4.2 EM Radiation and its interactions with matter Flashcards
What are the types of Absorption interactions between EM radiation and matter?
- Photoelectric effect
- Pair production
Less common:
- Photonuclear
What happens during the Photoelectric effect?
- Incoming photon interactions with inner shell orbital electrons and is completely absorbed (and no longer exists)
- If the energy of the photon is greater than the binding energy of the electron then the electron is ejected from the atom (this is a secondary electron)
When does the Photoelectric effect dominate?
(1/)E³ and Z³!!!
When photons are low energy - probability is proportional to 1/E³
(Low energy protons can more easily give all energy to a single electron in one interaction - the faster a photon is moving the less likely it is to hit an electron)
When Z number of material is high - Z³
When the material is dense - there are more electrons within the material so there is a higher chance of the photon interacting with one - this is what makes the contrast between bone and tissue and air on x-ray
When is the Photoelectric effect used in practice?
- kV imaging - planar x-ray and CT - images are made up of areas photons have passed through and not interacted with and areas they have interacted with and been absorbed
- Superficial kV ~50-150kVp
- Some brachytherapy ~50-500kV photons
Short range treatment because absorption is dominant at these energies - therefore the dose is more localised
What happens during Pair Production?
- The photon interacts in the electric field (Coulomb field) of the nucleus and an electron (-) - positron (+) pair is produced
- The positron then collides with an electron and annihilation occurs - the positron disappears and two 511kV are produced in opposite directions
When does the Pair production effect dominate?
- Very high energy photons - only if photon >1022kV (2x mass of the electron)
- When the Z number of a material is high - proportion to Z²
When does the Pair production effect appear in practice?
Not really inside the patient but may occur within the machine
What happens in Photonuclear interactions?
VERY HIGH ENERGY photons (10mEV+- absolute limit of what is used in RT)
interact with a nucelus and eject a nucleon (photon or neutron)
What are the types of Scatter interaction?
Compton scatter
What happens during Compton scatter?
- Incoming photon interacts with outer shell electron
- The photon transfers so energy to the electron which causes it to change direction and be scattered at an angle
- The energy transferred is sufficient to eject the outer electron and the
When does Compton scatter dominate?
High photon energy 1/E
Independent of Z (can be low)
Denser materials
When is Compton scatter used?
External beam radiation
High energy photons can travel further and reach deeper targets
Too much scatter to be useful for imaging
What is the Auger effect?
If an electron is removed from an atom a higher energy electron cascades down to fill the space it left
This releases excess energy as a photon
Sometimes this excess energy is transferred to another electron which then has enough energy to escape the atom
This is an Auger electron
How does radiation scatter?
Low energies scatter in all directions
Higher energies have forward scatter
The radiation beam is monodirectional
When the beam interacts with matter it scatters in all direction
How does the radiaiton actually interact with tissue?
The secondary electrons emitted from the interaction between the radiotherapy beam and the matter it passes through
These can travel outside the collimator beam and deposit energy as they travel through the material and interact with it
