The period during which half of the nuclei decay in a quantity of radionuclides. There are extreme variations in the half-lives of the various radionuclides, e.g. from 7.2·1024years for tellurium-128 down to 2·10-16 seconds for beryllium-8. The following relationships exist between the half-life T, the decay constant and the average lifetime:

decay curve of tritium (H-3) – Half-life 12.3 years