Radiation may be ionising or non-ionising, by virtue of its ability to cause ionisation of an atom; ie: cause the atom to lose an electron.
To be ionising, radiation must have the ability to knock an electron out of an orbital shell. This requires overcoming the binding energy of the electron. Therefore, many radiations may be both ionising or non-ionising, depending on their energy. A photon must have an energy of over 124 eV to be considered ionising. Atoms can be ionising at very low energy, particularly if they are electron acceptors.
Therapeutically used radiations are all ionising, falling into two broad categories.
- Directly ionising radiations are charged particles that interact with orbital electrons to cause ionisiation
Charged particles include electrons (negatively charged), protons and heavy ions (positively charged).
- Indirectly ionising radiations are neutral 'particles' that give their energy to charged particles, which then cause ionisation in a similar way to directly ionising radiation.
Photons and neutrons are two commonly used indirectly ionising radiations. Photons transfer energy to electrons in various ways, whereas neutrons transfer energy to protons.
Conceptualising types of ionising radiation
Electrons, protons and other charged particles will typically interact with most atoms they encounter, due to repulsive/attractive electromagnetic forces. They lose their energy slowly and constantly.
Photons do not interact strongly with charged particles, but may encounter them and transfer energy to them. If this happens, the generated electron will behave in a similar way to an electron generated through other means. Therefore, photons are a useful way of delivering dose to a point deep within a tissue, as it will not have to fight its way through all the charged particles in the way.