.

- 1S5 -

During the development, the emulsions which originally are

200 um ± 40 ym thick, shrink by a factor ^2.5 and undergo also

some deformation, except on the backplane, which is fixed to the glass

plate. This causes locally uniform distortions of the tracks, and

originally parallel tracks will all "curl" in the same way, as illustrated

in fig. 9.3. It is then easy to distinguish tracks which passed through

the emulsion under a quite different angle. The use of the angular

distribution of the tracks will be described further in sect. 9.2.4. In a

parallel exposure the shrinkage introduces a big uncertainty on the area

during exposure. In order to obtain the optimal area definition the

emulsions are generally exposed perpendicular to the beam.

Fig. 9-3 Drawing of tracks in emulsion which are seen with

the the

microscope within a square reticle of calibrated dimensions. The positions and curvatures of the tracks are approximate, but there is a clear distinction between the perpendicular short tracks and large angle tracks (in narrow band beam). Also a heavily ionizing track passes through this field of view, in the upper right part.

EMULSION! : vz.c.vt

(thick - ZCQÏ )

In the course of time the procedure of counting tracks has evolved

slightly, but still is a tedious job. As a rule, twenty consecutive

fields of view on the microscope, from 40 x 40 um2 to 120 x 120 um 2,

depending on the magnification used, are. scanned by two observers. They

draw all the tracks as they see them on a sheet of paper. At a later

stage, the drawing was made more precise by using a "camera clara" drawing

objective, and millimeter paper, as shown in fig. 9.4. A third observer

compares the drawings and, looking in the emulsion, verifies those tracks

which are found by a single scanner only. Occasionally, he may even find

a track which has been overlooked by both previous scanners, but in such a

case, these have both to agree that the track is clearly visible.

- 186 -

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4 U * L t*> Co OX.C..W ,='-&

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'••L

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«• 41 (F 7-2-M t»K 13 «.41 \r ?-#-•» C*R

Fig. 9.4 Two examples of an original scanning sheet. Using a "camera clara" additional drawing objective, the millimeter paper is seen simultaneously with the tracks in the emulsion, and an exact projection can be drawn on the paper (top). The coordinates are used in a computer program to calculate the angular distribution around the prevailing main direction. The same field of view, after treatment by the program is shown at the bottom. A number of "long" tracks was rejected.

- 187 -

The main criterion for accepting (and drawing) a track is that it can be followed all the way through the emulsion from an entry point to an exit point in the surface of the emulsion. The coordinates of the entry and exit points can be used to calculate the apparent angle of the track. Decisive for the counting is that the entry point should be inside the field of view, as defined by the reticle or the square on the millimeter paper. The exit point may be anywhere, and a large angle track may have to be followed over several fields of view, until its exit point is found.

The criterion, as it is formulated, excludes tracks by stopping muons, low energy electrons (< .3 MeV) and converting photons. On the other hand, a few dense tracks by heavily ionizing protons, nuclear fragments, etc. are included.

Different observers may see a slightly different field of view due to a difference in eye distance. The field of view in the third (comparison) scan is determining. The area of the field of view is precisely measured with an object micrometer, which is placed in the same position as the emulsion. The uncertainty on the area is less than 2%.

The result of the emulsion counting is then a number of tracks per cm2 , from which the cosmic ray background has to be subtracted. However, the tracks are not only muon tracks but include a certain number of electron tracks ("delta rays") from the electromagnetic cascade. In the next section this contribution is studied in detail. Once the number of muons is determined, this number can be compared to the signal V of the detector to be calibrated. Taking into account the amplifier gain G and a possible offset voltage 0, the charge developed in the detector can be directly compared to the number of muons N in the emulsion. The

u absolute value of the reciprocal detector sensitivity S , as defined in sect. 1.3.3, is then found with formula (1.6)

S D = N y x y-r-Q [cm"2 PC"1] (9.5)

9.2.3 Delta electron contribution in the emulsion The generation of secondary radiation, especially "delta-ray"

electrons, as a part of the muon energy loss process, has been discussed in detail in chapter 3. Also the influence of this secondary radiation on the silicon detector signal was studied there. In a nuclear emulsion electrons below 250 keV have a range < 200 ym, and although according to Sevier [9.2], they can be recognized individually, they do not satisfy our scanning criterion. Neither do the products of converting photons. The electrons of somewhat higher energy produce acceptable tracks, which

- 188 -

can be recognized by large angle scattering in the emulsion. High energy

electron tracks, however, cannot be distinguished from muon tracks, unless

they are followed over a considerable distance in the emulsion, so that

the mean angle of multiple scattering can be determined.

The method of multiple scattering has been employed by Eichten [1.3]

to find the electron contribution in the emulsion calibration in the PS

neutrino beam. An exposure of a thick emulsion stack has been made also

in the WNB, during run 116, to perform a precise determination of this

electron contribution. However, at least 1000 tracks have to be followed

to obtain sufficient statistics.

Contrary to the situation in the PS beam, with a mean muon energy

^ 2 GeV, the muon tracks in the SPS shield only make small angles with

the beam direction because of the much higher mean energy (20-200 GeV) .

So it is possible, even with a relatively thin emulsion, to use the

angular distribution of the tracks, instead of the multiple scattering.

9.2.4 Angular distribution of tracks in the emulsion

The coordinates of the entry point and the exit point of all tracks

found in a double scan of an emulsion are entered as data in a computer

program which first calculates the main direction of the tracks, including

only those which have a polar angle < 100 mrad with respect to the

average direction. Then it produces the angular distribution of all

tracks around this main direction. The curvature of the tracks is not

taken into account, and it is supposed that the distortion of the emulsion

is homogeneous over the scanned region, ^ 2 x .1 mm 2, corresponding to

20 fields of % 100 iitn.

In fig. 9.5 a histogram is shown of the angular distribution in an

emulsion, exposed in gap 2 during 200 GeV NNB (run 103) . The number of

tracks per bin of 2.5 mrad is shown as function of the angle, in mrad, on

a log scale. This presentation enables an easy comparison with a

projected angular distribution resulting from multiple scattering of an

initially parallel, monochromatic beam

dN % 6 -e2/2a2

âê = ô e ( 9- 6

9 is the scattering angle of an individual particle and o is the mean

scattering angle. In fig. 9.5 this distribution is plotted, using

cr = 6.5 mrad, and with this value of a it coincides with the measured

distribution in the region of the main peak. Two other contributions to

the measured distribution now appear more clearly. One is the "flat"

distribution of cosmic ray background (4%), the other the delta electron-

- 189 -

background. Burmeister [9.3] performed a Monte-Carlo simulation with EGS [3.20] of the propagation of electrons above 1 MeV through the gap. The initial electron energy distribution was obtained by generating electrons via the ionization formula (2.6) for 20 GeV muons in iron, and following these electrons until they leave the iron. The resulting angular distribution of these electrons at the emulsion plane is shown also in fig. 9.5, and is essentially flat. It was adjusted to the measured intensity in the region 40-300 mrad. Beyond 400 mrad considerable scanning losses occur, because these tracks sometimes have only a few grains in the initial field of view.

1000

o E

ID

00

co

<

UJ m

T

100

1, cosmic ray background r-

0.1 t J •—

10 100

ANGLE (mrad)

1000

Fig. 9.5 Histogram of the angular distribution of 614 tracks in emulsion V2.41.UP (+200 GeV NNB, run 103, 7 February 1980). The numbers are normalized to a total of 1000 tracks and histogrammed in bins of 2.5 mrad. The cosmic ray background in a dummy emulsion is shown at the bottom (3.8Z of tracks). A curve, derived from projected Gaussian multiple scattering (formula (9.6)) is fitted to the main peak. The result of the EGS Monte-Carlo simulation by Burmeister [9.3] of electrons in the gap is also histogrammed and adjusted to the average number of tracks in the region 40-300 mrad, where no muons are expected.

- 190 -

Instead of using the simple approximation (9.6), the angular distribution can be predicted with the help of the beam simulation program NUBEAM [3.8], and in that case a better agreement of calculation and measurement can be obtained, as remarked by Penfold and Wachsmuth [9.4]. The large angle tracks (in the NNB) can only be electrons (or cosmics) and by extrapolating their distribution towards the small angle region, the number of electrons in the main peak can be determined. They only represent ^ .5% of the tracks in this peak. Therefore, a negligible error is made if one takes directly all short, straight tracks for muons and all others for electrons. This has been the approach in the early scanning. The results obtained with this simple method and with the elaborate method are given in the next section.

In the wide band beam the muon angular distribution is much wider, hence it is more difficult to determine the electron contribution. For some time, only the total number of tracks was taken, and an arbitrary 20% electron contribution subtracted. It was expected that a more precise correction would be determined later. In fact, Burmeister et al., [9.3], Jongejans et al., [9.5] and Jongejans and Wachsmuth [9.6] describe the progress made in the determination of the delta electron correction factor. With more refined Monte-Carlo calculations it was established that there are quite a few large angle muons, e.g. in [9.5] it is shown that in gap 4 on axis about 4.5% of the muons have angles > 100 mrad. On the basis of these calculations, the muon calibration factors tend to become higher. But the improved scanning methods increase the overall number of tracks found, especially at large angles, and so the number of electrons still remains "v 20% of all tracks. Old and recent numbers will be compared in the next section. An example of the angular distribution of tracks in emulsion V5.0.0.UPS is shown in fig. 9.6. It is clearly wider than in the case of the narrow band beam, and the distinction between the muon peak and the flat background is less obvious.

The conclusion may be that the angular distribution provides a good separation criterion in the NNB. For the WNB the number of large angle muons is considerable, and therefore a good separation of electron and muon tracks is more complicated. An uncertainty of ^ 10% exists in the case of WNB. Either multiple scattering measurements or coincidence counting with electron absorbers should provide conclusive data with better precision.

9.3 Results 9.3.1 Results of absolute calibration in NNB Results of absolute emulsion calibrations of silicon detectors in the

NNB have been published by Cavallari et al. [7.1], Heijne [3.31] and most recently by Penfold and Wachsmuth [9.4]. In table 9.6 some of these

- 191 -

results are shown along with additional data, gathered over several years. However, care has to be taken in comparing these results, as will be explained in the following.

1000

0 1

I ITTTTTT

T-ru

J i * 10 100

Angle ( mrad )

1000

F i g . 9.6 Histogram of the angular d i s t r i b u t i o n of tracks in emulsion V5.00.UPS (400 GeV WNB, run 96 exposed in gap 5 ) . Superimposed i s the angular d i s t r i b u t i o n of muons, which was measured with coincidence counting ( s e c t . 9.4 and f ig . 9 . 8 ) . The dashed l i n e represen t s the cosmic ray backgound, as in f ig . 9 . 5 .

Due to the v a r i a t i o n with radius of the contr ibut ion by secondary r a d i a t i o n , which goes l a rge ly undetected through the emulsion, the s i l i c o n de tec tor c a l i b r a t i o n in the NNB is s t rongly pos i t ion dependent. In t ab le 9.4 i s shown tha t the c a l i b r a t i o n factor for a calbox detector on ax i s (0 cm) i s ^ 17% more than a t 30 cm, for a 200 GeV beam. In a 280 GeV beam the di f ference i s even 35%. Obviously, the pos i t ion dependence i s folded with an energy dependence. The proportion of recognizable e lec t ron t racks remains f a i r l y constant and is uncorrelated to the d i f fe ren t cond i t ions .

TABLE 9.4 Pos i t ion dependence of c a l i b r a t i o n factor in NNB

Gap Position Detector Signal in detector

Emu Is ion scan Electrons X

Muons calibration

(pC) rnuon flux Electron flux Electrons

X factor S

200 GeV, run 90 2 0 - 3 4 3 15 210 522 000 143 000 11 34.3 2 1 30 ] 343 7 472 219 800 54 000 24. 5 29.4 280 GeV, run 66 2 0 382 2 385 482 000 115 000 24 202 2 30 382 2 146 320 000 107 000 33 149

- 192 -

There is a possible cross check on the emulsion result. If the muon flux measured with the emulsions in various positions is normalized to the beam intensity, it can be compared to the true beam profile as measured with the absorber box, under the same beam conditions (see fig. 3.14a). In table 9.5 the values obtained with the iron absorber compare quite well (at 30 cm .049 times the intensity on axis) whereas the intensity in the normal profile is much higher (.067) due to the increased contribution from secondary radiation off-axis. Instead of calibrating all positions under all different conditions, it might therefore be sufficient to calibrate only in one or two positions and calculate the other calibration factors via the profile measurement with the absorber box.

TABLE 9.5 Muon flux profile in gap 2 measured with emulsions and with absorber box in 200 GeV NAB

(run 90)

Radial position (cm) 0 15 30 60

Emulsion Muon flux (cm" 2 ) Normalized to 10 1 3 protons Normalized to 0 cm

522 000 403 100 1

219 800 19 200 .048

18 600 1 625 .00403

Absorber box with Fe (12.4 c Charge (pC) Normalized to 0 cm

Tl)

22 200 1

8757 .394

1088 .049

Normal profile Charge (pC) Normalized to 0 cm

18 800 1.

7 880 .419

1 260 .067 .0051

This was judged too complex a procedure to yield reliable results, and therefore it was decided to calibrate all detectors at 15 cm radius in gap 2 and use only this measurement to renormalize the calculated neutrino intensity [9.7] .

The uncertainty on the combined result of the seven emulsions is 3.1% and is composed of [9.4]:

Track statistics Position uncertainty Area measurement Scanning inefficiency Detector signal fluctuation Delta ray subtraction

1.8% 1.6% 1% 1% 1% 1%

- 193 -

The spread on individual da ta , with ± 3.1% error bars , i s s h ° w n in f ig . 9.7 where the emulsi o n _determined f l u x i s CQmçaceâ with the f l u x P r o f i i e around the 15 cm c i r c l e , as measured with the fixed detectors and with the moving calbox. The ca l ibra t ion fac tors of the fixed detectors are normalized to the average resu l t of the emulsion ca l i b r a t i on , at 15 cm. A factor .87 has to be applied to the r e s u l t s based on ca l ib ra t ions in the centre of the gap. For the calbox scan prof i le the s e n s i t i v i t i e s are used as obtained from emulsion ca l ib ra t ions in the centre of the gap. At 15 cm radius , these give -v 15% higher r e s u l t s than the d i r e c t emulsion measurements, but show approximately the same flux p ro f i l e . The normalization of a l l the r e s u l t s , accumulated during several periods in two different runs, is done with respect to the i n t ens i t y indicated by the parent hadron BCT, which excludes a t l e a s t t a rge t t ing f luctuat ions . The four measurements reproduce the same p ro f i l e to within ± 5% and th is can be regarded as sa t i s fac tory agreement. Neither the shape of the prof i le nor the muon-flux to hadron BCT r a t i o are per fec t ly s t a b l e , and th is explains most of the observed f luc tua t ion .

103 • emulsions o fixed detectors

90 A fixed detectors + scan calbox

0 90 180 270 360

Angle ( degrees) Fig . 9.7 The muon i n t ens i t y in gap 2 for d i £ f e r e n t a n g i e s a t 15 cm dis tance of the t h e o r e t i c a l beam axis (+200 GeV parents ) - T h e emulsion r e s u l t s (• ) a r e compared with calbox scans (+) and with the i n t e n s i t i e s measured by the fixed de t ec to r s ( 0 , A ) . The calbox scan i s normalized to emulsion ca l ibra t ions on axis (0 cm), the i n t e n s i t i e s of the fixed de t ec to r s are normalized to the emulsion r e s u l t .

x 3

u.*o

0.^0

0.35

030

n ?7

- 194 -

Based on t h i s c a l i b r a t i o n , an a b s o l u t e n e u t r i n o c r o s s s ec t i on for r e a c t i o n s in BEBC f i l l e d with a neon-hydrogen m i x t u r e has been pub l i shed [9 .8] .

9 . 3 . 2 R e s u l t s of a b s o l u t e c a l i b r a t i o n in WHR

P r e l i m i n a r y r e s u l t s of t he emulsion c a l i b r a t i o n in the WNB were used for the o n - l i n e s e n s i t i v i t y f a c t o r s in the NFM sys tem. More d e f i n i t i v e r e s u l t s were p u b l i s h e d by t h e au thor [3.31] which were based on t o t a l t r ack c o u n t . The s t i l l undetermined d e l t a - r a y c o n t r i b u t i o n was a r b i t r a r i l y e s t i m a t e d to be 20%, and s u b t r a c t e d . The work of Burmeis ter e t a l . [9 .3] was the f i r s t r e a l a t tempt to s e p a r a t e the muons and t h e r e b y determine the d e l t a - r a y c o n t r i b u t i o n . An e x t e n s i v e e f f o r t has been made by Jongejans e t a l . [9 .5] and t h e work of Jonge jans and Wachsmuth [5 .6 ] has been a r e - a n a l y s i s of h i s r e s u l t for one of the emuls ions (run 9 5 , V4 .0 ) . They f i n a l l y c a l c u l a t e d de t ec to r c a l i b r a t i o n f a c t o r s which a r e \ 6% higher than t h e p r e l i m i n a r y o n - l i n e s e n s i t i v i t y f a c t o r s . The u n c e r t a i n t y on t h i s r e s u l t i s e s t i m a t e d a t 4 .2%, i . e . 3% s t a t i s t i c a l , 2% for d e l t a s u b t r a c t i o n , 1% for the measurement area and 2% for d e t e c t o r s i g n a l f l u c t u a t i o n s .

The r e s u l t s ob ta ined from s e v e r a l c a l i b r a t i o n s a r e shown in t a b l e 9.6 for t h r e e d e t e c t o r s in the r e f b o x , which were o f t en c a l i b r a t e d . However, the Jongejans-Wachsmuth r e - a n a l y s i s concerns d i f f e r e n t d e t e c t o r s . In t a b l e 9.7 a l l a v a i l a b l e r e s u l t s a re shown for one of them, d e t e c t o r 364. Comparison shows t h a t the f l u c t u a t i o n s l a r g e l y exceed the quoted ( s t a t i s t i c a l ) u n c e r t a i n t i e s . For the s e p a r a t e d muon count the f l u c t u a t i o n s , exp re s sed by t h e s t a n d a r d d e v i a t i o n , a r e b igge r than for t h e t o t a l t r a c k c o u n t . This i n d i c a t e s t h a t the t o t a l t r a c k count i s more c o n s i s t e n t . In o the r words, the u n c e r t a i n t y in f i n d i n g the t r a c k s i s sma l l e r than t h e u n c e r t a i n t y i n a s s i g n i n g the muon or e l e c t r o n s i g n a t u r e , which depends t o some ex t en t on assumptions or c a l c u l a t i o n s of angular d i s t r i b u t i o n . However, the t o t a l t r a c k count in [9.3] i s low and t h e t o t a l t r a c k c o u n t in [9.6]) i s high compared to t h e o t h e r d a t a . The f a c t t h a t t he more r e c e n t r e s u l t s t end to be higher i n d i c a t e s a l e a r n i n g f a c t o r in the scann ing and e v a l u a t i o n p r o c e s s . T h e r e f o r e , the u n c e r t a i n t y i s n o t only de te rmined by s t a t i s t i c s bu t probably more by s y s t e m a t i c s . In t h i s c a s e one can e i t h e r r e l y ° n " t h e b e s t r e s u l t " ( i . e . the r e - a n a l y s i s [9 .6 ] ) or choose the ave rage r e s u l t of as many da t a as p o s s i b l e , supposing t h a t in t h e long te rm l e a r n i n g e f f e c t s and other e l u s i v e f a c t o r s cance l ou t .

J o n g e j a n s e t a l . [9.5] s t a t e as one of t h e i r conc lus ions t h a t i n t e r n a l f l u c t u a t i o n s up t 0 1 0 % are i n h e r e n t i n t h e emulsion c a l i b r a t i o n method. On s e v e r a l occas ions i t was found t h a t s i m u l t a n e o u s l y exposed emuls ions g i v e r e s u l t s w i th in the s t a t i s t i c a l e r r o r , bu t t h a t emulsions

- 195 -

from a different exposure lead to a somewhat different result, whereas the relative detector signals seem to be the same. However, as is clear from table 9.6, these fluctuations are well below 10%.

