Directly Ionising Radiation Flashcards
Why can electrons scatter off other electrons?
They are approximately (only approx due to relativistic effects) the same sizes
What are terms a and b
a is Atomic radius (10^-10m)
b is impact parameter, measure of distance of incoming electron from atomic nucleus
What kind of electron interaction processes are there?
Collisional (excitation, ionisation)
Radiative (brem, cerenkov)
When do you have excitation?
b»a
When do you have ionisation?
a~b
What happens during excitation?
Closest approach of electron is large
Incoming electron loses small amount of energy to atom
Electron scattered by small angle
Electron in inner shell moves to outer shell
Only needs a few eV
Excited atom dissipates energy by emitting light photon if gas, or as heat in solid
What happens during ionisation?
Distance of closest approach same order as atomic dimension
Incoming electron loses a larger amount of energy to atomic electron
Electron in inner shell gets enough energy to overcome binding energy and is removed
Incoming electron loses energy and is deflected
What amount of energy transfer is more frequent in ionisation?
Small energy transfer
What is the name given to the ejected electron in ionisation and how much energy does it have?
Delta ray
delta E > 100eV
Has sufficient energy to produce ionisation and excitation at a distance from path of original incoming electron
Quantum mechanics dE/dl predictions
e^2
1/v^2
rho
Z/A
What is dE/dl
Linear stopping power
Rate of kinetic energy loss per unit path length of the particle
S
(S/rho is mass stopping power)
How many events will an electron undergo?
10^4 - 10^6 interactions before all energy is gone, depositing few eV of energy each time
When would Bremsstrahlung occur?
b«a
radiative
What does electron interact with in Bremsstrahlung?
Coulomb field of nucleus
What happens during Bremsstrahlung?
Electron interacts with coulomb field, decelerates and changes direction
Photon emitted with energy between 0 and E
What is cerenkov radiation?
When charged paticles passing through dielectric at velocity greater than phase velocity of light of that medium emits photons. Light emitted in optical region of spectrum,, only small amount, usually ignored
What is approximate ratio of radiative to collisional?
EZ/800
E in MeV
Total stopping power
Stot = Scoll + Srad
Effect of Z and velocity on Scoll
Rate of energy loss greatest when particle moving slowly
Rate of energy loss greater in low Z materials
Why is energy loss greater in low Z materials in Scoll?
They have higher electron density than high Z materials
Electrons are more loosely bound compared to higher Z materials
High Z materials have more tightly bound electrons not available to this type of interaction
Bragg peak rate of energy loss
Inversely proportional to square of velocity of charged particle
Greatest when particle is moving slowly
Why is Bragg peak not observed in electron PDD?
Energy and range straddling
Factors affecting radiative stopping power Srad
Z^2
1/m^2
E
path length vs range for electrons
Path length is total distance travelled before coming to rest
Range is sum of individual path lengths in the original direction of travel
How does mass angular scattering power vary with Z and E for electron beam?
Z^2/E^2
How does variation of mass angular scattering power affect shape of isodose?
Causes bulging of isodoses which can be clinically significant - dose under skin spreads out
What is origin of electron build up?
Electron beam at surface predominantly forward scattered
Increasing lateral scatter with depth effectively increases electron pathlength through deeper layers
What is the practical range?
The depth of the intersection of the linear portion of the curve with the bremsstrahlung tail
What does general expression for absorbed dose in medium exclude?
Electron interactions which deposit dose at distance from a point - radiative energy loss and energy transferred to delta rays
What is absorbed dose per unit mass of gas?
J.W/e
where J is total charge of one sign resulting from ionisation (not exposure, which is photons)
W is mean energy to create ion pair in gas
e is electronic charge
What are conditions of Bragg Gray Cavity Theory?
Infinite uniform medium
Small gas filled cavity
Cavity sufficiently small not to perturb electron fluence or electron energy fluence
Electrons only lose a small fraction of energy crossing cavity
Absorbed dose in cavity is solely due to charged particles crossing cavity
Assumptions for Bragg Gray in electrons
No secondary electrons (delta rays) produced in cavity
No electrons stopped in cavity
Mass stopping power ratios energy independent
Discount energy lost by electrons for Brem
Ratio of dose in gas and medium in Bragg Gray
Ratio of dose deposited in gas cavity and in medium in absence of gas cavity is same ratio as mass stopping power of gas and medium respectively
Dmed = Smed_gas.Dgas
Equation for Dmed, Bragg Gray
Dmed = Smed_gas . JW/e
Special consideration Bragg Gray
Energy spectrum of electrons as they slow down (average stopping power ratios over energy spectrum)
Density or polarisation effect (effect of polarisation of medium near to charged particle tracks which affect stopping powers)
What is mass stopping power?
S/rho = 1/ rho . dE/dl
What is linear energy transfer
L_delta = (dE/dl)_delta
Restricted to interactions where losses less than delta, as delta rays deposit energy at a distance (often delta 10keV)
What is continuous slowing down approximation
Can consider electrons as continuously slowing down as they undergo many interactions before losing all of their energy, losing energy each time
Why, for high energy electrons, is the magnitude of de/dl greater for liquids than for gases
Relativistic electrons cause polarisation of atoms in medium, screening E field which reduces strength of interactions and reduces dE/dl
Magnitude of effect increases with increasing density
dE/dl is therefore higher in vapour than condensed phases
What is J?
Total charge of one sign per unit mass from ionisation
dQ/dm
Not exposure which is specific to photons in air
What is W?
Mean energy required to create one ion pair
What is equation for Dgas?
J.W/e
