Property of certain substances to convert without external effect, emitting a characteristic Energy dispersion through matter or space. In atomic physics.... Radioactivity was discovered for Natural radioactive element with the atomic number 92. The n... by Unit of activity of a radionuclide named after Henri Becquer... 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 Unstable nuclide spontaneously decaying without external eff... is characterized by its The period during which half of the nuclei decay in a quanti..., 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 The period during which half of the nuclei decay in a quanti... of 7.2·1024 years) to the millionth of a second (Po-212) are known. The Energy dispersion through matter or space. In atomic physics... emitted during The spontaneous conversion of a nuclide into another nuclide... and its Ability to do work or diffuse heat. The unit of energy is th... is also characteristic. For example, radium-226 decays emitting alpha Energy dispersion through matter or space. In atomic physics..., while iodine-131 emits beta rays.