Interaction of Radiation with Matter

Question 1

Examples of charged radiation?

Answer 1

Heavy charged particles (eg protons, alphas, muons, large ions) and electrons

Question 2

Examples of uncharged radiation?

Answer 2

Gamma rays or neutrons

Question 3

Why is the energy transferred by a heavy charged particle per collision small?

Answer 3

Their mass is much greater than electron mass, so due to conservation of momentum the electron will only be transferred a small amount of energy.

Question 4

Stopping Power?

Answer 4

S = -dE/dx and is the loss of energy per unit length.

Question 5

What does the Bethe-Bloch Stopping Power Model describe?

Answer 5

The stopping power of a particle of charge ze moving with speed v through a material with atomic number Z, mass number A and density ρ.

Question 6

Up to what velocity can the relativistic β terms in the Bethe-Bloch equation be ignored?

Answer 6

v/c ~ 0.5

Question 7

How is the mass stopping power different to normal stopping power?

Answer 7

It is equal to the stopping power over the density, so it removes material density dependance.

Question 8

What is a common approximation of the BB equation?

Answer 8

S ∝ z^2/v^2

Question 9

What causes the Bragg Peak in a Bragg Curve?

Answer 9

As the ionising particle slows down, stopping power increases so the number of ionisations rapidly increases, before dropping off as the particle completely stops.

Question 10

What is straggling?

Answer 10

Once the energy of a particle is low enough, it is subject to directional changes that would not be possible before, visible in cloud chamber tracks.

Question 11

Key differences between heavy charged particles and electrons for ionisation?

Answer 11

Due to their lighter mass, electrons travel much faster and are much more susceptible to directional changes. They lose their energy over a much greater range of distances, and can also lose energy via radiative processes such as Bremsstrahlung.

Question 12

How is the stopping power for electrons represented?

Answer 12

A sum of contributions from collisional processes and radiative processes.

Question 13

What are the three key mechanisms by which photons interact with matter?

Answer 13

Photoelectric effect, Compton scattering and Pair Production

Question 14

What happens in the photoelectric effect?

Answer 14

The photon is absorbed and its energy is transferred to a photoelectron that has energy equal to the difference in the photon energy and the material work function.
Commonly with tightly bound, inner shell electrons.
The atom is left with an inner shell vacancy which is quickly filled by cascading electrons which emit x-rays or Auger electrons.

Question 15

What happens in Compton scattering?

Answer 15

An incident photon collides with a ‘free’ (loosely bound) electron, causing both the photon and electron to be scattered.

Question 16

What does the Klein-Nishina equation describe?

Answer 16

The angular distribution of γ rays Compton scattering from a single free electron.

Question 17

What is the shape of the Klein-Nishina distribution?

Answer 17

For low energies, the distribution is symmetrical for both 0 and 180 degrees. At high energies, the distribution is greatly forward peaked towards θ = 0.

Question 18

What happens in pair production?

Answer 18

For γ energies higher than 2m_e, it is possible for the photon to covert into an electron/positron pair. Due to energy conservation this interaction has to occur in the proximity of an atom.

Question 19

What is the microscopic cross-section σ?

Answer 19

The probability of a neutron interacting with a single atom.

Question 20

What is the macroscopic cross section Σ?

Answer 20

The product of number density n and the microscopic cross section σ.

Question 21

3 common neutron interactions?

Answer 21

Scattering, Radiative Capture and Fission, each with their own cross sections.

Question 22

What happens in neutron scattering?

Answer 22

A neutron simply collides with a nucleus and energy is transferred. The collision can be both elastic or inelastic, and the total cross section is just the sum of the elastic and inelastic cross sections.

Question 23

What happens in radiative capture?

Answer 23

A neutron collides with a nucleus and is absorbed. The release of binding energy causes γ ray emission.

Question 24

What does it mean if a nucleus is fissionable?

Answer 24

It can be made to fission with the absorption of a high energy neutron.

Question 25

What does it mean if a nucleus is fissile?

Answer 25

It can be made to fission with the absorption of a neutron of any energy, so they are less stable.

Question 26

Radiative capture and fission are both types of what?

Answer 26

Absorption, characterised by the absorption cross section σ_a = σ_γ + σ_f

Question 27

What does it mean if a neutron can be described as thermal?

Answer 27

A thermal neutron is one which has similar energy to its surroundings. They can now be modelled as a gas rather than a beam.

Question 28

What does the collision parameter α describe?

Answer 28

It depends on the mass of the nucleus relative to the neutron, and quantifies the ability of the material to slow down neutrons: higher α means greater slowing ability.

Question 29

What is the mean log energy decrement ξ?

Answer 29

ξ = ln(E_0) - ln(E_1) (average)