The spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nucleus of an unstable isotope. Also, the rate at which radioactive material emits radiation. Measured in units of becquerels or disintegrations per second.
RADIOACTIVITY
Meaning of RADIOACTIVITY in English
NRC Nuclear Energy English Glossary. Английский глоссарий ядерной энергии NRC (Национальный научно исследовательский совет). 2012