Journal Article

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

Beeks, Kjeld, Chhetri, Premaditya, Van Duppen, Piet, Correia, João G M, Chrysalidis, Katerina, Thirolf, Peter G, Van Den Bergh, Paul, Verlinde, Matthias, Heinke, Reinhard, Moens, Janni, Merckling, Clement, Claessens, Arno, Pereira, Lino M C, Sels, Simon, Ferrer, Rafael, Tunhuma, Shandirai Malven, Wahl, Ulrich, Kudryavtsev, Yuri, Geldhof, Sarina, Kraemer, Sandro, Athanasakis-Kaklamanakis, Michail, Huyse, Mark, Magchiels, Goele, Köster, Ulli, Manea, Vladimir, Witte, Hilde De, Lica, Razvan, Schumm, Thorsten, Raeder, Sebastian, Bara, Silvia, Cocolios, Thomas E, Villarreal, Renan, Vantomme, André, Hosseini, Niyusha, Laatiaoui, Mustapha

2023

Overview

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 . This nuclear clock will be a unique tool for precise tests of fundamental physics . Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older , the proof of existence has been delivered only recently by observing the isomer's electron conversion decay . The isomer's excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured . 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). By performing vacuum-ultraviolet spectroscopy of Th incorporated into large-bandgap CaF and MgF crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements and the uncertainty is decreased by a factor of seven. The half-life of Th embedded in MgF 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.