TABLE 9.6

Detector s e n s i t i v i t y S„ (N cm"2 pC"') from emulsion c a l i b r a t i o n D y

for 3 refbox de tec to r s

Run beam (GeV) Detector 308 Detector 343 Detector 381

Narrowband (Anti)Neutrino Beam LVity

Muons Pos. Sensitivity


Muons Pos. Sensiti LVity



Muons only only only

54 -200 3/0 a lso ref. 100. 2/0 35.7 5 7 -200 3/0 [ 7.1] 108. 2/15 92.6 66 +275 3/0 113. 87 -200 5/0 102. 5/0 36.8 3/0 no. 90 -200 2/0

2/30 97.4 80.5

2/0 2/30

34.3 28.3

3/0 3/30

95.7 97.2

97 -200 3/0 3/30

3/0 3/30

3/30

100 +300 2/30 85.4 2/30 30.4 3/15 103 +200 5/0 5/0 2/30

2/15 Indire ctly ref [9.4]

Wideband (Anti)Neutrino Beam ivity

Muons ivity Muons Pos.

S en s i t All

ivity Muons Pos.

Sensitivity All Muons Pos.

Sensit All

ivity Muons

tracks only tracks only tracks only

73 +350 3/0 121. 97. 2/75 2/90

44.1 36.5

35.3 29.2

2/75 2/90

120. 101.

96. 80.

76 -330 3/0 121. 97. 3/0 3/90

42.7 40.8

34.2 32.7

2/90 3/90

117. 114.

93. 91.

81 -330 3/0 120. 96. 5/0 42.3 33.9 5/0 113. 90. 83 +350 3/0

[9.3] 116. 111.

93. 3/90 39.8 31.9 3/0 [9.3] 3/90

112. 107. 110.

90. 87.7<*> 88.

95 +400 5/0 [9.5]

119. 116.

95. 104.(*>

5/0 [9.5]

42.2 41.1

33.7 37.0(*)

96 -400 4/0 [9.5] 4/90

130.

115. 122.(*>

114.<*>

4/0 [9.5] 4/90

45.4 40.8

42.8 ( < r )

40.5<*>

3/30 [9.5] 3/100

119. 114.

113.<*) 107.<*)

WNB average 118.8 101. 41.6 35.1 112.7 93.6 s tandard deviation 5.3 10.5

. 2.4 4.0 5.7 9.7

(*) The number of muons is determined from the angular d i s t r i b u t i o n ; in the other cases i t is .8 x " a l l " .

- 1*6 -

TABLEJK7

D e t e c t o r s e n s i t i v i t y S (N cm" 2 p C " ' ) from e m u l s i o n c a l i b r a t i o n 3 D v

for detector 364. The finally adopted value is S D = 38.1 ± 1.6

Run Beam P o s . Sens A l l t r a c k s

i t i v i t y Muons o n l y Method R e f e r e n c e

73 + 350 4 / 0 44 .0 35.2 ± 2 . A l l t r a c k s

81 - 3 3 0 4 / 9 0 44 .5 35-6 + 2 . A l l t r a c k s

95 +400 4 / 0 41 .2 4 2 , 4 4 3 . 1 47 .0

33.8 ± 4 0 . 1 ± 37.2 ± 38.1 t

2 . 2 . 1.5 1.6

Angular d i s t r i b u t i o n S i n g l e scan Double scan R e - a n a l y s i s

N a t a l i [ 9 . 3 ] J o n g e j a n s [ 9 . 5 ] J o n g e j a n s [ 9 . 5 ] J o n g e j a n s + Wachsmuth [ 9 . 6 ]

96 - 4 0 0 4 / 0 4 / 9 0

47 .0 4 2 . 1

44.4 ± 41.7 +

2 . 6 2 . 1

Double scan S i n g l e scan

J o n g e j a n s [ 9 . 5 ] J o n g e j a n s [ 9 . 5 ]

Average

S t a n d a r d D e v i a t i o n

43 .9

2 . 2

38.3

3.6

The best known value, from the re-analysis, for calbox detector 364 is 38.1 ± 1.6, to which 20% delta electrons can be added. To be useful for further calculations, this absolute calibration factor for detector 364 has to be related to the sensitivities for detectors in the reference box. In table 9.8 all the coefficients between det 364 and refbox detectors 308, 343 and 381 are listed, together with the resulting S D. Now one can compare these S_ with those independently obtained for the same detectors in other emulsion calibrations, (table 9.6). It appears that the double scan of Jongejans et al. in v run 95 (pit 5) gives an identical result, but all scans for run 96 are % 10% higher, in spite of the fact that total track counts deviate much less. The earlier results, based on total track count give all ^ 10% less. One may suppose that in these scans only ^ 13% of the tracks represent electrons, in which case equivalent results are obtained. Jongejans/Wachsmuth have searched better for large angle electrons, which may explain that finally they find 16%. electrons up to 400 mrad (20% including even larger angles).

For the reference box detectors the best known calibration factors are then:

- Detector 308 105. ± 5. - Detector 343 37. ± 1.5 - Detector 381 103. ± 5.

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TABLE 9-8

Coef f i c i en t s from r e l a t i v e c a l i b r a t i o n s over four years and corresponding absolu te s e n s i t i v i t i e s S

Detec tors 308, 343 and 381 are compared to de tec tor 364 Absolute c a l i b r a t i o n by emulsions gave for 364 a value S D = 38.1 ± 1.6

Run Slope Det. 308 Intercept

mV SD Slope Det. 343 Intercept

mV SD Slope Det. 381 Intercept

mV SD 67

80

86

96

102

105

106

2.769 2.788 2.778

2.740

2.636

2.627

2.661

12.6 9.3

- 8.8

7.5

14.7

16.5

1.8

105.5 106.2 105.8

104.4

100.4

100.1

101.4

.9779

.9808

.9736

.9776

.9812

.9796

.9791

.9664

.9660

.9582

.9638

.9900

.9588

2.0 - 3.0 1.0 .5

- .2 1.6 1.3

.5

1.5

1.3

-1.0

- 1.7

1.7

37.3 37.4 37.1 37.2 37.4 37.3 37.3

36.8

36.8

36.5

36.7

37.7

36.5

2.674 2.706 2.674 2.677 [2.902 2.700 2.677 2.631 2.712

2.7 - 10.3

1.6 4.0

- 29.6] 3.5 9.7

9.1 - .3

101.9 103.1 101.9 102.0

102.9 102.0 100.2 103.3

Average Standard deviation %

2.714

.070

2.6%

103.4 .9733

.0097

1.0%

37.1 2.681

.0256

.9%

102.1

The uncer ta in ty quoted corresponds to the 4.2% already mentioned, and which i s mainly s t a t i s t i c a l . I t seems necessary, however, to allow a l so for some uncer t a in ty in the method i t s e l f . The magnitude of t h i s uncer ta inty i s inherent ly unknown. A conservative es t imate would be 10% but i t i s l i k e l y to be l e s s . For example, the s e n s i t i v i t y of detector 343 was f i r s t determined to be 35. The work of Burmeister e t a l . [9.3] led to a value of 34. (-3%), but i t was based on an inco r rec t muon angular d i s t r i b u t i o n . The "best value" 37. i s 5.7% above 35. If there remains a systematical uncer ta in ty of 6%, the t o t a l uncer ta inty i s 7.3%:

The l i s t of "reciprocal s e n s i t i v i t i e s " in Appendix C for the de tec tors in wide band operation i s based on t h i s absolute c a l i b r a t i o n r e s u l t .

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9.4 Absolute calibration with counting detectors Single particle counting with electronic detectors is simply-

impossible in the fast extraction (23 us) , used for the NNB. In the fast resonant extraction (2-3 ms) of the WNB it can be done, but even if small counters ("o l cm2) are used, considerable dead time corrections have to be made, as has been shown in detail in sect. 3.3.3. A comparison was made between integrating silicon detectors and small silicon counters in fig. 3.12. An effective dead time of 90 ns can be used for the silicon counters.

On several occasions, emulsions were exposed together with silicon counters, aligned with the integrating detectors. In table 9.9 the results from these tests are listed. The data from the counters are of the same magnitude as those from the emulsions, but far less accurate because of the dead time correction and because of "v 20% uncertainty on the sensitive area. The effective area also varies slightly with the discriminator threshold, which was set at the lower edge of the Landau energy distribution (see fig. 1.9). In a circular silicon detector the edge region with partial charge collection represents quite a large fraction of the total area, e.g. a 500 um thick detector with nominal area of .25 cm2 has an effective area of -v, .35 cm2 and and edge region with partial collection of ^ .10-.20 cm2 depending on the silicon resistivity and the applied reverse bias voltage. A guard-ring or "key-hole" structure can improve the area definition, but the space between main diode and guard-diode should be as narrow as possible. So far, these special structures were not available in a size, small enough for muon counting.

TABLE 9.9

Comparison of emulsion exposures and electronic counting

Flux in Area of Counts per Flux with Coincidences Run Gap emulsion silicon counters pulse dead-time (corrected)

(per pulse) (cm 2) (cm- 2) correc tion cm"2 1

76 5 1993 Small .50 Big .65

1713 1732

1513 88.

81 5 1988 Small .30 Big .35

2024 2088

1599 79.

95 5 82100 Small .06 Big .35

63033 38249

76000 96200

70200 92.

- 199 -

9.4.1 Scintillator telescopes In 1982, both during the beam dump experiment and in the 200 GeV NNB

a 100 ms long slow extraction to the WANF has been available for a short time, to do absolute calibrations with scintillator telescopes. In both cases, these consisted of two thin, well defined scintillators, separated by a tungsten absorber. Size and distance were different, however.

In the case of the narrow-band beam, the scintillators and the integrating silicon detectors are gated for a 4 ms period, during which the beam line magnets are at the correct field strengths. They cannot be powered for the whole 60-100 ms spill duration.

In the case of the beam dump, the silicon detector gate is made 100 ms long, but a number of precautions were taken to reduce the effect of the leakage current and to diminish the noise on the signal. This was described in some detail in sect. 8.1.4. Preliminary results [9.9] seem to indicate a somewhat lower muon flux than what is obtained via the emulsion calibration. If confirmed, this would imply that some of the tracks in the emulsion are electrons instead of muons.

9.4.2 Measurement of muon angular distribution The muon angular distribution plays an important role in the absolute

calibration via nuclear emulsions. Therefore it is of interest to obtain independent information on this angular distribution, via coincidence counting.

A small counter (1 mm2) is fixed on the upstream iron wall in gap 5. The absorber box, mounted on the lift, 343 mm downstream, can be moved in steps of .3 mm, and the coincidences between the upstream detector and the detectors in the absorber box can be measured as function of the position. Also it is possible to place an iron absorber or a lead absorber (32 mm) in front of the downstream detector (60 mm 2). Only this downstream detector (diameter 8.7 mm) can be used, because on the smaller middle detector (1 mm 2, diameter 1.1 mm) the number of true coincidences is negligibly small in any position. The angle, subtended by the bigger detector is 28 mrad, that by the smaller one is only 6 mrad, which is apparently too small compared to the beam divergence. The step of .3 mm corresponds to a resolution of 1 mrad but the resolution is also influenced by the size and shape of the bigger detector. At angles > ± 100 mrad the number of true coincidences becomes much lower than the random coincidences, and no sensible numbers can be given under this condition.

The results are plotted in fig. 9.8 and show a FWHM of 26 mrad. This value is somewhat higher than the FWHM of 15 mrad of the angular distribution measured in emulsion V5.00 UPS, exposed at the same position

- 200 -

(fig. 9.6). The distribution measured with the coincidence counters is also reported in fig. 9.6 and is clearly wider than the emulsion result, due to the integrating effect of .the big detector. So the emulsion result is roughly confirmed but not improved by this different method.

+ x beam upstream

Fig . 9.8 Angular d i s t r i b u t i o n of p a r t i c l e s in gap 5 on a x i s , measured by coincidence between d e t e c t o r s "upstream" and "downstream", as sketched in the i n s e r t . An iron (+) or lead (x) absorber can be placed in front of the "downstream" d e t e c t o r . This does not change the coincidence r a t e . This angular d i s t r i b u t i o n was measured in the 400 GeV wide band beam.

With iron or lead absorbers the same coincidence r a t e and the same angular d i s t r i b u t i o n i s found as in a i r . This proves tha t the coincidences are p r a c t i c a l l y a l l caused by the muons and tha t e l ec t rons do not con t r ibu te to the coincidence r a t e when the de t ec to r s are small and a t some d i s tance from each o t h e r .

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10. THE MUON FLUX The muon flux measurement facilitates the continuous monitoring of

the neutrino beam, during its operation. Real time computer programs perform data acquisition and control the display "on-line" of several beam characteristics. The operator can activate special scans of the moving boxes, to measure the beam profile and he has options to modify parameters of the detector operation. The first part of this chapter will describe these "on-line" aspects of the muon flux measurement.

In the second part of this chapter the off-line muon flux analysis will be discussed and typical results will be shown.

Some introductory remarks have been made already in sects 8.4 and 8.5, and the electronics and computer hardware were also described in chapter 8. To obtain correct values for the muon flux, one has to use calibration factors ("reciprocal sensitivities" S ) for the detectors. Their determination has been discussed in chapter 9.

10.1 Beam monitoring "on-line" 10.1.1 General description The real time FORTRAN program OPCOM, once it is started and has

received its basic data, in the form of tables (initiation of the program, see next section) , can perform the data acquisition and some other functions without human intervention. However, it only delivers muon flux data, on-line or on magnetic tape, and if these muon data indicate shortcomings in the neutrino beam, corrective action must be taken by a human operator.

Up to and including the target, the beam steering is under control of the SPS Main Control Room (MCR) . Although the muon flux provides a measure for beam quality and target efficiency, the muon flux data are not directly available to the MCR and this part of the steering is done, using the SPS beam instrumentation, described in sect. 6.2.2. In fact, the target split foils immediately show beam asymmetries, and are more easily used for the steering of the proton beam. The MCR operator can move the beam horizontally and vertically, focus it and he can tilt the target and the proton beam in order to steer the parents beam and optimize the yield of parent particles.

The magnets in the beam, downstream of the target, are under control of the neutrino users, via the NBC computer. Faults in the magnets, i.e. large deviations from the required magnet currents, are signalled by a visible and acoustic magnet alarm, and can also be noted as a drop in the muon flux intensity.

- 202 -

Detector calibrations and beam profile scans with the calibration boxes are regularly performed automatically, but outputs have to be requested by the operator. The operator may in addition make high precision scans.

Errors may occur in the NFM system, which require operator intervention.

A description of the on-line commands is given by G. Cavallari, in the form of a "user guide" [8.5] .

10.1.2 Initiation of the program

The program OPCOM computes the amplifier gains necessary to produce signals around 1 V, given an estimate of the actual proton intensity. It needs detector layout tables and a table of the nominal expected flux (per 10 * 3 protons on target) for the type of beam in use. The following tables have therefore to be created and stored as files in the computer memory :

(a) Amplifier table: for each amplifier number the file contains the basic gain [V/101* pC] and the amplifier offset [mV] . The offsets have all been zero so far. Amplifier numbers may range from 1 to 510.

(b) Detector table: for each detector the file contains the reciprocal sensitivity S [muons cm" 2pC''] and the operation bias voltage [V] . Detector numbers range from 1 to 510, and correspond to those used in Appendix C.

(c) Flux table: the nominal flux [muons cm" 2 per 1 0 1 3 protons] is entered for gaps 1 to 6, and for 7 circles, from 0 to 90 cm radius, in steps of 15 cm.

(d) Position table: for each position the file contains the detector number and the amplifier number, if there is one installed. The first 21 channels in each gap are conventionally attributed to the horizontal axis (with seven detectors), the first and the second symmetric circle (each also with seven detectors), as has been described in extenso in sect. 7.2.1. The calibration detectors are attributed to the last 5 channels, i.e. 36 to 40. Non-standard detectors can be defined in the channels 22 to 35.

(e) Beam axis coordinates: for each detector support plate the reference coordinates have to be entered, for the WB or the NB axis (table 7.3).

( f ) Counter table: up to 20 detectors may be defined as counters, instead of integrating detectors. One must enter the address of the scaler which contains the result of the counting.

- 203 -

Once the tables are defined, the detector bias voltage can be set at its nominal value. This is accomplished in 30 steps with a pause of 20 s between the steps. The leakage current compensation is established at the same time. The actually applied bias voltage cannot exceed the setting of the "overbias" switch on the amplifier module. This switch is sometimes used to limit the applied voltage in cases of degrading detectors with too high leakage current.

The support plates may have to be moved to the theoretical beam position. Afterwards, the power supply to the motors is cut to avoid accidental displacements (due to power breaks, etc.).

Finally, the gains are computed and the run can be started.

10.1.3 Display of muon flux The main beam parameters are displayed continuously under control of

the NFM computer on a television screen and refreshed after each beam pulse, unless the measured intensity did not exceed a threshold value, which is set on a hardware trigger module. The analog signal of one of the detectors, generally the central detector in gap 2 (channel 41), is used for this trigger. The flash trigger for the bubble chamber is derived from this trigger, to avoid picture taking without beam. If the signal stays below the threshold, the TV screen is not refreshed, but an additional message "no beam" is added.

The TV screen shows the date, run number (as in table 9.1) and type of beam, the BEBC (and GGM) film/frame numbers, the proton intensity (in units of 10 1 0) from SEM, target multiplicity and BCT reading (proton BCT in WNB, hadron BCT in NNB).

Extraction timing data, the spill length and the horn/reflector currents (WNB) are displayed on the fifth line of the screen.

The middle of the screen gives for each gap (1-6) the muon flux on axis, normalized to 10 * 2 protons, the ratio of the flux at 15 cm radius to that on axis, and the position of the calibration box in the gap using polar coordinates (mm and degree) referring to the centre of the gap.

On the bottom of the screen the result of the beam axis calculation is shown again for all gaps. The apparent beam axis displacement (horizontal and vertical, in cm) is calculated, using 8 detectors on the indicated symmetry circle. Finally, the beam profile measured by these 8 detectors is displayed by numbers, which show the intensity in each

- 204 -

Fig. 10.1 Graphic display of a beam profile (WB400 GeV v) , as plotted on-line on the operator's screen:

(a) The radial intensity profile in gaps 1-5, as measured by the fixed detectors along the left horizontal radius (180°). The abscissa is the detector position in units of 15 cm. The ordinate is the muon flux [c m - 2 ] , normalized for 10 1 2 protons on target. The scale factor is indicated below each graph (E5 means x 10 s, etc.). The star in the right corner shows the circular symmetry, because each of the 8 legs is proportional to the muon intensity in the corresponding point of the circle of detectors.

* ,Mr. *

I I I. I I. I. I '. GSP H l /E33

¥

I 1 1 J 1 I 1 GSP 3 C/EHJ

(b) In a similar way as in (a) the intensity is plotted, as a percentage of the expected/standard intensity for the given beam. It can be seen that the muon flux is "harder", i.e. there are more high energy muons. The flux in gap 5 is •*• 150%, and the flux in gap 1 is * 70% of the standard.

I I I, I .t.

r GBP 2

F-H? : GBP t

E H M E=F

I I 1 I I

i i t

! t. I. I

1 8 G .

Ah

-GSP H

-•.

- •

-~ I 1 -4-1.-J.

OT> 3

(c) In the longitudinal plot the tendency remarked in (b) is made very obvious. It presents the percentage as in (b) for the central detectors, plotted for all gaps.

