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
- 11. Summary
- 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
- 22. W e- Tungsten nucleus X-ray Bremsstrahlung Radiation Intensity depends on Z2 , 1/M2 energy loss is important in high Z & light particles Intensity of Bremsstrahlung radiation increases with energy
- 23. Bremsstrahlung
- 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
- 28. Bragg Curve Alpha Particle Beta Path Bragg Curve - Proton
- 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
- 31. 1. Slow neutron interaction (0.025 eV – 100 eV) Radioactive capture Charged particle emission Nuclear fission 2. Fast neutron interaction (100 eV - 20 MeV) Elastic scattering Inelastic scattering Neutron Interaction
- 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)
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