The interacting electron is known as the incident electron. Incident electrons may interact with:
- The nucleus of an atom
- The orbital electrons of an atom
Interactions may be
- elastic, resulting in no loss of energy
- inelastic, where the kinetic energy of the incident electron changes
There are four possible scenarios:
- Elastic collisions with the nucleus result in scattering of the electron with no loss of energy. The amount of scattering that occurs is dependent on the atomic number of the nucleus - higher atomic numbers cause more scattering
- Elastic collisions with the orbital electrons of an atom, also leading to scattering of the incident electron but no transfer of energy
The two elastic collisions are less important as no energy is deposited in the tissue, and therefore there is no dose from these interactions. They do have a role to play in attenuation of the beam due to scattering of electrons; this causes changes in dose distribution particularly at the junction between two mediums with different atomic numbers (eg. muscle and bone).
- Inelastic collisions with orbital electrons result in loss of energy from the incident electron to the orbital electron. If this energy is sufficient for the electron to ascend to a higher shell it is known as excitation. The space in the lower shell is rapidly filled by another orbital electron, which releases a photon. This photon may generate characteristic x-rays if the binding energy of the shell is sufficiently large (seen with heavier elements such as lead or tungsten), or visible light for less tightly bound electrons.
- If sufficient energy is transferred to an orbital electron it may be able to escape the atom, leaving the atom ionised. This atom is often highly reactive and quickly reacts with a neighbouring atom to acheive a stable state. The freed electron may cause further ionisations before it loses its energy and is captured by an atom.
Inelastic collisions with the nucleus result in bremsstrahlung. Bremsstrahlung is German for "braking radiation". As the electron interacts with the nucleus, it slows down and changes direction. The energy that is lost by the electron is released as a photon.
In radiation therapy physics:
- Excitation is important as it explains characteristic x-rays
- Ionisation is important as this is the primary means of electrons causing damage to the DNA molecule
- Bremsstrahlung is important as this is the method of generating x-rays in an x-ray tube or a linear accelerator. It is also responsible for photon contamination of therapeutic electron beams (the 'bremsstrahlung tail').
Heat
Heat is thermal energy that is lost to the media when electrons interact. The amount of thermal energy generated is very small and insufficient to cause changes in the body.
