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Interaction of radiation with matter

How different types of matter interacts with different media. Includes interaction of photon, neutron, proton, electron and alpha particles.

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The complete description of the energy and depth of penetration of the moving electrons at any point in the medium is complicated by the fact that the electrons are very much lighter than the atomic nuclei. As a result, the electron can lose a very large fraction of its energy in a single process and thus can be deflected by very large angles. This means that even if the electron beam is monoenergetic when first impinging on a medium, there will be a large variation among all the moving electrons as to where in the medium each will stop. This is referred to as range straggling.

- 1. Interaction of radiation with matter Dr. BIKRAMJIT CHAKRABARTI, MD, DNB Physics Lecture 2 Dr BIKRAMJIT CHAKRABARTI
- 2. Dr BIKRAMJIT CHAKRABARTI
- 3. X-ray Gamma ray Production Extra-nuclear Nuclear Source Artificial Natural Electron Beta ray Production Extra-nuclear Nuclear Source Artificial Natural Dr BIKRAMJIT CHAKRABARTI
- 4. NATURE VELOCITY PENETRATION POWER UP TO IONISATION ALPHA Heavy, positive charged particle 1/10 of light Paper STRONG BETA Light, negative charged particle 9/10 of light Plastic WEAK GAMMA Electro- magnetic radiation, neutral. 100% of light Lead MODERATE Dr BIKRAMJIT CHAKRABARTI
- 5. A. Electro-magnetic radiation
- 6. B. Particle radiation PARTICLE SYMBOL CHARGE MASS PHOTON hv, γ 0 0 ELECTRON e, e-, β- -1 5.49 X 10-4 amu POSITRON e+, β+ +1 5.49 X 10-4 amu PROTON p, 1H1 +1 1.007277 amu NEUTRON n, 0n1 0 1.008665 amu ALPHA α, 2He4 +2 4.002604 amu NEUTRINO v 0 <1/2,000 mo PI MESONS π+ , π- , π0 +1, 0, -1 273 mo, 264 mo, MU MESON µ+ , µ- +1, -1 207 mo K MESON K+ , K- , K0 +1, 0, -1 967 mo,,973mo 27 -31Dr BIKRAMJIT CHAKRABARTI
- 7. Interaction depends on Radiation: Energy, charge, rest mass Media: Atomic configuration, density Dr BIKRAMJIT CHAKRABARTI
- 8. Dr BIKRAMJIT CHAKRABARTI
- 9. • High Z material • Photon energy is low enough that the quantum effects of the interaction are unimportant and the bound electron(s) can be regarded as essentially “free,” • EM wave passes near electron Oscillating electron re- irradiates energy of same frequency and wavelength. Coherent / classical scattering 1. Thomson scattering (single orbital electron) 2. Rayleigh scattering (group of electrons)Dr BIKRAMJIT CHAKRABARTI
- 10. Photon with specific energy Photo-electric effect Z3 specific differential attenuation causes contrast in X-ray and CT images High Z material (lead) used for protection 1.Photo-electron: • E= Ep-Eb • Direction of emission depends on Ep 2. Characteristic (fluorescent) X-ray: • Energy depends on Z & shell specific Eb. 3. Auger- electron Probability = attenuation τ/ρ = Z3 /E3 Probability peaks when Ep is just greater than Eb ↑ Increasing energy Dr BIKRAMJIT CHAKRABARTI
- 11. Dr BIKRAMJIT CHAKRABARTI
- 12. Photon with high energy The binding energy of the electron is insignificant (considered ‘free’) compared with the incident photon’s energy Maximum energy for photon during • scatter at right angle = 0.511 MeV • back-scatter = 0.255 MeV θ Remember, angle φ for photon! Probability = attenuation σc/ρ = •Independent of Z •Decreases with increasing E •Proportional to electron/gm which is essentially same for all atoms (except H) •Denser material (high gm/cc) will have smaller volume for same attenuation. Compton effect Therapeutic energy range MV images are blurred m0c2 = rest energy of electron = 0.511 MeV Dr BIKRAMJIT CHAKRABARTI
- 13. Pair production along with annihilation Energy of photon > 1.02 MeV Photon 0.51 MeV Photon 0.51 MeV e+ e- The probability of pair production (π/ρ) • increases rapidly with incident photon energy above the 1.02-MeV threshold • proportional to Z2 per atom, Z per electron, and approximately Z per gram. Dr BIKRAMJIT CHAKRABARTI Energy converted to mass (positron) Mass (positron-electron) converted to energy (annihilation)
