Directly ionising radiation Flashcards
What are the main interactions for electrons?
Coulomb interactions:
collisional losses,
Bremsstrahlung interactions,
Cerenkov Radiation
What are the sources of coulomb interactions?
Elastic with orbital electrons or nuclei
Inelastic with orbital electrons
Radiative with nuclei
What is the impact parameter?
Measure of distance from incoming electron and nucleus = b
What is the process of collisional loses
b is large, the incident electron loses some energy to an electron on the atom. This energy excites the orbital electron. This can either lead to ionisation, a delta ray, through which the ejected electron causes more ionisation or the electron will fall back to its ground state releasing a photon of the same energy.
What energy is needed to produce a delta ray
Greater than 100eV
Collisional loses: What are the energy transferred, momentum gained, energy gained, energy loss along a path dl proportional to?
Energy transferred prop coulomb field of atomic electron
Momentum prop time of interaction 1/v
Energy gained prop 1/v^2 and 1/E
Energy loss along a path dl prop electron density of material
Collisional loses: What is dE/dl proportional to?
dE/dl prop e^2, 1/v^2, density, Z/A
Bremsstrahlung: What are acceleration, radiative energy loss, and emission proportional to?
Acceleration, f, prop Z/m
Radiative energy loss prop E
Emission prop (Z/m)^2
What is Cerenkov Radiation?
A ‘sonic boom’ for photons, when light is emitted in the visible spectrum as the phase velocity of the photons > c
What is the ratio of radiative losses to collisional losses?
EZ/800
What is the electron fluence?
phi = dN/da
What is the electron energy fluence
psi = dN/da . average energy
What is linear stopping power?
S=dE/dl, it is the rate of kinetic energy loss per unit path length of the particle. Consider energy deposited in the irradiated material
What is linear energy transfer?
Ldelta = (dE/dl)delta, the linear stopping power restricted to E of a delta ray = 100eV
What is the mass stopping power?
S/density = 1/density . dE/dl
What is the total linear stopping power?
Stot = Srad + Scoll
What is Srad’s rate of energy transfer proportional to?
Z^2/m^2 . E
What is the electron path length?
The total distance travelled before coming to rest. p(I) = sum(p(i))
What is the electron range?
The sum of the distance travelled in the direction of the original path length. t=sum(p(i)cos(theta))
What distribution describes the angular and spatial spread of an electron beam?
Gaussian
What is the mean square scattering angle?
Theta^2 prop density.l
What is the mass angular scattering power?
theta^2/density.l = Z^2/E^2
What causes build up in electron beams?
The electron beam at the surface is mainly forward directed. There is increased lateral scatter at depth so the electron ranges are shorter. This creates charged particle equilibrium at depth.
What is the practical range of an electron beam?
The depth of intersection between the linear section of the curve and the bremsstrahlung tail
What is the absorbed dose in medium?
Energy deposited per unit mass of a medium. It depends on the energy fluence and mass collision stopping power.
What is the equation for absorbed dose?
D=Intergal(psi.Scoll.dE)
What does the absorbed dose exclude?
Dose deposited at a distance from the interaction due to radiative losses or delta rays.
What is the mean energy required to create an ion pair in gas?
W = E/N
What is the dose deposited per unit mass of gas?
D = JW/e
What is the Bragg Cavity Theory?
The ratio of the dose deposited in a gas filled cavity and a medium, if the cavity is not present, is the same as the ratio of the mass stopping powers of the medium and the gas respectively.
What is the equation associated with the Bragg Cavity Theory?
Dmed = Smed,gas . Dgas, where Smed,gas = ratio of the mass stopping powers
What is required for the Bragg Cavity Theory to hold?
An infinite medium with a gas filled cavity
The cavity doesn’t perturb the electron fluence or the electron energy fluence
The absorbed dose is entirely due to charge particles crossing the cavity
What assumptions are made for the Bragg Cavity Theory?
No secondary electrons are produced or stopped in the cavity
Secondary charged particles only undergo collisional losses - no delta rays
Ignore energy lost through radiative losses as they will deposit dose far from the cavity
Mass stopping power ratios are independent of energy
What should also be considered in the Bragg Cavity Theory?
The energy spectrum of electrons as they slow down - need to take an average of the stopping power ratios over the energy spectrum
Density or polarisation effects
