Interactions Flashcards
Name the 4 types of interactions
- None
- Change of direction with no energy loss - classical scattering
- Change of direction and lose energy
- Compton scattering - Absorbed - photoelectric effect, pair production, photonuclear disintegration
Explain classical scattering
Photon and electron interact
The photo is completely absorbed by electron but the binding energy or electron is not higher than photon energy.
This means there’s a change in direction but no ionisation so no energy loss.
The photons are scattered forwards and gets remitted in a diff direction but same energy
State the 5 interactions from lowest to highest energy, start with lowest
Classical
Photo electric - density cubed
Compton scattering
Pair production
Photonuclear disintegration
What is photoelectric absorption
Photon interacts with inner shell electron.
BE = to photon energy.
Photon is absorbed so electron is either ionised or excited.
Excited means it escapes the shell or moves to higher shell. This gives of extra energy which is emitted as a xray photon
Ionisation means ion pairs are created.
This cannot occur unless the binding energy is higher than photon.
This increases proportionally to Z cubed.
What’s the probability of photoelectric absorption in relation to Binding energy
BE > electron no interaction
BE = electron - creates interaction
BE < electron - rapidly decreases.
What is the relationship between density and PEE
Density cubed is proportional to PEE
Explain Compton scattering
Photon and electron interact on outer shell.
Photons energy is given to electron and recoil electron is ejected.
The photon is scattered with a change of direction and loss of energy.
The energy of recoil energy is equal to the binding energy and KE.
This scattered photon is absorbed photo electrically.
The recoil electron will drop into a hole of an other shell atom creating an ionising event.
This scatter is dependant on electron density.
Explain the Compton scattering angles.
The photon moving at 0 degrees can mean no energy loss.
The photon moving at 180 can mean more energy is transferred to the electron.
What happens if the mass density of the absorber increases in Compton
Is increases Compton scatter
What’s the advantages of Compton
Higher energy and greater penetration
Explain pair production
Photon with enough energy goes through electron cloud and interactions with electron close to nucleus. Energy must be above 1.022MeV
The electrostatic force makes the photon disappear into electron and positron.
They have the same mass and same energy of 511keV
The electron will interact with any electron and the positron will cancel out when interacting with an electron. The energy cannot be destroyed so it creates 2 X-rays of an energy is 511Kev each
What is Braggs peak
Shows the rate that energy is being deposited in the tissue. It only releases to electrons and protons.
When ionised the electron will whizz off and brags peak will represent the likeliness of interaction.
What 2 ways energy is lost
Electrons interact a with another electron and bumps into eachother.
Or id they pass closely to another electron and change direction
What is KERMA
KE RELEASES INTO THE MATERIAL
there’s an exponential decay of kerma with depth.
It allows for skin sparing as electrons are moving forward and give dose deeper into the surface
Explain the concept of build up region.
Elections are released to the surface of the skin.
The elected in high energy are scattered forward.
So it moves in a forward direction and takes the dose under the skin.
So braggs peak says it releases dose at the end of the track