- 14. Pair production along with annihilation Energy of photon > 1.02 MeV Photon 0.51 MeV Photon 0.51 MeV e+ e- The probability of pair production (π/ρ) • increases rapidly with incident photon energy above the 1.02-MeV threshold • proportional to Z2 per atom, Z per electron, and approximately Z per gram. Dr BIKRAMJIT CHAKRABARTI Energy converted to mass (positron) Mass (positron-electron) converted to energy (annihilation)
- 15. Photo-disintegration Low energy neutrons emitted Neutron contamination Photon energy > 10 MV Dr BIKRAMJIT CHAKRABARTI
- 16. Attenuation coefficients • Linear attenuation coefficient (μ) (unit = cm-1 ), • Mass attenuation coefficient (μ/ρ) (unit = g-1 cm2 ), • Mass energy-transfer coefficient (μt/ρ), • Mass energy-absorption coefficient (μen/ρ). – Division by ρ, the physical density of the medium, makes the coefficient medium independent. N = N0e-µx Dr BIKRAMJIT CHAKRABARTI
- 17. 30 KeV – 24 MeV 10-150 KeV 1.02 MeV and higher Dr BIKRAMJIT CHAKRABARTI
- 18. LET Stopping power Explanation Energy deposition per unit length Ability of medium to stop fluence of radiation Unit KeV/µm J/m or Mev/cm (linear) J/(kg/m2 ) or MeV/g/cm2 ) (mass) Dr BIKRAMJIT CHAKRABARTI Exposure = output Dose Kerma Explanation Ionization/unit mass Energy absorbed/ unit mass Energy released SI unit C/kg Gy (J/kg) Gy (J/kg) Other units R (esu/cm3 at STP) rad (100 ergs/g) - Relation 1 R = 2.58 X 10-4 C/kg 1 Gy = 100 rad = 0.876 R (air) - Equivalent dose Effective dose Unit is Sv (J/kg) Energy absorbed to volume of tissue Energy absorbed to whole body Radiation WF (WR) Tissue WF (WT)
- 19. Interaction of electrons 1. Elastic collision (excitation): With atomic electron OR nuclei → No loss of kinetic energy, only change in direction of incident electron. 2. In-elastic collision: – Ionisation of atom (with orbital electron) → Ejected electron (if produces further ionisations, are known as δ ray. – Bremsstraughlung X- ray = radiative loss (with nucleus) Dr BIKRAMJIT CHAKRABARTI
- 20. Interaction of heavy, charged particles 1. Ionization and excitation 2. Interaction of coulomb forces → radiative loss 3. Nuclear reactions producing radio- active nuclei Proton: Hydrogen ion Alpha particle: Helium ion Carbon ion Meson Dr BIKRAMJIT CHAKRABARTI
- 21. Why Bragg peak? • Stopping power (rate of energy loss / unit length) • Also depends on electron density of media. • The range of a charged particle is the distance it travels before coming to rest. Range proportional to (charge)2 X rest mass. • The mass stopping power of a material is obtained by dividing the stopping power by the density ρ. Dr BIKRAMJIT CHAKRABARTI
- 22. Dr BIKRAMJIT CHAKRABARTI
- 23. Interaction of NEUTRONS (High LET) Main energy loss occurs when interacts with hydrogen atom = Recoil proton Therefore, excess damage to hydrogen containing tissues (fat), nerve cells. Hydrogenous material is good for shielding Nuclear disintegration . Dr BIKRAMJIT CHAKRABARTI Proton Neutrons Deuterium γ
- 24. HIGH LET (High RBE, low OER) [Useful for hypoxic tissue / low α:β tumors] BRAGG PEAK (No exit / lateral dose) [Useful for tumors at close proximity to OAR] NEUTRON PROTON & other heavy, charged particles CARBON IONS Dr BIKRAMJIT CHAKRABARTI
- 25. Physico-chemical event • Excitation followed by ionization of water molecule: H2O → H2O+ + e- • Production of free radicals H2O+ → H+ + OH* Dr BIKRAMJIT CHAKRABARTI
- 26. Cellular effects of radiation - DNA Dr BIKRAMJIT CHAKRABARTI
- 27. Cellular effects of radiation – cell structure Damage to • Membranes • Lysosome Bystander effect Dr BIKRAMJIT CHAKRABARTI
- 28. Lecture 3 • Clinical radiation generators Dr BIKRAMJIT CHAKRABARTI

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