4 — * •

LONGITiJDIKM. BE-GBP NUtflER

EFF S I - •i-;'?'-

- 205 -

detector relative to the average flux value around the circle. This average value is normalized to 50 so that 2% above the average becomes 51, 2% below is 49, etc.

A detailed display of the muon flux can be obtained from the NFM computer with the command "DISPLAY" which produces a complete list of the flux in all detectors, for one or more selected beam pulses. It should be noted that the detector signals used to calculate the flux on-line are never corrected for possible offset. This offset correction can only be applied in the off-line analysis.

A summary of muon flux intensities is printed on a teletype, integrated over 100 beam pulses, which corresponds to -v» 20 min. of beam operation. These summaries can be used immediately to estimate the integrated flux for some period of the run, without analyzing the magnetic tapes.

A graphics output is available to show the radial beam profile as measured by the fixed detectors on the left-hand (180°) radius. At the same time, the symmetry is indicated by a star with 8 points, where the distance of each point to the centre is proportional to the measured flux in that direction. An example of this graphics output is shown in fig. 10.1(a). The same program produces two other graphs, which represent the same data, but this time divided by the expected muon flux, so that the actual beam quality can be compared to an "ideal" standard. The first graph (b) gives again the radial profile in all gaps separately, the second graph (c) is a cross section through the gaps along the beam axis, using the flux measured by the central detector in each gap.

The beam profile, measured by the moving calibration box can be displayed as a plot of the intensities measured along the horizontal cross section (-60 to +60 cm in steps of 15 cm) in all gaps (fig. 10.2(a)) and also as a plot of intensities measured around circles, (radii 15, 30 and 45 cm, steps of 45") with the theoretical beam axis as centre (fig. 10.2(b)). These scans are scheduled by the OPCOM program, and automatically executed every 8 hours. Cross sectional scans along other directions, with smaller steps, and circular scans with smaller steps can be asked for, and displayed in the same way. These allow a very precise determination of the flux profile. By numerical integration over the whole plane the total flux can be obtained.

10.1.4 Periodic checking by the operator Apart from the periodic checks of the beam symmetry, which have been

mentioned in the previous section, several other checks have to be performed regularly.

- 206 -

2

3

"J

.- *~ "S---*"' --^ S S'" " " " • . . ^ ' v

s

x v ' y . • * '

- 1 y'

t / \ N. f / ' f ". \ v

' / "~ -. w i i : - \ \ t i "-. \ v

\ \ i i

i i : ï i • i l

! " M 1 ; 4 : i *

t » : • î /

/ / ' • / /

* \ ' •" / / ^ \ "• .- i*

s \ . _ _ .- / i

\ ^ ' / / s. \ •- * * \ \ '-. • / * v V^ '. *~ s

X<"---. .••'' ' s '

^ >» " —- —***. - "* *" ""*" ~* r. •-Tri" 15f 330 450

0206 1052 RHO.SCAN RT EDEG § 0620 1506 CIA.SCAN IN GRP

F i g . 10.2 Graphie d isp lay of the muon i n t e n s i t y d i s t r i b u t i o n measured in a scan by the c a l i b r a t i o n boxes.

(a) Radial scan in gaps 1-5 during 200 GeV NNB, along the ho r i zon ta l ax i s ( 0 ° ) , with s t eps of 5 cm from -60 cm ( l e f t ) to +60 cm ( r i g h t ) . In each pos i t ion the muon flux i s normalized to the proton i n t e n s i t y inc ident on the t a r g e t . For one point (+ 25 cm) the e f f i c iency dropped temporar i ly . The v e r t i c a l i n t e n s i t y sca le is loga r i thmic . This p r o f i l e can be compared to the p r o f i l e s shown in f igs 3.13 and 3.14, which show the same "hump" a t + 35 cm.

(b) Circular scan in gap 3 during 400 GeV WNB, with steps of 15°, along c i r c l e s with r a d i i 150 mm, 300 mm and 450 mm. The r ad ius of the p lo t ted c i r c l e ( logar i thmic sca le ) i s p ropor t iona l to the measured i n t e n s i t y . The scan a t 150 mm is therefore represented as the ou te r c i r c l e , e t c . I t can be seen t ha t the p ro f i l e i s s l i g h t l y shi f ted to the r i g h t .

The t a r g e t eff ic iency i s cont ro l led by the SPS MCR, but the muon to proton r a t i o i s an ul t imate check. Generally, the ef f ic iency i s gradual ly improved during a run, as i l l u s t r a t e d in f ig . 10.3(a) (a few per cent improvement) . The muon i n t e n s i t y in the centre of gap 2 divided by the proton i n t e n s i t y on t a rge t i s p lo t t ed for a complete wide band run. Each point represen ts an average over a few hours of beam operat ion. In the same f igure a lso the spectrum "hardness" , i . e . the r a t i o of high energy muons to low energy muons i s p l o t t e d . I t is expressed as the muon i n t ens i t y in gap 5, divided by the i n t ens i t y in gap 1. A more de t a i l ed information on the hardness a t a given moment i s contained in the p lo t shown al ready in f i g . 1 0 . 1 ( c ) . In f ac t , for f i g . 10.3 the o f f - l i n e analysed data were used ra ther than the on- l ine ca lcu la ted flux. For the monitoring, however, the on - l ine flux i s su f f i c i en t ly p r e c i s e .

- 207 K

1.3

x106 1-2 f

1.1

1.0

(a) TV"/

808 811 BEBC ROLL 816 821

I

1.7 r

21 ,22 MARCH, 23 , 24 FRI SAT

25 SUN MON TUE

,26 WED

826

27 THUR

831 836

28 FRI

29 SAT

30 SUN

, 31 MARCH 1980 MON

1.6

1.5

L

1.4 -

1.3 L

(b)

1

Fig . 10.3 Target e f f ic iency and spectrum hardness for WHB run 106. Each data po in t r ep re sen t s an average for a 1/4 r o l l of BEBC.

(a) The t a rge t e f f i c iency is expressed as the muon flux in the cent re of gap 2 , divided by the proton i n t e n s i t y . I t v a r i e s around 1.2 x 10 s muons cm"2 per 1 0 1 2 p ro tons .

(b) The spectrum hardness i s the i n t e n s i t y in gap 5 divided by tha t in gap 1. I t v a r i e s around 1.55 x 10" 3 .

A gradual improvement of the t a r g e t e f f ic iency can be seen towards the end of the run . The f l u c t u a t i o n s , however, a re not more than a few %. The hardness f luc tua tes more, and a so f te r spectrum i s sometimes c o r r e l a t e d with a b e t t e r e f f i c i ency in gap 2 . But sometimes both hardness and ef f ic iency become low. In th i s plot obviously wrong cond i t ions , l i k e " r e f l e c t o r off" a re excluded.

For a per iodic d iagnos t i c s of the s i l i c o n d e t e c t o r s , the operator should check the b ias vo l t ages , bias cur ren t s and average measured o f f s e t s . A high (:> 100 mV) of fse t indicates e i t he r incorrec t compensation or too much noise in the detector or ampli f ier . Furthermore, the r e s u l t s of the successive automatic ca l ib ra t ions by the ca l i b r a t i on boxes can be inspected to ver i fy the s t a b i l i t y of the d e t e c t o r s . Once again, however, the o f f se t measurement i s not avai lable o n - l i n e , and not accounted for in these c a l i b r a t i o n r e s u l t s .

In the NNB a per iodic check of the magnet cur rents and the parent beam p r o f i l e s i s done. In the WNB i t i s e spec ia l ly the timing of the horn and r e f l ec to r magnets with respect to the ex t rac t ion which requi res a t t e n t i o n .

The s t a tu s of the var ious data l inks should be checked p e r i o d i c a l l y .

10.1.5 NFM e r ro r s Some errors should be noticed by the fact that usual operations are

not executed any more, like TV display or magnetic tape writing. Other

- 208 -

errors are brought to the attention of the operator via a small printer (Axiom). In the user guide the various error conditions are mentioned, as well as their severity, and the action to be taken.

10.2 Data analysis "off-line" The original magnetic tapes or the master tapes (sect. 8.4.2) can be

read, using the subroutine FQMIN [10.1], or any similar procedure. An easily available HYDRA program (NFMPLY) was designed by Klein and de Wolf [8.3]. A simple FORTRAN program (NFMSFS) has been used to obtain the results described here. The on-line calibration factors and position information written on the tapes are replaced, however, by corrected data, available on the so-called "lay-files". The construction of these lay-files is a lengthy procedure, involving the relative and absolute detector calibration, described in chapter 9, and a thorough checking of the detector gains, offsets and bias currents. After this updating the lay-files reflect the true NFM layout and the best available calibration factors (S ) . Sometimes, the value of S for a detector takes different values during one run, for example if the applied bias voltage had to be modified.

For the calculation of the beam symmetry and the integral flux which is described in sect. 10.2.2 it is essential to use precise calibration factors.

10.2.1 Neutrino fluxes from muon fluxes It is outside the scope of this report to discuss the neutrino flux

calculation, for which the muon flux measurement is only one ingredient. The other ingredients are the production spectra [6.8] and a beam simulation program, which predicts neutrino fluxes fitted to the measured muon fluxes.

Three beam simulation programs exist for the WNB. Besides Visser's program NUBEAM [3.8], which has been mentioned in sect. 3.1, there are DISMUNU by H. Wachsmuth (CERN) and NUFLUX by W. Venus (Rutherford-Appleton Laboratory, Oxford). A practical introduction has been written by Venus and Wachsmuth [10.2]. The NNB can be calculated, using an adapted version of NUBEAM, or using TURTLE [10.3] extended with routines to follow the muons in the shield, as done by Powell [6.13].

Once the relation between neutrinos and muons is established, the summing of the muon flux over the period of interest (run, film, roll, etc.) gives the neutrino intensity.

- 209 -

10-2.2 Flux measurement if beam is not centred In the neutrino beam simulation programs it is assumed that the muon

flux has a circular symmetry around the beam axis. In the wide band beam this is the ideal situation, if proton beam, target, horn and reflector are all aligned along the same axis. In the narrow band beam a circular symmetry can be assumed as first order approximation, but from figs 3.13 and 3.14 it has become clear that there are irregularities in the profile. If the NNB is simulated with TURTLE, these irregular features are correctly taken into account.

Apart from the complications arising from a non-symmetric beam, also the measurement planes with the fixed detectors may not be aligned with the actual beam axis and thus cause some distortion in the muon flux measurement. The flux measured by the central detector in the gap is lower than the true maximum whenever the beam centre is displaced. The calculation of the integral flux can give quite different results if the beam centre is displaced instead of being at the centre of the final detector arragement, because the wrong radii are then used. A study was made of the importance of these effects in the WNB [10.4].

By fitting parabolic functions through the logarithmic beam profile, using the central detector and two pairs of diametrically opposed detectors, the position of the true maximum can be determined. In fact, this position can be calculated four times, using two circles, each with 8 detectors. In table 10.1 the result is given of such a calculation in the WNB and it can be seen that the fitted maximum flux is not more than 1% higher than the measured value. The beam displacement was exceptionally large in this run. The integral flux, calculated with the parabolic fits is several % different from the integral flux obtained with the trapezium formula integration, using the measured flux values of the fixed detectors.

TABLE 10.1 WNB position calculation - Run 95, 30 June 1979, 14h00-16h40

Gap 1 1 2 2 3 3 4 \ 5

Symmetry c i r c l e radius (cm) 15 30 15 30 30 45 45 45

Horizontal 23 17 - 35 49 40 32 92 displacement (mm) 22 17 39 28 48 37 35 60

Ver t i ca l - 2 8 - 1 4 - 2 4 - 1 0 - 1 8 - 1 0 - 8 8 displacement (mm) - 2 3 -17 - 2 7 -19 - 1 8 - 1 7 -12 - 7 0

F i t t e d flux in maximum 24.6 24.3 - 8.31 1.76 1.75 0.336 0.0462 x 10 s muons cm"2 24.2 24.2 8.30 8.24 1.74 1.74 0.336 0.0458 Measured flux in cen t ra l de tec to r x 10s muons cm"2 24.2 8.24 1.74 0.335 0.0457 In tegra ted flux from f i t x 109 muons 13.3 13.5 - 5.64 1.59 1.56 0.385 0.0719 In tegra ted flux from fixed de t . x 109 muons 12.8 5.17 1.45 0.402 0.0684

- 210 -

The integral flux calculation would be somewhat easier if the detector plates were positioned exactly on the true beam axis. Although this has been common practice in the PS neutrino beam, and was tried on one occasion in the SPS, it is preferred to leave the detector systems on the theoretical beam axis, and steer the beam as closely as possible along this axis. Afterwards, beam deviations can be easily recognized.

10.2.3 Typical flux distributions Several examples of measured flux distributions are shown in

figs 10.4 to 10.7. In fig. 10.4 a cross section of the flux profile is plotted for the 400 GeV wide band neutrino and antineutrino beams, as measured along the horizontal. The flux in the centre of the gaps, on the beam axis, decreases by somewhat less than an order of magnitude between two gaps. The profile becomes gradually broader.

i 1 1 i i 1 1 1 1 i 1 i i i i i i ! r

w l i i I i ! 1 I l l I I I I I ! 1 1 1 1 -100 -80 -60 -40 -20 0 20 UO 60 80 100

Radial distance from axis (cm) Fig. 10.4 Cross section of the muon flux profile in the 400 GeV wideband neutrino ( ) and antineutrino ( ) beam. The profiles were measured with the calibration boxes, moving along the horizontal, in steps of 15 cm. The arrows indicate the difference in intensity between the neutrino and antineutrino beams, in the centre of each gap.

- 211 -

In fig. 10.5 a similar graph is shown for the case of the 200 GeV narrow band beam. It is obvious that not only the energy band is narrow, but also the size of this beam spot. The width in the gaps deep in the shield is dominated by the multiple scattering of the muons. Some further examples of narrow band beam profiles will be given in chapter 11, where measurements at various beam momenta are described.

IU I I i i i i [ i l l i i i i I i i

-

105 /

/ /

/ / /

1 1 \

1

\ \ \ \

\ \

200 GeV v 200 GeV v

—

l/> / / 2 \ \ -c: o / / \ \ \ -o l— i n \ \ _ • L i /

i \ \ X

\ *h> 104 / // J__ \ —

c_ 104

/ 11/^~ \ Z cu CL - / / if \

\ -

E - / _k_ V \ -— • / X / V \ \ 3 / " A ^ \

a— " / YK \ o / \ \ \ \ \ 3

103 — y

/ /

5

\\X \ —

102 I i / I i i i i i i 1 1 1 1 ! 1

-100 -80 -60 -40 -20 0 20 U0

Radial distance from axis (cm)

60 80 100

F i s - 10-5 Cross sec t ion of the ho r i zon t a l muon flux p ro f i l e in gaps 1-5 for the 200 GeV an t ineu t r ino narrow band beam, and for the 200 GeV neutr ino NNB in gap 1 only. The p r o f i l e s were measured with the c a l i b r a t i o n boxes, moving in s t eps of 5 cm. Similar p ro f i l e s obtained on- l ine were shown in f ig . 10.2(a)

In f i g . 10.6 a comparison i s made on l inear sca les between the p r o f i l e s of WB and NB in gap 1. The fu l l width at half maximum (FWHM) i s 59 cm respec t ive ly 11 cm. The beam prof i l e in the NB is not symmetric, however, so t h a t a FWHM depends somewhat on the d i r e c t i o n . In f ig . 10.7 the i n t e n s i t y around a 15 cm c i r c l e i s p lo t ted for a l l 6 gaps, and a c lear

"i r "i 1 r- 1 — i — I I I—I—I r

GAP 1

FWHM 59 cm

J L

-90 -70 -50 -30 -10 0 10 30 50

Radial distance (cm)

<10"

11

10

i/>

9 c o o (_ O -

R rsi

O

' 7 C _

OJ a.

rvi

F 6 LJ

X 5 1

u_ Z o

U = 3 U i -

3

0 70 90

Fig. 10.6 Comparison of WB and NB beam profile in gap 1. The linear scale for the WB is given on the left vertical axis, the scale for the NB is on the right vertical axisand is 20 x expanded. The full width at half maximum (FWHM) is indicated for both profiles.

x

o

5 I*

3

2.3 2.2 2.1 2.0

GAP 1

i 1 1 r

200 GeV NB

GAP 3

GAP K

GAP 5

GAP 6

_i_ _i_ _i_ 60 120 180 240

ANGLE ( DEGREES ) 300 360

Fig • 10.7 Intensity around the circles with 15 cm radius in gaps -200 GeV NB. The step size was 30° in all gaps. Note that the

scale changes for each gap.

- 213 -

increase is seen in the direction of 270° (bottom), which shifts to 330° (bottom right) for the deeper gaps 4, 5 and 6. This plot was made during a 200 GeV v run. A similar drawing for the profile in gap 2 during 200 GeV v has been presented in fig. 9.7. Although such profiles present always the same qualitative features, quantitatively they are changing continuously, depending on the beam steering and focusing.

A representation of several longitudinal beam profiles is given in fig. 10.8. The muon flux (muons cm"2) intensity measured in the centre of each gap is plotted as a function of the shielding thickness in g cm"2 in front of this gap. A number of WB and NB examples are shown. The muon intensity on axis decreases steeply with depth in the shield, which reflects several processes like the production spectrum, multiple scattering and decay of the muons. The integral number of muons over the whole plane drops less rapidly.

214

i/> c o o a.

ai a.

X ZD

O

1 1 1 1 1 1 1 1 1 ! 1 ( 1 _ MUON FLUX IN CENTER OF GAPS IN THE SHIELD -

107

1 1

1 1

1

106 =v 1 1

1 1

1

105

N N N \ V NN.

1 1

I 1

1 1

, N N N \ V NN.

10*

( g K \ \ \ \ [a ) • v v \ \ \

-

103 ••••...(h) \ ^ S \ \ : ""•••.. \ \ \ < c ) - •** \

N \ X \ 1

1 1

1 1

102

\ x _ \ s \ \ \ \ \ \ \ \

101 — \ \ \ \ —

MEASURING GAPS

\ \ \ \ \ \ \ \ -

" V1 V2 V3- V4 V5 V6 V7 \ N \ V 8 -1 _ |_ i LJ L_LJ ( 1—p L | — L \ • ^ 1

U 6 8 10

SHIELDING THICKNESS ( g cm" 2 )

12 14

xlO*

Fig. 10.8 The muon flux intensity in the centre of a gap as a function of the shielding thickness. Wide band beam: (a) 400 GeV v, run 95. 4 July 1979 (b) 400 GeV v, run 109, 14 May 1980 (c) 400 GeV v, run 107, 20 April 1980 Cd) 350 GeV v, run 83, 4 July 1978

Narrow band beam (e) 200 GeV v, run 85, 8 September 1978 (f) 200 GeV v, run 87, 20 October 1978.

(g) 300 GeV v, run 99, 17 November 1979. (h) 200 GeV v, run 85, closed collimators, 3 September 1978.

- 215 -

11. MUON FLUX MEASUREMENTS AT VARIOUS BEAM MOMENTA The presence of a large number of muon detectors imbedded in the

homogeneous iron shield, creates a nearly ideal situation for the measurement of the range of high energy muons in iron. Of course the muons in the narrow band beam are not monoenergetic, but in principle their momentum distribution can be precisely calculated. With the help of Monte-Carlo simulation programs, like the ones mentioned in 10.2.1, the muons can be tracked through the shield, and the resulting intensity in the measurement planes can be calculated. The algorithms used for the muon energy loss, the multiple scattering and the muon range straggling are determining the result of the simulation. The measurement therefore provides a check on the correctness of these algorithms and can reveal shortcomings in the programs or other uncertainties.

A three-day test has been devoted to run the narrow band antineutrino beam at various momenta between 60 GeV and 300 GeV (on 7-9 December 1979), and the data from this measurement are presented here. A detailed comparison with simulation is not given.

