Interactions

Question 1

What can photons interact with?

Answer 1

Atomic nucleii
Orbital electrons
E field surround above

Question 2

What is coherent scattering also known as?

Answer 2

Rayleigh scattering

Question 3

What is coherent scattering?

Answer 3

Photon passes near bound electron
Bound electron is tightly bound and atom absorbs recoil
Interaction sets electron into oscillation
Photon re-emitted with same frequency at different angle
Energy conserved

Question 4

How does coherent scattering vary with Z, E

Answer 4

Proportional to Z^2.5
Proportional to E^-2
Most probable at low photon energies with high Z
Only contributes a few percent or less to total attenuation coeff.

Question 5

What is pair production?

Answer 5

Incoming photon interacts with EM field of atomic nucleus
All energy used to create electron positron pair

Question 6

What is minimum energy for pair production?

Answer 6

1.02 MeV
Rest mass of the two particles 2mc^2 = 2 x 0.511MeV

Question 7

How is energy shared in pair production?

Answer 7

hv0 - 2mc^2 shared between two particles
On average 50/50 at RT energies
At higher energies becomes 75/25
In extreme cases, all excess goes to one particle

Question 8

Direction of electron positron pair?

Answer 8

Mean direction of pair is close to that of incoming photon

Question 9

How do electron and positron interact?

Answer 9

Excitation
Ionisation
Bremmstrahlung

Question 10

What is annhilation radiation?

Answer 10

At end of range of positron, combines with free elctron and produces two annihilation photons, with 0.511MeV each.
As momentum is conserved, emitted in opposite directions

Question 11

Dependence of pair production on E/Z

Answer 11

Proportional to E above 1.02MeV
Proportional to Z

Question 12

What is triplet production?

Answer 12

Occurs when incoming photon decelerated in field around electron and three particles result
Created electron and positron and original electron

Question 13

Threshold for triplet production

Answer 13

2.04MeV

Question 14

What is photo nuclear effect?

Answer 14

Occurs when energy of incident photon exceeds binding energy of nucleon
Photon absorbed by nucleus
Emission of either neutron or proton and transformation of nucleus into radioactive reaction product

Question 15

Minimum photon energy and likelihood of photo nuclear effect

Answer 15

Few MeV, rises to maximum between 12 and 24MeV and decreases rapidly at higher energies
Probability is low, less than 5% of total attenuation cross section at 15-20Mev

Question 16

What are the practical consequences of the PN effect in RT?

Answer 16

Radiation protection:
Neutron flux found with higher energy photon beams (greater than 10MV)
Induced radioactivity has implications for maintenance of linac
Induced radioactivity in air means regular air changes in bunker

Question 17

What is mass attenuation the sum of?

Answer 17

PE
CS
PP
Coherent
Triplet

Question 18

What is the cross section?

Answer 18

Apparent area that an interaction centre presents to incoming radiation which if traversed by radiation gives rise to interaction

CS = probability of intersection / unit particle fluence

Question 19

Cross section and mass attenuation coeff.

Answer 19

mu/rho = Na.sigma / M

Question 20

Relationship between PE effect and E/Z

Answer 20

E^-3
Z^4

Question 21

What happens in the photoelectric effect?

Answer 21

Photon absorbed by atom
Atomic electron emitted
Atom rearranges by emission of characteristic radiation or release of Auger electrons

Question 22

What is mean angle between incoming photon and created particle in PP? What is relevance with increasing E?

Answer 22

theta = mc^2 / E
where E = 1/2.(hv0 - 1.02)MeV

Angle between the two gets smaller with increased E - more forward peaked

Question 23

What is the mass energy transfer coefficient?

Answer 23

Product of the mass attenuation coefficient and the fraction of energy transferred to charged particles as KE by interacting incident photons

Question 24

What interaction does not contribute to energy transfer?

Answer 24

Coherent scattering

Question 25

Equation for mu_tr

Answer 25

mu_tr = _E_tr / hv0 . mu

Question 26

Equation for mu_en

Answer 26

mu_en = mu_tr (1-g)

Where g is fraction of energy deposited at a distance from site of interaction due to Brem (includes positron annihilation and characteristic x-rays).

Question 27

Why is mass attenuation coefficient an attractive quantity in mixtures?

Answer 27

Independent of density of absorber but dependent on atomic omposition of medium, good for use in compounds such as the body

Question 28

X, fluence relationship

Answer 28

X = lambda (mu_en / rho) (e/W)

Question 29

KERMA X relationship

Answer 29

KERMA = X/(1-g) . (W/e)

Question 30

How does a free air ionisation chamber work

Answer 30

Photon beam passes between parallel plates without hitting anything
Produces ionisation
Polarising voltage applied
Ions collected and measured

Question 31

Geometry of a free air ionisation chamber

Answer 31

Accurately known area defined by diaphragm
Guard electrodes required to ensure only ionisation in known volume is collected
Dimensions of box mean electrons released lose their energy before reaching the electrodes or walls
Distance between plates must be twice range of secondary electrons

Question 32

What is exposure in free air ionisation chamber?

Answer 32

X = Q/A.L.rho

Question 33

How can dose in air be related to dose in medium with mass energy absorption coefficients?

Answer 33

DOSEmed = X(W/e) . ((mu_en/rho)med/(mu_en/rho)air)

Question 34

Equation for cross section

Answer 34

probability of interaction/
unit particle fluence

Question 35

Equation for B

Answer 35

B = NA. rho / M

B is number of interaction centres per unit volume