Photon Interactions Flashcards
Name the types of photon interactions:
- Photo Electric (PE)
- Compton Scattering (CS)
- Pair Production (PP)
What is the energy range for each type of photon reaction?
- PE = 50KeV
- CS = 25KeV to 25MeV
- PP = >25MeV
How is each type of photon reaction related to Z?
- PE = Z^3
- CS = independent
- PP = Z^2
How is each type of photon reaction related to E?
- PE = 1/E^3
- CS = as E increases, CS decreases
- PP = roughly independent
What photon reaction(s) contributes most at 25KeV?
PE (50%) and CS (50%)
What photon reaction(s) contributes most at 25MeV?
CS (50%) and PP (50%)
What photon reaction is advantageous for imaging?
photoelectric effect (PE)
What is the target in PE?
bound electron
In PE, as photon energy increases, electron distribution ____?
is more forward
What are secondary emissions of PE?
auger e’, characteristic x-ray, and electrons
PE takes place in what shells?
any: K, L, M, N
What must happen for PE to take place?
Photon energy must exceed binding energy of electrons
What photon reaction is not advantageous to imaging?
Compton Scattering (CS)
What is the target in CS?
unbound electron
What is the energy distribution is CS?
Low inbound E photon: most energy goes to photon
High inbound E photon: most energy goes to e’
In CS, a direct hit of an inbound E photon results the electron going in ___ degrees, and the photon in ___ degrees?
0 degrees; 180 degrees
A CS photon at 90 degrees has an energy of ___?
0.511 MeV
A CS photon at 180 degrees has an energy of ___?
0.255 MeV
What is the E threshold for PP to occur?
1.022 MeV
At what E is PP predominant?
E > 25 MeV
How is PP related to Z?
proportional to Z^2
What is the target in PP?
the nucleus
Pair Production results is what?
2 electrons, e- and e+, that share energy (1/4: 3/4)
*similar to beta- and beta+, but NO decay takes place
What photon reaction results produces annihilation radiation?
PP
How is annihilation radiation produced, and what are the resulting energies?
when an e+ recombines with an e- as a result of PP. 2 511 KeV photons are emitted at 180 degrees apart
Define HVL
Width of material to reduce the intensity to half its original value
Attenuation characteristics of monoenergetic photons, and what is the formula for the attenuation coefficient?
NO beam hardening: HVL 1 = HVL 2 = HVL 3;
u = ln2/HVL = .693/HVL
u = attenuation coefficient
Define TVL
Avg amount of material needed to absorb 90% of all radiation to reduce the intensity to a tenth of its original value
Equation to determine TVL
TVL = 3.32 x HVL
Attenuation characteristics of polyenergetic photons
Beam hardening: HVL 1 < HVL 2 < HVL 3, etc…
*barriers must get thicker to reduce intensity by 1/2 due to beam hardening;
u (poly) < u (mono)
PE will always be less than what energy?
< 100 keV
At what energy does PE = CS
25 keV
At what energy does CS = PP
25 MeV
