Gamma Decay is a type of radioactivity in which an unstable atomic nucleus dissipates energy by gamma emission, producing gamma rays. A radioactive process in which an atomic nucleus loses energy by emitting a gamma ray (a stream of high-energy photons). When an element undergoes gamma decay its atomic number and mass number do not change. Gamma rays are ionizing radiation, and are thus biologically hazardous. They are classically produced by the decay from high energy states of atomic nuclei (gamma decay), but are also created by other processes.