Gamma Ray

Gamma radiation, also known as gamma rays or hyphenated as gamma-rays (especially in astronomy, by analogy with X-rays) and denoted as γ, is electromagnetic radiation of high frequency (very short wavelength). Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei (gamma decay). Important natural sources are also high-energy sub-atomic particle interactions resulting from cosmic rays. Such high-energy reactions are also the common artificial source of gamma rays. Other man-made mechanisms include electron-positron annihilation, neutral pion decay, fusion, and induced fission. Some rare natural sources are lightning strike andterrestrial gamma-ray flashes, which produce high energy particles from natural high-energy voltages. Gamma rays are also produced by astronomical processes in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. Gamma rays are ionizing radiation and are thus biologically hazardous.

A classical gamma ray source, and the first to be discovered historically, is a type of radioactive decay called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately on formation, although isomeric transition can produce inhibited gamma decay with a measurable and much longer half-life. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard's radiation was named gamma rays by Ernest Rutherford in 1903.

Gamma rays typically have frequencies above 10 exahertz (or >10¹⁹ Hz), and therefore have energies above 100 keV and wavelength less than 10 picometers, less than the diameter of an atom. However, this is not a hard and fast definition but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay commonly have energies of a few hundred keV, and almost always less than 10 MeV. On the other side of the decay energy range, there is effectively no lower limit to gamma energy derived from radioactive decay. By contrast, energies from astronomical sources can be much higher, ranging over 10 TeV (this is far too large to result from radioactive decay).