Photons may be generated through:
- Electron - nucleus interactions (bremsstrahlung)
- Electrons filling vacancies in orbital shells, releasing characteristic x-rays
- Release of energy from a nucleus in a high energy state (gamma rays)
Bremsstrahlung is an electron-nucleus interaction. Negatively charged electrons are attracted and slowed by the positively charged nucleus. The kinetic energy lost by the electron is released as a photon. The amount of energy lost by the electron is not a constant, so this type of interaction gives rise to a continuous spectrum of photon energies. Bremsstrahlung is more likely to occur when nuclei have a higher charge (ie. a high atomic number) and when electrons have significant starting energies.
A vacancy in an electron shell may be filled by a free electron or by an electron in a more distant shell. As this electron falls into the vacancy in the lower shell, it releases kinetic energy as a photon. The energy of the released photon is dependent on the energy difference between the two shells, and therefore photons are released at discrete energies (unlike the continuous spectrum of bremsstrahlung radiation).
Vacancies in electron shells may be generated through collisions of free electrons with orbital electrons. When electrons are used to generate bremsstrahlung photons, there is often some contamination of the spectrum by characteristic x-rays arising from electron-electron interactions. The characteristic x-rays are usually not desirable as they have lower energy than the bremsstrahlung x-rays, and are usually filtered out.
When an electron and its corresponding anti-lepton (the positron) combine, they are annihilated and release their energy as a pair of photons. The photons contain energy equivalent to the rest mass of both particles - 511 keV each. Positrons are obtained through pair production (from high energy photon-nucleus interactions) or through radioactive decay.
Gamma rays are photons that arise from the nucleus itself (rather than through electrons), typically as a result of nuclear decay.
Much like the electrons shells of an atom, the nucleus of an atom has various energy states. The lowest energy state is known as the ground state. When a radioactive nucleus decays, through release of alpha particles, beta particles or electron capture, the resulting nucleus is typically in a higher energy state. This state is unstable and the nucleus will normally release this energy as a gamma photon to acheive the ground state. This process is often incredibly fast but there are some elements (such as 99mTc) that have a long period before gamma decay occurs.
The energy of the gamma rays released by the nucleus is dependant on how much energy is used in the original decay process, and whether this energy is transferred to the escaping particle (if present). Elements that decay through electron capture, where an orbital electron is absorbed by the nucleus, typically have discrete energies of photon release. If energy is imparted to a radioactive particle in a more random way, then a spectrum of photon energies may result.
Use of Photon Generation Techniques in Radiation Oncology
Bremsstrahlung is the most commonly employed photon generation technique in modern radiation oncology departments. Linear accelerators and older x-ray tubes both utilise bremsstrahlung to generate a spectrum of photon energies.
Characteristic x-rays are not used, but must be accounted for and appropriately filtered out.
Gamma rays are used for many brachytherapy treatments. Gamma rays were also used for older teletherapy machines which have been replaced by linear accelerators.
Annihilation quanta are not used for treatment (although may result when positrons are generated through pair production). They are an important concept for positron emission topography (PET) scanning, as positron emitting compounds can be targeted to certain metabolic processes (such as hypoxia or glucose metabolism.