The unstable nuclei of certain atoms disintegrate spontaneously: some release their excess energy by emitting an alpha particle, others do so by emitting a beta particle. These disintegrations are very often accompanied by the emission of gamma radiation.

The unstable nucleus simultaneously ejects two neutrons and two protons, which correspond to a helium nucleus. The emission of gamma photons is a secondary reaction that occurs a few thousandths of a second after the disintegration.

The unstable nucleus ejects an electron. The emission of gamma photons is a secondary reaction that occurs a few thousandths of a second after the disintegration.