A gamma ray is a penetrating very short-wavelength electromagnetic radiation, similar to an X ray, but of a higher energy
that is emitted spontaneously by some radioactive substances.
Gamma radiation also originates in the decay of certain
Subatomic Particles, and in particle-antiparticle
Annihilation (also see
Antimatter). Gamma rays can initiate nuclear fission, can be absorved by ejection of an electron, and can be scattered by free
electrons.
