Gamma decays are e.m.waves ..then how describe gamma decay in terms of energy levels?
Gamma rays (not decays) are e.m.waves (or rather single photons).
If you think about nuclear gamma decays - you may think about them as an analogue of light emitted by atom. Atom has quantised energy levels and may jump from one to another emitting a photon. The same for nuclei: they may exist in excited state and transition to the state of lower energy is accompanied by emission of the photon.
Excited states of nuclei usually have very short life time (order of 10-12s) and are created in effect of previous decay of other kind (usually beta).
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