Interactions Flashcards
What can photons interact with?
Atomic nucleii
Orbital electrons
E field surround above
What is coherent scattering also known as?
Rayleigh scattering
What is coherent scattering?
Photon passes near bound electron
Bound electron is tightly bound and atom absorbs recoil
Interaction sets electron into oscillation
Photon re-emitted with same frequency at different angle
Energy conserved
How does coherent scattering vary with Z, E
Proportional to Z^2.5
Proportional to E^-2
Most probable at low photon energies with high Z
Only contributes a few percent or less to total attenuation coeff.
What is pair production?
Incoming photon interacts with EM field of atomic nucleus
All energy used to create electron positron pair
What is minimum energy for pair production?
1.02 MeV
Rest mass of the two particles 2mc^2 = 2 x 0.511MeV
How is energy shared in pair production?
hv0 - 2mc^2 shared between two particles
On average 50/50 at RT energies
At higher energies becomes 75/25
In extreme cases, all excess goes to one particle
Direction of electron positron pair?
Mean direction of pair is close to that of incoming photon
How do electron and positron interact?
Excitation
Ionisation
Bremmstrahlung
What is annhilation radiation?
At end of range of positron, combines with free elctron and produces two annihilation photons, with 0.511MeV each.
As momentum is conserved, emitted in opposite directions
Dependence of pair production on E/Z
Proportional to E above 1.02MeV
Proportional to Z
What is triplet production?
Occurs when incoming photon decelerated in field around electron and three particles result
Created electron and positron and original electron
Threshold for triplet production
2.04MeV
What is photo nuclear effect?
Occurs when energy of incident photon exceeds binding energy of nucleon
Photon absorbed by nucleus
Emission of either neutron or proton and transformation of nucleus into radioactive reaction product
Minimum photon energy and likelihood of photo nuclear effect
Few MeV, rises to maximum between 12 and 24MeV and decreases rapidly at higher energies
Probability is low, less than 5% of total attenuation cross section at 15-20Mev
What are the practical consequences of the PN effect in RT?
Radiation protection:
Neutron flux found with higher energy photon beams (greater than 10MV)
Induced radioactivity has implications for maintenance of linac
Induced radioactivity in air means regular air changes in bunker
What is mass attenuation the sum of?
PE
CS
PP
Coherent
Triplet
What is the cross section?
Apparent area that an interaction centre presents to incoming radiation which if traversed by radiation gives rise to interaction
CS = probability of intersection / unit particle fluence
Cross section and mass attenuation coeff.
mu/rho = Na.sigma / M
Relationship between PE effect and E/Z
E^-3
Z^4
What happens in the photoelectric effect?
Photon absorbed by atom
Atomic electron emitted
Atom rearranges by emission of characteristic radiation or release of Auger electrons
What is mean angle between incoming photon and created particle in PP? What is relevance with increasing E?
theta = mc^2 / E
where E = 1/2.(hv0 - 1.02)MeV
Angle between the two gets smaller with increased E - more forward peaked
What is the mass energy transfer coefficient?
Product of the mass attenuation coefficient and the fraction of energy transferred to charged particles as KE by interacting incident photons
What interaction does not contribute to energy transfer?
Coherent scattering
Equation for mu_tr
mu_tr = _E_tr / hv0 . mu
Equation for mu_en
mu_en = mu_tr (1-g)
Where g is fraction of energy deposited at a distance from site of interaction due to Brem (includes positron annihilation and characteristic x-rays).
Why is mass attenuation coefficient an attractive quantity in mixtures?
Independent of density of absorber but dependent on atomic omposition of medium, good for use in compounds such as the body
X, fluence relationship
X = lambda (mu_en / rho) (e/W)
KERMA X relationship
KERMA = X/(1-g) . (W/e)
How does a free air ionisation chamber work
Photon beam passes between parallel plates without hitting anything
Produces ionisation
Polarising voltage applied
Ions collected and measured
Geometry of a free air ionisation chamber
Accurately known area defined by diaphragm
Guard electrodes required to ensure only ionisation in known volume is collected
Dimensions of box mean electrons released lose their energy before reaching the electrodes or walls
Distance between plates must be twice range of secondary electrons
What is exposure in free air ionisation chamber?
X = Q/A.L.rho
How can dose in air be related to dose in medium with mass energy absorption coefficients?
DOSEmed = X(W/e) . ((mu_en/rho)med/(mu_en/rho)air)
Equation for cross section
probability of interaction/
unit particle fluence
Equation for B
B = NA. rho / M
B is number of interaction centres per unit volume
