Radioactivity, spontaneous disintegration of unstable atomic nuclei, accompanied by the emission of alpha particles (weakly penetrating helium nuclei), beta rays (more penetrating streams of electrons), or gamma rays (electromagnetic radiation capable of penetrating up to 4 in/100 mm of lead). In 1896, Antoine Becquerel noticed the spontaneous emission of energy from uranium compounds (particularly pitchblende). The intensity of the effect depended on the amount of uranium present, suggesting that it involved individual atoms. The Curies discovered further radioactive substances such as thorium and radium; about 40 natural radioactive substances are now known. Their rates of decay are unaffected by chemical changes, pressure, temperature, or electromagnetic fields, and each nuclide (nucleus of a particular isotope) has a characteristic decay constant, or half-life (amount of time for half of a substance to decay). Rutherford and F. Soddy suggested in 1902 that a radioactive nuclide decays to a further radioactive nuclide, a series of transformations that ends with the formation of a stable “daughter” nucleus. A large number of induced radioactive nuclides have been formed by nuclear reactions taking place in accelerators or nuclear reactors.
See also: Radiation.