radioactivity

Spontaneous change of the nuclei of atoms accompanied by the emission of radiation. Such atoms are called radioactive. It is the property exhibited by the radioactive isotopes of stable elements and all isotopes of radioactive elements, and can be either natural or induced. See radioactive decay.

A radioactive material decays by releasing radiation, and transforms into a new substance. The energy is released in the form of alpha particles and beta particles or in the form of high-energy electromagnetic waves known as gamma radiation. Natural radioactive elements are those with an atomic number of 83 and higher. Artificial radioactive elements can also be formed.

Devices such as the Geiger–Muller tube, a photographic plate, or an electroscope can detect radioactivity. An electronic counter on the Geiger–Muller instrument displays a digital reading of the amount of radiation detected.