Half-life of Th232 and the branching ratio of Bi212

The half-life of Th²³² has been calculated by determining an absolute gamma-disintegration rate for Tl²⁰⁸ in equilibrium with Th²³² for three old thorium nitrate salts and one natural thorite sample. The branching ratio, α/(α+β), for Bi²¹², a necessary parameter in the calculation, was also measured. The half-life of Th²³² was found to be 1.42×10¹⁰ years within an estimated error of 5%, which is essentially in agreement with the presently accepted value. The branching ratio, α/(α+β), of Bi²¹² was found to be 0.362±0.006, about 7.4% higher than the currently accepted value.