Lifetime of the singly charged $^{229}$Th nuclear isomer

Abstract: The nucleus of thorium-229 ($^{229}$Th) has an exceptionally low-energy isomeric state ($^{229\mathrm{m}}$Th). Because its excitation energy is close to the energy levels of valence electrons, the lifetime of $^{229\mathrm{m}}$Th varies substantially depending on its electronic state. Although internal conversion and radiative decay were recently observed, the electronic bridge decay of $^{229\mathrm{m}}$Th, a higher-order decay process through an electronic transition, has not yet been confirmed. A promising candidate to search for this decay channel is singly charged $^{229\mathrm{m}}$Th$^+$ in the electronic ground state. Here we produce $^{229\mathrm{m}}$Th$^+$ by a charge exchange reaction in an ion trap and detect the isomers through measuring the electrons emitted from internal conversion processes. We determined the half-life of $^{229\mathrm{m}}$Th$^+$ to be 0.46(8) s. Our result differs by several orders of magnitude from the half-lives of internal conversion and radiative decay, indirectly suggesting the existence of the electronic bridge decay of $^{229\mathrm{m}}$Th. This will enable the direct observation and further investigation of the electronic bridge process, which will contribute to understanding nuclear-electron interactions and accelerating nuclear deexcitation in the operation of a thorium-based nuclear clock.