Theoretical evaluation of decay mode of ²²⁹ᵐ Th in solid samples

The excitation energy of ²²⁹ᵐ Th is extremely low at 8.4 \, 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 $ γ$-ray transition are not consistent among the previous experiments. In this study, we investigate the chemical states of ²²⁹ᵐ Th by DFT calculations. Based on these results, we evaluate the relationship between the experimental half-life for each sample and the electronic state of Th. The calculation results indicate that ion trap method, Ca F₂ model and Mg F₂ one decay only via the $ γ$-ray transition, whereas Li Sr Al F₆ decay via the $ γ$-ray transition and has a possibility of decay via internal conversion, and electron bridge.