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.

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