Observation of internal-conversion electrons emitted from Th229m produced by β decay of Ac229

The first nuclear-excited state of $^{229}\mathrm{Th}, ^{229\mathrm{m}}\mathrm{Th}$, is known to have extremely low excitation energy at 8.28 \ifmmode\pm\else\textpm\fi{} 0.17 eV, and the nuclear decay mode and half-life of $^{229\mathrm{m}}\mathrm{Th}$ are considered to depend on its chemical environment. Recently, internal-conversion (IC) electrons from $^{229\mathrm{m}}\mathrm{Th}$ produced from a $^{233}\mathrm{U}$ source were detected for the first time, and the half-life of $^{229\mathrm{m}}\mathrm{Th}$ 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 $^{229\mathrm{m}}\mathrm{Th}$ through the \ensuremath{\beta} decay of $^{229}\mathrm{Ac}$ and measured low-energy IC electrons from $^{229\mathrm{m}}\mathrm{Th}$ using the coincidence measurement technique between high-energy electrons (mainly \ensuremath{\beta} particles and high-energy IC electrons) and all electrons (including IC electrons from $^{229\mathrm{m}}\mathrm{Th}$) emitted from an $^{229}\mathrm{Ac}$ electrodeposited source. The $^{229}\mathrm{Ac}$ nuclide was produced by the $^{232}\mathrm{Th}(p,\ensuremath{\alpha})^{229}\mathrm{Ac}$ 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 $^{229\mathrm{m}}\mathrm{Th}$, were detected using the coincidence measurement technique. The half-life of $^{229\mathrm{m}}\mathrm{Th}$ in the Ac electrodeposited source was determined to be 10(8) \ensuremath{\mu}s, which is close to the previous experimental value. The method established in this study lays the foundations to study the IC-decay property of $^{229\mathrm{m}}\mathrm{Th}$ as a function of the chemical environment.