11.1 The narrow band beam at various momenta The momentum of the secondary beam was varied by scaling all

currents, starting from the "200 GeV optics" in the region 60 GeV to 230 GeV, and from the "275 GeV optics" for the momenta 260, 275, 300 GeV (sect. 6.3).

Each time the steering of the beam had to be done in order to obtain a reasonably symmetric beam. In fig. 11.1 the radial muon profiles are shown, measured by the calibration box in gap 2. It can be seen that the usual symmetry is generally obtained. At 60 GeV the profile has become very wide due to enhanced multiple scattering, as gap 2 is then at the end of the range of the muons.

Once the beam was stable and well on axis, the muon flux was meassured during 5-10 minutes, with all calibration boxes in the central positions, in order to have more reliable data. When afterwards a symmetry scan was performed, another 20 minutes were required at the given beam momentum.

11.2 Results of the measurements A careful relative calibration of all fixed detectors was the first

part of the off-line analysis. It involved the narrow band runs 97 to 104, because of the comparisons with the reference box. The procedure of the calibration has been described in chapter 9. The result of the

216

c o o a.

O ZD

RADIAL DISTANCE FROM AXIS (cm)

Fig. 11.1 The horizontal profiles of the NB antineutrino beam, for various beam momenta, as indicated. These profiles were measured in gap 2, by the calibration box, which moved in steps of 5 cm.

absolute calibration with nuclear emulsions, performed during run 103 (sect. 9.3.1, fig. 9.7) has been used to fix the scale of sensitivities in an absolute way.

The muon intensities are normalized to 10 1 2 incident protons on target, as measured by the proton BCT. The analog proton signal ("PNFM"), directly available to the NFM system has been used. Comparison with the numbers ("PBCT") sent via the WEXTR computer links (sect. 6.2.2) shows a ^ 30% difference between the first and second part' of the test. Such a change is not observed if one compares the muon intensity and the hadron BCT ("HBCT") intensity with PNFM. The absolute proton normalization is therefore questionable.

The muon flux in the centre of each gap is given in table 11.1, for all beam momenta used. For comparison, the data for +200 GeV/c and + 300 GeV/c are also shown. The uncertainty on the measured flux is -v 5% because of the varying beam conditions, and the rather low signal levels. Above 230 GeV/c the. signals were so low (10-100 mV) that the uncertainty is even bigger.

- 217 -

Using the fitted beam centre and the intensities measured by the fixed detectors, the integral muon flux over an area with a radius of 90 cm has been calculated, also in each gap. The precision of the result of this calculation is 10-15%, again due to the low signal levels at large radii.

The highest integral muon flux in gap 1 is obtained at the lowest beam momentum because the yield of parent pions and kaons decreases exponentially with energy. The highest central intensity in gap 1 is found at ^ 100 GeV/c. At lower momentum the flux profile broadens, the beam itself becomes wider and the multiple scattering is increased.

In table 11.2 the same data have been normalized, so that the relative behaviour is more evident. In the first columns the central and integral intensities in gap 1 are normalized to the intensity of the parent particles, as measured by HBCT. The integral number of muons is 7.3% of the measured number of parent particles at -60 GeV/c and it drops to ^ 4% at -200 GeV/c. Some fraction of the muons in the shield consists of muons from decays wich occurred upstream in the beam line, before momentum selection and final focusing (see also sect. 6.3.2). Bosetti et al. [9.8] mention such a fraction of (7.6 ± 1.5)% on the 15 cm radius in gap 2, for the 200 GeV beam. An important part of this contribution can be measured when the beam collimators are closed (see fig. 10.8, curve (h)).

In the second part of table 11.2 the central intensities in gaps 2 to 6 are normalized to an intensity of 10 6 muons cm"2 in the centre of gap 1. Above -150 GeV/c beam momentum the intensity in gap 2 remains around 45% of that in gap 1, with some fluctuations. Above -180 GeV/c the intensity in gap 3 stabilizes around 13% of that in gap 1, etc. The fluctuations are mainly caused by displacements of the beam, which result in a lower flux in the central position in gap 1, where the beam profile is very steep, especially at the higher momenta. For 275 and 300 GeV the beam was even a few cm off axis, and therefore the values measured are obviously not corresponding to the true beam axis. Also, the muon intensity for -275 and -300 GeV/c becomes very low, both because of lower yield in the production and a longer decay length.

In the third part of table 11.2 the total muon fluxes, integrated over a circular area with a radius of 90 cm, are normalized to 10 9 muons in gap 1. Below "v 110 GeV there is still a loss of muons between gaps 1 and 2, but for higher momentum the same number of muons is measured in both gaps. The fluctuations are less than 5%, excepted again the higher momenta (230 GeV/c and above). Above ^ 140 GeV/c the number of muons in gap 3 stabilitzes around 73% and above T- 180 GeV/c the limit in gap 4 becomes 60% of the initial integral intensity.

TABLE 1 1 . 1

Beam HBCT PBCT Central muon intens ity x 10" Integral muon intensity x 10" Momentum PNFM PNFM Muon s cm*2 p er 10 1 2 protons PNFM Muons pe r 10' 2 protons PNFM GeV/c x 10"3 Gap 1 Gap 2 Gap 3 Gap 4 Gap 5 Gap 6 Gap 1 Gap 2 Gap 3 Gap 4 Gap 5

- 60 9 Dec. 2.72 1.30 12.0 1.67 0.004 2.0 1.1 - 70 2.91 1.29 13.0 2.75 0.023 1.9 1.4 0.05 - 80 2.95 1.16 13.5 3.60 0.176 1.8 1.7 0.22 - 90 2.97 1.26 14.5 4.51 0.43 1.8 1.8 0.4 0.05 - 100 2.92 1.27 14.5 4.70 0.66 0.008 1.55 1.35 0.55 0.06

- 110 2.59 1.18 14.6 5.05 0.86 0.035 1.35 1.3 0.62 0.07 - 120 9 Dec. 2.69 1.31 14.4 5.08 1.05 0.092 0.001 1.2 1.2 0.68 0.15

8 Dec. 2.55 1.01 13.5 4.91 0.98 0.085 1.15 1.15 0.66 0.14 - 130 2.20 1.11 13.3 5.06 1.12 0.156 1.05 1.02 0.68 0.23 - 140 2.06 1.31 12.9 5.15 1.22 0.219 0.003 0.95 0.90 0.66 0.29 - 150 1.78 1.32 11.2 4.71 1.19 0.257 0.007 0.79 0.79 0.55 0.32 0.01

- 160 9 Dec. 1.64 1.32 9.97 4.80 1.21 0.310 0.016 0.66 0.64 0.50 0.35 0.03 7 Dec. 1.65 1.01 10.9 4.65 1.25 0.295 0.017 0.72 0.66 0.53 0.35 0.03

- 170 1.44 1.31 9.61 4.25 1.15 0.315 0.028 0.0018 0.62 0.62 0.46 0.34 0.05 - 180 1.24 1.30 9.01 3.90 1.09 0.314 0.037 0.002 0.53 0.52 0.39 0.32 0.06 - 190 1.07 1.27 7.31 3.50 1.0 0.308 0.043 0.0028 0.46 0.45 0.33 0.29 0.065 - 200 9 Dec. 1.03 1.32 7.46 3.26 0.925 0.295 0.048 0.004 0.37 0.36 0.30 0.25 0.07

8 Dec. 0.92 1.00 7.27 3.26 0.928 0.297 0.051 0.004 0.35 0.35 0.28 0.25 0.07

- 220 0.64 1.24 5.49 2.46 0.716 0.243 0.052 0.007 0.27 0.27 0.19 0.16 0.06 - 230 8 Dec. 0.54 1.01 4.48 2.05 0.615 0.208 0.045 0.008 0.21 0.28 0.16 0.14 0.06 - 260 7 Dec. 0.23 0.99 2.2 0.98 0.29 0.099 0.026 0.004 0.12 0.1 0.1 0.07 0.04 - 275 0.19 0.99 0.45 0.43 0.18 0.07 0.020 0.006 0.09 0.08 0.06 0.05 0.03 - 300 0.14 0.91 0.25 0.20 0.09 0.04 0.006 •0.004 0.07 0.07 0.05

+ 200 12.4 1.34 21.8 9.30 3.06 0.990 0.177 0.014 1.5 1.5 1.0 0.84 0.23 + 300 10.2 3.88 1.77 0.575 0.232 0.075 0.025 0.30 0.15 0.11 0.09 0.06

TABLE 1 1 . 2

Beam Momentum GeV/c

G< Central

HBCT x 10"*

sp 1 Integral

HBCT x 10"'

Central muon intensity x 10 3

(normalized to 106 muons cm - 2 in gap 1) Gap 1 Gap 2 Gap 3 Gap 4 Gap 5 Sap 6

x 10 3 cm"2

Integral muon intensity (normalized to 109 muons in gap 1) Gap 1 Gap 2 Gap 3 Gap 4 Gap 5

(x 10", on 2.54 m 2)

- 60

- 70 - 80 - 90 - 100

0.441

0.447 0.458 0.488 0.497

0.73

0.65 0.61 0.61 0.53

1000.

1000. 1000. 1000. 1000.

139.

211. 267. 311. 324.

0.3

1.8 13. 30. 46. 0.5

10. 10. 10. 10. 10. 10.

5.5 7. 7.4 9.4

10. 8.7

0.3 1.2 2.2 3.6

0.3 0.4

- 110 - 120

- 130 - 140 - 150

0.564 0.535 0.529 0.605 0.626 0.629

0.52 0.45 0.45 0.48 0.46 0.44

1000. 1000. 1000. 1000. 1000. 1000.

346. 353. 364. 380. 399. 420.

59. 73. 73. 84. 95. 106.

2.4 6.4 6.3 11.7 17. 23.

0.07

0.2 0.6

10. 10.

10. 10. 10.

9.6 10.

9.7 9.5 10.

4.6 5.7

6.5 7.0 7.0

0.5 1.2

2.2 3. 4. 0.1

- 160

- 170 - 180 - 190 - 200

0.608 0.661 0.667 0.727 0.683 0.724 0.790

0.40 0.44 0.43 0.43 0.43 0.36 0.38

1000. 1000. 1000. 1000. 1000. 1000. 1000.

481. 427. 442. 433. 479. 437. 448.

121. 115. 120. 121. 137. 124. 128.

31. 27. 33. 35. 42. 40. 41.

1.6 1.6 2.9 4.1 5.9 6.4 7.0

0.2 0.2 0.4 0.5 0.5

10.

10. 10. 10. 10.

9.7

10. 9.8 9.8 9.7

7.6

7.4 7.4 7.2 8.1

5.3

5.5 6.0 6.3 6.8

0.4

0.8 1.1 1.4 1.9

- 220 - 230 - 260 - 275 - 300

0.858 0.83 0.96 0.24 0.18

0.42 0.39 0.52 0.47 0.5

1000. 1000. 1000. 1000. 1000.

448. 457. 445. 900. 800.

130. 137. 132. 400. 360.

44. 46. 45. 150. 160.

9.5 10.0 12. 40. 24.

1.3 1.8 1.8

13. 16.

10. 10. 10. 10. 10.

10. 13. 8.3 8.9

7.0 7.6 8.3 6.7 10.

5.9 6.7 5.8 5.6 7.1

2.2 2.9 3.3 3.3

+ 200 + 300

0.176 0.038

0.12 0.03

1000. 1000.

427. 456.

140. 148.

45. 60.

8.1 19.

0.6 6.4

10. 10.

10. 5.

6.7 3.7

5.6 3.0

1.5 2.0

- 220 -

11.3 Muon range straggling It is not possible to interpret the measurement results directly in a

muon range-energy relation, because the muons are not monoenergetic. Also, it is outside the scope of this work to simulate precisely the muon production and transport in the shield, using Monte-Carlo programs. However, it is possible to make a superficial comparison with available calculations performed by Kopp [11.1] for monoenergetic muons in iron, for 20, 50, 100, 150 and 200 GeV/c.

A typical range curve, for 100 GeV/c muons, is shown in fig. 11.2. It can be understood on the basis of the processes, discussed in chapter 3. Due to interactions involving large energy transfers a small fraction of the muons has a much shorter range. In the simulation shown in fig. 11.2 already .5% of the muons has stopped at 1/3 of the mean range, 1.2% has stopped before 1/2 of the beam range and 4% before 2/3 of the mean range. The modal ("most probable") range of 100 GeV/c muons is 7.5% longer than their mean range, and the maximum extrapolated range is even 17.5% longer.

1 1

MUON RANGE IN IRON

1 !

100 GeV/c

i

2800 - -21.00 n -2000 - L _

-1600 -1200 _ MODAL MAXIMUM

- ' RANGE RANGE -800 - -

« A N ' 1.00

i _ i 1 — • 1 1

RANGE

1

2 3

Shielding thickness ( g cm"? I

F i g . 11.2 The number of stopping muons in iron as a function of the th ickness in g cm"2 . This i s the r e s u l t of a s imulat ion by Kopp [ 1 1 . 1 ] , for 10'' incoming muons of 100 GeV/c. The mean range of a l l muons i s 43770 gem" 2 , the maximum ext rapola ted range i s 51400 gem"2 and the modal range ("most probable range") i s 47000 gem" 2 .

The width of the range s t raggl ing d i s t r i b u t i o n becomes wider and wider for higher energ ies , because of the r i s i n g cross sect ion for r ad i a t i ve i n t e r a c t i o n s , involving large energy t r ans fe r s (see f i g s . 3.1 and 3 .3 ) . The maximum extrapola ted range then becomes considerably longer than the mean range. This tendency is apparent already in f ig . 11 .3 , where some of the measured i n t e n s i t i e s from tab le 11.1 are p lo t ted as a function of the shie lding th i ckness . At the same t ime, some of the numbers obtained by Kopp are shown as " s t ragg l ing l i n e s " , which ind ica te the region between mean range and maximum range. These "s t raggl ing l i ne s " are given for monoenergetic muons. The muons in the neutr ino beam have

- 221 -

lower average energy, expecially those which originate from the kaon

decays (see sect. 6.2) , although these arrive less frequently along the

beam axis, because of their larger decay angles (fig. 6.7).

Ï6 10°

a. < 13

O (X

X

c o o a.

ai a.

10 5 -

10* -

103 h

10z

20 GeV

I 1 1 1—~r MUON RANGE IN IRON

T T"

50 GeV 100 GeV

150 GeV

CENTRAL MUON FLUX

*

Z) O 10 8 -

LU h -

ai CL

< CM

ce b ID -J" LU LO 1— rsi

10'

106

10b

- i 200 GeV

z INTEGRATED MUON FLUX

V1

-M-V2 V3

I I I

MEASURING GAPS

V4 V5 V6

J L _ h |_|_L V7 V8

10 12 14

SHIELDING THICKNESS (g cm - 2 , x1(T

Fig. 11.3 The muon flux for a few parent beam momenta (in GeV/c) as a function of shielding thickness (in g cm" 2). In the lower part of the figure the integrated muon flux is plotted (taking all muons within a radius of 90 cm, i.e. an area of 2.54 m 2 ) . In the middle, the muon flux (muons cm" 2) measured by the central detectors is given. On the top of the figure some "straggling lines" are drawn which result from a simulation by Kopp [11.1] for monoenergetic incident muons in iron. The left point on each line represents the mean range, the right point is the maximum extrapolated range and the point in between is the modal range. On the curves measured in the neutrino beam shielding, the mean range for monoenergetic muons is indicated by 0. In reality, the average energy of the muons in the neutrino beam is much lower than the nominal parent beam momentum.

- 222 -

In fig. 11.3 it is indicated where the mean range for the maximum momentum of the muons intersects with the measured intensity profiles. The intensity at this point is of the order of 1% of the initial control intensity and 5-10% of the integrated intensity.

The measurements show clearly that many muons have a longer range than the mean range for the maximum available momentum. Even compared with the maximum range which follows from Kopp's calculation, the measured intensities appear rather high. Only more precise simulations would enable stronger conclusions.

The most likely reason for discrepancies between calculation and measurement is an underestimation of large energy transfers, which does not influence very much the mean value, but may change the straggling considerably. Other causes for discrepancy could be an overestimation of the shielding density (taken here as 7.25 g cm"3) or too high muon detector calibration factors. However, both these have been very carefully determined. So far no other causes have been identified to explain the possible discrepancy.

- 223 -

Acknowledgements The construction and running of the muon flux measurement system NFM

has been a part of the assignment of the EF neutrino beam group, directed by Dr. P. Lazeyras. Most of the work described in this report has been organized by G. Cavallari. The electronics was designed by P. Jarron and M. Musso. The electromechanical systems were made in collaboration with the groups of P. Quéru and G. Amato. Many CERN collaborators, from the EF, EP, HS and SPS Divisions contributed in various ways to the successful operation of the neutrino flux monitoring. J.P. Avondo, A. Girardet, V. Munda, A. Poget and A. Wasem climbed countless times up and down the pits, always under the vigilant eye of a "pompier". The nuclear emulsions were skilfully handled by O. Mendola and scanned by Mrs. G. Ley and Mrs. B. Masson. Mrs. D. Zeller calibrated the amplifiers. K. Knudson and H.J. Klein cared for the off-line data analysis.

The user's point of view has been expressed all the way long by Dr. H. Wachsmuth. The essential features of the NFM layout were proposed by him and Dr. W. Venus. Many enlightening discussions with them, and also with Dr. J.B. Pattison, are gratefully acknowledged. The numerous contacts with the users, on and off shift, made work pleasant.

Assistance with the radiation tests was given by Mr. F. Coninckx and Dr. H. Schônbacher. Many of the measurements were performed in the Centre de Recherches Nucléaires in Strasbourg, and the assistance and hospitality of Dr. P. Siffert and Dr. J.C. Muller is very much appreciated. Also Dr. C. Ammerlaan contributed in various ways to this work.

Some of the work related to the anomalous injection could be done in the electron beam of MEA thanks to the interest of Dr. J. Bailey and Dr. R. von Dantzig of the Pi-Mu group. The help of Dr. J. Wisse and Dr. P. Koldewijn is gratefully acknowledged.

Dr. W.R. Nelson and Dr. T. Jenkins contributed significantly to the tiny telescope test, both by providing beam time at SLAC and by participating in calculations and measurements.

The collaboration with Dr. B. Jongejans and C. Visser helped to clear up the situation regarding the absolute emulsion calibrations.

The contribution of the commercial detector manufacturers, without mentioning names, has been essential, of course. Not only they provided the detectors, but they also helped with advice and answered many questions.

Finally the writing of this report would not have been possible without the appreciated support of the EF Division Leader, Dr. A. Minten, and the stimulating interest of Professor A.G. Tenner and Dr. J. May,, who followed during a long time the progress of the report or the lack of it, and who gave decisive advice on many different subjects.

The drawings were made by Mrs. C. Plumettaz and Mrs. C. Rollinger and the layout and typing was in the hands of Miss E. Delucinge.

IRON PIUG LATER MODIFICATIONS

U 9 ? 9 )

BLOG 2 74

0 10m

PIT ?

SHIELDING MAGNET

(19791 n

BEAM DUMP CAVE ( 1 9 8 2 )

=0 J IL IL

GGM with E M I

QUARK SEARCH COUNTER

r~-, j — J BUILDING

w.lh I—AIR—| — EARTH S H I E L D - - - * - — - IRON SHIELD with - - * • E P L E M ' | MEASURING PITS

VACUUM DECAY TUNNEL — U N E U T R I N O CAVE — i

-131 I? *

E X T R A C T E D PROTON BEOM

Scale drawing of the West Area Neutrino Facility WANF. The region of the iron shield with flux measurement pits is also shown enlarged. Indicated are modifications in 1979, when a magnet just after pit 5 and additional shielding behind building 274 were installed, and in 1982, when a cave for the proton beam dump was built.

