1. Chapter 5 - Interactions of Ionizing Radiation
Chien-Cheng Sung

2. 游離(ionization) 激發(excitation) Basic PhysicsK L M

3. 輻射偵測的原理 電荷 發光 變色
．．．
偵檢器
(充氣式)
(閃爍式)
(半導體) 放大 計讀 評估 校正

4. 充氣式偵檢器 - +
離子對

5. 固體 (含雜質) 的電子能階
傳導帶
價電帶
雜質引起的電子能階
多餘電子

6. 熱發光劑量計與輻射作用的原理
傳導帶(空)
電子能階
(電子捕獲)
(電洞捕獲)
電洞能階
價電帶(滿)

7. 熱發光劑量計的計讀原理
傳導帶(空)
電子能階
(電子)
(電洞)
電洞能階
價電帶(滿)

8. Ionization radiation Directly ionizing radiation (charge particle)
electrons, protons, and  particles
sufficient kinetic energy to produce ionization
Ionization
 ray
Excitation
Stopping power (S)
Indirectly ionizing radiation (EW)
neutrons and photons
to release directly ionizing particles from matter when they interact with matter
Attenuation coefficient ()

9. Interactions of charged particles
Charged particles interact principally by ionization
and excitation
Stopping power (S)
the rate of kinetic energy loss per unit path
length of the particle (dE/dx)
Heavy charged particles
the rate of energy loss caused by ionizing radiation
interactions for charged particles is proportional to
the square of the particle charge and inversely
proportional to the square of its velocity.
 Bragg peak

10. Stopping Power dx
-dE
M, ze v
gas, r
ion pair (ip)

11. Stopping Power and Linear Energy Transfer (LET)
Delta-ray cutoff energy = D
Bremsstrahlung (Radiation)
S = Scol + Srad
= collisional SP + radiative SP
Scol = Scol,<D + Scol,>D
LET = Scol,<D
LET (linear energy transfer)
= restricted collisional stopping power

12. Extending the dose in depth – the ‘Spread-out-Bragg-peak’ (SOBP)
Treatment delivery
Energy Loss
Extending the dose in depth – the ‘Spread-out-Bragg-peak’ (SOBP)
target
Dose per proton (nGy)
Depth in water (cm)

13. Dose Distribution Comparison
Ideal dose distribution

14. Interaction of photons with matter
Coherent scattering
Photoelectric effect
Compton effect
Pair production
Photo disintegration (> 10MeV)
Total attenuation coefficient

15. Coherent Scattering (Rayleigh scattering)
Interaction of photons with matter (I)
Coherent Scattering (Rayleigh scattering)
No energy is transferred
The coherent scattering is probable in high atomic number materials
& with photon of low energy

16. Interaction of photons with matter (II)
Photoelectric effect
a photon interacts with an atom and ejects one of the orbital
electrons from the atom
α Z3 / E3

17. Interaction of photons with matter (III)
Compton effect
The photon interacts with an atomic electron as though it
were a “free” electron.
σ/ ρ α 1/ E

18. Interaction of photons with matter (IV)
Pair production
The threshold energy of pair production = 1.02 MeV
Annihilation radiation
π/ρ α Z2

19. Interaction of photons with matter (V)
Photo disintegration
An interaction of a high energy photon with an atomic
nucleus can lead to a nuclear reaction and to the emission of one or more nucleons.
e.g Cu + γ  62 Cu + 1n

20. Beam Quality Ideal way to describe radiation quality
Specify the spectral distribution of x-ray beam
e.g. energy fluence in each energy interval
Difficult to measure
Not necessary in most clinical situations
HVL is crude but simpler way
More interest in penetration of the beam into the patient
The penetrating ability of radiation is often described as the quality (energy) of the radiation

21. Be measured under narrow-beam or “good” geometry condition.
Measurement of Half-Value Layer
Be measured under narrow-beam or “good” geometry condition.
The exposure reading is a result of the photons that are transmitted through the absorber without interaction and no scattered photons are detected by the chamber.

22. Be measured under broad beam geometry condition.
Measurement of Half-Value Layer
Be measured under broad beam geometry condition.
B is the buildup factor, which is always greater than 1.
Be defined as the ratio of the intensity of the radiation, including both the primary and scattered radiation, at any point in a beam, to the intensity of the primary radiation only at that point.

23. Thank you for your attention
THE END