gammadecay

Gamma decay is a mode of radioactive decay in which an excited nucleus releases one or more gamma photons, thereby lowering its energy without changing its atomic number or mass. This process often follows another decay, such as beta or alpha decay, or arises from a nucleus produced in an excited state after neutron capture or nuclear reactions. Gamma decay can occur directly to the ground state or as part of a cascade through intermediate nuclear levels; in some cases a metastable state (an isomer) decays by emitting gamma radiation with a characteristic delay.

The emission of gamma rays involves transitions between discrete energy levels of the nucleus. The energy of the emitted photon equals the difference between the initial and final nuclear states, typically ranging from keV to MeV. Transitions are described by their multipolarity (for example E1, M1, E2) and are subject to angular-momentum and parity selection rules. Internal conversion can compete with gamma emission, in which the transition energy is transferred to an atomic electron instead of producing a photon.

Gamma decay leaves the nucleus with very little overall change in its charge or mass, but the nucleus recoils slightly in the opposite direction to conserve momentum. The probability of gamma emission is characterized by a partial decay constant or half-life for the specific transition; some nuclei exhibit delayed gamma decay from isomeric states, which can persist for microseconds to hours.

Gamma rays interact with matter primarily by photoelectric absorption, Compton scattering, and pair production, depending on energy and material. Applications include gamma-ray spectroscopy, medical imaging and radiotherapy, sterilization, and industrial nondestructive testing. Common examples include the 1.17 and 1.33 MeV gamma rays from cobalt-60 and the 0.662 MeV gamma ray from cesium-137’s decay chain.