5 Ramma Rays Int

7/28/2019 5 Ramma Rays Int

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PNRA Course onRadiation Safety

Dr. Muhammad Tufail(Izaz-i-Fazeelat)

1

Lecture 5

INTERACTION OF GAMMA

RAYS


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Interaction of Gamma Rays

Lecture contents

Interaction Mechanisms

Photoelectric absorption

Compton scattering

Pair production Gamma ray Attenuation

Attenuation coefficient

Absorber mass thickness

Buildup


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Interaction Mechanism

Interaction processes

Major processes of interaction of-rays withmatter are Photoelectric absorption

Compton scattering

Pair production

Gamma rays interact with Atom as whole,

Electron in atom shell, or

Nucleus of atom


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Interaction Mechanism conti.

All these processes lead to

partial or complete transfer of-rays photonenergy to electron energy

sudden or abrupt changes in -ray photonhistory

either complete disappearance of photons, or scattering with less amount of energy

Unlike charged particles, ray energytransfer is abrupt


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Photoelectric Absorption

Process

A photon undergoes an interaction withabsorber atom, the photon is completelyabsorbed.

A photoelectron is ejected by the atom from

one of its bound shells. Interaction can not take place with free

electrons

Interaction takes part with atom as a whole(to conserve energy and momentum)


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Photoelectric Absorption conti.

Energy of photoelectron

Ee-= hv

Eb hv = energy of gamma raysE

b= electron binding energy

The most probable origin of photoelectron is

K-shell of the atom

in 80%cases ejection is from K-shell

Plot of vs E and absorption edges If photon energy is equal to the binding energy

of K-shell electron, then there is sudden drop inprobability

K edge and L edge are the prominent


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Photoelectric cross section ( ) Approximate relation for is

Z = At No. of absorber

n = 45 depends on ray energy Interaction prob. is more for

Low energy photons visible light, x rays & soft -rays

High Z material (Pb, U etc)

5.3tan

E

Ztcons

n


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The number of photoelectric interactions/

cm2.s is

whereI = intensity of gamma rays incident on target

[s/cm2.s]N = Atoms per unit volume of target [atoms/cm3]

pe = photoelectric interaction cross section peratom

peIN


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Associated processes

Ionization & excitation by photoelectron

Emission of characteristic X-rays

Further photoelectric absorption and emission of

secondary photoelectron Emission of Auger electrons

Ionization & excitation by Auger electrons


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Compton Scattering

Process

In this process an incident photon transfers

a part of its energy to an orbital electrons of

absorbing material.

The electron is ejected from the atom. The photon is scatted with less energy at

some angle with the initial direction of the

incident photon.


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Compton Scattering conti.


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Using the simultaneous equations for the

conservation of energy and momentum, energyof the scattered photon is

Probability per atom of absorber depends on

number of electrons

Prob. of Compton scattering falls off gradually

with increase of energy

)cos1(12

cm

hv

hvvh

o


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In Compton scattering, the photon interacts

with individual electron, therefore cross sectionis defined per electron, eC

The maximum value ofeC is 2/3 b at E=0, whichis called Thomson cross section, T

The Compton cross

section per atom is

Overall cross section

is given by eC as a function of energy is shown in figure

CeC Z

E

Z

C


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The Compton cross section per electron as a function

of gamma ray energy


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Associated processes

Multiple interactions of

Scattered photon by photoelectric effect

Recoil electrons

Back scattering of initial or scatteredphotons.

Include all the processes associated with

photoelectric effect.

Multiple scattering


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Pair Production

Process

In this process a

photon disappears

and an electron pair

(a positron andelectron) is created

The threshold energy for the process is 2moc2

= 1.022 MeV

Pair production can take place in the vicinity of

a Coulomb field, in most of the case it is the

field of nucleus

Z

e+

e-


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Pair Production conti.

Practically E must be greater than 2 MeV

Kinetic energy of electron and positron

Excess energy is shared between electronand positron as kinetic energy

022.1 EEE ee


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Cross section

The dependence of pair production crosssection is follows:

When E > 1.022 MeV, cross section for pairproduction pp increases steadily withincreasing energy, and atomic number ofmedium atoms

Cross section vs. Energy is showngraphically

2022.1 ZEpp


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The pair production cross section of lead as afunction of gamma ray energy


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Annihilation

At the end of its path, the positronannihilates with electron and twoannihilation photons of energy 0.511 MeVare emitted in opposite direction


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Interaction of annihilation photons

Photoelectric effect Compton scattering

Accompanied processes

All the processes of photoelectric and pair

production interactions


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Three types of Gamma ray interactions


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Gamma Ray Attenuation

Transmission Experiment

Gamma ray photons are removed from thebeam due to absorption or scattering.

Photons are removed under a fixed prob.

per unit path length called linear attenuationcoefficient The removal prob. is given by

).()().( PPComptonEP


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Gamma Ray Attenuation conti.


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The intensity of transmitted photons is

given by

I transmitted intensity(

s/cm2 s)

Io intensity without an absorber(s/cm2 s)t absorber thickness (cm) linear attenuation coefficient (cm-1)

Probability to attenuate gamma rays per unitpath length

teII

0


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Mass attenuation coefficient

varies with density of the absorber eventhough the materials is same.

m= / (cm2/g) does not change withphysical state of material e.g. it is same forice, H2O and steam

mfor a compound or mixture of elements

can be calculate from

i

iw


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Mean free path

The average distance traveled in theabsorber before an interaction can take

place (cm).

Half value thickness

It is the distance in which the radiation

intensity is reduced to half of the value w/o

absorber (cm)

1

2ln2/1 t


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Absorber mass thickness

tm = t: is called mass thickness of absorber(g/cm2)

The absorber with equal mass thickness

carry equal # of electrons. For charged particles the stopping power

and range are roughly same for materials

that do not differ greatly in Z.


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Buildup Factor (B)

With reference to transmission experiment,buildup factor is given by

Transmitted intensity in this case is

1U S U

U S

B

0

tI BI e


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Photon Build up


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