Fundamentals of Radiation Damage- Lecture 2 Flashcards
What is the primary knock-on atom?
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
Conditions for whether an atom is displaced or not
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.
What else can happen if the energetic particle has energy less than Ed?
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).
What happens when the displaced atom has energy greater than Ed?
It can go on to displace other atoms in the material and so on. This is called a collision cascade
How long does the energetic particle carry on for?
Continues to displace atoms as it travels through the material until its energy is less than Ed
What does displacement energy (Ed) depends on?
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
Order of radiation damage processes caused by incident energetic particle
- Interaction of incident energetic particle with a lattice atom.
- Transfer of KE to the atom producing a PKA.
- Displacement of the PKA from its lattice site.
- PKA travels through lattice creating additional knock-on atoms.
- Production of a displacement cascade (collection of point defects created by the PKA).
- Termination of the PKA as an interstitial.
What events happen after the production of a displacement cascade and what do they result in?
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
What is dynamic annealing (description)?
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.
What is a Frenkel pair?
A vacancy and interstitial pair created when an atom is displaced from its lattice site and moves to an interstitial position
Displacements per atom (dpa) definition and what it’s used for
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).
Fluence definition
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.
Flux compared to fluence
Flux=fluence/time
How does yield stress vary with dpa?
Increases steady exponential with increasing dpa.
Why isn’t dpa used alone?
It doesn’t give information about the ion (particle) producing the damage. So fluence is also used.
Will a particle with E greater than Ed always displace an atom?
No. E.g Kinchin-Pease model
The three interactions which stop particles
Elastic (nuclear)
Inelastic (electronic)
Radiation (we ignore as mainly for thing like beta radiation)
Nuclear stopping
The energy is removed from the PKA through impact collisions. Elastic interaction (ballistic interaction). Kinetic energy conserved.
Electronic stopping
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.
Why is electronic stopping an inelastic interaction?
As energy is lost during the process (not used to displace atoms)
What do collisions in electronic stopping result in?
Excitation of bound electrons of the medium
Excitations of the electron cloud of the ion
What favours electronic/nuclear stopping over the other one?
High mass or low energy means nuclear dominates.
Low mass of high energy means electronic dominates.
How does damage caused and particle range depend on whether nuclear or electronic stopping dominates?
For increased nuclear stopping (lower energy or heavier ion), the damage is more severe (greater dpa) and particle range is less.
What is mean projected range?
The average range an energetic particle travels through a material. Symbol Rp
What does mean projected range depend on?
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).
What do neutrons cause (in neutron radiation)?
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
What is the neutron absorption cross section, its unit and what its value means?
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
What are the two outcomes of neutron absorption?
Radiative capture σγ
Neutron induced fission σf
σa=σγ+σf
Scattering cross section
The probability of a neutron being scattered.
Symbol σs
Total cross section and formulae for probability that neutron is absorbed or scattered
σt=σa+σs
Probability neutron absorbed=σa/σt
Probability neutron scattered=σs/σt
Difference between thermal and fast neutrons and how this effects their interactions with nuclei
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
Types of scattering and absorption interactions with neutrons and their notations and what they mean
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
