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Pakistan Institute of Engineering & Applied Sciences (PIEAS)
Dr. Nasir M Mirza
Deputy Chief Scientist,
Department of Physics & Applied mathematics,
PIEAS, P.O. Nilore, 45650, Islamabad.
Email: [email protected]
Ph: +92 51 9290273 (ext: 3059)
Lectures onRadiation Detection
Delivered in Professional Training Course onRadiation Safety & RWM at PNRA (April, 2008)
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Gamma ray spectroscopy
Recommended Text Books
1. Glenn F Knoll sRadiation Detection & Measurement (recentedition).
Lecture 8:
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Spectroscopy with Scintillators
Spectroscopy:
Measurement of (radiation ) energy
Gamma-ray spectroscopy with solid portable mediumwas 1st time done with Na1(Tl) detector in 1950s
Na1(tl) detector characteristics
Extremely good light yield
Excellent linearity
High atomic number of iodine; high efficiency forcounting
They have good energy resolution
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General Considerations in -Ray Spectroscopy
Gamma rays interactions
There in general no direct ionization
Interact first with electrons of atoms
Transfer full or partial energy in absorbing (Detector) Medium
Gamma-rays generate fast electrons
Electrons slow down and loose their energy
This results in Excitation and Ionization of medium atoms
A detector to serve as a gamma -ray spectrometer must carry out two
distinct functions
1st it must act as a conversion medium in which ray could interactand produce electrons
2ndit must function as a detector for the electrons
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General Considerations in -Ray Spectroscopy (Contd.)
Assume that detector is sufficiently large (of the
order of centimeters) to avoid the escape of
secondary electrons and bremsstrahlung;
Low stopping power of gases and the requirementof full energy absorption for the secondary
electrons generally rules out gas filled detectors
for -Ray spectroscopy
Remember at STP, the penetration distance of 1MeV
electron is of the order of several meters
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Gamma Ray Interactions
Significant ways of gamma-ray interaction are:
Photoelectric absorption
This process is dominant at low energy
Compton Scattering
Predominant for low energy
Pair Production
Probable for high energies (5-10 MeV)
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Photoelectric Absorption
Photoelectric Effect
The interaction in which ray photon disappearsand a photoelectron is produced with energy
Energy utilizationElectrons kinetic energy;
Recoil atomic energy is negligible;
X-rays or Auger electron energy is alsoproduced (Auger electrons may escape or X-rays may reabsorb in the medium to cause PEinteraction with less tightly bound electron ofabsorber atom )
Bremsstrahlung emission.
beEhE
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Photoelectric Absorption (Contd.)
Photoelectric effect is therefore an ideal process to measure the
original (complete) energy of gamma ray.
Detector Response
If nothing escape from the detector than response of thedetector represents a single peak of gamma ray in the energy
spectrum.
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Compton Scattering
Compton Scattering Process
Gamma interaction results in
a recoil electron and a
scattering -ray photon
Energy is divided b/w them
depending on the scattering
angle;
)cos(cm
hv
hvvh
o
112
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Compton Scattering (Contd.)
K.E. of recoil electron:
Two extreme cases:
2
2
(1 cos )
1 (1 cos )
o
e
o
hv
m cE h hv
hv
m c
0 and 0e
h h E
2
2 2
2
( ) and ( )2 2
1 1
oe
o o
hv
m chvhv E
hv hv
m c m c
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Compton Scattering (Contd.)
All scattering angles are possible in a detector so a continuum
of energies can be transferred to electrons from 0 to maximumat =
The gap energy is given by:
2
2
2
( )2
1
2
0.256 MeV2
c e
o
o
o
c
hvE h E
hv
m c
h m c
m cE
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Compton Scattering (Contd.)
For anyone specific gamma-ray energy, the electron energy
distribution has the general shape shown in the sketch below.
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Pair Production
In this process, incident photon is completely disappeared,
create an electron-positron pair
Energy of electron positron pair:
A problem
Positron slow down after depositing its energy andannihilate to give two photons of 0.511 MeV each
These annihilation radiation appears in virtual coincidencewith the original pair production interaction (as time to slowdown and annihilate for a positron is very small)
22 cmhEE
oee
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Pair Production (Contd.)
A plot of the total (electron + positron) charged particle kinetic
energy created by the incident gamma ray is again a simple deltafunction, but it is now located 2moc
2below the incident gamma-ray
energy, as illustrated in the sketch below.