225

APPENDIX A

LAYOUT OF WEST AREA NEUTRINO FACILITY (WANF)

A l l d i s t a n c e s a r e g i v e n in m e t r e s , p a r a l l e l t o t h e s l o p e o f t h e Wide Eand N e u t r i n o Beam. The o r i g i n of t h i s N e u t r i n o Be air,

C o o r d i n a t e S y s t e m i s a h y p o t h e t i c a l p o i n t i n s p a c e o n t h e Wide Band N e u t r i n o Bean a x i s , 2 . n b e f o r e t h e p o i n t " R " , i . e . 1 . 4 5 IT.

b e f o r e T9

L e n g t h B e g i n L e n g t h Mass o f s e c t ion

I n t e g r a t e d r r a s s g Cm' :

( 1 9 7 9 - 1 9 8 0 )

NEUTRINO CAVE

T a r g e t WNB T9 p o i n t " T 9 " a t 1 .45

T a r g e t NNB T i l

H a d r o n s t o p p e r WNB

WNB c o l l i m a t o r WNB h o r n p o s . B R e f l e c t o r A R e f l e c t o r B

1 2.0 2 2.0 3 1.6 1 .9 2 .9 3 .9

12 49 83 73 92 03

125 77 126 39 125 77

6 . 5 1 6 . 5 4 6 . 5 4

11 x . 1 0 3 mm 6 x . 1 0 3 mm 1 x 160 (J 30 m

5 x . 1 0 2 mm 5 x . 1 0 3 mm 5 x . 1 0 10 mm

0 8 nm

. 1 . 1 . 1

Be Be

Be Be Be

E v e n l y d i s t r i b u t e e o v e r t h e 2 . m One p i e c e

D u r i n g a n t i n e u t r m o o n l y

A c c e p t a n c e 8 mrad

DECAY TUNNEL

E n t r a n c e w i n d o w P o i n t " Y " D e c a y t u n n e l 1

D e c a y t u n n e l 2

H a d r o n s t o p p e r G a r a g e p o s i t i o n End p o s i t i o n

E x i t w i n d o w

.002

1 2 9 . 2 7 3 9 6 . 8 1

3 . 3.

1.2 m

2 . 7 c o n c r e t e 2 . 2 v a c u u m W a l l 8 mm Fe

1.2 v a c u u m W a l l 5 mm F e

Special

1.2

Ti

Vacuum 5 Torr

Vacuum 5 Torr

F e p

F e

7 . 8

S u r v e y p o i n t

2 3 4 0 . O f f a x i s

IRON SHIELD

U n t i l 1981

C a l o r i m e t e r cave C a l o r i m e t e r

P o i n t "W" S h i e l d

3 . 8 1 1 .50

4 1 9 . 3 9 4 1 9 . 3 9

Dump c a v e Sh i e l d

4 1 5 . 5 8 4 2 4 . 2 2

U n t i l 1979

P i t 1 A n t i c a l o r i m e t e r

S h i e l d P i t 2 S h i e l d P i t 3 S h i e l d P i t 4 S h i e l d

2.7 .5 x .5

2.7 concrete 2.5 iron

Air, NBM apparatus Fe - first 3 discs have hole 0 .95

depth 1.2 m on NNB axis

Fe o - 7 . 2 5 2 3 d i s c s

S u r v e y p o i n t

6 6 7 0

8 . 6 4 4 . 4 2

431.29 451.81 452.79 472.94 473.99 494.56 495.49

Pit 5 Shield

519.72 520.74

Pit 5 Toroid magnet

Space

S h i e l d P i t 6 S h i e l d P i t 7 S h i e l d S p a c e

20.52 .98

2 0 . 1 5 1 . 0 5

2 0 . 5 7 . 9 3

2 4 . 2 3

1 . 0 2 1 6 . 10

2 . 0 2 1 0 . 0 0 2 . c o r e n o f i e l d

6 . f i e l d r e g i o n

A i r , c o p p e r dumps 11 d i s c s

F e D - 7 . 2 5

Air, NBM, NFM Fe, compensates for

calorimeter (out 51 discs Air, NBM, NFM 50 discs Air, NFM 51 d i s c s A i r , NFM 60 d i s c s

A i r , NFM 4 0 d i s c s

A i r , NFM Fe D - 7 . Fe A i r

536.74 8.45 545.29 1.10 546.39 25.28 571.57 1.84 573.51 26.58 600.09 .3

21 d i s c s A i r , NFM 6 3 d i s c s A i r 66 d i s c s A i r

h o l e s i n from 1 9 8 2 )

14 7 9 0 .

14 5 0 0 .

14 7 9 0 .

17 4 0 0 .

11 6 0 0

b 0 9 0 .

18 2 7 0 .

19 1 4 0 .

21 4 6 0 .

35 9 6 0 .

50 7 5 0 .

68 1 5 0 .

82 0 4 0 .

100 3 1 0 .

119 4 5 0

"EARTH SHIELD"

2 2 3 . 0 3 ! B u i l d i n g 274 E a r t h , p = 2 . 4 10 6 0 0 . 130 0 5 0 .

\ ! E a r t h - s h i e l d

j P i t 8 ) : S h i e l c

E a r t h - s h i e l d I B u i l d i n g E2

644 644

39 69

677 784

89 19

. 3 3 9 . 2

1 0 0 . 3 3 9 . 2 3

A i r v e r t i c a l s h a f t F e b l o c k s

E a r t h A i r , c o n e r e t e

C o u n t e r bu ' i l d i n g

Ga r g a m e l l e

Q u a r k s e a r c h b u i I d i n g

C e n t r e BEBC BfcBC EMI

H y d r o g e n t a r g e t WA1 WA1R

I GGM b u i l d i n g 9 4 5 , i C e n t r e CCM 9 5 4 ,

APPENDIX B

- 228 -

y TT

Hi MVB | |11107« [ [ l \ [ ] MVB j J J 111077 M f H MVB ' ^ 111079 j j I H«ooo* flAio

- - 1 -

HEFUECIEUB EN * ! T E H T t

SUPPOm Fi>£

Z)^i: ' K I E : 1 . . . ^ = = * Jlj- «

"7 7 ' ~Ç-G C^6bil

+ : - - i j f r - ••*<--- - 1*1

/ , / . . . . . . . . . . . . . . . . . I n i i i t t 34 m T B " _NfcUlR!N0

DETAILED LAY OUI SECTION %

C i i N LAB H CH m i « - « 1 , , 8 0 9 0 - 3 6 3 5 - 0

- 232 -

- 2 3 3 -

- 234 -

- 235 -

APPENDIX C

In this appendix a list is given of all silicon detectors used in the NFM system. Each detector has a unique number, which is engraved on its box (fig. 7.3). An additional code enables to know the detector manufacturer, the detector diode construction, and its size, as follows:

I 3 ^* serial number

standard size (table 7.1) diode type (chapter 1.2)

"-*• manufacturer - Manufacturers: L = LASCO/SCHLUMBERGER

0 = ORTEC/EG&G P = PHILIPS Q = QUANTRAD S = SIMTEC

- Diode type: B = surface barrier D = diffused diode 1 = ion-implanted diode L = lithium drifted diode

- Size: A, B, C, D (DD, S) DD is 25 mm 2, 1 mm thick S is special Note: the size of all SIMTEC detectors

is not conform table 7.1.

The reciprocal sensitivity S [number of muons cm"2 pC"1] has been calculated for each detector (Cale. SD) with formula (1.7), taking a mean energy deposit of 3.5 MeV cm"1 in Si. The sensitive volume was calculated as described in 1.2.2.

The following three columns indicate the S values obtained in the PS G-2 test (sect. 7.1.2), in the SPS wide band beam (mainly run 105) and in the narrow band beam (mainly 1979 NB) . The G-2 S and the WNB S are compared with the theoretically calculated S and the ratio is shown in the last two columns. It can be seen that there is a systematic difference between the measured and calculated values, depending on detector type and detector size, for all manufacturers, except SIMTEC; in this last case there is considerable spread, partly explained by the less well-known detector dimensions, partly by loss of charge via trapping (chapter 4).

- 236 -

The G-2 S D values for size C surface barrier detectors are (75.6 ± 1.9}% of the calculated value for all three manufacturers (L,0,P) . For the same detectors the WNB S D are (69.4 ± 1.9)% of the calculated value. The calculated values therefore are quite precise, although shifted in absolute value. For diffused detectors QDC a different behaviour is found, because of the different diode technology, which influences the sensitive volume. The "spray" of secondary radiation apparently causes 25-30% of the integrated signal (chapter 3).

After the manufacturers lists one finds a consecutive list of the same detectors, indicating the correspondence between box number and code number. In addition it contains the bias voltage needed for total depletion, the actual operating bias voltage, the actual leakage current measured (in the gap) and the r.m.s. detector noise voltage.

- 237 -

MANUFACTURER: LASCO/SCHLUMBERGER

BOX det. NO. Thickness microns Calc.SD G-2 SD WNB-SD NNB-SD G-2/

ratio WB/ ratio

320 LIA 3 100 2021.8 1050.0 998.8 915.0 .52 .49 305 LIA 4 100 2021.8 1220.0 840.0 0.0 .60 .42

301 LBB 1 330 161.2 117.0 107.7 99.6 .73 .67 302 LBB 2 295 190.5 140.0 122.9 112.0 .73 .65 307 LBB 3 310 155.4 108.0 98.2 87.0 .70 .63 309 LBB 4 320 163.2 124.0 .5 0.0 .76 .00

7 LBB 5 295 168.6 125.0 113.2 109.0 .74 .67 10 LBB 6 270 205.3 174.0 170.8 158.0 .85 .83 30 LBB 7 300 167.3 135.0 128.4 123.0 .81 .77 31 LBB 8 270 203.0 152.0 141.4 137.0 .75 .70

3 53 LBB 9 325 155.4 118.0 105.5 90.5 .76 .68 358 LBB 10 330 149.4 115.0 86.2 0.0 .77 .58 331 LBB 11 305 173.7 119.3 0.0 0.0 .69 0.00 368 LBB 12 295 183.1 130.0 0.0 116.0 .71 0.00

24 LBC 1 536 52.2 38.0 35.7 27.7 .73 .68 27 LBC 2 475 58.2 43.5 47.3 31.7 .75 .81 2 LBC 3 470 61.0 44.5 42.0 42.5 .73 .69

61 LBC 4 465 59.0 44.0 44.0 37.0 .75 .75 32 LBC 5 450 62.8 47.8 39.2 32.6 .76 .62 37 LBC 6 550 53.8 38.0 33.7 22.2 .71 .63 39 LBC 7 500 56.9 43.1 41.8 36.4 .76 .73 42 LBC 8 470 62.0 47.5 46.8 41.4 .77 .75 62 LBC 9 470 63.3 48.5 45.0 43.0 .77 .71 359 LBC 10 450 70.2 52.0 49.0 80.0 .74 .70

26 LBD 1 933 28.7 22.5 21.8 19.8 .79 .76 29 LBD 2 925 28.8 22.5 22.3 21.5 .78 .77 37 LBD 3 927 28.6 22.4 22.8 15.3 .78 .80 14 LBD 4 930 28.8 22.5 22.2 22.0 .78 .77 46 LBD 5 950 27.9 23.0 23.2 22.5 .83 .83 49 LBD 6 936 29.0 22.0 20.7 14.3 .76 .71 34 LBD 7 965 26.4 19.8 19.4 19.2 .75 .73 349 LBD 8 1045 24.7 19.5 18.7 18.3 .79 .76 35 LBD 9 1025 24.7 17.5 19.7 12.2 .71 .80

378 LBD 10 930 28.9 23.5 20.7 12.8 .81 .72 392 LBD 11 950 26.9 0.0 18.0 18.0 0.00 .67 393 LBD 12 950 28.2 0.0 22.0 17.0 0.00 .78 394 LBD 13 1050 26.3 0.0 0.0 0.0 0.00 0.00

200 LBDD 2 1048 113.3 154.0 0.0 0.0 1.36 0.00 204 LBDD 3 900 137.3 125.0 0.0 0.0 .91 0.00 100 LBDD 4 938 135.9 115.0 0.0 0.0 .85 0.00

- 238 -

MANUFACTURER: ORTEC INC.

Box Det. No. Thickness microns Cale. SD G-2 SD WNB-SD NNB-SD G-2/

ratio WB/ ratio

111 OIA 2 93 2605.7 1300.0 891.1 867.0 .50 .34

304 OIB 1 316 183.4 120.0 92.5 86.7 .65 .50 306 OIB 2 304 191.5 135.0 130.6 112.0 .71 .68 18 OIB 3 305 190.8 130.0 113.2 107.0 .68 .59 21 OIB 4 308 188.7 120.0 100.6 100.0 .64 .53

342 OIB 5 305 190.8 130.0 0.0 0.0 .68 0.00 308 OIB 6 299 195.0 132.0 101.8 92.1 .68 .52 347 OIB 7 300 194.3 133.0 112.1 107.0 .68 .58 381 OIB 8 305 190.8 130.0 105.4 95.4 .68 .55

65 OIB 9 320 180.9 0.0 0.0 0.0 0.00 0.00 362 OIB 10 312 186.0 0.0 101.2 96.0 0.00 .54

68 OIB 11 339 169.7 0.0 93.6 71.7 0.00 .55 69 OIB 12 349 164.3 0.0 86.0 83.7 0.00 .52

348 OBC 1 524 54.3 41.5 36.7 34.3 .76 .68

38 OBC 2 498 57.5 42.0 39.2 33.0 .73 .68

41 OBC 3 496 57.8 42.5 36.8 25.5 .74 .64

40 OBC 4 451 64.2 50.0 42.4 38.8 .78 .66

369 OBC 5 520 54.8 44.0 0.0 41.0 .80 0.00

16 OBD 1 1000 25.5 18.0 17.4 17.2 .71 .68

36 OBD 2 1011 25.2 17.5 18.4 18.2 .69 .73

345 OBD 3 1010 25.2 19.5 17.6 15.7 .77 .70

94 OBD 4 1008 25.3 19.0 19.0 .0 .75 .75

355 OBD 5 1006 25.3 20.0 22.3 18.0 .79 .88

389 OBD 6 1000 25.5 0.0 17.1 14.7 0.00 .67

390 OBD 7 1007 25.3 0.0 17.2 15.6 0.00 .68

391 OBD 8 1007 25.3 0.0 16.9 16.1 0.00 .67

- 239 -

MANUFACTURER: PHILIPS NV


ratio WB/ ratio

81 PBA 1 107 1103.0 1020.0 550.0 0.0 .92 .50

9 PBC 1 486 58.8 44.0 41.2 37.8 .75 .70 12 PBC 2 480 56.2 42.5 38.5 34.4 .76 .69 1 PBC 3 490 56.2 42.0 38.1 32.0 .75 .68 13 PBC 4 486 56.9 43.0 38.5 37.7 .76 .68 23 PBC 5 492 53.5 40.0 37.1 26.8 .75 .69 25 PBC 6 495 53.1 41.2 35.0 27.3 .78 .66 28 PBC 7 492 54.9 42.0 40.1 31.0 .76 .73 343 PBC 8 478 55.2 42.0 37.0 32.5 .76 .67 63 PBC 9 488 54.0 42.0 39.1 37.0 .78 .72 364 PBC 10 476 55.3 42.5 38.5 36.0 .77 .70 354 PBC 11 513 51.1 39.0 36.0 30.3 .76 .71 67 PBC 12 505 51.3 37.5 40.0 0.0 .73 .78

4 PBD 1 970 27.6 21.0 17.5 15.4 .76 .63 98 PBD 2 970 24.4 0.0 19.6 15.80 .00 .80 5 PBD 3 970 24.5 19.0 18.6 18.3 .77 .76 3 PBD 4 980 25.9 19.0 21.8 14.8 .73 .84 6 PBD 5 980 24.1 18.0 18.5 18.2 .75 .77 8 PBD 6 980 24.1 19.5 18.0 19.2 .81 .75

11 PBD 7 980 24.0 19.0 19.8 17.1 .79 .82 370 PBD 8 980 24.2 19.0 18.2 19.0 .78 .75 360 PBD 9 980 25.5 20.0 17.4 17.5 .78 .68 376 PBD 10 995 23.7 0.0 19.3 14.90 .00 .81 15 PBD 11 907 26.5 20.5 20.7 19.5 .77 .78 344 PBD 12 908 26.6 19.5 20.7 20.2 .73 .78 110 PBD 13 890 27.1 19.0 0.0 0.0 .70 0.00 365 PBD 14 883 27.3 19.0 20.2 19.0 .69 .74 95 PBD 15 877 27.9 21.0 20.5 20.8 .75 .73 350 PBD 16 878 27.5 19.5 19.6 18.2 .71 .71 45 PBD 17 876 27.8 21.0 20.7 20.7 .75 .74 47 PBD 18 871 27.8 21.0 20.7 0.0 .76 .74 48 PBD 19 871 27.8 20.0 22.3 22.2 .72 .80 50 PBD 20 870 33.0 19.5 20.2 20.9 .59 .61 91 PBD 21 998 27.0 0.0 19.6 15.30 .00 .72 92 PBD 22 1000 26.7 0.0 20.9 18.20 .00 .78

395 PBD 23 876 31.5 0.0 22.1 18.90 .00 .70 396 PBD 24 870 31.5 0.0 22.7 21.80 .00 .72 397 PBD 25 874 31.6 0.0 23.3 22.00 .00 .74 96 PBD 26 871 31.7 0.0 24.6 14.80 .00 .77

109 PBDD 1 980 159.7 86.5 0.0 0.0 .54 0.00 103 PBDD 2 980 154.6 81.0 0.0 0.0 .52 0.00 379 PBDD 3 985 148.7 84.0 0.0 84.0 .57 0.00 104 PBDD 4 985 136.2 87.0 0.0 0.0 .64 0.00 105 PBDD 5 1005 135.9 81.0 0.0 0.0 .60 0.00

240 -

MANUFACTURER : QUANTRAD CORP.

