Hybridoma Technology ( Production , Purification , and Application )
Interaction_photon_electron_with_matter.ppt
1. Interactions of Photon …
Types -
Photoelectric effect
Compton scattering
Pair production
Attenuation of Photons
HVT
TVT
Summary
2. Photoelectric Effect
• PE process involves bound electrons
• All the energy of photon is transferred to an atomic electron and the
photon is completely absorbed
• The electron is ejected from the atom and known as photo-electron
• Kinetic energy of ejected photoelectron (Ec) is equal to incident photon
energy (E0) minus the binding energy of the orbital electron (Eb)
Ec = Eo – Eb
3. Photoelectric Effect
• The vacancy created by the ejected electron is filled by
the outer orbital electron and in this process
characteristic X-rays are emitted
• There is also possibility of emission of Auger electron by the
absorption of characteristic X-rays
• The probability of PE effect decreases with increase of
energy of photon, but increases with the atomic number
of the medium
• PE process is predominant when Z (atomic number) of
material is high e.g. Pb (lead) and energy of photon
(incident) is lower than 100 keV
4. Photoelectric Effect
• Probability of ejection of an electron is maximum when
the photon energy is equal to or greater than the
B.E. of electron
• Probability of PE process varies with energy,
approximately as Z4 (of absorber ) / E3 (of radiation)
• As photon energy increases the probability for
photoelectron to be ejected in the forward
direction increases
5. Compton Scattering
• Compton scattering involves a free electron (outermost
electrons of an atom which have very low binding energies)
• The incident photon transfers a part of its energy to the free
electron and gets scattered with reduced energy
• Since Compton scattering involves these free electrons,
the process is independent
of the atomic number of the
medium
• Energy given to Compton electron is
ultimately absorbed in the medium
6. Compton Scattering
• This process is independent of Z of the medium.
• The probability of this interaction, decreases with increase
in energy of the incident photon.
• In this interaction, some energy is absorbed and the rest is
scattered. The energy of scattered photon depends upon
the angle of scattering (inversely)
• In soft tissues, this interaction is more predominant in
energy range of 100 keV to 10 MeV
• As the energy of incident photon increases, more electron
get scattered (ejected) in forward direction and it will carry
larger portion of the energy.
7. Pair Production
• This interaction occurs between a photon and the nucleus.
• Intense electric field close to nucleus converts energetic photon
into a positron-electron pair.
• Threshold energy for this process is 1.02 MeV.
• Excess energy is shared as K.E. between the electron and positron
• Probability for this process increases with the square of the
atomic number (Z2) of the medium and increases with increase in
energy of the photon.
8. Annihilation
• Positron at the end of its track encounters an electron
& the two particles annihilate to produce two photons
each of energy 0.511 MeV.
9. Summary
• Photoelectric Effect
• low energy photons and high Z
• Photon completely absorbed
• photon bound electron
• Compton Scattering
• medium energy photons
• Independent of Z
• Photon gets scattered
• photon free electron new photon
• Pair Production
• photon ( 1.02 MeV)
• Photon gets absorbed
• photon e- + e+
10. Interaction of Photons in low Z
Photon Energy (MeV)
Pair
Production
Compton
Photoelectric
Combined
Probability
WATER
12. Attenuation Of Photons
I0
X
I
I = I0 e-x
I0 - intensity of the incident radiation
I - transmitted intensity,
X - thickness of the material
- linear attenuation coefficient
The amount of radiation
transmitted through matter
decreases with thickness
and can be described by the
relation
13. Attenuation Of Photons
• If x is expressed in cm, is expressed in 1/cm (cm-1) and
is called linear attenuation coefficient.
• Dependency of probability of interaction on number of
atoms per volume can be overcome by normalizing
linear attenuation coefficient for density of material.
• The quantity /ρ is called mass attenuation coefficient;
where ρ is the density of the medium. It is expressed
in units of cm2/g
14. Attenuation Of Photons
• Thus all these three interactions PE, CS & PP result in the emission of
electron by absorption of photon energy in the medium
• When x rays traverses matter it undergoes all the three interactions
with varying probabilities
• Part of the energy may be absorbed, part scattered and part may be
transmitted without undergoing any interaction
• Which process would dominate will depend on the energy of the
radiation and the nature of medium
• There is a certain probability for occurrence of each process and this
probability is referred as photoelectric, Compton and pair
production attenuation coefficients
• The total attenuation coefficient is the sum of these three
coefficients ( = e + s + p)
15. Half Value Thickness (HVT)
• Thickness of material required to reduce intensity of a
Photon beam to one-half of its initial value.
• A second half value layer will permit ½ of the incident
radiation (already reduced by ½) to pass so that only ¼ of
the initial radiation (½ x ½) is permitted to pass.
• If “n” half value layers are used, (½)n of the initial
radiation is permitted to pass. “n” may be any number.
• Relationship between and HVT: HVT = 0.693/
16. Tenth Value Thickness (TVT)
• Thickness of material required to reduce intensity of a
Photon beam to one-tenth of its initial value.