In our simple model, this amount of energy will be deposited each time
a pair production interaction occurs within the detector.
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Predicted Response Functions
Depends upon the size of the detector so we broadly
classify it into three classes
Small Detector
Large Detector
Intermediate Detector
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Predicted Response Function for Small Detectors
Small detector means size of detector is small as
compared with the mean free path of the secondaryradiations produced in interaction of the original gammarays
Secondary radiations mean Compton scattered gamma
rays and annihilation photons
Mean Free Path of secondary rays is typically of theorder of several centimeters, so small detector meansthat the detector dimensions are less than 1 or 2 cm
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Predicted Response Function for Small Detectors (Contd.)
Now there can be two possibilities:
E < 1.02 MeV
E > 1.02 MeV
CASE 1: E < 1.02 MeV
For energy, E below the value at which pair production issignificant.
Spectrum contains Compton scattering and photoelectric
absorption
The ratio of area under photo peak to the area under Comptoncontinuum is the same as the ratio of the photoelectric crosssection to the Compton cross section in the detector material
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Predicted Response Function for Small Detectors (Contd.)
Figure 10.2 The "small detector" extreme in gamma-ray spectroscopy.
The processes of photoelectric absorption and single Compton scattering
give rise to the low-energy spectrum at the left when E < 1.02 MeV.
MeV1.02: E
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Predicted Response Function for Small Detectors (Contd.)
CASE 2: E > 1.02MeV
For small detectors, only the electron and positron kinetic
energies deposited
Annihilation radiation escapes
Result of annihilation radiation escape is double escape peak
located at an energy 1.02 MeV below the photo peak
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Predicted Response Function for Small Detectors (Contd.)
Figure 10.2 The "small detector" extreme in gamma-ray spectroscopy.
The processes of photoelectric absorption and single Compton scattering give rise
to the low-energy spectrum at the left. At higher energies, the pair production
process adds a double escape peak shown in the spectrum at the right.
CASE 2: E > 1.02MeV
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Large Detectors
Detector dimensions are sufficiently large so that all secondary
radiation also interact within the detector;
none escapes from the surface; all energy is deposited within the
detector;
For typical gamma rays energies, to fulfill the requirement of
depositing all the energy in the detector requires detector
dimensions to be of the order oftens of centimeters (unrealistically
large for practical cases)
Predicted Response Funct ion
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Response Function for Very Large Detectors (Contd.)
The detector response is the same as
if the original gamma-ray photon hadundergone a simple photoelectric
absorption in a single step.
The name Full energy peak is used here
for photo-peak as it covers the
contributions of all other events as well.
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Predicted Response Function for Large Detectors (Contd.)
The detector response is the same as if the original gamma
ray photon had undergone a simple photo electric absorption
in a single step
The assumption of single step is being used at the time
involved in the whole process of depositing total energy is
very small as compared to inherent response time of
virtually all practical detectors used in gamma ray
spectroscopy;
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Intermediate Size Detectors
Predicted Response Function
Real detectors of the sizes in common use for gamma ray
spectroscopy are neither small nor large by the standards
discussed before
So the response of real detectors will be mixture of features
discussed in two previous cases plus some additional
features
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Predicted Response Function for Intermediate Size Detectors (Contd.)
For the case of E
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Predicted Response Function for Intermediate Size Detectors (Contd.)
E >> 2moc2
In addition to things discussed for E
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Predicted Response Function for Intermediate Size Detectors (Contd.)
Figure 10.4 The case of intermediate detector size in gamma-ray spectroscopy. In
addition to the continuum from single Compton scattering and the full-energy
peak, the spectrum at the left shows the influence of multiple Compton events
followed by photon escape. The full-energy peak also contains some histories that
began with Compton scattering.
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Predicted Response Function for Intermediate Size Detectors (Contd.)
Figure 10.4 The case of intermediate detector size in gamma-ray spectroscopy. At the
right, the single escape peak corresponds to initial pair production interactions in which
only one annihilation photon leaves the detector without further interaction. A double
escape peak as illustrated in Fig. 10.2 will also be present due to those pair production
events in which both annihilation photons escape.
