Interaction of ionising radiation with tissues and detectors Flashcards
what are the 3 types of interaction of photons with matter?
absorbing : photoelectric effect, pair production
reflecting/ scattering: compton scatter
what is the cross section of interaction?
formula: probability of interaction (absorption/scatter) / number of particles passing through a unit area (unit particle fluence/flux)
what does the attenuation coefficient depend on?
the energy of the incoming beam
the thickness of the beam: a thicker beam will allow photons to scatter and deflect into the beam, so more photons will be detected
what mathematical model does the removal of photons follow?
a negative exponential curve
what does the number of transmitted photons depend on?
initial number of photons in the narrow beam
the thickness of the material
the linear attenuation coefficient
relationship between final number of photons which are detected by the detector at the other end of the material and the linear attenuation coefficient
the higher the linear attenuation coefficient, the more photons are absorbed by the material, so the fewer photons are detected
relationship between final number of photons detected at the other end of the detector and the thickness of the material
the thicker the material, the more photons are attenuated (absorbed) and the lower the final number of those detected by the screen
for which conditions is the equation of removal of photons only valid under?
there is a narrow beam of incoming radiation
from a single energy source
how to mathematically calculate the linear attenuation coefficient?
add the cross section of interaction of each interaction process
1. photoelectric effect
2. compton scatter
3. coherent scatter
4. pair production
what is the mass attenuation coefficient equal to mathematically?
the mass probability of:
- coherent scatter and compton scatter
- pair production
- photoelectric interaction
Photoelectric effect
An incident photon with energy hf is given to a single orbital electron (bound electron), which is sometimes found in the k-shell 80% and sometimes in the l-shell 20%
The bound electron gains kinetic energy equal to the energy of the photon - binding energy of the bound electron
Almost all the energy is transferred to the photoelectron meaning that the binding energy of the bound electron is much lower in comparison to the incident energy
There is now a vacancy in the shell
Which is filled by outer electrons
Explain the k-shell and l-shell differences
k-shell is inner shell
l-shell is outer shell
which explains why there is a higher likelihood that the photon colliding with the inner electron will release a high energy photoelectron as the higher the energy level, the higher the binding energy, so that the kinetic energy of the photoelectron will decrease with an increase in shell number
what is another name for the photoelectron?
the auger electron
what does the cross sectional probability of the photoelectric effect depend on?
the density, the atomic number and the energy
what does the atomic number affect?
the capacity of absorption of incident x-rays of the material. The higher the atomic number, the higher the absorption capacity
what is the difference between the density and the atomic number?
atomic number is the number of protons or electrons in an atom
density is the number of atoms that can be packed into a unit volume
how does energy affect the probability of photoelectric absorption?
the higher the energy, the lower the probability of photoelectric absorption
this is because the auger electron, or the photoelectron, will have a lot more kinetic energy to penetrate through the material
how does the energy affect the attenuation of the photoelectric absorption?
the higher the energy, the lower the attenuation
what are the units for the linear attenuation coefficient?
it is cm^-1
what are the units for mass attenuation coefficient?
process: divide the linear attenuation coefficient by the density
units are in cm^2/g
Describe the compton scatter
incident photon collides with electron in the shell (characteristic/ bound electron). Photon leaves with a new, lower energy and is scattered at a certain angle. The bound electron/ characteristic electron exits the shell and is now called a recoil electorn with a certain energy equal to the subtraction of the energy of the incoming photon - energy of the scattered photon
how does the cross-sectional probability of the compton scatter effect absorption change with energy of the incident photon?
the probability increases as the incident energy of the photon decreases
how does the cross sectional probability of the compton scatter absorption change with atomic number?
the linear attenuation coefficient is almost linearly dependent of the atomic mass
but it is dependent on density because the higher the density the more scattering sites there are, so the higher the cross sectional probability
how does the attenuation change with the energy of an incoming photon the compton scatter effect?
at mid-range energies, the attenuation is the highest
as energy decreases or increases past that range, the attenuation decreases
so at a certain range of energies, the number of photons absorbed from the compton scatter is the highest is dependent on the angle resulting from the energy of the incident photon.
describe pair production
incident photon collides with bound electron
producing a positron electron pair (the same bound electron)
which are antimatter
undergo annihilation
and produce radiation
there is a threshold energy of 1.02 MeV
how does the energy affect the probability of interaction in pair production?
it increases proportionally to energy
how does the probability of interaction in pair production increase with atomic number?
high dependancy on the atomic number
increase- increase
what is the relative importance of the photon interactions in different energy ranges?
in diagnostic imaging, photoelectric
in nuclear medicine, compton scatter
in radiotherapy, pair production
order of energy of interaction processes from lowest to highest
photoelectric interaction
compton scatter
pair production
what is the denominator of the cross section of interaction?
unit particle fluence which is the number of particles passing through a unit area. In this case, it is a unit area of the detector
what is equal to the number of photons absorbed/scattered, i.e those that are not detected by the detector?
the initial number - number of photons transmitted
