Interaction Ionizing Radiation with Matter

1. Interaction Ionizing Radiation with Matter BNEN 2012-2013 Intro William D’haeseleer

2. Ionizing particles • Directly ionizing particles alpha (He-4++) & beta (e-/e+) • Indirectly ionizing particles Gamma or X rays/photons & neutrons

3. Ionizations Energetic ionizing particles move around in sea of electrons, ions & nuclei • Leads to ionizations i.e., creation of i/e pairs Excitations in atoms and nuclei

4. Ionizations

5. Directly ionizing particles

6. Ionization - alphas • Alpha particles4He++(~ 4-8 MeV) • Very massive and ++ • Create ample i/e pairs per unit distance • Loose on ave 34 eV per e/i pair in air 38 eV per e/i pair in water • Create ample local damage • Are very easily stopped in air & matter • E.g., in air ~ Range 3 to 7 cm water ~ Range 0.03 to 0.09 mm

7. Ionization - betas • Beta particlese- / e+(~ keV …10 MeV) • Very light and + (elect) or + (posit) • Create “some” i/e pairs per unit distance • Create some local damage • Are quite easily stopped in air & matter • Range less precisely defined (straggling)

8. Ionization - betas • Beta particlese- / e+(~ keV …10 MeV) • … • E.g., 3 MeV particles • Alpha in air R ~ 3 cm … 4000 i/e pairs/mm • Beta in air R ~ 10 m … 4 i/e pairs/mm • Beta 1.0 keV in water Range ~ μm • Beta 1.7 MeV in water Range 6cm in air Range 4.5 m

9. Indirectly ionizing particles

10. Ionization - Gammas • X & Gamma / Photon interactions (~ eV …10 MeV) • Photoelectric effect • Compton scattering • Pair formation

11. Ionization - Gammas

12. Photons / Gammas Consider beam of impinging photons with intensity I0 Intensity (2) detector (1) (3) (1) detected; not yet interacted(2) & (3) disappear from original beam as a consequence of interactions

13. Photons / Gammas Impinging intensity (or flux) = I0particles/(m2s) At location x still I particles/(m2s) remaining from original beam Between 0 and x, some of the particles have deviated from the original path due to interactions

14. Photons / Gammas Call:μ the probability for an interaction per m Hypothesis: μ = uniform ≠ f(x) a particle at location x has the same probability to undergo an interaction within the next 1 cm as a particle at the location 0 would have between 0 and 1 cm.

15. Photons / Gammas Probability for interaction of a particle within the interval dx = μ dx Suppose at place x I particles/(m2s), then the number of particles that undergoes an interaction (on average) per m2s is = I μ dx

16. Photons / Gammas Hence, the decrease in number of particles (from originally parallel beam): dI = -I μ dx So that: I = I0 e-μx or

17. Photons / Gammas Or, alternatively μ≡ linear attenuation coefficient [1/m](=probability for interaction per m) μ/ρ≡ mass attenuation coefficient [m²/kg]

18. Photons / Gammas Hence, the attenuation coefficient is a measure for attenuation of the originally parallel beam = fraction that has not yet interacted

19. Ionization - Gammas Half length for attenuation X1/2 = ln2 / μ After 3 X1/2 factor 8 attenuation After 10 X1/2 factor 1024 attenuation

20. Photons / Gammasa. Photo-electric effect Fig. 3.1. Photoelectric effect in lead -- Ref: Schaeffer

21. Ionization - Gammas Photoelectric effect

22. Photons / Gammasb. Compton effect Microscopic cross section Ref: Lamarsh & Baratta

23. Photons / Gammasc. Pair Formation Ref: Schaeffer

24. Photons / Gammasc. Pair Formation Ref: Lamarsch & Baratta

25. Ionization - Gammas Pair formation

26. Ionization - Gammas Sum of all processes

27. Photons / Gammas Dose Rate Assume that upon interaction, an amount of energy E of the impinging particle will be transferred to the target material: deposited energy per interaction x RR E

28. Photons / Gammas Dose Rate Dose rate expressed per kg

29. Photons / Gammas Dose Rate In case of the Compton effect (see later for definition), not the total impinging energy will be deposited; only the fraction E = hv = energy of incoming photon E’ = hv’ = energy of scattered photon

30. Photons / Gammas Dose Rate Therefore, one writes: Note: actually, μa must be obtained through averaging over all angles mass absorption coefficient

31. Photons / Gammas Dose Rate If one takes this μasystematically, one no longer has to bother about the actually absorbed energy!

32. Ionization - Gammas Total attn coeff metals

33. Ionization - Gammas Total abs coeff metals

34. Ionization - Gammas Total attn coeff low-Z materials

35. Ionization - Gammas Total abs coeff low-Z materials

36. Ionization - Neutrons • Interactions with neutrons (~ eV …8 MeV) • Elastic scattering • Inelastic scattering • Absorption (n,γ) Billiard ball collision Collision with nucleus left in excited state - recoil nucleus - gamma from de-excitation Neutron absorbed in nucleus which becomes highly excited - recoil nucleus - gamma from de-excitation - extra n moves nucleus up one step in N,Z plot  new nucleus may be radioactive

37. Ionization - Neutrons with Macroscopic cross section fcn (target material, En)

38. Ionization - Neutrons Half length for attenuation X1/2 = ln2 / Σ After 3 X1/2 factor 8 attenuation After 10 X1/2 factor 1024 attenuation

39. Ionization - Neutrons Neutron attenuation

40. Ionization - Neutrons • Interactions with neutrons (~ eV …8 MeV) • Elastic scattering • Inelastic scattering • Absorption (n,γ) Neuton absorbed in nucleus which becomes highly excited - some absorption in U-233 U-235 and Pu-239 can lead to fission

41. Ionization Summary Ionizations & Range in tissue/water Ref. J. Shapiro

42. Ionization - Summary Ref. J. Shapiro

43. Shielding

44. Shielding

45. References • Some examples (a.o.)