Gamma radiation, also known as
gamma rays or hyphenated as
gamma-rays and denoted as
γ, is
electromagnetic radiation of high frequency. Gamma rays are usually naturally produced on Earth by decay of high energy states in
atomic nuclei. 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 strikes and
terrestrial 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.