Gamma decay is the easiest form of nuclear decay to understand. You learned that an atomic system can emit a photon with $E_{\text{photon}}=\text{Δ}{E}_{\text{atom}}$ when an electron undergoes a quantum jump from an excited energy level to a lower energy level. Nuclei are no different. A proton or a neutron in an excited nuclear state, such as the one shown in FIGURE 42.22, can undergo a quantum jump to a lower-energy state by emitting a high-energy photon. This is the gamma-decay process.

FIGURE 42.22 Gamma decay.

The spacing between atomic energy levels is only a few eV. Nuclear energy levels, by contrast, are typically 1 MeV apart. Hence gamma-ray photons have $E_{\text{gamma}}\approx 1\text{MeV}$. Photons with this much energy have tremendous penetrating power and deposit an extremely large amount of energy at the point where they are finally absorbed.

Nuclei left to themselves are usually in their ground states and thus cannot emit gamma-ray photons. However, alpha and beta decay often leave the daughter nucleus in an excited nuclear state, so gamma emission is usually found to accompany alpha and beta emission.

The cesium isotope ${}^{137}\text{Cs}$ is a good example. We noted earlier that ${}^{137}\text{Cs}$ is used as a laboratory source of gamma rays. Actually, ${}^{137}\text{Cs}$ undergoes beta-minus decay to ${}^{137}\text{Ba}$. FIGURE 42.23 shows the full process. A ${}^{137}\text{Cs}$ nucleus undergoes beta-minus decay by emitting an electron and an antineutrino, which share between them a total energy of 0.51 MeV. The half-life for this process is 30 years. This leaves the daughter ${}^{137}\text{Ba}$ nucleus in an excited state 0.66 MeV above the ground state. The excited Ba nucleus then decays within a few seconds to the ground state by emitting a 0.66 MeV gamma-ray photon. Thus a ${}^{137}\text{Cs}$ sample is a source of gamma-ray photons, but the photons are actually emitted by barium nuclei rather than cesium nuclei.

FIGURE 42.23 The decay of ${}^{137}\text{Cs}$ involves both beta and gamma decay.