Determination of the half-life of the ground state of 229Th by using 232U and 233U decay series

The half-life of the ground state of Th-229 (Th-229(g)) has become an important factor in nuclear technology, for example, in the geological disposal of nuclear spent fuel. However, the values reported in two previous studies are not in agreement. This study reevaluates the half-life of Th-229(g) by using a simple and reliable method. The U-232/U-233 activity ratio of a U-232,U-233 sample was measured by high-resolution alpha-particle spectrometry. Next, the Th-228/Th-229(g) activity ratio of the Th sample, which was grown from the U-232,U-233 sample, was also measured. The half-life of Th-229(g) was calculated from these activity ratios, the growth time, and the half-lives of U-232, U-233, and Th-228. From the results of these five measurements, the half-life of Th-229(g) is determined to be 7932 +/- 55 yr at a confidence level of 2 sigma.