${}^{229}\mathrm{Th}$ has an isomeric state of unusually low energy, whose adopted value is by now 3.5(10) eV. This value was determined indirectly, based on several very precise γ-ray energies, between 25 and 217 keV, from the $\alpha$ decay of ${}^{233}\mathrm{U}$. Two recent works suggest that the decay pattern of the transitions that link the low-energy levels of ${}^{229}\mathrm{Th}$ is different from that assumed in earlier works, but there also is a difference between them. In this article we investigate the effect on the value determined for the excitation energy of that isomeric state if those different assumptions regarding the γ-ray transitions in ${}^{229}\mathrm{Th}$ are considered. We use published data and a statistical method that takes into account both variances and correlations between data. Adopting the statistically most acceptable assumption regarding the decay pattern of ${}^{229}\mathrm{Th}$, we deduced the value of 5.5(10) eV for the excitation energy of that isomeric state, with consequences for both theoretical and experimental studies related to that level.

• Received 25 March 2004

DOI:https://doi.org/10.1103/PhysRevC.71.044303