When electrons make transitions between orbits in the atom, photons are emitted with energies on the order of an eV. Nucleons in nuclei also reside in specific states. Neutrons and protons each have their own set of orbits. When a nucleon makes a transition from one state to another, a photon is emitted. This photon has an energy on the order of MeV and is called a g ray.
Please note that g ray energies are very high as compared to visible light. The red line in the Balmer series of the hydrogen atom, for example, has an energy of 1.88 eV. The typical energies for photons emitted in nuclear transitions are a million times higher!
g rays are observed with discrete energies much like the sharp spectral lines from the hydrogen atom.
Other forms of radioactive decay often leave the daughter nucleus in an excited state. From this state, the nucleus can g decay to lower energy levels. Thus g decay often accompanies a and b decay. The drawing on the left shows schematically the b- decay of 60Co to an excited state of 60Ni which then g decays to another excited state. 60Ni then g decays again to the ground state. In this example, a b- and two g rays are observed.
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