Interactions

Question 1

what are the types of photon interactions?

Answer 1

1. None - good for imaging as Xrays do not deposit energy. they reach detector unaffected
2. change direction, no energy loss
3. change direction, lose energy
4. Absorption

Question 2

talk about absorption.

Answer 2

good for treatment - dose goes into body and affects cells -> to cell death

good for imaging - different materials attenutate x rays differently so Number of Xrays reaching detector for different materials - can tell apart tissues

bad for imaging - gets dose in areas you dont want it in, justified as it is CT scan but you get dose in healthy regions

Question 3

what is the beer lambert law?

Answer 3

Interaction of x rays with matter is exponential so it never hits 0

Question 4

what is the linear attenuation coefficient

Answer 4

Fraction of beam of x ray or gamma rays that is absorbed/scattered per unit of thickness of absorber.

thickness remains same throughout

Question 5

what does the linear attenuation coefficient account for?

Answer 5

• number of atoms in cubic cm of material
• probabilty of a photon being scattered or absorbed from nucleus or electron of one of these atoms

Question 6

What does linear attenuation coefficient depend on?

Answer 6

Density and energy of photon

Question 7

what are the conditions for linear attenuation coefficient?

Answer 7

mono-energetic beam
narrow beam geometry

Question 8

what is a half value layer?

Answer 8

thickness of material needed to reduce its intensity of a beam, to 1/2 of its initial value

can be used as a measure of beam geometry

Question 9

what does narrow beam geometry do?

Answer 9

reduces impact of scatter in measurement

reduces penumbra

Question 10

what does a mono energetic beam do?

Answer 10

Avoid beam hardening

Question 11

when does classical scattering occur?

Answer 11

when energy of the photon is smaller than the binding energies of the electrons

no energy transferred

Question 12

describe classical scattering

Answer 12

1. low energy photon interacts
2. photon interacts with electrons
3. there is a change in direction
4. no energy loss, no ionisation
5. photon absorbed by electron
6. energy causes it to vibrate
7. energy is re-emitted as new photon
8. new photon identical to original

occurs with low energy x rays (little importance)

Question 13

what happens in a photoelectric interaction

Answer 13

1. low energy photon interacts with electron
2. photon energy absorbed
3. electron raised to higher shell or escapes the atom becoming a photoelectron
4. leaves a hole in lower shell
5. electrons move down from outer shells to fill hole - give off excess energy in form of x ray and this is called valency filling

Question 14

when does photoelectric effect occurs?

Answer 14

photon energy must be same to binding energy of electron

Question 15

what energy does photoelectron have?

Answer 15

KE equal to difference between incident x ray and binding energy of the electron

Question 16

what is the probability of a photoelectric effect occuring?

Answer 16

photon energy less than binding energy -> cannot occur

photon energy = to binding energy
-> at its greatest

photon energy higher than binding energy -> rapidly reducing

inversely proportional to third power of photon energy

directly proportional to third power of atomic number of absorbing material

Question 17

when does compton scatter happen?

Answer 17

energy of photon much higher than BE of electron

change direction lose energy

Question 18

describe compton scatter

Answer 18

1. photon collides with electron
2. photon scattered - change direction, loses energy
3. energy given to recoil electron
4. atom ionised and ion pairs created
5. scattered photon lost energy

Question 19

scattered photon?

Answer 19

any direction
0 degree = no energy loss
180 degree = more energy to electron
higher the energy of incoming photon, the smaller the angle of deflection

go through more compton interactions
eventually absorbed photoelectrically

secondary electron creates further ionisation events

Question 20

summary of compton scatter

Answer 20

as x ray energy increases:
- penetration in tissue without interaction increases
- increased compton relative to PE
-reduced total scatter

as atomic no of absorber increases:
- no effect

as mass density of absorber increases:
- proportional increase in compton

Question 21

when does pair production occur?

Answer 21

when photon has enough energy, can go through electron cloud and come close enough to nucleus to come under influence of strong electrostatic field of nucleus.

Question 22

describe pair production

Answer 22

photon is converted to mass
2 particles created
electron and positron

1.022MEV needed
- shared between two particles as KE
- electron interacts as normal
- positron is antimatter so more damaging
- positron loses energy as it is near the end
- combines with electron
- annihilate each other
- mass -> energy
- 2 photons created travelling at 180 to each other

Question 23

when does photonuclear disintegration occur?

Answer 23

higher energies (>10MEV)

Question 24

what is photonuclear disintergration?

Answer 24

nucleus absorbs high energy photon and becomes excited
immedietaly decays by emitting subatomic particles

can produced short lived radionuclides