Gamma Rays (denoted as γ) are electromagnetic radiation of high frequency (very short wavelength). They are produced by sub-atomic particle interactions such as electron-positron annihilation, neutral pion decay, radioactive decay, fusion, fission or inverse Compton scattering in astrophysical processes. Gamma rays typically have frequencies above 1019 Hz, and therefore have energies above 100 keV and wavelength less than 10 picometers, often smaller than an atom. Gamma radioactive decay photons commonly have energies of a few hundred keV, and are almost always less than 10 MeV in energy.
Because they are a form of ionizing radiation, gamma rays can cause serious damage when absorbed by living tissue and, are therefore a health hazard.
Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Alpha and beta "rays" had already been separated and named by the work of Ernest Rutherford in 1899, and in 1903 Rutherford named Villard's distinct new radiation "gamma rays."
In the past, the distinction between X-rays and gamma rays was based on energy (or equivalently frequency or wavelength), the latter being considered a higher-energy version of the former. However, high-energy X-rays produced by linear accelerators ("linacs") and astrophysical processes now often have higher energy than gamma rays produced by radioactive gamma decay. Because of this overlap in energy ranges, the two types of electromagnetic radiation are now usually defined by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus (that is, produced by gamma decay), or from other particle decays or annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet and even lower energy photons produced by these processes would also be defined as "gamma rays". In certain fields such as astronomy, gamma rays and X-rays are still sometimes defined by energy, as the processes which produce them may be uncertain.