radioactivity

The property of particular atom nuclei (radionuclides) to transmute into other atomic nuclei without any external influence and emit ionising radiation in the process. The measured quantity is the activity, i.e. the number of nuclear decay processes in a radionuclide or a mixture of radionuclides during a particular time. The unit is the becquerel (Bq), which corresponds to one nuclear decay process per second. There are both natural radionuclides in the natural world and artificial radionuclides formed by nuclear physical processes. Each radionuclide has a half life.