What happens when a nucleus is left in an excited state after decay?

• A. It remains in excited state indefinitely.
• B. It decays via alpha emission only.
• C. It becomes non-radioactive immediately.
• D. It emits a gamma ray to de-excite to ground state.

Answer: (D)

When a nucleus is left in an excited state after a decay, it has excess energy that it needs to shed without changing its number of protons or neutrons. The natural way to do this is by emitting a gamma ray, a high-energy photon, which carries away the energy difference between the excited state and a lower state (often the ground state). This de-excitation happens extremely quickly, so you don’t see a long-lived excited nucleus in practice. Sometimes the nucleus de-excites in steps, emitting more than one gamma ray in a cascade until it reaches the ground state. For example, certain beta decays produce a daughter nucleus that emits two gamma rays in sequence to reach ground state.