Measuring Very Long Half-Lives some radioisotopes such as samarium (149Sm) and gadolinium (152Gd) have half lives that are much longer than the age of the universe, so we can't measure their half-lives by watching their decay rate decrease. Luckily, there is another way of calcula1ing the half-life, using Eq. (43.16). Suppose a 12.0-g sample of 149Sm is observed to decay at a rate of 2.65 Bq. Calculate the half-life of the sample in years.