Photon Interactions Flashcards
What happens in photoelectric absorption?
The incoming photon must have enough energy to overcome the binding energy of the inner orbital electron
Photon is completely absorbed
Residual energy is given as kinetic energy to the electron, allowing it to escape
Atom is left with a vacancy, so an electron from a higher shell drops down to fill this space
Does photoelectric absorption depend on atomic number?
Yes.
Absorption of xrays is proportional to Z^3
Are more photons absorbed by photoelectric absorption in soft tissue or bone? Why?
Bone.
Because bone has a higher atomic number.
What is the most dominant interaction process at radiotherapy energies?
Compton scatter
What happens in compton scatter?
Incident photon transfers SOME of it’s energy to the electron
Electron is ejected from atom
Photon continues with less energy in a different direction
Does compton scatter depend on atomic number?
No
At what energies does pair production occur?
Above 1.022 MeV
What happens in pair production?
Photon interacts with the NUCLEUS
Energy is transferred to the atom, annihilating the photon
An electron and a positron are created
Positrons are short lived and disappear when they meet an electron and are annihilated (forming 2 photons each of 0.51 MeV)
Electrons cause damage until captured by an atom
Does pair production depend on atomic number?
Yes.
As is is dependent on the nucleus, a higher Z means an increased probability of pair production
What is the concept of photons as ‘carrier waves’ in radiotherapy?
Photons generate electrons at depth within the patient
It is the electrons that cause damage and kill the tumour
How do photons damage tumour cells?
They don’t
They generate electrons at depth within the patient
The electrons then cause damage
What is ionisation?
The process by which a neutral atom gains a charge by:
Removal of an orbital electron (making the atom positively charged)
OR
In some cases an atom can acquire an electron (making the atom negatively charged)
What is excitation?
The energy given to an orbital electron is insufficient to eject it from an atom, but raises it to a higher energy level
When this electron drops back to fill the vacancy, it will release the excess energy as a photon
Why does a single photon interaction cause a chain of ionising events?
Electrons are small, light, and have charge, so have a higher probability of interacting
Each interaction can release another electron that can go on to interact, and so forth
Why is most of an electron’s energy deposited locally (in the neighbourhood of the initial photon interaction)?
Because every time it interacts, it loses energy and slows down
Is a photon beam monoenergetic or polyenergetic?
What does this mean?
Polyenergetic
It consists of a large number of photons with a spectrum of energies
What is fluence?
What is the equation?
What are the units?
The number of photons passing through a sphere of cross sectional area ‘a’ in space
Φ = dN/da
unit = m^-2
What is fluence rate?
What is the equation?
What are the units?
Fluence per unit time
dΦ/dt
unit = m^(-2)s^(-1)
What is energy fluence?
What is the equation?
What are the units?
The sum of all the energies of all the photons passing through a sphere of cross sectional area ‘a’
It is the total energy carried by the photons
Ψ = dE/da
unit = Jm^-2
What is the inverse square law?
The intensity of the radiation field is inversely proportional to the square of its distance from the source
I α 1/d^2
What is attenuation?
The reduction in intensity of the beam due to interactions
What are the 3 possible outcomes of attenuation (with regards to tumour kill)
Absorption (possible tumour kill)
Scatter (intensity reduced but no tumour kill)
Exit (beam intensity maintained)
What are the 2 main factors affecting fluence at a point in tissue?
Inverse square law
Attenuation
Does the chance of interactions increase or decrease as the photon beam is attenuated?
Why?
Decrease
Because there are less photons
What is the linear attenuation coefficient?
What equation can we use to find it?
It describes the fraction of attenuated photons per unit thickness
I = I0e^(-μx)
Where I0 is the initial intensity, and μ is the linear attenuation coefficient
What is the mass attenuation coefficient?
How can it be calculated?
The attenuation per unit mass rather than unit length
Calculated by dividing the linear attenuation coefficient by density
What is the difference between primary radiation and scatter?
Primary radiation is the radiation reaching and depositing energy directly at our point of interest
Scatter arrives indirectly
Does scattered radiation contribute dose to the patient?
Yes
Is primary radiation dependent on field size?
No
What are the sources of scatter?
Head scatter/leakage Patient scatter (scatter in the patient due to interaction processes)
Is head scatter the same for all linacs?
It varies between manufacturers
Is scattered radiation dependent on field size?
Yes
The higher volume irradiated means more chance of scattered photons
Can scatter be good?
Yes
It contributes dose to the tumour and gives is skin sparing
