1. Chapter 5 Interactions 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 of charged 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. Energy imparted of photon
1 MeV
(Initial Energy of
Compton Electron, Ee)
(Bremsstrahlung)
(Scattered Photon)
0.3 MeV
0.7 MeV
0.2 MeV
(Incident Photon, hn)
Etr = ? Een=?

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

12. Coherent scattering Classical scattering or Rayleigh scattering
Coherent scattering  K L M
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 K L M h  
h’
Free electron
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)  = h0/m0c2 = h0/0.511 E h0 h’ By (1), (2), (3)
Free electron  
h’
By (1), (2), (3)
 = h0/m0c2 = h0/0.511

17. Special cases of Compton effect
The radiation scattered at right angles (=90°) is independent of incident energy and has a maximum value of MeV.
The radiation scattered backwards is independent of incident energy and has a maximum energy of 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. Positron annihilation
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. hn E- E+ + -
0.51 MeV
Positron annihilation

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

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

23. The relationships between tr and enhn
PE effect hn
Compton effect Ee 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 the atomic 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. Heavy charged particles
Bragg peak 能量 深度
The particle slows down
energy loss 
ionization or absorbed
dose 

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

29. Interactions of neutrons
Recoiling protons 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…