Photon Interaction processes

Question 1

Name 4 photon interaction processes

Answer 1

1. Photoelectric effect
2. Compton Scattering
3. Rayleigh scattering
4. Pair Production

Question 2

Describe Photoelectric effect

Answer 2

1. X-ray photon collides and transfers all energy to k-shell electron (Ephoton = binding energy of electron + KE of photoelectron)
2. Energetic k-shell electron is emmited, leaving a vacancy behind in k-shell
3. Outer shell electron fills in vacancy, emitting photon with characteristic energy (Ep = difference in energy levels)

Question 3

How does photoelectric effect interact with soft tissue (low z) and metals (high z)

Answer 3

Low z - k-shell beinding energies are low (<1keV) meaning the characteristic x-ray sproduced are absored in the material and don’t make it out

High z - characteristic radiation can escape, this is why you use metal targets in x-ray tubes.

Question 4

What is an Auger electron?

When are they produced?

What is fourescent yield?

Do high or low materials produce more Auger electrons?

Answer 4

• Auger electrons are produced when the k-shell vacancy is filled and an outer shell electron is emitted (instead of the usual photon).
• Flourescent yield wk, = number of k x-ray photons / number of k shell vacancies i.e. what fraction of the vacancies when filled produce photons.Think about it, it makes sense, it’s in the name.
  
  • wk =1 = no auger electrons
  • wk = 0 = all Auger eectrons
• low z materials are almost all auger electrons.

Question 5

What is a photoelectric mass absorption coefficent>

How is it related to u/P?

Answer 5

Tau/P

u/P is proportional probability photon is removed from beam by any process

t/p is proportional to probability photon is removed from the beam by PE alone.

Question 6

How does probability of PE depend on:

Z - atomic number

E - photon energy

Answer 6

T/P ∝ Z³/E³

Question 7

Should you use higher or lower energy x-rays for Mammography?

Why?

Answer 7

Lower energy x-rays

Difference in z between normal and pathological calcifications is very small. Want to maximise the amount of absorption by PE to increase contrast, hence you need to lower the energy.

Less attenuating, more easily stopped in soft tissue in breast.

Question 8

How do you choose ideal energy of x-ray beam

Answer 8

Comprimise between:

Higher energy = more penetrating

Lower energy = higher proabibility of interaction via PE, hence better contrast

Question 9

Describe Compton Scattering

Answer 9

• Photon collides with FREE electron
• Photon’s path is altered (scattered) with reduced energy
• Electron supplied with energy recoils

Question 10

Is Compton Elastic or inelastic?

Answer 10

Inelastic - momentum is preserved, but energy is not. Loss of energy for incoming photon.

Question 11

For compton scattering:

What does the change in photon energy depend on?

Answer 11

• initial photon energy
• angle of scatter

Question 12

Compton scattering:

Change in wavelength?

Answer 12

Delta lambda = h/m_ec * (1 - cos(theta))

• Best just to remember this, otherwise you have to do that nasty derivation.*
• remember the 1-costheta becase:*
• when theta = 0 ,costheta =1, delta lambda = 0*

Question 13

Compton scattering:

Change in photon energy?

Small energy approximation?

Answer 13

Delta E = E_o * (a(1-costheta))/(1+a(1-costheta))

a = E_o/0.511 with Eo in MeV

Don’t know if i need to know this?

Small Energy:

Delta E = Eo²/m_ec^{2 *} (1-costherta)

How much more energetic was the incident photon than the rest mass of the elctron that it collided with? How has that been modified by the angle it’s scattered through

Question 14

How does Compton scattered photon energy depend on intial photon energy?

Answer 14

• Higher energy photon: Greater proportion of photons energy is transferred to electron
• Probabilty of compton interaction decreases with increasing energy
• At low energies, most of the energy is retained by the photon

Question 15

3 reasons why scatter is bad

Answer 15

1. Low energies - scattered photons also have low energy and don’t make it out of the patient. Still contributing to dose but not to image.
2. Higher energies - scattering is less likely, but scattered photons have higher energies and may escape more easily & contribut to image (noise).
3. Higher energies - scattered radiation with high enough energy to escape the patient can contribute to staff dose

Question 16

What is Compton mass energy coefficent?

Answer 16

Probability that photon is removed form the beam by compton effect alone

Question 17

How does probabiity of compton interaction (sigma/rho) depend on:

E

Z

Answer 17

Sigma/P indpendent of z

Sigma/P ∝ 1/E

Increased energy, lower probability of scatter.

Question 18

Why is compton independent of Z?

Answer 18

Compton actually depends on Electron density

• No atoms in A grams of material (A = atomic mass)
• Natoms/mass = No/A
• Nelectrons/atom = z
• Electron density = Nelectrons/mass = No (z/A)
• BUT Z/A is approximately costant hence it’s independent of Z

Question 19

How do you measure bacscatter factor?

Typical value?

Answer 19

• Dose at skin with scatter/ Dose without scatter
  
  • i.e. work out the dose without scatter, then put your dosemeter there and measure what you actually get.
• Typically 1,3

Question 20

Is it better to have the tube above or below the couch?

Answer 20

Under is better (although less practical) because the backscatter from the patient is aimed at your feet or attenuated by the bed

If you have an overcouch situation, the backscatter is directed at your head

Only really makes a difference for flouro

Question 21

Rayleigh Scattering:

• Alternate name
• Describe

Answer 21

Elastic scattering

• Whole atom absorbs photon
• Bound electrons resonate at the incoming frequency
• Electrons re-radiate energy out as photon (same energy)
• Scattering in forward direction
• (<10% of interactions) not very important

Question 22

How does the probability of elastic scattering depend on Z & E

Answer 22

e/Rho ∝ z²/E³

^{<em>Very stronG energy dependence because you need to hit that resonant frequency of the electrons. </em>}

Question 23

Describe energy dependence of mass attenuation coefficent in

water

lead

Answer 23

Water

• Rapid fall at low energies due to PE (1/E³)
• Compton (1/E) effect dominates at higher enegies and flattens out

Lead

• Similar to above
• Additional k-edge discontinuties (increase in stopping power when energy of incoming photon matches energy of k-shell electrons (increased PE)

At higher energies, COMPTON IS INDEPENDENT OF Z

Question 24

Explain why you get a k-edge?

Answer 24

E< K edge binding energy

• K-electrons can’t be emitted, less absoption of electrons
• Beam is more pennetrating
• Lower u/P

E> K edge binding energy

• K electrons can be emitted
• greater absorption and less penetration
• higher U/P

Question 25

How do you take advantage of the k-edge for imaging?

Answer 25

• Match the k-edge approcximately to peak of x-ray spectrum
• Maxmimise absorption in the detector
• lower dose required for image, lower dose to patient
• NB// use Iodine or Barium k-edge as contrast agents

Question 26

Describe Pair production & Anihilation

Do we care?

Answer 26

PP

• High energy photon absorbed in the nucleus
• Produces positron and electron
• need twice the rest mass of electron 2*0.511 meV = 1.02 mEv

Anihilation

• Slow positron his free electron and is anihilated
• Preduces two 511keV photons in opposite directions

Not important at diagnostic energy levels (but is proportional to z)

Question 27

How do you make u/P

Sum all the components?

Multiply all the component elements?

Answer 27

Sum them, remember that it’s an exponential multiplication so they add in the exponent.

Question 28

Summarise Interaction mechanisms

All 4 interaction mechanisms:

What do they interact with?

Energy dependence

z dependence

Answer 28

Question 29

Summarise absorption and attenuation of scattering mechanisms.

For all four scattering mechanisms:

Absorption?

Attenuation?

Answer 29