Fundamentals of Radiation Damage- Lecture 2

Question 1

What is the primary knock-on atom?

Answer 1

The first atom that the incident radiation particle collides (interacts) with and displaces. This can then go on to displace other atoms in the material. Aka PKA

Question 2

Conditions for whether an atom is displaced or not

Answer 2

If the energetic particle has energy greater than Ed then the atom will be displaced from its lattice site. If it less than Ed then the atom won’t be displaced.

Question 3

What else can happen if the energetic particle has energy less than Ed?

Answer 3

The atom it interacts with can vibrate. Can therefore get localised heating which may locally raise the temperature of the material above its melting point and cause localised melting (amorphous region).

Question 4

What happens when the displaced atom has energy greater than Ed?

Answer 4

It can go on to displace other atoms in the material and so on. This is called a collision cascade

Question 5

How long does the energetic particle carry on for?

Answer 5

Continues to displace atoms as it travels through the material until its energy is less than Ed

Question 6

What does displacement energy (Ed) depends on?

Answer 6

The atom being displaced and the types of bonds it makes with surrounding atoms (depends on elements the surrounding atoms are). Highest around 90eV for T and W. Lowest around 25eV for Pb and Al

Question 7

Order of radiation damage processes caused by incident energetic particle

Answer 7

1. Interaction of incident energetic particle with a lattice atom.
2. Transfer of KE to the atom producing a PKA.
3. Displacement of the PKA from its lattice site.
4. PKA travels through lattice creating additional knock-on atoms.
5. Production of a displacement cascade (collection of point defects created by the PKA).
6. Termination of the PKA as an interstitial.

Question 8

What events happen after the production of a displacement cascade and what do they result in?

Answer 8

Energy dissipation, spontaneous recombination and clustering results in stable Frenkel pairs and defect clusters.
Defect reactions by thermal migration (later) result in single interstitial and vacancy recombination, clustering, trapping, defect emission

Question 9

What is dynamic annealing (description)?

Answer 9

During collision cascade the displaced atoms may have enough energy to move into vacancies, healing the damaged structure. When this defect annihilation occurs during collisions and is because of the energy given to the atoms by the collisions (and not thermal energy) it is called dynamic annealing.

Question 10

What is a Frenkel pair?

Answer 10

A vacancy and interstitial pair created when an atom is displaced from its lattice site and moves to an interstitial position

Question 11

Displacements per atom (dpa) definition and what it’s used for

Answer 11

The number of times an atom is displaced for a given fluence. 1dpa means that on average every atoms has been displaced from its lattice site once. Used to normalise the amount of radiation damage that different reactors produce (different types of radiation in different materials).

Question 12

Fluence definition

Answer 12

Defined as the total number of particles that intersect a unit area over a specific time interval of interest. Units number of ions/cm^-2.

Question 13

Flux compared to fluence

Answer 13

Flux=fluence/time

Question 14

How does yield stress vary with dpa?

Answer 14

Increases steady exponential with increasing dpa.

Question 15

Why isn’t dpa used alone?

Answer 15

It doesn’t give information about the ion (particle) producing the damage. So fluence is also used.

Question 16

Will a particle with E greater than Ed always displace an atom?

Answer 16

No. E.g Kinchin-Pease model

Question 17

The three interactions which stop particles

Answer 17

Elastic (nuclear)
Inelastic (electronic)
Radiation (we ignore as mainly for thing like beta radiation)

Question 18

Nuclear stopping

Answer 18

The energy is removed from the PKA through impact collisions. Elastic interaction (ballistic interaction). Kinetic energy conserved.

Question 19

Electronic stopping

Answer 19

The energy is removed from the PKA through transfer to the electronic structure. Is the slowing down of a particle/ion due to the inelastic collisions between bound electrons in the material, and the particle/ion moving through it.

Question 20

Why is electronic stopping an inelastic interaction?

Answer 20

As energy is lost during the process (not used to displace atoms)

Question 21

What do collisions in electronic stopping result in?

Answer 21

Excitation of bound electrons of the medium
Excitations of the electron cloud of the ion

Question 22

What favours electronic/nuclear stopping over the other one?

Answer 22

High mass or low energy means nuclear dominates.
Low mass of high energy means electronic dominates.

Question 23

How does damage caused and particle range depend on whether nuclear or electronic stopping dominates?

Answer 23

For increased nuclear stopping (lower energy or heavier ion), the damage is more severe (greater dpa) and particle range is less.

Question 24

What is mean projected range?

Answer 24

The average range an energetic particle travels through a material. Symbol Rp

Question 25

What does mean projected range depend on?

Answer 25

The energy of the particle (greater travels further).
The size of the particle (lighter travels further for same energy).
The density of the material (higher slows particles down more).

Question 26

What do neutrons cause (in neutron radiation)?

Answer 26

Although small they do cause atomic displacements and significant levels of damage. Also induce/undergo other effects:
Induce collision cascades - scattering.
Absorption to induce fission.
Absorption - producing γ radiation or β- particle

Question 27

What is the neutron absorption cross section, its unit and what its value means?

Answer 27

The probability of neutron absorption by a nucleus.
Unit is barn=10^-24 cm^2.
The larger the cross section the more likely the neutron will be absorbed by the nucleus.
Symbol σa

Question 28

What are the two outcomes of neutron absorption?

Answer 28

Radiative capture σγ
Neutron induced fission σf
σa=σγ+σf

Question 29

Scattering cross section

Answer 29

The probability of a neutron being scattered.
Symbol σs

Question 30

Total cross section and formulae for probability that neutron is absorbed or scattered

Answer 30

σt=σa+σs
Probability neutron absorbed=σa/σt
Probability neutron scattered=σs/σt

Question 31

Difference between thermal and fast neutrons and how this effects their interactions with nuclei

Answer 31

Thermal neutron has less kinetic energy and so is slower than fast neutron. Faster travelling neutron spends less time near a nucleus of an atom so the scattering, capture and fission cross sections are generally lower for a given isotope

Question 32

Types of scattering and absorption interactions with neutrons and their notations and what they mean

Answer 32

Scattering: elastic (n,n), inelastic (n,n’).
Absorption: EM (n,γ),
charged (n,p), (n,α), (n,β),
neutral (n,2n), (n,3n), (n,4n),
fission (n,f)
The left letter is what goes in and the right letter is what comes out