Photon Interactions

Question 1

Photon beam attenuation

Answer 1

Incident beam photons may be scattered, absorbed or transmitted resulting in an attenuated beam.

Question 2

What are photon interactions dependent on?

Answer 2

1. Atomic number Z of material
2. Photon energy E

Question 3

What are the 3 main photon interactions?

Answer 3

1. Photo electric absorption
2. Compton scattering
3. Pair production

Question 4

What are the other 2 photon interactions?

Answer 4

1. Rayleigh scattering
2. Photonuclear reactions

Question 5

Photoelectric absorption

Answer 5

Type = Orbital electron interaction (tightly bound electron).
Occurrence: Predominates low photon energies. Decreases as energy increases and increases as atomic number increases. Photon energy must be equal or higher than electron binding energy.
Process:
- photon interacts with tightly bound electron
- photon is absorbed completely
- orbital electron is ejected
- vacancy is left behind
- characteristic radiation follows as higher orbital electron fills vacancy
Kinetic energy of electron = incident photon energy - electron binding energy

Question 6

Compton Scattering

Answer 6

Type = Orbital electron (loosely bound).
Occurrence = Predominates at intermediate photon energies. Probability decreases with increasing photon energy.
Process:
- Photon interacts with loosely bound electron
- part of photon energy transferred to electron
- electron is emitted from atom as a Compton electron at an angle relative to photon direction
- photon is scattered at an angle and energy is lower than at the start
Kinetic energy of Compton photon = incident photon energy - energy transferred to electron.

Question 7

Pair Production

Answer 7

Type = Nuclei interaction (electrostatic field).
Occurrence = Predominates at high photo energies. Probability increases with increasing atomic number and increasing energies. Requires 1.022 energy to occur as this is equal to at rest electron masses.
Process:
- photon interacts with strong electric field near nucleus
- photon disappears
- photon energy converted into pair of particles (electron and positron)
- positron slows quickly through collisions with electrons and eventually undergoes annihilation

Question 8

Rayleigh Scattering

Answer 8

Type = Orbital electron (whole atom)
Occurrence = low photon energies, high atomic number
Process:
- interaction of a photon with the total atom where photon is absorbed and re-emitted in a slightly different direction.

Question 9

Photonuclear Reactions

Answer 9

Type = Direct photon/nucleus interaction
Occurrence = Requires a energy threshold of 10 Mev or higher.
Process:
- High energy photon absorbed by nucleus of an atom, neutron or proton emitted.