Interactions Flashcards
what are the types of photon interactions?
- None - good for imaging as Xrays do not deposit energy. they reach detector unaffected
- change direction, no energy loss
- change direction, lose energy
- Absorption
talk about absorption.
good for treatment - dose goes into body and affects cells -> to cell death
good for imaging - different materials attenutate x rays differently so Number of Xrays reaching detector for different materials - can tell apart tissues
bad for imaging - gets dose in areas you dont want it in, justified as it is CT scan but you get dose in healthy regions
what is the beer lambert law?
Interaction of x rays with matter is exponential so it never hits 0
what is the linear attenuation coefficient
Fraction of beam of x ray or gamma rays that is absorbed/scattered per unit of thickness of absorber.
thickness remains same throughout
what does the linear attenuation coefficient account for?
- number of atoms in cubic cm of material
- probabilty of a photon being scattered or absorbed from nucleus or electron of one of these atoms
What does linear attenuation coefficient depend on?
Density and energy of photon
what are the conditions for linear attenuation coefficient?
mono-energetic beam
narrow beam geometry
what is a half value layer?
thickness of material needed to reduce its intensity of a beam, to 1/2 of its initial value
can be used as a measure of beam geometry
what does narrow beam geometry do?
reduces impact of scatter in measurement
reduces penumbra
what does a mono energetic beam do?
Avoid beam hardening
when does classical scattering occur?
when energy of the photon is smaller than the binding energies of the electrons
no energy transferred
describe classical scattering
- low energy photon interacts
- photon interacts with electrons
- there is a change in direction
- no energy loss, no ionisation
- photon absorbed by electron
- energy causes it to vibrate
- energy is re-emitted as new photon
- new photon identical to original
occurs with low energy x rays (little importance)
what happens in a photoelectric interaction
- low energy photon interacts with electron
- photon energy absorbed
- electron raised to higher shell or escapes the atom becoming a photoelectron
- leaves a hole in lower shell
- electrons move down from outer shells to fill hole - give off excess energy in form of x ray and this is called valency filling
when does photoelectric effect occurs?
photon energy must be same to binding energy of electron
what energy does photoelectron have?
KE equal to difference between incident x ray and binding energy of the electron
what is the probability of a photoelectric effect occuring?
photon energy less than binding energy -> cannot occur
photon energy = to binding energy
-> at its greatest
photon energy higher than binding energy -> rapidly reducing
inversely proportional to third power of photon energy
directly proportional to third power of atomic number of absorbing material
when does compton scatter happen?
energy of photon much higher than BE of electron
change direction lose energy
describe compton scatter
- photon collides with electron
- photon scattered - change direction, loses energy
- energy given to recoil electron
- atom ionised and ion pairs created
- scattered photon lost energy
scattered photon?
any direction
0 degree = no energy loss
180 degree = more energy to electron
higher the energy of incoming photon, the smaller the angle of deflection
go through more compton interactions
eventually absorbed photoelectrically
secondary electron creates further ionisation events
summary of compton scatter
as x ray energy increases:
- penetration in tissue without interaction increases
- increased compton relative to PE
-reduced total scatter
as atomic no of absorber increases:
- no effect
as mass density of absorber increases:
- proportional increase in compton
when does pair production occur?
when photon has enough energy, can go through electron cloud and come close enough to nucleus to come under influence of strong electrostatic field of nucleus.
describe pair production
photon is converted to mass
2 particles created
electron and positron
1.022MEV needed
- shared between two particles as KE
- electron interacts as normal
- positron is antimatter so more damaging
- positron loses energy as it is near the end
- combines with electron
- annihilate each other
- mass -> energy
- 2 photons created travelling at 180 to each other
when does photonuclear disintegration occur?
higher energies (>10MEV)
what is photonuclear disintergration?
nucleus absorbs high energy photon and becomes excited
immedietaly decays by emitting subatomic particles
can produced short lived radionuclides
