Worlds of David Darling
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The spontaneous breaking apart, or decay, of unstable atomic nuclei. The three types of radioactive emission are alpha particles (nuclei of helium, which corresponds to a decrease in mass of the nucleus by four units), beta particles (fast-moving electrons, which corresponds to an increase in atomic number of the nucleus by one unit), and gamma rays (high-energy photons, which corresponds to a loss in energy by the nucleus). Radioactive nuclei are formed in supernova explosions. Radioactivity is measured in units of becquerels or disintegrations per second. See also activity and ionizing radiation.

There are less common types of radioactive decay. Positron emission results in a decrease by one unit in the atomic number; K capture involves the incorporation of one of the extranuclear electrons into the nucleus, the atomic number is again decreased by one unit.

Radioactivity was discovered for uranium by Henri 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.

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Source: European Nuclear Society