Energy of the 3/2+ state of 229Th reexamined -: art. no. 044303

Th-229 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 gamma-ray energies, between 25 and 217 keV, from the alpha decay of U-233. Two recent works suggest that the decay pattern of the transitions that link the low-energy levels of Th-229 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 gamma-ray transitions in Th-229 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 Th-229, 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.