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

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 older¹⁰, the proof of existence has been delivered only recently by observing the isomer's electron conversion decay¹¹. 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 (^229mTh). By performing vacuum-ultraviolet spectroscopy of ^229mTh 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 ^229mTh 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.