• Tags: gamma, excited-states, isomeric
• Difficulty: Foundation

Gamma rays are high-energy photons emitted when an excited nucleus transitions to a lower energy state. Unlike alpha and beta decay, gamma emission changes neither Z nor A — it only changes the nuclear energy state. Gamma emission typically follows alpha or beta decay, since the daughter nucleus is often left in an excited state. Example: Co-60 undergoes beta-minus decay to excited Ni-60, which then emits two gamma photons (1.17 MeV and 1.33 MeV) to reach the ground state. Gamma energies are discrete (line spectrum), reflecting the quantized nuclear energy levels. Gamma photons have no charge and no rest mass, travel at the speed of light, have the highest penetrating power (can pass through several cm of lead), and the lowest ionizing power among the three radiations. Internal conversion is an alternative to gamma emission: the nuclear energy is transferred directly to an orbital electron, which is ejected from the atom.