Defect Dynamics and Isomer Quenching in ${}^{229}$Th:CaF${}_2$ Crystals

Abstract

Thorium-229 doped calcium fluoride (${}^{229}$Th:CaF${}_2$) is a leading candidate for a solid-state nuclear clock, owing to the unusually low energy of the ${}^{229}$Th isomer transition at 8.3557\,eV and the very high banggap of CaF${}_2$ around 12\,eV. Here we report a temperature-dependent study of ${}^{229}$Th:CaF${}_2$ luminescence, covering radioluminescence, photoluminescence, thermoluminescence, and afterglow. Using synchrotron x-ray excitation together with a cryo-vacuum system, we resolve the thermal quenching of the self-trapped exciton (STE) scintillation into its triplet and singlet components, identify eleven thermoluminescence glow peaks together with their associated emission bands, and separate the temperature-dependent components that make up the long-lived afterglow. The trap depths obtained in this way are closely correlated with the temperature dependence of x-ray-induced isomer quenching, including the enhanced isomer yield near \SI{-60}{\celsius} and the minimum near \SI{-80}{\celsius}, supporting the carrier-trapping picture in which trapped carriers are unavailable to quench the nucleus. Taken together, these results connect the optical and nuclear observables of ${}^{229}$Th:CaF${}_2$ through a unified picture of electron defects and Th-related traps, while providing a comprehensive characterization of the material relevant to solid-state nuclear clock operation.

12 pages, 7 figures, 2 tables