Observation of internal-conversion electrons emitted from Thm229 produced by β decay of 229Ac

The first nuclear-excited state of ²²⁹Th, ^Thm₂₂₉, is known to have extremely low excitation energy at 8.28 ± 0.17 eV, and the nuclear decay mode and half-life of ^Thm₂₂₉ are considered to depend on its chemical environment. Recently, internal-conversion (IC) electrons from ^Thm₂₂₉ produced from a ²³³U source were detected for the first time, and the half-life of ^Thm₂₂₉ on the nickel-alloy surface of a microchannel plate detector was determined. In this study, to determine the half-lives for other chemical environments, we produced ^Thm₂₂₉ through the β decay of ²²⁹Ac and measured low-energy IC electrons from ^Thm₂₂₉ using the coincidence measurement technique between high-energy electrons (mainly β particles and high-energy IC electrons) and all electrons (including IC electrons from ^Thm₂₂₉) emitted from an ²²⁹Ac electrodeposited source. The ²²⁹Ac nuclide was produced by the ²³²Th(p ,α )²²⁹Ac reaction and then was separated from a large amount of the reaction byproducts utilizing chemical separation techniques. Electron signals, which correspond to the IC electrons from ^Thm₂₂₉, were detected using the coincidence measurement technique. The half-life of ^Thm₂₂₉ in the Ac electrodeposited source was determined to be 10(8) μs, which is close to the previous experimental value. The method established in this study lays the foundations to study the IC-decay property of ^Thm₂₂₉ as a function of the chemical environment.