Observation of the radiative decay of the Th-229 nuclear clock isomer

Abstract

The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks(1-3). This nuclear clock will be a unique tool for precise tests of fundamental physics(4-9). Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older(10), the proof of existence has been delivered only recently by observing the isomer's electron conversion decay(11). The isomer's excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured(12-16). In spite of recent progress, the isomer's radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (Th-229m). By performing vacuum-ultraviolet spectroscopy of Th-229m incorporated into large-bandgap CaF2 and MgF2 crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements(14-16) and the uncertainty is decreased by a factor of seven. The half-life of Th-229m embedded in MgF2 is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus.