The nucleus of an atom constitutes the majority of its mass and is held together by high binding energy. The stability of the nucleus, its qualitative potential energy, depends on the number and the ratio of its neutrons and protons. A nucleus with high potential energy is unstable and likely to decay.
All decays result in a nucleus with higher binding energy and lower potential energy.
In some decays the daughter nucleus is left in an excited state. It descends to a lower, more stable energy state by the emission of a photon called a gamma ray. The resulting gamma rays have energies of 1.173 MeV or 1.332 MeV in the majority of the Co-60 decays. Gamma rays with this energy are capable of passing through several centimeters of lead. By using a Geiger counter the number of gamma rays being emitted from a source of Co-60 can be found.