• Relationship between and TVT:
• TVT = ln(10)/
• TVT = 3.323 x HVT
17. HVT and TVT
HVL (cm) TVL (cm)
Isotope Photon E
(MeV)
Concrete Steel Lead Concrete Steel Lead
137Cs 0.66 4.8 1.6 0.65 15.7 5.3 2.1
60Co 1.17,
1.33
6.2 2.1 1.2 20.6 6.9 4
192Ir 0.13 to
1.06
4.3 1.3 0.6 14.7 4.3 2
226Ra 0.047 to
2.4
6.9 2.2 1.66 23.4 7.4 5.5
18. Summary
• At low energies, the main interaction is photoelectric
• In the medium energy region, the major interaction is
Compton process
• The pair production starts at 1.02 MeV and increases with
energy
• There is a dip in the curve, which corresponds to a transition
from decreasing predominance of Compton interaction and
increasing predominance of pair production
• Total linear attenuation coefficient depends upon both the
energy of photon and the atomic number of attenuating
medium
19. Collision with bound electron
Large energy loss & change in direction due to same mass
Heat, light, characteristic radiation (X-rays)
Collision losses are seen with low Z & at low energy
Prominent process of absorption of energy even at high
energy (10 MeV) for low Z
* With bound electrons (excitation & ionization)
* With nucleus (Bremsstrahlung)
Collision loss / Ionization loss
Charged Particle Interaction
20. When a charged particle interacts with an atom, it may:
traverse in close proximity to the atom
( “hard” collision)
traverse at a distance from the atom
(“soft” collision)
A hard collision will impart more energy to the material
Particle Interactions
21. • Ionizing radiation removes
orbital electrons from atoms.
• This creates an ion pair – an
electron and the atom that has
lost an electron.
• The no. of ion pairs produced
per unit path length is called
specific ionisation.
• Specific ionisation is proportional
to charge of the particle and
inversely proportional to its velocity.
Ionization
24. The fraction of electrons producing
bremsstrahlung follows the relationship:
F = 3.5 x 10-4 (Z)(E)
Empirical Relationship
25. The amount of energy deposited will be the sum of
energy deposited from hard and soft collisions
The “stopping power,” S, is the sum of energy
deposited for soft and hard collisions
Most of the energy deposited will be from soft
collisions since it is less likely that a particle will
interact with the nucleus
Stopping power is a function of charge, energy, medium
of interaction
Stopping Power
26. LET is the energy absorbed per unit path
length of the particle.
It is expressed in keV/µm.
Alpha particle – heavy mass - high specific
ionization - high LET
Beta particle – low specific ionization - low
LET
Range of beta particles in any medium
depends upon the energy of beta particle
and the density of the medium.
Linear Energy Transfer (LET)
27. Low Energy Beta
Little shielding is required to absorb completely in comparison to
gamma radiation (particles range is less than the thickness of the
material)
Glass container gives complete absorption (beta emitters in
solution), plastic (low Z material) shielding is effective.
High Energy Beta
Absorption results in production of Bremstrahlung radiation
Low Z material produces less Bremsstrahlung (plastic, rubber,
aluminum)
To shield the Bremsstrahlung X-rays, high Z materials (ex: lead,
tungsten) need be placed on the inner side of the Low Z material.
Shielding of Beta
29. Little penetrating power due to large mass
& charge
Paper, unbroken dead layer of skin cell few
inches of air is sufficient to shield the -
particles
External contamination only, internal health
hazard
Alpha Shielding
30. Shielding
High z & high density material
(lead, concrete, depleted uranium, steel, tungsten) is best
material for gamma ray shielding
32. Slow neutron interaction
Radioactive capture (n,)
• neutron is captured by nucleus and excess energy
emitted as - radiation
• occurs for nuclides from low to high mass no.
• extensively used for production of isotopes in
reactors
• increase in n/p ratio, the product becomes a beta –
emitter
27Co59 + 0n1 27Co60 + (1.25 MeV)
1H1 + 0n1 1H2 + (2.2 MeV)
33. Charged particle emission
neutron is absorbed into nucleus & a charged
particle is emitted from target nucleus
(n, p), (n, d), (n, ), type reaction
7N14 + 0n1 6C14 + 1H1
5B10 + 0n1 3Li7 + 2He4
Slow neutron interaction
34. Slow neutron interaction
Nuclear Fission
absorption incident neutron splits the target nucleus into two
parts
source of energy for nuclear reactors & nuclear weapons
some fission occurs at thermal neutron energies but there is
a threshold energy eg. 235U requires neutron energy =1.1 MeV
U235 + n Y95 + I139 + 2n + Energy
(30 different ways that fission may take place with production of
about 60 primary fission fragments)
ejected neutron used to activate stable materials to produce
radioisotopes
slow neutron by several heavy atomic nuclei
35. Fast neutron interaction
Elastic Scattering (n, n)
occurs when a neutron strikes a nucleus approximately
same mass
depending on the size of nucleus, neutron can transfer
much of its KE
hydrogen causes greatest energy loss to neutron
fraction of energy loss is greater, if mass of scattering
nucleus smaller
billiard ball type collision
momentum & kinetic energy both conserved
36. Fast neutron interaction
Inelastic Scattering
occurs when a neutron strikes a heavy nucleus
formation of compound nucleus (neutron penetrate for a short
period of time)
nucleus is left in an excited state, emitting γ-radiation
which can cause ionization and/or excitation
(n, n), (n, 2n), (n, γ) type of reactions
loss of energy of neutron is more
Example :
(n, n)
45Rh103 (n, n)45Rh103m 45Rh103 + γ
(n, 2n)
11Na23 (n, 2n)11Na22 10Ne22 + +
37. Shielding of Neutrons
hydrogenous material are most effective for slowing down fast neutron
(water, paraffin, concrete).
should contain sufficient additive (boron, cadmium) to absorb slow
neutron
borated polyethylene (excellent material for fast neutron)