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Complications in Response Functions
Secondary Electron Escape
Bremsstrahlung Escape
Characteristic X-ray Escape
Secondary Radiations Created Near the Source
Effect of Surrounding Material
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Secondary Electron Escape
Secondary electrons (electrons from PE, CS, PP)
may escape the detector without full deposition ofenergy
Response function is distorted by
Moving events to lower amplitude
Height of Compton continuum is reduced
Photo-fraction will be reduced
The ratio of the area under photo-peak (or full energypeak) to that under the entire response function is calledphoto-fraction
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Bremsstrahlung Escape
Slowing of secondary electron, production of
bremsstrahlung
Process is dominant for high -ray energies and high
Zmaterials
Electron producing bremsstrahlung may stop in medium butsome of bremsstrahlung may escape
The escape of bremsstrahlung leads to
Change in shape of response function but additional peaks orsharp features are not introduced
Similar as secondary electron escape
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Characteristic X-Ray Escape
Characteristic X-ray Escape
Photoelectric absorption
Vacancy in atom shell
Characteristics x-ray
Photoelectric effect near detector boundary Characteristic X-Ray may escape
Energy deposited = h - Ex
X-Ray Escape peaks most prominent at
At low incident -ray energies
For detectors whose S/V ratio is large
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Secondary Radiations Created Near the Source
Major contributors to secondary radiations created near the
source are
Annihilation Radiation
Bremsstrahlung
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Secondary Radiations Created Near the Source (Contd.)
Annihilation radiation
Source consists of isotope emitting positron
Annihilation peak observed at 0.511 MeV energy
superimposed an the -ray spectrum
For detector geometries in which it is possible to detect bothannihilation photons from a single decay simultaneously
(Example: Well Counter), then a peak at 1.022 MeV may
also be observed in the recorded spectrum
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Secondary Radiations Created Near the Source (Contd.)
Bremsstrahlung
Produced from -emitting sources
Some external absorber used to stop beta particles to reachdetector
Bremsstrahlung produced in such absorbers can complicate the
spectrum
No peaks are formed as bremsstrahlung in continuous form butcant be subtracted simply as back ground so errors inquantitative measurement
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Effects of Surrounding Material
Various materials surrounding detector give rise to the secondary
radiations Detector Encapsulation (a barrier against moisture and light)
Detector Shield (for reduction of natural background)
Source Encapsulation
Secondary radiation
Back scattered gamma rays
X-rays
Photoelectric Absorption in Material
Require Graded Shield
Annihilation peak
Pair Production in Materials
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Effects of Surrounding Material (Contd.)
Figure 10.6 Influence of surrounding
materials on detector response. In additionto the expected spectrum (shown as a
dashed line), the representative histories
shown at the top lead to the indicated
corresponding features in the response
function.
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Spectrum from a 86Rb Source
Figure lO.U Pulse
height spectrarecorded from
NaI(TI) scintillation
detectors. (a) A
spectrum for a 86Rb
source (1.08 MeV
gamma rays)showing the
contribution at the
lower end of the
scale from
bremsstrahlung
generated in
stopping the beta
particles emitted by
the source.
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Spectrum from a 60Co Source
(b) Spectrumfrom a 60Cosource (1.17and 1.33 MeVgamma raysemitted incoincidence)
taken underconditions inwhich the solidangle subtendedby the detectoris relatively
large. enhancingthe intensity ofthe sum peak at2.50 MeV.
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Spectrum from a 24Na Source
Figure 10.13 Pulse height spectrum from a NaI(Tl) scintillator for gamma raysemitted by 24Na at 1369 and 2754 keV. The single and double escape peakscorresponding to pair production interactions of the higher energy gamma raysare very apparent, as is the annihilation radiation peak at 511 keV due to pairproduction interactions in surrounding materials.
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Resolution of NaI(tl) based System
In many practical cases, the
statistical broadening of thepeak predominates over otherpotential sources of resolutionloss.
In that event, the variation ofthe resolution with gamma-rayenergy can be predicted simplyby noting that the FWHM of thepeak is proportional to thesquare root of the gamma-rayphoton energy.
The average pulse heightproduced is directly proportionalto the gamma-ray energy.Therefore, from the definition ofenergy resolution:
Therefore, a plot of InR versus E
should be a straight line with slope
of .
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Resolution
Figure 10.17
Experimentally measured
resolutionR from a
NaI(Tl) scintillation
detector for various
gamma-ray energiesE.
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Peak Efficiency of Detector
Figure: Intrinsic
peak efficiencies
for BGO and
NaI(Tl)
scintillators of
equal 38 x 38 mm
size. Radioisotope
sources used for
various photon
energies are
indicated.
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