Gamma ($\gamma$) decay occurs when a nucleus in an excited state transitions to a lower energy state or its ground state by emitting high-energy electromagnetic radiation (photons). Unlike $\alpha$-decay (which decreases the mass number by 4 and atomic number by 2) or $\beta$-decay (which changes the atomic number by 1), $\gamma$-emission involves only a change in the energy state of the nucleus. Since no nucleons (protons or neutrons) are emitted or transformed during this process, both the mass number ($A$) and the atomic number ($Z$) of the element remain unchanged .