Explain `gamma` - decay ?

Explanation

1. It is the phenomenon of emission of gamma ray photon from a radioactive nucleus.
2. Like an atom, a nucleus has discrete energy levels in the ground state and excited states.
3. When a nucleus in an excited state spontaneously decays to its ground state (or to a lower energy state), a photon is emitted with energy equal to the difference in the two energy levels of the nucleus. This is the so called gamma - decay.
4. The energy (MeV) corresponds to radiation of extremely wave length, shorter than the hard X-ray region.
5. A Gamma ray is emitted when `alpha` or `beta` decay results in a daughter nucleus in an excited state.
6. The `bar(beta)`-decay of `._(27)CO^(60)` transforms it into an excited `._(28)Ni^(60)` nucleus. This reaches the ground state by emission of `gamma` -rays of energy 1.17 MeV and 1.33 MeV. This is shown in fighure.