Every radionuclide emits radiation at its own specific rate, which is measured in terms of half-life. Radioactive half-life is the time required for half of the radioactive atoms present to decay. Radioactive decay is when a radioisotope transforms into another radioisotope; this process emits radiation in some form. Some radionuclides have half-lives of mere seconds, but others have half-lives of millions of years.