Box Det. No. Thickness microns Calc. SD G-2 SD WNB-SD NNB-SD G-2/

ratio WB/ ratio

341 QDA 1 100 2008.9 1500.0 1443.8 1330.0 .75 .72 346 QDA 2 100 2008.9 1540.0 1457.9 0.0 .77 .73 351 QDA 3 100 2008.9 1600.0 1523.2 1320.0 .80 .76 357 QDA 4 100 2008.9 1630.0 1555.8 1400.0 .81 .77 361 QDA 5 100 2008.9 1480.0 1410.0 1320.0 .74 .70 366 QDA 6 100 2008.9 1600.0 1479.7 0.0 .80 .74 367 QDA 7 100 2008.9 1560.0 1400.0 0.0 .78 .70 314 QDA 8 100 2008.9 1550.0 1320.0 0.0 .77 .66 313 QDA 9 100 2008.9 1400.0 1467.0 1339.0 .70 .73 323 QDA 10 100 2008.9 1800.0 1486.0 1339.0 .90 .74 315 QDA 11 100 2008.9 1600.0 1321.9 0.0 .80 .66 112 QDA 12 100 2008.9 1500.0 1360.0 1272.0 .75 .68 113 QDA 13 100 2008.9 1550.0 1327.4 0.0 .77 .66 114 QDA 14 100 2008.9 1550.0 1577.6 1020.0 .77 .79 115 QDA 15 100 2008.9 1550.0 1577.6 1089.0 .77 .79 116 QDA 16 100 2008.9 1550.0 1595.0 1500.0 .77 .79 117 QDA 17 100 2008.9 1550.0 1607.0 1021.0 .77 .80 118 QDA 18 100 2008.9 1650.0 1653.8 1257.0 .82 .82 119 QDA 19 100 2008.9 1600.0 1642.9 1310.0 .80 .82 120 QDA 20 100 2008.9 1600.0 1610.2 1297.0 .80 .80 121 QDA 21 100 2008.9 1550.0 1651.6 0.0 .77 .82 122 QDA 22 100 2008.9 1600.0 1490.0 0.0 .80 .74 123 QDA 23 100 2008.9 1550.0 1617.9 0.0 .77 .81 124 QDA 24 100 2008.9 1570.0 1472.0 0.0 .78 .73 125 QDA 25 100 2008.9 1630.0 1640.7 0.0 .81 .82 126 QDA 26 100 2008.9 1650.0 1674.4 0.0 .82 .83 127 QDA 27 100 2008.9 1550.0 1440.0 0.0 .77 .72 128 QDA 28 100 2008.9 1500.0 1588.5 0.0 .75 .79 129 QDA 29 100 2008.9 1550.0 1440.0 0.0 .77 .72 130 QDA 30 100 2008.9 1650.0 1640.7 0.0 .82 .82 131 QDA 31 100 2008.9 1500.0 1504.7 1205.0 .75 .75 132 QDA 32 100 2008.9 1650.0 1635.3 1435.0 .82 .81 133 QDA 33 100 2008.9 1650.0 1567.8 1387.0 .82 .78 134 QDA 34 100 2008.9 1540.0 1474.2 0.0 .77 .73 135 QDA 35 100 2008.9 1640.0 1575.4 1387.0 .82 .78 136 QDA 36 100 2008.9 1570.0 1529.7 1262.0 .78 .76 137 QDA 37 100 2008.9 1600.0 1588.5 1292.0 .80 .79 138 QDA 38 100 2008.9 1540.0 1512.3 0.0 .77 .75 139 QDA 39 100 2008.9 1650.0 0.0 0.0 .82 0.00 140 QDA 40 100 2008.9

-1550.0 1388.0 0.0 .77 .69

- 2 4 1 -

MANUFACTURER: QUA JTRAD CORP. (Con t'd)


ratio WB/ ratio

141 QDA 41 100 2008.9 1460.0 0.0 0.0 .73 0.00 142 QDA 42 100 2008.9 1500.0 0.0 0.0 .75 0.00 143 QDA 43 100 2008.9 1550.0 0.0 0.0 .77 0.00 144 QDA 44 100 2008.9 1600.0 0.0 0.0 .80 0.00 145 QDA 45 100 2008.9 1600.0 0.0 0.0 .80 0.00 146 QDA 46 100 2008.9 1500.0 0.0 0.0 .75 0.00 147 QDA 47 100 2008.9 1550.0 0.0 0.0 .77 0.00 148 QDA 48 100 2008.9 1530.0 0.0 0.0 .76 0.00 149 QDA 49 100 2008.9 1550.0 0.0 0.0 .77 0.00 150 QDA 50 100 2008.9 1600.0 0.0 0.0 .80 0.00 151 QDA 51 107 1881.3 1800.0 0.0 0.0 .96 0.00 152 QDA 52 107 1881.3 1655.0 0.0 0.0 .88 0.00 153 QDA 53 100 2008.9 1500.0 1270.8 1218.0 .75 .63 154 QDA 54 100 2008.9 1500.0 1501.4 0.0 .75 .75 155 QDA 55 100 2008.9 1500.0 1378.0 0.0 .75 .69 156 QDA 56 100 2008.9 1500.0 1567.8 0.0 .75 .78 157 QDA 57 100 2008.9 1500.0 0.0 0.0 .75 0.00 163 QDA 63 100 2008.9 1500.0 1563.5 0.0 .75 .78

17 QDB 1 290 201.6 220.0 179.5 189.0 1.09 .89 19 QDB 2 290 201.6 182.0 177.3 165.0 .90 .88 20 QDB 3 290 201.6 178.0 179.5 187.0 .88 .89 22 QDB 4 290 201.6 199.0 155.6 134.0 .99 .77 85 QDB 5 290 201.6 206.0 156.7 0.0 1.02 .78 86 QDB 6 290 201.6 300.0 159.9 150.0 1.49 .79 87 QDB 7 290 201.6 177.0 174.1 0.0 .88 .86 88 QDB 8 290 201.6 166.0 163.2 0.0 .82 .81 310 QDB 9 290 201.6 179.0 165.4 108.0 .89 .82 312 QDB 10 290 201.6 188.0 162.1 161.7 .93 .80 90 QDB 11 310 187.4 170.0 165.4 164.0 .91 .88 82 QDB 15 300 194.3 0.0 0.0 0.0 0.00 0.00 83 QDB 16 300 194.3 0.0 0.0 0.0 0.00 0.00 84 QDB 17 300 194.3 0.0 0.0 0.0 0.00 0.00

43 QDC 1 492 59.7 51.0 63.3 40.0 .85 1.06 33 QDC 2 492 59.7 51.0 51.1 37.0 .85 .86 44 QDC 3 492 59.7 49.0 47.3 44.6 .82 .79 64 QDC 4 492 59.7 49.0 49.8 53.0 .82 .83 66 QDC 5 492 59.7 51.5 46.3 39.0 .86 .78 89 QDC 6 492 59.7 50.0 54.3 41.2 .84 .91 386

i

QDC 7 492 59.7 50.0 50.0 50.0 .84 .84

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MANUFACTURER: SIMTEC


ratio WB/ ratio

404 SDA 1 100 1843.4 1468.0 0.0 0.0 .80 0.00 405 S DA 2 0 0.0 0.0 0.0 0.0 0.00 0.00 406 SDA 3 0 0.0 0.0 . 0.0 0.0 0.00 0.00 352 SDA 4 100 1843.4 0.0 960.0 0.0 0.00 .52 407 SDA 5 0 0.0 0.0 0.0 0.0 0.00 0.00 385 SDA 6 100 1843.4 1585.0 740.0 0.0 .86 .40 408 SDA 7 0 0.0 0.0 0.0 0.0 0.00 0.00 409 SDA 8 0 0.0 0.0 0.0 0.0 0.00 0.00 410 SDA 9 100 1843.4 0.0 0.0 0.0 0.00 0.00 322 SDA 10 45 2809.3 2590.0 1330.0 1369.0 .92 .47 411 SDA 11 0 0.0 0.0 0.0 0.0 0.00 0.00 412 SDA 12 0 0.0 0.0 0.0 0.0 0.00 0.00 413 SDA 13 0 0.0 0.0 0.0 0.0 0.00 0.00 414 SDA 14 0 0.0 0.0 0.0 0.0 0.00 0.00 58 SDA 15 300 372.8 358.0 208.9 201.0 .96 .56 59 SDA 16 300 372.8 347.0 230.7 186.0 .93 .62 70 SDA 17 300 372.8 401.0 243.7 173.0 1.08 .65

415 SDA 18 0 0.0 0.0 0.0 0.0 0.00 0.00 416 SDA 19 0 0.0 0.0 0.0 0.0 0.00 0.00 417 SDA 20 0 0.0 0.0 0.0 0.0 0.00 0.00 418 SDA 21 0 0.0 0.0 0.0 0.0 0.00 0.00 78 SDA 22 0 0.0 0.0 0.0 0.0 0.00 0.00 57 SDA 23 300 372.8 291.0 238.3 184.0 .78 .64 79 SDA 24 0 0.0 0.0 0.0 0.0 0.00 0.00 419 SDA 25 0 0.0 0.0 0.0 0.0 0.00 0.00 75 SDA 34 300 372.8 355.0 282.9 193.0 .95 .76 76 SDA 35 0 0.0 0.0 0.0 0.0 0.00 0.00 77 SDA 36 0 0.0 0.0 0.0 0.0 0.00 0.00 71 SDA 37 300 372.8 406.0 264.4 212.0 1.09 .71 72 SDA 38 300 372.8 382.0 279.6 227.0 1.02 .75 73 SDA 39 300 372.8 358.0 285.1 273.0 .96 .76 74 SDA 40 300 372.8 353.0 286.1 261.0 .95 .77 387 SDA 42 0 0.0 0.0 0.0 0.0 0.00 0.00 356 SDA 44 100 1843.4 1301.0 810.0 0.0 .71 .44 382 SDA 48 300 372.8 322.0 266.6 205.0 .86 .72 383 SDA 49 300 372.8 247.0 188.8 165.0 .66 .51 384 SDA 50 300 372.8 306.0 203.5 202.0 .82 .55 51 SDB 1 300 194.3 0.0 129.5 0.0 0.00 .67 388 SDB 2 0 0.0 0.0 0.0 0.0 0.00 0.00 303 SDB 3 300 194.3 186.3 0.0 0.0 .96 0.00 60 SDB 4 300 194.3 0.0 134.9 117.0 0.00 .69

401 SDB 5 300 194.3 0.0 0.0 18.5 0.00 0.00 54 SDB 6 300 194.3 0.0 108.0 0.0 0.00 .56 402 SDB 7 0 0.0 0.0 0.0 70.4 0.00 0.00 311 SDB 8 300 194.3 0.0 152.0 0.0 0.00 .78 55 SDB 9 300 194.3 0.0 115.3 108.0 0.00 .59 403 SDB 10 300 194.3 0.0 0.0 0.0 0.00 0.00 56 SDB 11 300 194.3 0.0 163.2 0.0 0.00 .84 53 S DC 1 500 109.1 0.0 63.8 60.2 0.00 .58 52 S DC 2 500 109.1 0.0 111.0 97.0 0.00 1.02 363 S DC 3 500 109.1 84.9 67.3 66.0 .78 .62

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List of all detectors, in order of box numbers

Box Det. NO. V-depl Volt

Opbias Volt

Leakage crt UAmp

Noise RMS V Remarks

1 PBC 3 75.0 92.5 .32 .004 2 LBC 3 76.0 38.5 .32 .037 3 PBD 4 220.0 176.2 1.01 .031 4 PBD 1 110.0 155.2 1.67 .031 REFB 2 5 PBD 3 180.0 88.1 2.27 .017 BREAKDOWN 6 PBD 5 180.0 191.7 1.20 .035 7 LBB 5 35.0 48.1 .21 .006 8 PBD 6 180.0 208.8 .62 .005 9 PBC 1 83.0 99.7 .18 .006

10 LBB 6 78.0 48.4 .10 .006 ABS.BOX MI 11 PBD 7 200.0 86.6 1.39 .112 BREAKDOWN 12 PBC 2 60.0 95.2 .45 .011 13 PBC 4 75.0 148.0 .47 .006 BOX C 14 LBD 4 160.0 147.6 .29 .007 15 PBD 11 200.0 193.3 .43 .013 16 OBD 1 280.0 207.0 .72 .029 17 QDB 1 60.0 93.3 .66 .044 18 OIB 3 48.0 150.4 .30 .004 19 QDB 2 80.0 147.0 .57 .016 20 QDB 3 80.0 104.4 .64 .041 21 OIB 4 45.0 55.2 .34 .005 22 QDB 4 400.0 105.8 4.19 .435 150V 23 PBC 5 85.0 146.7 .35 .005 24 LBC 1 100.0 105.3 .28 .005 UPSTREAM 25 PBC 6 76.0 144.1 .24 .006 26 LBD 1 220.0 174.1 .24 .010 27 LBC 2 80.0 76.6 .22 .006 28 PBC 7 100.0 150.6 .21 .003 BREAKDOWN 29 LBD 2 200.0 178.8 .32 .012 30 LBB 7 100.0 49.8 .10 .006 ABS.BOX UP 31 LBB 8 30.0 23.0 .28 .036 32 LBC 5 80.0 84.2 .35 .008 UPSTREAM 33 QDC 2 250.0 142.4 1.23 .020 34 LBD 7 180.0 191.5 .33 .012 35 LBD 9 200.0 173.6 2.53 .226 36 OBD 2 200.0 198.1 .70 .005 37 LBC 6 60.0 59.9 1.16 .006 60 38 OBC 2 85.0 94.7 .51 .004 CAL X 1 39 LBC 7 60.0 72.8 .21 .010 UPSTREAM M 40 OBC 4 35.0 46.9 .36 .016 UPSTREAM M 41 OBC 3 80.0 87.9 .56 .007 42 LBC 8 90.0 62.5 .23 .009 BOX D 43 QDC 1 280.0 199.6 1.00 .019 44 QDC 3 280.0 193.9 .42 .006 45 PBD 17 200.0 145.4 .75 .018 46 LBD 5 140.0 141.4 .24 .004 47 PBD 18 190.0 95.1 7.68 .128 REP PH 48 PBD 19 200.0 121.4 .45 .013 49 LBD 6 190.0 177.2 .41 .044 50 PBD 20 190.0 143.4 1.15 .017 51 SDB 1 90.0 29.6 .42 .025 BAD? 52 SDC 2 600.0 151.1 5.44 .098 53 SDC 1 70.0 93.7 .01 .001 54 SDB 6 100.0 0.0 .00 .000 55 SDB 9 160.0 92.1 .82 .006 56 SDB 11 200.0 45.1 .56 .002 57 S DA 23 90.0 86.8 .27 .004 /92 58 S DA 15 220.0 48.5 .86 .019 /32 59 S DA 16 220.0 144.6 .78 .006 /51 60 SDB 4 50.0 56.8 .48 .006 61 LBC 4 85.0 27.6 .36 .007

List of all detectors, in order of box numbers (Cont'd)


Opbias Volt

Leakage crt liAmp

Noise RMS V Remarks

62 LBC 9 80.0 92.7 .33 .004 63 PBC 9 100.0 147.1 .55 .011 64 QDC 4 250.0 23.6 .42 .005 UPSTREAM M 65 OIB 9 31.0 0.0 0.00 0.000 BACK ORTEC 66 QDC 5 250.0 141.5 3.26 .296 67 PBC 12 115.0 0.0 0.00 0.000 SPILL DET 68 OIB 11 400.0 137.6 .45 .006 REPL 69 OIB 12 240.0 214.6 .52 .006 REPL 70 S DA 17 90.0 87.7 .57 .011 24V /38 71 S DA 37 240.0 58.6 3.08 .036 12V /52 72 SDA 38 150.0 51.1 .47 .010 /55 73 S DA 39 200.0 57.7 3.92 .021 /54 74 SDA 40 160.0 73.2 5.56 .023 /48 75 SDA 34 250.0 74.9 1.39 .031 /35 76 SDA 35 0.0 0.0 0.00 0.000 77 SDA 36 0.0 0.0 0.00 0.000 78 SDA 22 0.0 0.0 0.00 0.000 79 SDA 24 0.0 0.0 0.00 0.000 81 PBA 1 11.0 0.0 0.00 0.000 82 QDB 15 150.0 95.2 .67 .001 83 QDB 16 150.0 116.4 .54 .003 84 QDB 17 150.0 0.0 0.00 0.000 85 QDB 5 140.0 88.1 .90 .004 86 QDB 6 140.0 100.3 1.24 .012 60V-100V 87 QDB 7 130.0 87.3 .63 .010 88 QDB 8 140.0 89.1 1.24 .006 89 QDC 6 300.0 205.0 .92 .160 90 QDB 11 72.0 96.1 .75 .072 PROTOTYPE 91 PBD 21 90.0 177.0 .55 .002 92 PBD 22 120.0 147.2 .34 .042 93 LBD 13 280.0 172.0 .18 .015 WAS 394 94 OBD 4 280.0 161.7 .44 .010 3 95 PBD 15 210.0 201.8 .45 .019 96 PBD 26 180.0 207.5 .36 .005 98 PBD 2 110.0 96.2 2.01 .020

100 LBDD 4 200.0 0.0 0.00 0.000 COUNTING 103 PBDD 2 200.0 0.0 0.00 0.000 BAD 104 PBDD 4 190.0 0.0 0.00 0.000 NA-4 105 PBDD 5 170.0 0.0 0.00 0.000 UNUSED 109 PBDD 1 180.0 0.0 0.00 0.000 SPILL DET 110 PBD 13 190.0 0.0 0.00 0.000 TIMING 111 OIA 2 15.0 41.2 .14 .004 112 QDA 12 35.0 44.0 .06 .004 BOX F 113 QDA 13 39.0 0.0 0.00 0.000 BOX F 114 QDA 14 39.0 46.2 .12 .004 115 QDA 15 37.0 46.1 .14 .004 116 QDA 16 35.0 48.9 .11 .009 117 QDA 17 30.0 46.9 .32 .011 118 QDA 18 37.0 46.6 .10 .004 119 QDA 19 37.0 46.5 .08 .006 120 QDA 20 41.0 46.6 .12 .005 121 QDA 21 24.0 43.5 .96 .002 A'DAM 122 QDA 22 35.0 0.0 0.00 0.000 A'DAM 123 QDA 23 30.0 43.0 1.11 .012 A'DAM 124 QDA 24 42.0 0.0 0.00 0.000 A'DAM 125 QDA 25 35.0 43.2 .83 .003 A'DAM 126 QDA 26 30.0 36.5 .78 .003 A'DAM 127 QDA 27 32.0 0.0 0.00 0.000 V-CAVE 128 QDA 28 37.0 0.0 0.00 0.000 V-CAVE 129 QDA 29 35.0 43.7 .72 .002 V-CAVE 130 QDA 30 35.0 44.2 .64 .002 A'DAM


V-depl Opbias LK-CUR Noise Box Det. NO. Volt Volt yAmp RMS V Remarks

131 QDA 31 37.0 43.4 .05 .005 V-CAVE 132 QDA 32 39.0 44.1 .05 .008 V-CAVE 133 QDA 33 39.0 29.1 .07 .009 V-CAVE 134 QDA 34 42.0 43.3 .14 .050 135 QDA 35 35.0 28.7 .05 .004 136 QDA 36 35.0 29.8 .06 .005 137 QDA 37 35.0 29.1 .07 .012 138 QDA 38 40.0 42.6 .10 .002 139 QDA 39 39.0 0.0 0.00 0.000 LENGELER 140 QDA 40 40.0 0.0 0.00 0.000 141 QDA 41 40.0 -.5 .00 .002 142 QDA 42 40.0 0.0 0.00 0.000 CAL S 4 143 QDA 43 39.0 0.0 0.00 0.000 CAL V 5 144 QDA 44 39.0 0.0 0.00 0.000 CAL W 5 145 QDA 45 30.0 0.0 0.00 0.000 146 QDA 46 39.0 43.7 .06 .002 147 QDA 47 43.0 43.8 .06 .001 148 QDA 48 39.0 43.2 .04 .001 149 QDA 49 35.0 43.0 2.07 .003 150 QDA 50 37.0 43.1 .23 .002 151 QDA 51 23.0 0.0 0.00 0.000 PROTOTYPE 152 QDA 52 18.0 0.0 0.00 0.000 PROTOTYPE 153 QDA 53 37.0 35.0 .10 .005 154 QDA 54 39.0 43.1 .10 .002 155 QDA 55 39.0 0.0 0.00 0.000 156 QDA 56 30.0 42.9 .23 .002 157 QDA 57 40.0 0.0 0.00 0.000 163 QDA 63 35.0 42.9 .08 .002 166 QCA 6 0.0 0.0 0.00 0.000 COUNTING 167 QCA 7 0.0 0.0 0.00 0.000 COUNTING 168 QCA 8 0.0 0.0 0.00 0.000 169 QCA 9 0.0 0.0 0.00 0.000 COUNTING 200 LBDD 2 110.0 0.0 0.00 0.000 COUNTING 204 LBDD 3 100.0 0.0 0.00 0.000 301 LBB 1 25.0 23.2 .18 .003 BOX A 302 LBB 2 25.0 29.9 .69 .006 BOX A 303 SDB 3 200.0 0.0 0.00 0.000 OLD 7 304 OIB 1 48.0 53.8 4.08 .346 V-CAVE 305 LIA 4 22.0 44.1 .69 .003 306 OIB 2 31.0 14.6 4.34 .069 CAL S 2 307 LBB 3 45.0 43.2 .85 .228 BOX NORM 308 OIB 6 33.0 46.5 1.58 .006 REFB 4 309 LBB 4 40.0 0.0 0.00 0.000 ABS.B0X DO 310 QDB 9 150.0 88.2 .85 .005 BOX 201 311 SDB 8 135.0 0.0 0.00 0.000 312 QDB 10 140.0 147.3 .77 .009 BOX C 313 QDA 9 39.0 45.3 .08 .005 BOX E 314 QDA 8 23.0 0.0 0.00 0.000 CAL T 4 315 QDA 11 42.0 0.0 0.00 0.000 BOX F 320 LIA 3 25.0 29.1 .09 .005 CAL S 3 321 QDA 49 0.0 0.0 0.00 0.000 322 S DA 10 20.0 21.8 1.15 .005 BOX E /17 323 QDA 10 42.0 42.0 .08 .007 BOX E 331 LBB 11 65.0 0.0 7.00 0.000 PROTO 341 QDA 1 38.0 45.6 .14 .004 CAL S 5 342 OIB 5 42.0 0.0 0.00 0.000 BACK 343 PBC 8 66.0 151.8 .69 .003 REFB 3 ST 344 PBD 12 200.0 115.8 2.26 .036 345 OBD 3 280.0 193.3 .71 .006 CAL S 1 346 QDA 2 39.0 46.8 10.00 8.793 CAL U 4 347 OIB 7 35.0 27.4 2.88 .041 A



