The half-life of ${\mathrm{Th}}^{232}$ has been calculated by determining an absolute gamma-disintegration rate for ${\mathrm{Tl}}^{208}$ in equilibrium with ${\mathrm{Th}}^{232}$ for three old thorium nitrate salts and one natural thorite sample. The branching ratio, $\frac{\alpha }{(\alpha +\beta )}$, for ${\mathrm{Bi}}^{212}$, a necessary parameter in the calculation, was also measured. The half-life of ${\mathrm{Th}}^{232}$ was found to be 1.42×${10}^{10}$ years within an estimated error of 5%, which is essentially in agreement with the presently accepted value. The branching ratio, $\frac{\alpha }{(\alpha +\beta )}$, of ${\mathrm{Bi}}^{212}$ was found to be 0.362±0.006, about 7.4% higher than the currently accepted value.

• Received 9 July 1956

DOI:https://doi.org/10.1103/PhysRev.104.1629