Property of certain substances to convert without external effect, emitting a characteristic radiation. Radioactivity was discovered for uranium by Becquerel in 1896. If the substances, or to be more precise, the radionuclides occur in nature, one refers to natural radioactivity; if they are a product of nuclear conversions in nuclear reactors or accelerators, one refers to artificial radioactivity. More than 2,750 radionuclides are known today. Each radionuclide is characterized by its half-life, the time during which half the atomic nuclei convert in a given quantity. Half-lives of several billion years (uranium-238; even still longer-lived is tellurium-128 with a half-life of 7.2·1024 years) to the millionth of a second (Po-212) are known. The radiation emitted during decay and its energy is also characteristic. For example, radium-226 decays emitting alpha radiation, while iodine-131 emits beta rays.