Opbias Volt

LK-CUR yAmp

Noise RMS V Remarks

348 OBC 1 100.0 49.1 .45 .002 CAL U 2 349 LBD 8 180.0 171.1 .55 .015 CAL Y 2 350 PBD 16 200.0 146.1 2.34 .054 REP 150V 351 QDA 3 37.0 42.4 .51 .005 CAL T 5 352 SDA 4 20.0 0.0 0.00 0.000 /2 353 LBB 9 40.0 44.1 .22 .003 CAL T 3 354 PBC 11 140.0 131.3 1.29 .023 PROTO T 2 355 OBD 5 400.0 211.2 .92 .016 D 356 SDA 44 25.0 0.0 0.00 0.000 /101 357 QDA 4 31.0 47.0 .21 .035 REFB 5 358 LBB 10 40.0 0.0 0.00 0.000 359 LBC 10 70.0 68.2 .50 .007 CAL V 3 360 PBD 9 140.0 80.4 1.90 .077 361 QDA 5 42.0 44.0 .05 .009 V-CAVE 362 OIB 10 42.0 56.9 1.61 .003 CAL V 4 363 S DC 3 125.0 74.5 .82 .003 /10 4 364 PBC 10 83.0 96.5 .28 .003 CAL V 2 365 PBD 14 180.0 145.5 1.50 .031 BOX D 366 QDA 6 40.0 43.5 .21 .001 CAL U 5 367 QDA 7 31.0 0.0 0.00 0.000 CAL U 3 368 LBB 12 110.0 73.8 .05 .004 CAL W 4 369 OBC 5 180.0 173.7 .34 .011 CAL W 3 370 PBD 8 170.0 92.0 .34 .024 376 PBD 10 180.0 106.2 .99 .008 CAL U 1 377 LBD 3 180.0 136.6 .25 .004 ABS.MI 378 LBD 10 200.0 140.6 .43 .011 CAL Y l 379 PBDD 3 180.0 0.0 0.00 0.000 GONIO-BOX 381 OIB 8 44.0 75.5 .66 .007 CAL S 4 382 SDA 48 100.0 47.1 .40 .002 /102 383 SDA 49 40.0 44.0 .75 .002 ,103 384 SDA 50 100.0 45.5 1.08 .007 ,1 385 SDA 6 20.0 0.0 0.00 0.000 /4 386 QDC 7 280.0 192.2 1.04 .022 387 SDA 42 0.0 0.0 0.00 0.000 388 SDB 2 0.0 0.0 0.00 0.000 389 OBD 6 200.0 205.7 .95 .063 REFB 1 390 OBD 7 200.0 176.6 .63 .016 CAL S 2 391 OBD 8 200.0 195.1 .71 .018 CAL W 1 392 LBD 11 330.0 36.7 10.00 9.999 393 LBD 12 350.0 206.2 .15 .054 CAL X 2 394 LBD 13 280.0 0.0 0.00 0.000 NOW 93 395 PBD 23 180.0 131.3 .83 .079 CAL T 1 396 PBD 24 175.0 194.8 .33 .008 CAL V 1 397 PBD 25 180.0 159.4 .33 .022 CAL W 2 401 SDB 5 145.0 89.4 0.00 .005 402 SDB 7 0.0 144.4 .36 .084 403 SDB 10 160.0 90.5 .44 .102 404 SDA 1 10.0 0.0 0.00 0.000 /91 405 SDA 2 0.0 0.0 0.00 0.000 406 SDA 3 0.0 0.0 0.00 0.000 407 SDA 5 0.0 0.0 0.00 0.000 408 SDA 7 0.0 0.0 0.00 0.000 409 SDA 8 0.0 0.0 0.00 0.000 410 SDA 9 20.0 0.0 0.00 0.000 411 SDA 11 0.0 0.0 0.00 0.000 412 SDA 12 0.0 0.0 0.00 0.000 413 SDA 13 0.0 0.0 0.00 0.000 414 SDA 14 0.0 0.0 0.00 0.000 415 SDA 18 0.0 0.0 0.00 0.000 416 SDA 19 0.0 0.0 0.00 0.000 417 SDA 20 0.0 0.0 0.00 0.000 418 SDA 21 0.0 0.0 0.00 0.000 419 SDA 25 0.0 0.0 0.00 0.000 500 LBAS 66 0.0 0.0 0.00 0.000 PROTO T 3 566 LBAS 66 50.0 0.0 0.00

J 0.000 OFF,TEST

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APPENDIX D

GLOSSARY OF ABBREVIATIONS AND MNEMONICS

AC Alternating current (signal coupling via capacitance) ADC Analog to digital conversion ASTRA Austrian research reactor, with SNIF BA7 "Bâtiment auxiliaire", auxiliary building at the SPS BCT Beam current transformer BEBC Big European Bubble Chamber CAMAC Modular instrumentation system for data handling CDC Control Data Corporation, manufacturer of the CDC 7600 central

computer system at CERN CDHS CERN Dortmund-Heidelberg-Saclay collaboration, WA1 CERN European Organization for Nuclear Research CHARM CERN-Hamburg-Amsterdam-Rome-Moscow Collaboration, WA18 DAC Digital-to-Analog conversion DATEL Manufacturer of the multiplexing ADC used in the NFM data

acquisition DISMUNU Computer simulation program for neutrino beam DC Direct Current (direct coupling of signal) DLTS Deep level transient spectroscopy EF CERN Division for Experimental Facilities EGS Electron-Gamma Shower computer simulation program ELS Electron energy loss spectroscopy EMI External muon identifier ENC Equivalent noise charge ENDOR Electron nuclear double resonance (extension of EPR) EP CERN Division for Experimental Physics EPB Extracted proton beam EPR Electrom paramagnetic resonance (ESR) ESR Electron spin resonance (EPR) FE Fast extraction of proton beam FET Field effect transistor FORTRAN Computer programming language FQMIN Set of programs to write and read minicomputer data tapes FRE Fast resonant extraction of proton beam FWHM Full width at half of the maximum GGM Heavy liquid bubble chamber Gargamelle HBCT Hadron beam current transformer HYDRA System of computer procedures for analysis programs in high

energy physics JFET Junction field effect transistor LCR Oscillator circuit with inductance L, capacitance C and

resistance R MA Main amplifier MC "Monte-Carlo" stochastic computer simulation program MCR SPS main control room MEA Medium energy electron accelerator in Amsterdam MOS Metal-oxide-semiconductor structure MOSFET Field effect transistor with MOS structure MUX Multiplexer of several inputs into one output, or vice versa NA20 etc. Experiments located in the CERN North Area have code NA NAB Narrow band antineutrino beam NB Narrow band beam NNB Narrow band neutrino beam NBC Neutrino beam computer system NFM Neutrino flux measurement/monitoring system NFMPLY Data reduction program for NFM data tapes

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NIM Standard for nuclear electronics instrumentation NIP n-type, intrinsic, p-type semiconductor structure NNB Narrow band neutrino beam NORD Norsk data computer NUBEAM Computer simulation program for neutrino beam in HYDRA NUFLUX Computer simulation program for neutrino beam Ni, N3 Codes for wide band and narrow band neutrino beam OPCOM Computer program for NFM system PBCT Proton beam current transformer PCB Printed circuit board PHD Pulse height defect PIN p-type, intrinsic, n-type semiconductor structure PNFM Proton intensity signal available in NFM PS CERN proton synchrotron accelerator PVC Polyvinylchloride, isolating plastic material, used in cables RF Radio frequency RMS Root of the average of the squared values (r.m.s.) SD Reciprocal sensitivity of silicon detector SEM Secondary emission monitor SINTRAN Operating system of NORD computer SLAC Stanford linear accelerator center SNIF Standard neutron irradiation facility at ASTRA SNR Signal-to-noise ratio SPS CERN super proton synchrotron accelerator SSD Solid state detector TBID Beam/target instrumentation downstream TBIU Beam/target instrumentation upstream TFA Timing filter amplifier TOMCAT Computer simulation program for muons in a shield TSC Thermally stimulated current measurement TURTLE Computer simulation program for charged particle beams T9, Til SPS proton beam target for WB and NB Vl, etc. Code for vertical access pits to NFM measurement gaps in shield WA Experiments located in the CERN West Area have code WA WAl See CDHS WA18 See CHARM WA44 Quark search experiment in neutrino beam WAB Wide band antineutrino beam WANF CERN West Area neutrino facility WB Wide band beam WEXTR West Area extraction control computer WNB Wide band neutrino beam

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REFERENCES - Chapter 1 [1.1] D. Bloess, J.B.M. Pattison, G. Plass, D. Rusch, W. Venus and

H.W. Wachsmuth, Determination of the neutrino spectrum in the CERN 1967 Neutrino Experiment, Nucl. instr. Meth. 91 (1971) 605.

[1.2] Th. Eichten, Messung und Kontrolle des CERN Neutrino-experiments durch Registrierung der Begleitmyonen in Halbleiterdetektoren, Thesis, III. Phys. Institut Tech. Hochschule, Aachen, 1974.

[1.3] W.A. Venus and H.W. Wachsmuth, Remarks on neutrino fluxes and their measurement in 200 and 400 GeV neutrino experiments using wide-band and narrow-band beams, CERN internal report TCL/73-2 (1973) .

[1.4] R.D. Ryan, Precision Measurements of the Ionization Energy and its temperature variation in high purity silicon radiation detectors IEEE Trans. Nucl. Sci. NS-20 No. 1 (1973) 473.

[1.5] D. Pines, Elementary Excitations in Solids, chapter 4.3, W.A. Benjamin Inc. New York, 1963.

[1.6] G. Bertolini and A. Coche éd., Semiconductor Detectors, Chap. 1.4, North-Holland Publ. Cy, Amsterdam 1968.

[1.7] A. Alberigi Quaranta, M. Martini and G. Ottaviani, The Pulse shape and the Timing Problem in Solid State Detectors - a review paper, IEEE Trans. Nucl. Sci. NS-16 No. 2 (April 1969) 35.

[1.8] S.M. Sze, Physics of Semiconductor Devices, Wiley-Interscience, Second edition, New York 1981.

[1.9] W. Schottky, Zur Halbleiter Théorie der Sperrschicht und Spitzengleichrichter, Z. Physik 113 (1939) 367.

[1.10] P. Siffert and A. Coche, Rectifying Process in Surface Barrier Detectors, IEEE Trans. Nucl. Sci. NS-11 No. 3 (1964) 244.

[1.11] J.P. ponpon and P. Siffert, Mechanism of Formation of Schottky diodes, Report CRN-CNPA 76/27, Strasbourg, 1976.

[1.12] H.M. Heijne, Solid State Detectors used in the Neutrino Flux Monitoring (NFM), CERN/EF/BEAM 77-1.

[1.13] W.K. Hofker, implantation of Boron in Silicon, Philips Res. Repts. Suppl. 1975, No. 8.

[1.14] J.C. Muller, R. Stuck, R. Berger and P. Siffert, Thermally stimulated current measurements on silicon junctions produced by implantation of low energy boron ions, Solid St. Electron. 17 (1974) 1293.

[1.15] See for example: J.C. Muller, A. Grob, J.J. Grob, R. Stuck and P. Siffert, Laser Beam annealing of Heavily Damaged implanted Layers on Silicon, Report CRN/PN 77-25 Energie, Strasbourg 1977.

[1.16] E. Laegsgaard, position-sensitive Semiconductor Detectors, Nucl. instr. Meth. 162 (1979) 93.

[1.17] E.H.M. Heijne, L. Hubbeling, B.D. Hyams, P. Jarron, P. Lazeyras, F. Piuz, J.C. Vermeulen and A. Wylie, A Silicon Surface Barrier Microstrip Detector designed for High Energy Physics, Nucl. Instr. Meth. 178 (1980) 331.

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REFERENCES - Chapter 1 (Cont'd) [1.18] J. Kemmer, Fabrication of low Noise Silicon Radiation

Detectors by the planar process, Nucl. Instr. Meth. 169 (1980) 499.

[1.19] J. Kemmer, P. Burger, R. Henck and E. Heijne, Performance and applications of passivated ion-implanted silicon detectors, IEEE Trans. Nucl. Sci. NS-29 (February 1982)

[1.20] D.A. Landis, F.S. Goulding, R.H. Pehl and J.T. Walton, Pulsed feedback techniques for semiconductor detector radiation spectrometers, IEEE Trans. Nucl. Sci. NS-18 (Feb. 1971) 115.

[1.21] T.D.S. Hamilton, Handbook of linear integrated electronics for research, London, 1977, Mc. Graw-Hill Ltd.

[1.22] E.H.M. Heijne, P. Jarron, P. Lazeyras, W.R. Nelson and G.R. Stevenson, A tiny telescope of Si-detectors for high energy muon flux measurement with electron rejection, IEEE Trans. Nucl. Sci. NS-27 (February 1980) 272.

[1.23] P. d'Angelo, A. Hrisoho, p. Jarron, P.F. Manfredi and J. Poinsignon, Analysis of low noise, bipolar transistor head amplifier for high energy applications of silicon detectors, Nucl. Instr. Meth. (193) 1982) 533.

[1.24] V. Radeka, Trapezoidal filtering of signals from large germanium detectors at high rates, IEEE Trans. Nucl. Sci. NS-16 (1972) 412.

[1.25] M.J. Turner, E.M. Rhoderick, Metal-Silicon Schottky barriers, Solid St. Electron. 11 (1968) 291.

[1.26] H.F. Wolf, Semiconductors, Wiley-Interscience, New York, 1971.

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REFERENCES - Chapter 2 [2.1] Extensive references can be found in:

R.M. Sternheimer, interaction of radiation with matter, chapter 1.1 in Methods of Experimental Physics, vol. 5A, eds L.C.L. Yuan and C.S. Wu, Academic Press, New York, 1961 and

H. Bichsel, Passage of charged particles through matter, ch. 8d, AIP Handbook, 3rd éd., McGraw-Hill, 1972.

[2.2] e.g. J.D. Jackson, chapter 13, Classical Electrodynamics, John Wiley, New York, 1962.

[2.3] H. Bichsel and R.P. Saxon, Comparison of calculational methods for straggling in thin absorbers, Phys. Rev. All (1975) 1286.

[2.4] C. Kittel, Chapter 8, Introduction to Solid State Physics, 3 rd ed. John Wiley, New York, 1968 and ref. [1.5].

[2.5] Nat, Ac. Sci. - Nat. Res. Council, publication 1133, Studies in penetration of charged particles in matter, chapter 6, Washington, 1964.

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[2.7] A. Crispin and G.N. Fowler, Density effect in the ionization energy loss of fast charged particles in matter, Rev. Mod. Phys. 42 (1970) 290.

[2.8] H. Esbensen et al., Random and channeled energy loss in thin germanium and silicon crystals for positive and negative 2-15 GeV/c pions, kaons, and protons, Phys. Rev. B18 (1978) 1039.

[2.9] B. Rossi, High Energy Particles, Prentice-Hall, Englewood Cliff, 1952.

[2.10] U. Fano, formula (88) in: Penetration of protons, alpha particles and mesons, Ann. Rev. Nucl. Sci. 13 (1963) 1.

[2.11] N. Laulainen and H. Bichsel, Energy removed by delta rays from finite volumes in passage of charged particles, Nucl. Instr. Meth. 104 (1972) 531.

[2.12] H. Raether, Excitation of plasmons and interband transitions by electrons, Springer Tracts in Modern Physics 88 (1980).

[2.13] J. Stiebling and H. Raether, Dispersion of the volume plasmon of silicon (16.7 eV) at large wave vectors, Phys. Rev. Lett. 40 (1978) 1293.

[2.14] C.A. Klein, Radiation ionization energies in semiconductors: speculations about the role of plasmons, Proceedings Int. Conf. Phys. Semiconductors, Kyoto 1966, in J. Phys. Soc. Japan 21 suppl. (1966) 307.

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[2.16] C.H. Chen, J. Silcox and R. Vincent, Electron-energy losses in silicon: Bulk and surface plasmons and Cerenkov radiation, Phys. Rev. B12 (1975) 64.

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REFERENCES - Chapter 2 (Cont'd)

[2.17] e.g. M.J. Berger and S.M. Seltzer, Tables of energy losses and ranges of electrons and positrons, NASA report SP-3012. Idem as chapter 10 in [2.5].

[2.18] L. Landau, On the energy loss of fast particles by ionization, J. Phys. (USSR) 8 (1944) 201.

[2.19] P.V. Vavilov, Ionization losses of high-energy heavy particles, English translation in: Soviet Physics JETP, 5 (1957) 749.

[2.20] W. Bôrsch-Supan, On the evaluation of the function ${\) =

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[2.21] B. Schorr, Programs for the Landau and the Vavilov distributions and the corresponding random numbers, CERN program Libary, long write-ups G110, Gill, Geneva 1974.

[2.22] H.D. Maccabee and D.G. Papworth, Correction to Landau's energy loss formula, Phys. Lett. 30A (1969) 241:

[2.23] J.M. Paul, The rate of energy loss and intrinsic resolution of silicon detectors, Nucl. Instr. Meth. 94 (1971) 275.

[2.24] N. Jarmie, M.S. Pindzola and H. Bichsel, Limits of validity for the Vavilov energy straggling calculation, Comp. Phys. Comm. 13 (1978) 317.

[2.25] G. Knop, A. Minten and B. Nellen, Der Energieverlust von 1 MeV-Elektronen in sehr diinnen Schichten, Z. physik 165 (1961) 533.

[2.26] G.D. Badhwar, Calculation of the Vavilov distribution allowing for electron escape from the absorber, Nucl. Instr. Meth. 109 (1973) 119.

[2.27] J.L. Matthews, D.J.S. Findlay and R.O. Owens, The distribution of electron energy losses in thin absorbers, Nucl. Instr. Meth. 180 (1981) 573.

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[2.29] M.D. Maccabee, M.R. Raju and C.A. Tobias, Fluctuations of energy loss by heavy charged particles in thin absorbers, Phys. Rev. 165 (1968) 469.

[2.30] D.W. Aitken, W.L. Lakin and H.R. Zulliger, Energy loss and straggling in silicon by high-energy electrons, positive pions and protons, Phys. Rev. 179 (1969) 393.

[2.31] F.R. Buskirk, J.N. Dyer, X.K. Maruyama and K.E. Woehler, The measured energy loss of high energy electrons in various metals, Z. Physik 271 (1974) 69.

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[2.34] Yung-Su Tsai, Pair production and bremsstrahlung of charged leptons, Rev. Mod. Phys. 46 (1974) 815.

[2.35] K.S. Kôlbig and B. Schorr, Asymptotic expansions for the Landau density and distribution functions. Int. Report CERN/DD/83-5.

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REFERENCES - Chapter 3

[3.1] W.R. Nelson and K.R. Kase, Muon shielding around high energy electron accelerators, part I, Theory, Nucl. Instr. Meth. 120 (1974) 401;

W.R. Nelson, K. R. Kase and G.K. Svensson, idem part II, Experimental investigation, ibidem 413.

