A particle or a photon with enough energy to knock off an electron from an atom, thus making it more reactive.
There are four basic types of ionizing radiation:
- alpha particles - helium nuclei
- beta particles - electrons
- neutrons - neutrons are not themselves ionizing but their collisions with nuclei lead to the ejection of other charged particles that do cause ionization.
- gamma rays - note that x-rays, are generally identical to gamma rays except for their place of origin.