5 ramma rays int
TRANSCRIPT
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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 Absorption conti.
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Photoelectric Absorption conti.
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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Photoelectric Absorption conti.
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Photoelectric Absorption conti.
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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Photoelectric Absorption conti.
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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Compton Scattering conti.
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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Compton Scattering conti.
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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Compton Scattering conti.
The Compton cross section per electron as a function
of gamma ray energy
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Compton Scattering conti.
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Compton Scattering conti.
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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Pair Production conti.
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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Pair Production conti.
The pair production cross section of lead as afunction of gamma ray energy
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Pair Production conti.
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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Pair Production conti.
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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Gamma Ray Attenuation conti.
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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Gamma Ray Attenuation conti.
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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Gamma Ray Attenuation conti.
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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Gamma Ray Attenuation conti.
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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Gamma Ray Attenuation conti.
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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Gamma Ray Attenuation conti.
Photon Build up
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