Theoretical evaluation of decay mode of $ {}^{229m} \mathrm{Th} $ in solid samples

Abstract

The excitation energy of $ {}^{229m} \mathrm{Th} $ is extremely low at $ 8.4 \, \mathrm{eV} $; thus, this isotope exhibits changes in its decay modes depending on the chemical state, specifically the outermost electronic states. However, the reported half-lives of the $ \gamma $-ray transition are not consistent among the previous experiments. In this study, we investigate the chemical states of $ {}^{229m} \mathrm{Th} $ by density functional theory calculations. Based on these results, we evaluate the relationship between the experimental half-life of each sample and the electronic state of $ \mathrm{Th} $. The calculation results indicate that ion trap method, $ \mathrm{Ca} \mathrm{F}_2 $ model and $ \mathrm{Mg} \mathrm{F}_2 $ one decay only via the $ \gamma $-ray transition, whereas $ \mathrm{Li} \mathrm{Sr} \mathrm{Al} \mathrm{F}_6 $ one decays via the $ \gamma $-ray transition and has a possibility of decay via internal conversion and electron bridge.