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.
Categories: physics.atom-ph cond-mat.other nucl-ex nucl-th physics.chem-ph