PNM Flashcards
Which are the materials used in a reactor?
Fuel, cladding, moderator, coolant
Which function do we want to use materials for in a nuclear reactor?
- control
- shield
- electronics
- superconductor (accelerators)
Type of radiations?
- photons (we study ionizing photons)
- charged particles, like e- or ions
- neutrons
- elastic scat(energy and momentum conservation)
- inhelastic scat (momentum conservation)
- nuclear reaction (n,gamma)
What does a photon carry?
-Energy (h*nu)
- momentum (mc^2 = pc = sqrt(pc^2+m^2c^4) )
ES: if lambda = 400 nm = c/nu => E = 3 ev
What is the crosssection?
It’s the probability of a certain phenomenon.
photoeletric cross section?
photoeletttic means that a photon hits and releases an electron.
sigma_pe #= E^-3 (decreasing)
We have in order L edge and K edge
Also sigma_pe #= Z^n where
n = 4 @ 0.1 Mev
n = 4.5 @ 3 Mev
What happen if photon moves at E>1.02 Mev?
pair production, electron and positron, each with 511 kev
sigma_pp #= Z^2
What is compton scattering?
Scattering between photon and electron. Results in both a scattered electron and photon.
Decreases with energy
sigma_c #= z
Coherent vs incoherent compton scattering?
It’s coherent (compton scattering) when the photons interact with the entire e- surrounding the atom, causing the cloud to vibrate momentarily. The photons doesn’t change direction.
It’s predominant at low energy
Incoherent scattering is more common at high E and it’s a scatter where a single electron is ejected due to the photons
monopoles, dipoles and quadrupoles electric field dependence on distance r?
Due to series expansion:
- monopoles
E propto 1/r^2 (energy of electric field)
V propto 1/r
- dipoles
E propto 1/r^3
V propto 1/r^2
- quadrupoles
E propto 1/r^4
V propto 1/r^3
so higher poles have a shorter “range of action”
What is a Schottky pair?
It’s when I remove two ions with different charge from a lattice.
What’s a Frenkel pair?
It’s when I remove an ion from the lattice so that a pair (interstitial and vacnacy) is formed. This causes the creation of a dipole.
What is spallation?
(p,n) reaction or (n,2n) reaction, on nuclei
How can photon be emitted by atoms (x-ray)?
by fluorescnece (an internal e- is emitted so an external e- takes his place and emits a x ray
by brehmsstralung electron are slowed down by matter and emits photon
What are L edge or K edge in photoelectric crossection profile?
They are a sudden increase of crossection due to reaching the threeshold for the L or K elettronic states
most probable effect of photon interaction with atom for increasing energy?
- at low energy: photoeletric
- at medium energy. Compton
- at high energy: pair production
What is the stopping power?
The energy loss per unit path dE/dx
What is a vacancy from an electrical pov?
it’s a cumulation of charge so it’s a monopole
are monopoles easier to create?
No. they are associated with higher energy (E #= 1/r^2) so they are harder to create than frenkel or shotty pair.
What a vacancy creation causes thermodinamically? and an interstitials?
It increases enthalpy and entropy.
This is because we start from 4 bounds and getting the atom to the boundary create only 2 bounds (less energy, less stable)
Similarly with an interstitials but the energy changes due to elastic deformation on of the crystal to accomodate the interstitials
How much energy is needed for frenkel pair creation (scatter out of lattice)?
Temperature is not enough (around 10 ev)
What is PKA?What’s a collisional cascade?
Is the Primary knock on atom. It generates a collisional cascade. Primary knocked atom can knock secondary knocked on atom (SKA). It happens for E>E_d (displacement energy)
There is the displacement spike. In the core region there are more vacancies, inthe perifery there are more interstitials
There is the thermal spike where the energy is below the E_d and all the energy is given as thermal oscillation.
All the phenomena happen in picoseconds (very fast)
What has a bigger collisional cascade?
Ion is smaller becuase it immmeidately loses energy also with interactions with electrons.
Neutron do not interact directly with e- but PKA do.
What happens when a neutron is absorbed?
The nucleus moves with the recoil momentum.
momentum balance
m v = (m + M) V
so V = m/(m+M) v = 1/(1+A) v
energy of final nucleus:
T = 0.5 (M+m) V^2 =T_in * m / (m+M)
It can cause collisional cascade only if it’s an epithermal. But epithermal have low absorption crosssection so we have colllsional cascades only due to scattering