gamma decay

An atom that has been excited drops back down by emitting a photon of visible or ultraviolet light. A nucleus does exactly the same thing, only the energies are a million times larger. When a nucleus is left in an excited state, it de-excites by emitting a very high-energy photon, a gamma ray. Gamma decay is that release of electromagnetic energy from a nucleus rearranging itself internally.

In gamma decay the nucleus makes a transition from a higher to a lower energy level, and the photon carries off the energy difference: E_gamma = E_initial - E_final (with a tiny recoil correction). Crucially, neither the proton number Z nor the mass number A changes; the nuclide keeps its identity and only sheds excitation energy. Because nuclear energy levels are typically spaced by tens of keV to several MeV, gamma rays are far more energetic and penetrating than X-rays. The transitions obey angular-momentum and parity selection rules, classified as electric or magnetic multipole radiation (E1, M1, E2, and so on). A competing process, internal conversion, lets the nucleus hand its energy directly to an atomic electron instead of emitting a photon.

Gamma emission almost always follows an alpha or beta decay, because those decays frequently leave the daughter nucleus in an excited state that then relaxes by gamma emission, usually within picoseconds. The sharp, characteristic gamma energies act as nuclear fingerprints and are the basis of gamma-ray spectroscopy for identifying isotopes, medical imaging (technetium-99m), and radiotherapy.