Gamma Decay

Gamma ($\gamma$) radiation is high-energy electromagnetic radiation, and it is almost always emitted TOGETHER WITH an alpha or a beta decay, rather than occurring entirely on its own. The reason is that when a parent nucleus emits an alpha or beta particle, the resulting daughter nucleus is often left behind in an energetically EXCITED state, not immediately in its lowest-energy (ground) state; the daughter then relaxes down to its ground state by emitting the extra energy as one or more gamma-ray photons.

A concrete illustration from the source: uranium-238 decaying by alpha emission, $238$-$92$-$U \rightarrow 234$-$90$-$Th + 4$-$2$-$He + \gamma$, is observed to emit alpha particles of two distinct energies -- 4.147 MeV (23% of decays) and 4.195 MeV (77% of decays). When the lower-energy alpha (4.147 MeV) is emitted, the resulting thorium-234 daughter is left in an excited state, and it de-excites down to its ground state by emitting a gamma-ray photon of energy 0.048 MeV -- accounting for the energy 'missing' from that lower-energy alpha branch com …