The energy of ionising radiations

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.)