Radioactivity

Characteristic of certain atomic nuclei (radionuclides) to convert into other atomic nuclei or change from a high-energy (metastable) state to a lower-energy state without extraneous cause, emitting ionising radiation.

Measurand is the activity, i.e. the number of nuclear transformations of a radionuclide occurring per time unit. The unit of the activity is the becquerel (Bq), which corresponds to one nuclear transformation per second.

There are both radionuclides occurring in nature and artificial radionuclides produced by nuclear-physical processes. Each radionuclide is characterised by its half-life and the type and energy of the ionising radiation emitted.