### Gamma Radiations:

There are energy levels in a nucleus, just like there are energy levels in atoms. When a nucleus is in an excited state, it can make a transition to a lower energy state by the emission of electromagnetic radiation. As the energy differences between levels in a nucleus are of the order of MeV, the photons emitted by the nuclei have MeV energies and are called gamma rays.

### Units of Radioactive Decay

1 Becquerel = 1Bq = 1 decay per second

1 Rutherford = 1 rd = 10^{6} decay per second

1 curie = 1 Ci = 3.7 × 10^{10} decays per second

### Mass-Energy Equivalence

Einstein showed from his theory of special relativity that it is necessary to treat mass as another form of energy.

Einstein gave the famous mass-energy equivalence relation

E = mc^{2}

Here the energy equivalent of mass m is related by the above equation and c is the velocity of light in vacuum and is approximately equal to 3×10^{8} ms^{-1}.