Photon Interactions

Question 1

Name the types of photon interactions:

Answer 1

1. Photo Electric (PE)
2. Compton Scattering (CS)
3. Pair Production (PP)

Question 2

What is the energy range for each type of photon reaction?

Answer 2

1. PE = 50KeV
2. CS = 25KeV to 25MeV
3. PP = >25MeV

Question 3

How is each type of photon reaction related to Z?

Answer 3

1. PE = Z^3
2. CS = independent
3. PP = Z^2

Question 4

How is each type of photon reaction related to E?

Answer 4

1. PE = 1/E^3
2. CS = as E increases, CS decreases
3. PP = roughly independent

Question 5

What photon reaction(s) contributes most at 25KeV?

Answer 5

PE (50%) and CS (50%)

Question 6

What photon reaction(s) contributes most at 25MeV?

Answer 6

CS (50%) and PP (50%)

Question 7

What photon reaction is advantageous for imaging?

Answer 7

photoelectric effect (PE)

Question 8

What is the target in PE?

Answer 8

bound electron

Question 9

In PE, as photon energy increases, electron distribution ____?

Answer 9

is more forward

Question 10

What are secondary emissions of PE?

Answer 10

auger e’, characteristic x-ray, and electrons

Question 11

PE takes place in what shells?

Answer 11

any: K, L, M, N

Question 12

What must happen for PE to take place?

Answer 12

Photon energy must exceed binding energy of electrons

Question 13

What photon reaction is not advantageous to imaging?

Answer 13

Compton Scattering (CS)

Question 14

What is the target in CS?

Answer 14

unbound electron

Question 15

What is the energy distribution is CS?

Answer 15

Low inbound E photon: most energy goes to photon

High inbound E photon: most energy goes to e’

Question 16

In CS, a direct hit of an inbound E photon results the electron going in ___ degrees, and the photon in ___ degrees?

Answer 16

0 degrees; 180 degrees

Question 17

A CS photon at 90 degrees has an energy of ___?

Answer 17

0.511 MeV

Question 18

A CS photon at 180 degrees has an energy of ___?

Answer 18

0.255 MeV

Question 19

What is the E threshold for PP to occur?

Answer 19

1.022 MeV

Question 20

At what E is PP predominant?

Answer 20

E > 25 MeV

Question 21

How is PP related to Z?

Answer 21

proportional to Z^2

Question 22

What is the target in PP?

Answer 22

the nucleus

Question 23

Pair Production results is what?

Answer 23

2 electrons, e- and e+, that share energy (1/4: 3/4)

*similar to beta- and beta+, but NO decay takes place

Question 24

What photon reaction results produces annihilation radiation?

Answer 24

PP

Question 25

How is annihilation radiation produced, and what are the resulting energies?

Answer 25

when an e+ recombines with an e- as a result of PP. 2 511 KeV photons are emitted at 180 degrees apart

Question 26

Define HVL

Answer 26

Width of material to reduce the intensity to half its original value

Question 27

Attenuation characteristics of monoenergetic photons, and what is the formula for the attenuation coefficient?

Answer 27

NO beam hardening: HVL 1 = HVL 2 = HVL 3;
u = ln2/HVL = .693/HVL
u = attenuation coefficient

Question 28

Define TVL

Answer 28

Avg amount of material needed to absorb 90% of all radiation to reduce the intensity to a tenth of its original value

Question 29

Equation to determine TVL

Answer 29

TVL = 3.32 x HVL

Question 30

Attenuation characteristics of polyenergetic photons

Answer 30

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)

Question 31

PE will always be less than what energy?

Answer 31

< 100 keV

Question 32

At what energy does PE = CS

Answer 32

25 keV

Question 33

At what energy does CS = PP

Answer 33

25 MeV