Spectrum if all energy is captured in detector. Allows identification of gamma energy.

Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh).

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Spectrum if all energy is captured in detector.Allows identification of gamma energy.

• Large Detector (in depth?!).• Photo Peak or Full Energy Peak…?• Doppler.• Cost.• Crystal growth.

(with atoms, why??)

Photoelectric AbsorptionPhotoelectric Absorption.

Principles of Spectrometry

Auger


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• Spectroscopy, energy deposition.• Consider what might escape.• Size and material of detector.• Shield lining.

Considerations for shields and detectors?shields and detectors?



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HPGe

HW 15HW 15


• “Probability” for photoelectric absorption:

• Strong Z dependence.• Considerations for shields and detectorsshields and detectors.


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Practical

Theoretical



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Efficiency considerations.Efficiency considerations.



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NaI or BGO

HPGe

Efficiency vs. ResolutionEfficiency vs. Resolution



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Considerations for shields and detectors?

shields and detectors?



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cos11 20

'

cmhh

h

20

21cmh

hEc

cos11

cos1

20

20'

cmhcmh

hhhEe

In the figure:• Photoelectric suppressed.• Single Compton (effect of crystal dimensions).• Below 1.022 MeV.

Compton ScatteringCompton Scattering.



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Reproduce the graph.

cos11 20

'

cmhh

h



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• Compton scattering is predominant for energies typical of radioisotope sources.

• “Probability” per atom increases linearly with Z (Why).• Electron density.• Effect of binding energy.• Effect of polarized photons.• Compton fraction. Klein-Nishina formulaKlein-Nishina formula

Let the recoil electron be emitted at an angle , calculate cot() as a function of tan(/2).

HW 16HW 16

20cm

h



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Note tendency for forward Note tendency for forward scattering at high scattering at high

energies.energies.

Considerations for shields and detectors? Sources vs. accelerators???shields and detectors? Sources vs. accelerators???

HW 17HW 17 What is Thomson scattering.What is Thomson scattering.



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If you need to measure gamma ray yield from nuclear reactions (using particle accelerators), the photon energies could be as high as 10 MeV. What is the typical Ge detector to be used?

Prepared with Monte Carlo simulations.



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Single peak if electron and positron “kinetic” energies are captured by the

detector. But annihilation!2

02 cmhEEee

Rest mass of electron and positron. Gamma must have 1.022 MeV

minimum energy for PP to occur.

Two annihilation photons are then created when positron recombines with an electron. These photons may or may not be captured, causing single and double escape peaks in the spectrum (effect of crystal dimensions).

Pair ProductionPair Production.



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Low-energy gamma.Low-energy gamma. High-energy gamma.High-energy gamma.



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Probability of pair production.

Dependence on Z?

Recoil nucleus,Why?Why?

Effect of Effect of screening?screening?

HW 18HW 18



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Importance in Linacs?


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Very large detector. All secondary radiation, including Compton scattered gamma rays, Bremsstrahlung, x-rays and annihilation photons, are captured in detector volume

Large photofraction (fraction of full energy events) is desired in any detector



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Very small detectors. All secondary radiation, including Compton scattered gamma rays and annihilation photons, are not captured in detector volume (escaping).



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A typical detector.



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Effect of surrounding material. Detectors are normally shielded to minimize as much as possible the counting of ambient background radiation.

X-rays and

lining.


Spectrum if all energy is captured in detector. Allows identification of gamma energy.

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