# Interaction of Radiation with Matter Flashcards

1
Q

A

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

2
Q

A

Gamma rays or neutrons

3
Q

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

A

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.

4
Q

Stopping Power?

A

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

5
Q

What does the Bethe-Bloch Stopping Power Model describe?

A

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.

6
Q

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

A

v/c ~ 0.5

7
Q

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

A

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

8
Q

What is a common approximation of the BB equation?

A

S ∝ z^2/v^2

9
Q

What causes the Bragg Peak in a Bragg Curve?

A

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

10
Q

What is straggling?

A

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.

11
Q

Key differences between heavy charged particles and electrons for ionisation?

A

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.

12
Q

How is the stopping power for electrons represented?

A

A sum of contributions from collisional processes and radiative processes.

13
Q

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

A

Photoelectric effect, Compton scattering and Pair Production

14
Q

What happens in the photoelectric effect?

A

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.

15
Q

What happens in Compton scattering?

A

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

16
Q

What does the Klein-Nishina equation describe?

A

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

17
Q

What is the shape of the Klein-Nishina distribution?

A

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.

18
Q

What happens in pair production?

A

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.

19
Q

What is the microscopic cross-section σ?

A

The probability of a neutron interacting with a single atom.

20
Q

What is the macroscopic cross section Σ?

A

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

21
Q

3 common neutron interactions?

A

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

22
Q

What happens in neutron scattering?

A

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.

23
Q

A

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

24
Q

What does it mean if a nucleus is fissionable?

A

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

25
Q

What does it mean if a nucleus is fissile?

A

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

26
Q

Radiative capture and fission are both types of what?

A

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

27
Q

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

A

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

28
Q

What does the collision parameter α describe?

A

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.

29
Q

What is the mean log energy decrement ξ?

A

ξ = ln(E_0) - ln(E_1) (average)