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 ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray transition, whereas $\mathrm{Li}\mathrm{Sr}\mathrm{Al}{\mathrm{F}}_6$ one decays via the ${\hat{I}}^3$-ray transition and has a possibility of decay via internal conversion and electron bridge.

8 pages, 6 figures, 8 tables