Chapter 5 Interactions of Ionizing Radiation

1. Chapter 5Interactions of Ionizing Radiation

2. Ionization • The process by which a neutral atom acquires a positive or a negative charge • Directly ionizing radiation • electrons, protons, and  particles • sufficient kinetic energy to produce ionization •  ray • excitation • Indirectly ionizing radiation • neutrons and photons • to release directly ionizing particles from matter when they interact with matter

3. Photon beam description • Fluence () • Fluence rate or flux density () • Energy fluence () • Energy fluence rate, energy flux density, or intensity ()

4. Photon beam attenuation • x–the absorber thickness (cm) • –linear attenuation coefficient (cm-1) • I–intensity

5. Half-value layer (HVL) • x=HVL I/I0=1/2 A practical beam produced by an x-ray generator A mono-energetic beam

6. Coefficients (1) • Linear attenuation coefficient(, cm-1) • Depend on the energy of the photons the nature of the material • Mass attenuation coefficient(/, cm2/g) • Independent of density of material • Depend on the atomic composition

7. Coefficients (2) • Electronic attenuation coefficient(e, cm2/electron) • Atomic attenuation coefficient(a, cm2/atom) Z the atomic number N0 the number of electrons per gram NA Avogradro’s number AW the atomic weight

8. Coefficients (3) • Energy transfer coefficient(tr) • When a photon interacts with the electrons in the material, a part or all of its energy is converted into kinetic energy ofcharged particles. The average energy transferred into kinetic energy of charged particles per interaction

9. Coefficients (4) • Energy absorption coefficient(en) • Energy loss of electrons • Inelastic collisions lossesionization and excitation • Radiation lossesbremsstrahlung • en= tr(1-g) • g fraction energy loss to bremsstrahlung • increses with Z of the absorber the kinetic energies of the secondary particles

10. 0.3 MeV (Scattered Photon) (Incident Photon, hn) 1 MeV 0.7 MeV (Initial Energy of Compton Electron, Ee) 0.2 MeV (Bremsstrahlung) Energy imparted of photon Etr = ? Een=?

11. Interactions of photons with matter • Photo disintegration (>10 MeV) • Coherent scattering (coh) Photoelectric effect () • Compton effect (c) • Pair production ()

12.   K L M Coherent scattering • Classical scattering or Rayleigh scattering • No energy is changed into electronic motion • No energy is absorbed in the medium • The only effect is the scattering of the photon at small angles. • In high Z materials and with photons of low energy

13. Photoelectric effect (1) • A photon interacts with an atom and ejects one of the orbital electrons. h-EB

14. Photoelectric effect (2) 15 keV L absorption edge • /  Z3/E3 • The angular distribution of electrons depends on the photon energy. 88 keV K absorption edge

15. Compton electron Free electron h   h’ K L M Compton effect (1) • The photon interacts with an atomic electron as though it were a “free” electron. • The law of conservation of energy • The law of conservation of momentum …………(1) ………(2) …...…………(3)

16. Compton effect (2) E  = h0/m0c2 = h0/0.511 h0  Free electron  h’ By (1), (2), (3)

17. Special cases of Compton effect • The radiation scattered at right angles(=90°)is independent of incident energy and has a maximum value of 0.511 MeV. • The radiation scattered backwards is independent of incident energy and has a maximum energy of 0.255 MeV.

18. Dependence of Compton effect on energy • As the photon energy increase, the photoelectric effect decreases rapidly and Compton effect becomes more and more important. • The Compton effect also decreases with increasing photon energy.

19. Dependence of Compton effect on Z • Independent of Z • Dependence only on the number of electrons per gram electrons/g

20. E- 0.51 MeV hn E+ + - Positron annihilation 0.51 MeV Pair production • The photon interacts with the electromagnetic field of an atomic nucleus. • The threshold energy is 1.02 MeV. • The total kinetic energy for the electron-positron pair is(h-1.02) MeV.

21. The probability of pair production •   Z2/atom

22. Ee The relationships between  and tr hn PE effect hn Compton effect hn E+ PP production E-

23. Ee The relationships between tr and en hn PE effect hn Compton effect hn E+ PP production E-

24. The comparison between  and en

25. (/)total v.s energy

26. Interactions of charged particles • Coulomb force • Collisions between the particle and theatomic electrons result in ionization and excitation. • Collisions between the particle and the nucleus result in radiative loss of energy or bremsstrahlung. • Nuclear reactions • Stopping power (S) = • Mass stopping power (S/, MeV cm2/g)

27. Bragg peak 能量 深度 Heavy charged particles • The particle slows down energy loss  ionization or absorbed dose 

28. Ionization Bremsstrahlung Excitation Electrons • Multiple changes in direction during the slowing down process smears out the Bragg peak.

29. Interactions of neutrons • Recoilingprotons from hydrogen and recoiling heavy nuclei from other elements • A billiard-ball collision • The most efficient absorbers of a neutron beam are the hydrogenous materials. • Nuclear disintegrations • The emission of heavy charged particles, neutrons,and  rays • About 30% of the tissue dose

30. Comparative beam characteristics (1) • Neutron beams

31. Comparative beam characteristics (2) • Heavy charged particle beams

32. Comparative beam characteristics (3) • Electron beams & protons

34. Advantages of neutron, proton and heavy charged particle beams over the standard x ray and electron modalities: • Lower oxygen enhancement ratio (OER) for neutrons • Improved dose-volume histograms (DVHs) for protons and heavy charged particles. 􀀁 Disadvantage of neutron, proton and heavy charge particle beams in comparison with standard x ray and electron modalities: considerably higher capital, maintenance and servicing cost.

35. Thank you…