Week 3: Interaction of radiation with matter part 2

Question 1

What is the photoelectric effect?

Answer 1

In the photoelectric effect, the energy of the photon is used to free an electron from an atom and give it kinetic energy. The photoelectron moves away from the atom with energy:

T = E - B,

where B is the binding energy of the electron.

Question 2

Why can the photoelectric effect not occur with free electrons?

Answer 2

To conserve energy and momentum.

Question 3

Why is the photoelectric effect most likely to occur with inner shell electrons?

Answer 3

As they are the “least free”. This leaves the atom with an inner shell vacancy, filled by outer shell electrons, resulting in the emission of X-rays or Auger electrons.

Question 4

What is Compton scattering?

Answer 4

Occurs when a photon transfers some of its energy to an electron. After the interaction, the recoil electron moves away with some kinetic energy and the energy of the original photon is correspondingly decreased.

Question 5

Draw a diagram for Compton scattering.

Answer 5

(10)

Question 6

Using the conservation laws and the cosine rule, derive the relationship between energy and angle for Compton scattering.

Answer 6

(11)

Question 7

What is pair production?

Answer 7

Once the energy of the photon is greater than twice the electron rest mass, it becomes possible for the photon to convert into an electron - positron pair.

Question 8

For pair production to be possible, where must it occur and why?

Answer 8

The interaction must occur near to an atom in order or conserve energy and momentum.

Question 9

Why is there a Z dependence for all the electron processes?

Answer 9

As all three processes are required to occur in the electric field of an atom.

Question 10

What is the equation for the reaction rate of a neutron travelling through some material? Define all symbols used.

Answer 10

(12)

Question 11

What is the macroscopic cross section?

Answer 11

The product of the number density, n, and the microscopic cross section, lowercase sigma, is the macroscopic cross section, capital sigma. It has units of m^-1.

Question 12

What are the three common outcomes in neutron collisions?

Answer 12

Scattering
Radiative capture
Fission

Question 13

Explain neutron scattering and describe what happens to the neutrons energy.

Answer 13

Neutron scattering occurs when a neutron collides with a nucleus and energy is transferred.

In most cases, the neutron will have significantly more kinetic energy than the nucleus, so energy will be given to the nucleus, however for low energy neutrons, it is possible to gain energy from the nucleus instead.

Question 14

What is the difference between elastic and inelastic scattering?

Answer 14

If kinetic energy is conserved in this process, the it is referred to as elastic scattering.

If some of the kinetic energy is converted into excitation energy of the nucleus, then it is inelastic scattering.

Question 15

How is the probability of a scattering reaction occurring quantified?

Answer 15

Through the use of the scattering cross section. It is made up of the elastic and inelastic scattering cross sections.

Question 16

What is radiative capture?

Answer 16

Radiative capture occurs when a neutron collides with a nucleus and becomes part of it.

The addition of the neutron releases binding energy, which results in the production of a gamma ray.

The probability of this reaction taking place is measured by the radiative capture cross section.

Question 17

What is neutron induced fission?

Answer 17

When a nucleus absorbs a neutron and then splits into two large pieces, emitting neutrons and gamma radiation.

The probability of fission is measured by the fission cross section.

Question 18

Sketch a diagram depicting neutron induced fission.

Answer 18

(13)

Question 19

If a nuclide is fissile, what does that mean?

Answer 19

The addition of a neutron is sufficient to cause fission.

Question 20

When is a neutron thermal?

Answer 20

When a neutron has kinetic energy comparable to the atoms in its surroundings, it can be described as being thermal, as in thermal equilibrium.

At room temperature, this occurs at kT = 0.025 eV.

Question 21

How can the intensity of a gamma ray beam be modelled if it is travelling through a material of thickness x?

Answer 21

(14)

Question 22

What is the linear attenuation coefficient? What symbol is used for it?

Answer 22

It is the probability of interaction per unit length travelled and is presented a lowercase mu.

Question 23

How does the mass attenuation coefficient relate to the linear attenuation coefficient?

Answer 23

(15)

Question 24

Why is a build-up facto used?

Answer 24

As additional photons reach the detector due to there being uncollimated attenuation.

Question 25

Why are low mass numbers used for moderating neutrons? Give examples of typical materials used.

Answer 25

Moderating neutrons requires low mass number nuclides to ensure high energy loss per collision. Typical choices include: water, paraffin blocks. polythene and concrete.

Question 26

List the three main gamma ray interaction mechanisms in order of the energies where they are the dominant process from lowest to highest.

Answer 26

Photoelectric effect Compton scattering Pair production

Question 27

Explain why X-rays are emitted after a photoelectric effect interaction.

Answer 27

The photoelectric effect leaves an inner shell vacancy, which is filled by an electron from an outer shell, which emits X-rays as it de-excites.

Question 28

State the forms of radiation that will be produced following a pair production interaction.

Answer 28

Electron and positron, X-rays from positron annihilation.

Question 29

Explain what is meant by the term microscopic cross section as it refers to neutron interactions.

Answer 29

The microscopic cross section of an atom is the effective area it presents as a target for neutron interactions.

Question 30

Briefly describe the process of radiative capture.

Answer 30

A neutron is absorbed into a nucleus. The addition of the neutron releases binding energy, which is emitted as a gamma photon.

Question 31

Define the terms fissile and fissionable.

Answer 31

Fissile means that the nuclide can be made to fission by the absorption of a neutron of any energy. Fissionable means that there is a threshold kinetic energy for the neutron, above which it can induce the nuclide to fission.

Question 32

Briefly explain why two interaction processes are considered when designing neutron shielding.

Answer 32

Neutron absorption cross sections are larger at low energies, so the neutrons are moderated (slowed down) by scattering to make them easier to absorb.