Radiation: spontaneous emission of radiation, either directly from unstable atomic nuclei or as a consequence of a nuclear reaction.
A phenomenon resulting from an instability of the atomic nucleus in certain atoms where the nucleus experiences a spontaneous but measurably delayed nuclear transition or transformation with the resulting emission of radiation. The discovery of radioactivity by H. Becquerel in 1896 marked the birth of nuclear physics.
All chemical elements may be rendered radioactive by adding or by subtracting (except for hydrogen and helium) neutrons from the nucleus of the stable ones. Studies of the radioactive decays of new isotopes far from the stable ones in nature continue as a major frontier in nuclear research. The availability of this wide variety of radioactive isotopes has stimulated their use in many fields, including chemistry, biology, medicine, industry, artifact dating, agriculture, and space exploration.
A radioactive transition may be delayed by less than a microsecond or by more than a billion years, but the existence of a measurable delay or lifetime distinguishes a radioactive nuclear transition from a â€œpromptâ€ nuclear transition, such as in the emission of most gamma rays.
The most commonly found types of radioactivity are alpha, beta negatron, beta positron, electron capture, and isomeric transition. Each is characterized by the particular type of nuclear radiation which is emitted by the transforming parent nucleus. In addition, there are several other decay modes that are observed more rarely in specific regions of the periodic table.