Radiation Interaction - Detection

Radiation Detection and Measurement, JU, 1st Semester, 2008-2009 (Saed Dababneh).

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Radiation Interaction - Detection• Radiation detection requires that radiation interacts with detector material.• Energy deposition (not all detectors…!).• Charged particles continuously transfer their energy to the medium.http://www.srim.org

• Gammas and X-rays interact (if they do) catastrophically.http://physics.nist.gov/PhysRefData/Xcom/Text/XCOM.html

• Photon interactions produce secondary electrons.• Neutron interactions produce secondary heavy charged particles.

I0 I Usually corresponds to net area under the full energy peak.

If not, then

build-up.Good or

bad geometry.

and shielding!!

Collimation.

Could be secondary gamma, but for shielding.

http://www.srim.org/

http://physics.nist.gov/PhysRefData/Xcom/Text/XCOM.html


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Charged particles.

Gamma.

Radiation Interaction - Detection


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• Partial or complete transfer of photon energy to electrons.• Many types of interactions, but three (or four) are important.

Interaction of gammasInteraction of gammas

• Attenuation Attenuation coefficient and coefficient and

mass attenuation mass attenuation coefficient.coefficient.

• Compare to Compare to macroscopic cross macroscopic cross

section section for for neutrons.neutrons.

What is coherent scattering.What is coherent scattering.

xeII 0

HW 11HW 11


Buildup Factor


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


• Large Detector.• Photo Peak or Full Energy Peak…?• Doppler.• Cost.• Crystal growth.

(with atoms, why??):Photoelectric AbsorptionPhotoelectric Absorption.


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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 12HW 12


• “Probability” for photoelectric absorption

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


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Practical Theoretical



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Radiation Interaction - DetectionEfficiency considerations.Efficiency considerations.


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Radiation Interaction - DetectionNaI 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

'

cmh

hh

20

21cm

hh

Ec

cos11

cos1

20

20'

cmhcm

h

hhhEe

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

Radiation Interaction - DetectionCompton ScatteringCompton Scattering.


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Reproduce the graph.


cos11 20

'

cmh

hh


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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 13HW 13

20cm

h



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Note tendency for Note tendency for forward scattering forward scattering at high energies.at high energies.

Considerations for shields and detectors?shields and detectors?

HW 14HW 14 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.

202 cmhEE

ee

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. This photon may or may not be captured, causing single and double escape peaks in the spectrum (effect of crystal dimensions).

Radiation Interaction - DetectionPair 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.

Recoil nucleus,Why?Why?

Effect of Effect of screening?screening?

HW 15HW 15



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HPGeHPGe



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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.


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• Peak area determination.• Peak-to-total ratio.• Peak-to-Compton ratio.• Relative efficiency.• Absolute efficiency.• Total efficiency.• Solid angle.• Energy Resolution (effect of number of charge carriers).• Dead time.• Resolving time.• Anti-coincidences.• Coincidences.• Summation effects.• True coincidences.• Random coincidences.

• Attenuation coefficient.• Mass attenuation coefficient.• Mean-free path.



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Compton suppression.“Anti-coincidences”.

What if cascades are present?Coincidences.

Radiation Interaction - DetectionSelect events.


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Cascade Cascade selectionselection


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Compton spectrometerCompton spectrometer



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Linear attenuation coefficientLinear attenuation coefficient(photoelectric) + (Compton) + (pair production)

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0

0

dxe

dxxe

x

x

Mean free path

Mass attenuation coefficient Mass attenuation coefficient is independent of density and physical state.

Compounds and mixtures??Compounds and mixtures??Give examples.Give examples.Refer to XCOM.Refer to XCOM.

tt eIeII )/(00

Mass thicknessMass thickness


HW 16HW 16

THINKING QUIZTHINKING QUIZProbability of interaction or of

no-interaction?


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Self-AbsorptionSelf-Absorption

Photon AttenuationPhoton Attenuation

Valid for parallelphoton paths

or far geometries.


HW 17HW 17

t

eII

t

10

Build-upteII 0

teEtBII

),(0

Solutions to buildup problem?Solutions to buildup problem?

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Not realistic…!!Not realistic…!!


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Isotropic PhotonEmission


Solutions to self Solutions to self absorption at close absorption at close

geometries?geometries?

t

eII

t

10

MCMC


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Geometric correction factorGeometric correction factor

PointSource

Detector

ExtendedSource

Detector

THINKING QUIZTHINKING QUIZ


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N

Neutron Interaction

• Secondary radiation is different from that for gammas, it is mainly heavy charged particles (or gammas).• Utilize them for neutron detection (Chapters 14 and 15).• Slow neutrons: Thermalized through scattering then detected through reactions.• Fast neutrons: Recoil nuclei (elastic) and probable gammas (inelastic). (Shielding vs. detection).(Shielding vs. detection).

Macroscopic cross section

Nuclei per unit volume

...

0

absorptionscattertotal

ttotaleIIMean free path?Neutron flux?


Cd cutoff energy.Cd cutoff energy.

• Later in this courseLater in this course We will discuss neutron detection. neutron detection.

Radiation Interaction - Detection

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