Gamma rays are a form of electromagnetic radiation, just like visible light or x rays, but with a much higher energy. The gamma ray spectrum is usually defined as light having a frequency between 10¹⁸ and 10²¹ Hertz. Gamma rays are more energetic than x rays, but are less energetic than cosmic rays. Compared to ultraviolet light (light that corresponds in energy to electronic transitions within molecules and having a frequency of approximately 8 x 10¹⁴ Hertz), a gamma ray photon with of frequency of 10¹⁹ Hertz is approximately 10,000-fold more energetic. Gamma rays are sufficiently energetic to cause nuclear transitions within atoms and, thus, also correspond to the energy release that accompanies the transition of an excited nucleus to its ground (i.e. most stable) state.

The discovery of gamma rays generally is attributed to the French physicist Paul Villard (1860-1934). Following the discover of x rays by the German...