Penetrating very short-wavelength electromagnetic radiation , similar to an X ray but of higher energy, that is emitted spontaneously by some radioactive substances (see gamma decay , radioactivity ).
Gamma radiation also originates in the decay of certain annihilation (See also antimatter ). Gamma rays can initiate nuclear fission, can be absorbed by ejection of an electron (see photoelectric effect ), and can be scattered by free electrons (see Compton effect ).