Lec 8 Spectroscopy

7/28/2019 Lec 8 Spectroscopy

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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)
mailto:[email protected]:[email protected]


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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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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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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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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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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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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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E >> 2moc2

In addition to things discussed for E


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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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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.

Lec 8 Spectroscopy

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