Interaction of ionising radiation with tissues and detectors

Question 1

what are the 3 types of interaction of photons with matter?

Answer 1

absorbing : photoelectric effect, pair production
reflecting/ scattering: compton scatter

Question 2

what is the cross section of interaction?

Answer 2

formula: probability of interaction (absorption/scatter) / number of particles passing through a unit area (unit particle fluence/flux)

Question 3

what does the attenuation coefficient depend on?

Answer 3

the energy of the incoming beam
the thickness of the beam: a thicker beam will allow photons to scatter and deflect into the beam, so more photons will be detected

Question 4

what mathematical model does the removal of photons follow?

Answer 4

a negative exponential curve

Question 5

what does the number of transmitted photons depend on?

Answer 5

initial number of photons in the narrow beam
the thickness of the material
the linear attenuation coefficient

Question 6

relationship between final number of photons which are detected by the detector at the other end of the material and the linear attenuation coefficient

Answer 6

the higher the linear attenuation coefficient, the more photons are absorbed by the material, so the fewer photons are detected

Question 7

relationship between final number of photons detected at the other end of the detector and the thickness of the material

Answer 7

the thicker the material, the more photons are attenuated (absorbed) and the lower the final number of those detected by the screen

Question 8

for which conditions is the equation of removal of photons only valid under?

Answer 8

there is a narrow beam of incoming radiation
from a single energy source

Question 9

how to mathematically calculate the linear attenuation coefficient?

Answer 9

add the cross section of interaction of each interaction process
1. photoelectric effect
2. compton scatter
3. coherent scatter
4. pair production

Question 10

what is the mass attenuation coefficient equal to mathematically?

Answer 10

the mass probability of:
- coherent scatter and compton scatter
- pair production
- photoelectric interaction

Question 11

Photoelectric effect

Answer 11

An incident photon with energy hf is given to a single orbital electron (bound electron), which is sometimes found in the k-shell 80% and sometimes in the l-shell 20%
The bound electron gains kinetic energy equal to the energy of the photon - binding energy of the bound electron
Almost all the energy is transferred to the photoelectron meaning that the binding energy of the bound electron is much lower in comparison to the incident energy
There is now a vacancy in the shell
Which is filled by outer electrons

Question 12

Explain the k-shell and l-shell differences

Answer 12

k-shell is inner shell
l-shell is outer shell
which explains why there is a higher likelihood that the photon colliding with the inner electron will release a high energy photoelectron as the higher the energy level, the higher the binding energy, so that the kinetic energy of the photoelectron will decrease with an increase in shell number

Question 13

what is another name for the photoelectron?

Answer 13

the auger electron

Question 14

what does the cross sectional probability of the photoelectric effect depend on?

Answer 14

the density, the atomic number and the energy

Question 15

what does the atomic number affect?

Answer 15

the capacity of absorption of incident x-rays of the material. The higher the atomic number, the higher the absorption capacity

Question 16

what is the difference between the density and the atomic number?

Answer 16

atomic number is the number of protons or electrons in an atom
density is the number of atoms that can be packed into a unit volume

Question 17

how does energy affect the probability of photoelectric absorption?

Answer 17

the higher the energy, the lower the probability of photoelectric absorption
this is because the auger electron, or the photoelectron, will have a lot more kinetic energy to penetrate through the material

Question 18

how does the energy affect the attenuation of the photoelectric absorption?

Answer 18

the higher the energy, the lower the attenuation

Question 19

what are the units for the linear attenuation coefficient?

Answer 19

it is cm^-1

Question 20

what are the units for mass attenuation coefficient?

Answer 20

process: divide the linear attenuation coefficient by the density
units are in cm^2/g

Question 21

Describe the compton scatter

Answer 21

incident photon collides with electron in the shell (characteristic/ bound electron). Photon leaves with a new, lower energy and is scattered at a certain angle. The bound electron/ characteristic electron exits the shell and is now called a recoil electorn with a certain energy equal to the subtraction of the energy of the incoming photon - energy of the scattered photon

Question 22

how does the cross-sectional probability of the compton scatter effect absorption change with energy of the incident photon?

Answer 22

the probability increases as the incident energy of the photon decreases

Question 23

how does the cross sectional probability of the compton scatter absorption change with atomic number?

Answer 23

the linear attenuation coefficient is almost linearly dependent of the atomic mass
but it is dependent on density because the higher the density the more scattering sites there are, so the higher the cross sectional probability

Question 24

how does the attenuation change with the energy of an incoming photon the compton scatter effect?

Answer 24

at mid-range energies, the attenuation is the highest
as energy decreases or increases past that range, the attenuation decreases
so at a certain range of energies, the number of photons absorbed from the compton scatter is the highest is dependent on the angle resulting from the energy of the incident photon.

Question 25

describe pair production

Answer 25

incident photon collides with bound electron
producing a positron electron pair (the same bound electron)
which are antimatter
undergo annihilation
and produce radiation
there is a threshold energy of 1.02 MeV

Question 26

how does the energy affect the probability of interaction in pair production?

Answer 26

it increases proportionally to energy

Question 27

how does the probability of interaction in pair production increase with atomic number?

Answer 27

high dependancy on the atomic number
increase- increase

Question 28

what is the relative importance of the photon interactions in different energy ranges?

Answer 28

in diagnostic imaging, photoelectric
in nuclear medicine, compton scatter
in radiotherapy, pair production

Question 29

order of energy of interaction processes from lowest to highest

Answer 29

photoelectric interaction
compton scatter
pair production

Question 30

what is the denominator of the cross section of interaction?

Answer 30

unit particle fluence which is the number of particles passing through a unit area. In this case, it is a unit area of the detector

Question 31

what is equal to the number of photons absorbed/scattered, i.e those that are not detected by the detector?

Answer 31

the initial number - number of photons transmitted