Indirectly Ionising Radiation Flashcards
What are three interactions of high energy photons with matter?
Photoelectric
Compton
Pair production
What is primary dose vs scatter dose?
Primary dose is energy deposited by Compton electrons interacting adjacent to the site of the primary interaction
Scatter dose is energy deposited in interactions between scattered photons and electrons at a distance from the initial interaction
At low energies, what ends up with the majority of energy of incoming photon for CS?
Compton scattered photon
At high energies, what ends up with the majority of energy of incoming photon for CS?
Scattered electron
Compton interaction with photon energy
The cross section of the interaction falls slowly with energy at photon energy used in RT.
Compton interaction with atomic number
Cross section proportional to Z
What does Compton interaction depend on outside of Z and E?
Number of essentially free electrons in the medium.
Electron density
Ne = NA.Z/A
When is pair production significant?
Over 5MeV
Cross section of pair production with Z
Z^2
What is the effective atomic number?
The atomic number of an element which would interact with photons in the same way as the composite material
What is linear attenuation coefficient and symbol?
Energy removed from initial radiation beam
Fraction of incident beam of photons absorbed or scattered per unit thickness
mu
What is energy transfer coefficient and symbol?
Energy transferred to the medium
mu_tr
What is energy absorption coefficient and symbol?
Energy deposited in medium locally
mu_en
What is KERMA the sum of?
Initial KE of all charged particles produced as a result of radiation interactions
What does KERMA include?
Energy subsequently re-irradiated as Brem
Energy deposited outside ROI by particles created in it
NOT energy deposited in ROI from charged particles created outside of it
What does absorbed dose include?
Energy deposited in ROI from charged particles created outside of it
NOT
Energy subsequently re-irradiated as Brem
Energy deposited outside of ROI by particles created in it
How can electron energy function approximately be described?
Decreasing as an exponential function
What considerations are there for high energy photon beam?
ISL effect
Poly energetic beam, preferential attenuation of lower energy photons
Photon beam become harder and more penetrating
Fano theorum
In medium of given composition exposed to given flux of primary radiation, flux of secondary radiation is uniform and independent of density of medium as well as density variations from point to point
What is an interaction and how is it characterized?
Process by which direction and/or energy of photon is altered
Absorption or scattering process
What is KERMA dependent on and what do we need to describe it
The fluence of the initial beam
Need the mass energy transfer coefficient to describe it
What happens during PE?
(absorption process)
Energy of incoming photon greater than binding energy
All energy transferred to electron, which is removed, this is photo-electron
Outer electron relaxes into space created with emission of characteristic x-ray
What are discontinuities in PE interaction?
Change in number of electrons able to undergo photoelectric interactions as photon energy increases beyond binding energy of different shells.
How does free air ionisation chamber work?
Photon beam passes between parallel plates and ionise air
High polarising voltage present
Ions collected and measured
What is responsible for energy deposited at a distance from site of primary interaction?
Bremsstrahlung
Delta rays
Compton scattered photons
Number of electrons per unit mass
Ne = N_A . Z/A
(usually use relative electron density in practice where rho_e = Ne(substance)/Ne(water)
Relationship between mass energy absorption coeff. and mass energy transfer coeff.
mu_en / rho = mu_tr / rho x (1-g)
Energy transferred to KE of charged particles not all absorbed by medium as fraction g lost to photons - brem (includes positron annihilation and characteristic x-rays)
What is mass attenuation coefficient?
mu/rho
total mass attenuation coeff is sum of mass attenuation coeffs for individual interaction processes in material
What interaction does not contribute to energy transfer?
Coherent scattering
Why are we interested in stopping powers in Bragg Gray in MV photons?
Assume photon interactions in cavity gas negligable
Only dealing with secondary electrons
What are requirements for absorbed dose measurement in medium at MV energies
Small volume of air to be enclosed and measurement of charge
Chamber to be used to enclose known volume of air
Presence of chamber does not perturb energy fluence
What is secondary particle equilibrium?
The number, energy and directions of electrons leaving a volume must equal the number, energy and direction of those that enter it.
What drives absorbed dose curve if Brem is accounted for?
mu_en
lambda . mu_en/rho
