The energy of ionising radiations
Physics Narrative for 14-16
Energy in measured joules, but other units are sometimes convenient and so still in use
Energy is measured in joule – except when it's not. For historical reasons there are a number of other measures of energy for ionising radiations. Foremost among them is the electron-volt.
You'll often find the energies of, say, alpha particles measured in mega electron volt (MeV).
Electrons have a charge. The SPT: Electricity and energy topic shows how to calculate the energy shifted by a charge moving through a potential difference.
energy shifted = charge × potential difference
1 electron-volt is 1.6 × 10-19 joule
1 MeV is 1.6 × 10-13 joule.
Here we'll prefer joule. Alpha radiations from a particular source are mono-energetic: they are all emitted with a single energy.
Here are a sample range of values, for emissions from different nuclei: U235, 706.4 femtojoule ; Ra225, 713.6 femtojoule ; Hf174, 400.0 femtojoule . (These are quoted in femtojoules, where 1 femtojoule is 1.6 × 10-15 joule.)