Half-life (thing)

The constant time period required for the disintegration of half of the atoms in a sample of some specific radioactive substance. The half-life of cobalt-60, for example, is 5.3 years. Thus, after that interval of time, a sample originally containing 8 grams of cobalt-60 would contain only 4 grams of cobalt-60 and would emit only half as much radiation. After another interval of 5.3 years, the sample would contain only 2 grams of cobalt-60. Neither the volume nor the mass of the original sample visibly decreases, however, because the unstable cobalt-60 nuclei decay into stable nickel-60 nuclei, which remain with the still-undecayed cobalt-60. This term also applies to the decay of excited atoms by the emission of light