[3.2] R.G. Alsmiller, F.S. Alsmiller, J. Barish and Y. Shima, Muon transport and the shielding of high energy (< 500 GeV) proton accelerators, Proc. of Int. Congress on Protection against accelerator and space radiation, 1971, CERN report 71-16 (1971) 601.

[3.3] D. Theriot, NAL report TM229 (1970).

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[3.5] M. Ladu, M. Pelliccioni, P. Picchi and M. Roccella, Shielding for ultra-relativistic muons, Nucl. Instr. Meth. 104 (1972) 5.

[3.6] R. Hartmann and H. Leutz, A magnetized focusing iron shield against muons in high energy neutrino beams, Nucl. Instr. Meth. 126 (1975) 165.

[3.7] N.V. Mokhov, G.I. Semenova and A.V. Uzunian, Muon production and transport in matter in the energy range 10-105 GeV, Nucl. Inst. Meth. 180 (1981) 469.

[3.8] C. Visser, NUBEAM: neutrino beam simulator, HYDRA application library, long write up, CERN 1979.

[3.9] R.K. Adair and H. Kasha, Cosmic ray muons, ch. 4 in "Muon Physics" ed. V.W. Hughes and C.S. Wu, Ac. Press, Ney York 1977.

[3.10] A.A. Petrukhin and V.V. Shestakov, The influence of the nuclear and atomic form factors on the muon bremsstrahlung cross section, Proc. 10th Int. Conf. Cosmic Rays, 1967 (Can. J. Physics (Suppl.) 46 (1970) S 377.

[3.11] R.P. Kokoulin and A.A. Petrukhin, Analysis of the cross section of direct pair production by fast muons, Proc. 11th Int. Conf. Cosmic Rays, Budapest 1969 (Acta Phys. Hung. 29, Suppl. 4 (1970) 277.

[3.12] R.P. Kokoulin and A.A. Petrukhin, Influence of the nuclear form factor on the cross section of electron pair production by high energy muons, Proc. 12th Int. Conf. Cosmic Rays, Hobart, vol. 6, A2436 (1971) .

[3.13] S.R. Kelner and Yu.D. Kotov, Muon energy loss to pair production, Sov. J. Nucl. Phys. 7 (1968) 237 and: Pair production by muons in the field of nuclei, Can. J. Phys. (Suppl.) 46 (1968) S387.

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[3.17] L.R. Fortney et al., Direct electron-positron pair production by 200 GeV negative pions, Phys. Rev. Lett. 34 (1975) 907.

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[3.18] W. Constandt, W.D. Dau and H. Jokisch, The energy loss by nuclear interactions of cosmic ray muons, Proc. 16th Int. Conf. Cosmic Rays, MN 4 (1979) 233.

[3.19] H. Messel and D.F. Crawford, Electron photon shower distribution function, tables for lead, copper and air absorbers, Pergamon Press, Oxford 1970.

[3.20] R.L. Ford and W.R. Nelson, The EGS code system: computer programs for the Monte-Carlo simulation of electromagnetic cascade showers (version 3), SLAC report 210, Stanford 1978.

[3.21] D. Adler, B. Fuchs and K.O. Thielheim, Longitudinal development of electromagnetic cascades in lead, iron, copper and air, At. Nucl. Data Tables 20 (1977) 513.

[3.22] T. Tabata, R. I to, S. Okabe and Y. Fujita, Charge distribution produced by 4 to 24 MeV electrons in elemental materials, Phys. Rev. B3 (1971) 572.

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[3.24] P.J. Ebert, A.F. Lauzon and E.M. Lent, Transmission and backscattering of 4.0 to 12.0 MeV electrons, Phys. Rev 183 (1969) 422.

[3.25] D. Adler, B. Fuchs and K.O. Thielheim, Numerical integration of electromagnetic cascade equations, discussion of results for air, copper, iron, and lead, Nucl. Instr. Meth. 146 (1977) 601.

[3.26] G. Bassompierre, Développement de gerbes induites par des muons de haute énergie, Note interne CERN 1973.

[3.27] S. Wittig, Eine Universelle Monte-Carlo Méthode zur Lôsung von elektronen Transport Problemen, institut fiïr Kernphysik, Johann Wolfgang Goethe-Universitat, Frankfurt am Main, 1968, Report IKF 20.

[3.28] E. Heijne, P. Jarron, T. Jenkins, W.R. Nelson and H. Ing, Identification of GeV electrons via particle multiplicity in a silicon detector: measurement and EGS simulation, Nucl. instr. Meth. 205 (1983) 437.

[3.29] W.R. Nelson, G.R. Stevenson, E.H.M. Heijne, P. Jarron, Liu Kuei-Lin, M. Nielsen, Preliminary profile measurements of high energy muon beams in a soil shield, HS divisional report HS-RP/042, CERN 1979.

[3.30] R.D. Evans, The Atomic Nucleus, Chapter 28, McGraw Hill, New York, 1955.

[3.31] E.H.M. Heijne, Influence of muon induced secondary radiation on the muon flux measurement in the CERN neutrino beams, CERN/EF/BEAM 79-4.

[3.32] N. Chaudhuri, Direct electron pair production by muons in the energy-transfer range 3 MeV-10 GeV, 17th Int. Cosmic Ray Conf. MN 4.10, p. 78, Paris 1981.

- 255 -

REFERENCES - Chapter 4 [4.1] G. Cavalleri, E. Gatti, G. Fabri and V. Svelto, Extension of

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[4.2] S. Ramo, Currents induced by electron motion, Pro. IRE 27 (1939) 584.

[4.3] R.N. Williams and E.M. Lawson, Formation of current pulses in semiconductor nuclear radiation detectors, Nucl. Instr. Meth. 113 (1973) 597.

[4.4] A. Alberigi Quaranta, M. Martini, G. Ottaviani, G. Redaelli and G. Zanarini, Experimental results on the drift velocity of hot carriers in silicon and associated anisotropic effects, Solid-St. Electron. 11 (1968) 685.

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[4.6] R.N. Williams and E.M. Lawson, The plasma effect in silicon semiconductor radiation detectors, Nucl. Instr. Meth. 120 (1974) 261.

[4.7] E. Elad, C.N. Inskeep, R.A. Sareen and P. Nestor, Dead layers in charged-particle detectors, IEEE Trans. Nucl. Sci. NS-20 (February 1973) 534.

[4.8] H.C. Britt and H.E. Wegner, Response of semiconductor detectors to fission fragments, Rev. Sci. Instr. 34 (1963) 274.

[4.9] H.W. Schmitt, W.M. Gibson, J.H. Neiler, F.J. Walter and T.D. Thomas, Absolute energy calibration of solid state detectors for fission fragments and heavy ions, Proc. Symp. Physics and Chemistry of fission, vol. I, IAEA, Vienna, 1965, p. 531.

[4.10] J. Lindhard, V. Nielsen and M. Scharff, Approximation method in classical scattering by screened Coulomb fields (Notes on atomic collisions I ) , Kgl. Dan. Vid. Selsk. Mat. Fys. Medd. 36 (1968) No. 10.

[4.11] E.C. Finch, An analysis of the causes of the pulse height defect and its mass dependence for heavy-ion silicon detectors, Nucl. Instr. Meth. 113 (1973) 41.

[4.12] D.W. Potter and R.D. Campbell, Pulse height defect and window energy loss of low energy ions in surface barrier detectors, Nucl. Instr. Meth. 153 (1978) 525.

[4.13] E.C. Finch and A.L. Rodgers, Measurements of the pulse height defect and its mass dependence for heavy-ion silicon detectors, Nucl. instr. Meth. 113 (1973) 29.

[4.14] G.L. Miller and W.M. Gibson, Charge Collection in semiconductor radiation detectors, Nuclear Electronics, vol. 1, Belgrade Conference 1961, IAEA Vienna 1962 (p. 477).

[4.15] H.C. Britt and H.E. Wegner, Multiplication phenomenon for fission fragment response in semiconductor detectors, Rev. Sci. Instr. 34 (1963) 627.

[4.16] H.W. Schmitt and Frances Pleasonton, Evaluation of semiconductor detectors for fission fragment energy measurements, Nucl. Instr. Meth. 40 (1966) 204.

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REFERENCES - Chapter 4 (Cont'd) [4.17] F.J. Walter, Multiplication in the fission fragment pulse height

response of silicon surface barriers, IEEE Trans. Nucl. Sci. NS-11 (1964) 232.

[4.18] H.M. Heijne, E. Belcarz, J.C. Muller and P. Siffert, Charge Multiplication in silicon radiation detectors under dense irradiation, IEEE Trans. Nucl. Sci. NS-25 (1978) 378.

[4.19] E.D. Klema, J.X. Saladin, J.G. Alessi and H.W. Schmitt, Energy resolution of silicon surface-barrier detectors for alpha particles, oxygen ions, and fission fragments, Nucl. Instr. Meth. 178 (1980) 383.

[4.20] E. Belcarz, private communication. [4.21] H.W. Kraner, R. Beuttenmuller, T. Ludlam, A.L. Hanson, K.W. Jones,

V. Radeka and E.H.M. Heijne, Charge collection in silicon strip detectors, IEEE Trans. Nucl. Sci. NS-30 (1983) 405.

[4.22] S. Kirkpatrick, Modeling diffusion and collection of charge from ionizing radiation in silicon devices, IEEE Trans. El. Dev. ED-26 (1979) 1742.

[4.23] A.B. Campbell, A.R. Knudson, Charge collection measurements for energetic ions in silicon, IEEE Trans. Nucl. Sci. NS-29 (1982) 2067.

[4.24] F.B. McLean and T.R. Oldham, Charge funneling in n- and p-type Si substrates, IEEE Trans. Nucl. Sci NS-29 (1982) 2018.

[4.25] A. Hemmendinger, M.G. Silbert and A. Moat, Transient response of solid state detectors, IEEE Trans. Nucl. Sci. NS-12 (1965) 304.

[4.26] W.L. Lakin, A surface-barrier beam monitor, Nucl. instr. Meth. 102 (1972) 367.

[4.27] C. Canali, G. Ottaviani, A. Taroni and G. Zanarini, Experimental results on transient space charge limited currents in p-n junctions, Solid-St. Electron. 14 (1971) 661.

[4.28] P.A. Tove and L.G. Andersson, Transient space charge limited currents in light-pulse excited silicon, Solid-St. Electron. 16 (1973) 961.

[4.29] M.A. Green and J. Shewchun, Current multiplication in metal-insulator-semiconductor (MIS) tunnel diodes, Solid-St. Electron. 17 (1974) 349.

[4.30] M.A. Green, V.A.K. Temple, J. Shewchun, Frequency response of the current multiplication process in MIS tunnel diodes, Solid-St. Electron 18 (1975) 745.

[4.31] M.H. Woods, W.C. Johnson and M.A. Lampert, Use of a Schottky barrier to measure impact ionization coefficients in semiconductors, Solid-St. Electron. 16 (1973) 381.

- 257 -

REFERENCES - Chapter 5

[5.1] G.D. Watkins, A review of ESR studies in irradiated silicon, p. 97 in: Proceedings 3 of 7th Int. Conf. on Phys. of Semiconductors, Radiation Damage in Semiconductors, Dunod, Paris 1965.

[5.2] V.A.J, van Lint, T.M. Flanagan, R.E. Leadon, J.A. Naber and V.C. Rogers, Mechanisms of radiation effects in electronic materials, vol. I, Wiley-Inter science, New York 1980.

[5.3] ORTEC Inc., Instruction manual surface barrier detectors, ORTEC, Oak Ridge.

[5.4] J.W. Corbett, Electron radiation damage in semiconductors and metals, supplement 7 of the series "solid state physics", Ed. F. Seitz and D. Turnbull, Academic Press, New York 1966.

[5.5] D.V. Lang, Deep-level transient spectroscopy: A new method to characterize traps in semiconductors, J. Appl. Phys. 45 (1974) 3023.

[5.6] C.T. Sah, Bulk and interface imperfections in semiconductors, Solid-St. Electron. 19 (1976) 975.

[5.7] C.T. Sah, Detection of defect and impurity centers in semiconductors by steady-state and transient junction capacitance and current techniques, p. 868 in "Semiconductor Silicon 1977" Ed. H.R. Huff and E. Sirtl, The Electrochemical Society, Conf. Proceedings, vol. 77-2, Princeton, 1977.

[5.8] A.G. Milnes, Deep impurities in semiconductors, Wiley-Interscience, New York, 1973.

[5.9] J.C. Muller, Etude des défauts introduits par implantation d'ions bore dans le silicium, thesis, CRN/PN 76-2, Centre de Recherches Nucléaires, Strasbourg 1976.

[5.10] H.M. Heijne, J.C. Muller and P. Siffert, TSC defect level in silicon produced by irradiation with muons of GeV energy, Rad. Effects 29 (1976) 25;

An abstract of a poster presentation has been published in: J.C. Muller, P. Siffert and H.M. Heijne, Defects introduced in silicon by irradiation with muons of GeV energy, radiation defects in semiconductors 1976, p. 505, the Institute of Physics, Conference series number 31, Bristol 1977.

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[5.13] A.H. Kalma and J.C. Corelli, Photoconductivity studies of defects in silicon: divacancy-associaed energy levels, Phys. Rev. 173 (1968) 734.

[5.14] M. Hôfert and F. Coninckx, Mesures de fluence et de dose dans le blindage du faisceau neutrino (e8) au moyen de différents détecteurs à activation et de dosimètres, CERN Int. rep. HS-RP/IR/76-06 (1976).

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[5.15] Compilation of radiation damage test data, parts I, II, III, H. Schonbacher, A. Stolarz-Izycka, P. Beynel and P. Maier, CERN Yellow reports 79-04, 79-08 and 82-10.

[5.16] G. Backenstoss, E. Braunersreuther and K. Goebel, Radiation damage of semiconductors by protons, CERN yellow report 62-5, Genève 1962.

[5.17] E. Heijne, Radiation damage: experience with silicon detectors in high energy particle beams at CERN, Proc. Miniaturization of Detectors, Pisa Conference 1980, Ed. A. Stefanini, Plenum Press, London 1983.

[5.18] B.E. Deal, Charge effects and other properties of the Si-Si02

interface: the current understanding, p. 276 in "Semiconductor Silicon 1977", Ed. H.R. Huff and E. Sirtl, The Electrochem. Soc. Conf. Proceedings, vol. 77-2, Princeton, 1977.

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[5.20] E. Schibli, Deep level effects on the small signal capacitance of p-n junctions, Solid-St. Electron. 13 (1970) 392.

- 259 -

REFERENCES - Chapter 6 [6.1] CERN SPS Experiments Handbook, Ed. M. Reinharz, Second

edition, May 1981, Geneva. [6.2] W. Kalbreier, W.C. Middelkoop and P. Sievers, External targets

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[6.4] W. Kalbreier, A. Knezovic, P. Sievers and A. Warman, Performance Test of the Target Station T9, SPS Commissioning Report No. 62, SPS/ABT/PS/fv, Geneva 1977.

[6.5] A. Chapman-Hatchett, G. Cultrut, J. Dieperink, A. Fasso, W. Kalbreier, A. Muller, S. Peraire, M. Nielsen, A. Regelbrugge, G.R. Stevenson and D. Thomas, Calibration of the Secondary Emission Monitors TBIU and TBID of the North area targets Stations T2, T4 and T6 in TCC2 for Slow Extracted Protons of 400 GeV, CERN Int. Report SPS/ABT/Int. 79-1.

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muon flux measurements in the narrow band neutrino beam, CERN internal report CERN/NBU 81-7;

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[6.14] S. van der Meer, A Directive Device for Charged Particles and its Use in an Enhanced Neutrino beam, CERN Yellow Report 61-7 (1961) .

[6.15] A. Ball, program RHINO. See also: J.M. Flynn, Magnetic horn focusing for a low-energy neutrino beam, Int. rep. CERN/EF/BEAM 81-5.

[6.16] A.L. Grant, West Area Neutrino Facility-shielding Studies, Int. Rep. CERN/EF/BEAM 78-2.

[6.17] W. Kalbreier, Beam optics, beam instrumentation, and the vacuum system of the primary proton beam for the new proton beam dump experiment for prompt neutrino production, Internal note SPS/ABT/TR/Note Techn./80-2.

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REFERENCES - Chapter 7 [7.1] G. Cavallari, H. Heijne, P. Jarron, P. Lazeyras and M. Musso,

Solid state detectors used for the CERN neutrino flux monitoring (NFM), IEEE Trans. Nucl. Sci. NS-25 No. 1 (1978) 600.

[7.2] A.P. Bugorsky, V.N. Goryachev, V.I. Kochetkov, V.A. Krendelev, V.I. Kurbakov, A.I. Mukhin, V.I. Poletaev, V.G. Rybakav, V.N. Rychenkov, Yu. M. Sviridov, V. Ya. Uglekov, A.A. Volkov and A.S. Vovenko, Muon flux measuring system for neutrino experiments at the IHEP accelerator, Nucl. Instr. Meth. 146 (1977) 367.

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[7.4] G. Cavallari, NFM tape formats, Int. Report, latest revision 23.04.79.

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REFERENCES - Chapter 8 [8.1] Commision of the European Communities, "CAMAC, a modular

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[8.5] G. Cavallari, The "NFM User Guide" and the "NFM Mini Guide".

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REFERENCES - Chapter 9 [9.1] E. Heijne, Evaluation of a new calibration box support in

pit VI, Internal Note, 28 May 1980. [9.2] K.D. Sevier, Emulsion as a detector of individual electrons

for use in beta-spectroscopy, Nucl. Instr. Meth. 22 (1963) 345.

[9.3] H. Burmeister, H. Heijne, P. Iaselli, H. Lettenstr^m, S. Natali, J.B.M. Pattison, W. Venus and H. Wachsmuth, Present status of the absolute calibration of the muon detectors in the wideband neutrino beam using nuclear emulsions, CERN Int. Rep. CERN/EP/NBU 79-9, December 1979.

[9.4] C. Penfold and H. Wachsmuth, Emulsion calibration of the solid state detectors in the narrow band beam, CERN Int. Report CERN/EP/NBU 81-4, September 1981.

[9.5] B. Jongejans, A. Marzari and C. Visser, The calibration of a wideband (anti)neutrino beam at CERN with emulsion plates, CERN Int. Report CERN/EP/NBU 81-2.

[9.6] B. Jongejans and H. Wachsmuth, Re-analysis of an absolute calibration of muon flux detectors in the wideband neutrino beam (emulsion V4.00 from July 1979), CERN Int. Rep. CERN/EP/NBU 81-5.

[9.7] H. Wachsmuth, Narrowband beam flux calibration, CERN Int. Rep. CERN/EP/NBU 79-2.

[9.8] P. Bosetti et al., (ABÇLOS Collaboration), Total cross sections for v y and v„ charged current interactions between 20 and 200 GeV, Phys. Lett. HOB (1982) 167.

[9.9] F. Bergsma, Muon flux measurements in beam dump experiment, Internal note CHARM Collaboration, 1983.

REFERENCES - Chapter 10 [10.1] H.J. Klein, J. Zoll, Reading and writing Mini-computer Data Tapes,

HYDRA Library, FQM, CERN 1977. [10.2] W. Venus and H. Wachsmuth, Neutrino fluxes from muon fluxes, CERN

Int. Report CERN/EP/NBU 79-1. [10.3] K.L. Brown and Ch. Iselin, DECAY TURTLE (Trace Unlimited Rays

Through Lumped Elements), CERN Report 74-2 (1974). [10.4] E.H.M. Heijne, What happens to the measured muon flux in the

wideband neutrino beam, if the beam is off-axis, CERN Int. Report CERN/EP/NBU 80-1 (1980).

REFERENCE - Chapter 11 [11.1] R. Kopp, private communication.

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