Radiation Flashcards
How is radiation classed?
Ionizing - forms ions by knocking electrons out
Non-ionizing
What are the different types of radiation?
High energy photons (X-rays, gamma rays), electrons (beta), alpha (He nucleus), heavy ions (fissile and PKAs), neutrons, protons, mesons, positrons
What are the effects of neutron radiation?
Interact with materials by scattering (elastic, inelastic) or absorption (radiative capture)
What are the effects of alpha radiation?
Charged and ionising - pass materials and strip off electrons causing the formation of electron-hole pairs
What are the effects of beta radiation?
Ionising but with high atomic mass materials for shielding causes Bremsstrahlung radiation
What is B- decay?
atom -> atom+1 + B- (electron) + anti-neutrino
What is B+ decay?
atom -> atom-1 + B+(positron) + neutrino
What is the nature of the atoms in beta decay?
The three atoms are of definite momentum and so the spectrum is continuous.
What is the nature of the atoms in alpha decay?
The two particles have defined energy levels leading to a spectrum with discrete energy lines
How does the kinetic energy of beta decay vary with intensity?
Because beta particle is so light the nuclear recoil energy is practically zero, the decay energy is shared between the beta particle and the anti neutrino, hence the energy ranges from 0 to almost the total decay energy
What is the half life?
The time for the number of radio nuclides to drop too half the initial number
What is the dose and its units?
The energy deposited by radiation in a material, Gray (Gy) or J/kg
What is the unit of equivalent dose?
Sievert (Sv)
What are the different types of isotopes?
Long-lived - eg Biological
Short-lived - decay as excite
What is the half value layer?
The thickness of material required to reduce the intensity of radiation by 50%
What is the attenuation length?
the reciprocal of the linear attenuation coefficient or mean free path
What are the different stages of gamma radiation?
Prompt gamma - fission in reactor core
Decay gamma - decay of fission product
Activation gamma - capture of neutron
What are the main ways gamma radiation ionises?
Photoelectric effect, Compton scattering, pair production
What is the photoelectric effect?
Gamma photon interacts with and transfers its energy to an atom ejecting electrons
What is Compton scattering?
Incoherent scattering - incident photon loses enough energy to eject electron, remaining photon energy scattered, probability decreases with increasing photon energy
What is pair production?
interaction via coulomb force, energy of photon is converted into electron-positron pair, positron has very short range and recombines with a free electron and is converted into two gamma photons
What is the stopping power of a material?
The average energy loss of the particle per unit path length MeV/cm
What is electronic stopping?
The slowing down of the radiation due to inelastic collisions between bound electrons in the medium and the ions moving through it
What is nuclear stopping?
The slowing down of the radiation due to elastic collisions (ballistic) between the radiation particles and atom in the medium.
What is the bragg curve?
energy loss per unit length, density of ionisation increases until peak (bragg peak) then drops to 0
What is the range and what does it depend on?
The penetration depth, density, range and effective atomic mass
What is LET?
Linear energy transfer - the amount of energy an ionising particle transfers to the medium
What is radiation damage?
The disruption of the initial structure by high energy radiation
What are effects of radiation damage?
dimensional changes, phase changes, amorphization, optical
embrittlement, hardening, creep
phase separation, resolution, corrosion
What are some effects on reactor environments?
Possible dissociation and/or activation of coolant
Thermal - softening, creep of fuel cladding
Embrittlement of structural components e.g. RPV welds
Enhanced corrosion of cladding and RPV
Fuel/clad interaction
Fission gas release => increase rod internal pressure
Segregation of elements
What are the main steps of radiation damage?
Transfer of energy from the energetic incident radiation particle to a lattice atom to create primary knock on atom (PKA)
moment of PKA away from its lattice site through the lattice creating more PKA
Evolution of a displacement cascade: single vacancies and interstitials but also clusters of defects. The cascade reaches a maximum in damage during the ballistic phase after which there is usually some degree of recovery.
What are the different types of structure defects?
Point defects: include missing atoms (i.e. vacancies), incorrectly positioned atoms (e.g. interstitials) and chemically inappropriate atoms (dopants). Point defects may exist as single species or as ‘small’ clusters, bubbles or voids containing multiple species.
Line defects or dislocations extend through the crystal in a line or chain. The dislocation, line has a central core of atoms that are located well away from usual crystallographic sites (in ceramics this extends, in cylindrical terms a nanometre or so).
Planar defects extend in two dimensions. Many different types exist the most common of which is the grain boundary. Other common types include stacking faults, inversion domains and twins.
Precipitates are larger clusters and so extend too far to be point defects
What are the universal damage concepts?
Displacive radiation damage is initiated upon formation of Frenkel pair defects in the lattice
The concentration of Frenkel pair defects in an irradiated material far exceeds the equilibrium thermodynamic concentration of Frenkel defect pairs.
The radiation damage response of a material is determined by the fate of these excess Frenkel pairs in the lattice.
What is the criteria for radiation resistant materials?
Resistance to void swelling, amorphisation
What is fluence?
Total number of ions or neutrons incident on the surface
What is flux?
Rate at which ions or neutrons are incident on the surface
What is dose?
Total number of events per unit volume
Deposited energy is dose multiplied by energy per particle
What is the most radiation resistant material?
Er2Zr2O7
What are the critical amorphisation conditions?
The temperature or dose at which a material amorphises
What is chemical rate theory?
fate of irradiation-induced point defects
Harmless - defect annihilation
Medium - depends on compound
Harmful - defect aggregation
What is the Kinchin-Pease model?
estimation of the number of displaced atoms per PKA
