Characterization of the thorium-229 defect structure in CaF₂ crystals

Abstract

Recent advancements in laser excitation of the low-energy thorium-229 (²²⁹Th) nuclear isomeric state in calcium fluoride (CaF₂) single crystals render this system a promising candidate for a solid-state nuclear clock. Nonetheless, the precise experimental determination of the microscopic ion configuration surrounding the doped ²²⁹Th and its electronic charge state remains a critical challenge. Such characterization is essential for precisely controlling the clock transition and evaluating the performance of this solid-state nuclear clock system. In this study, we use x-ray absorption fine structure spectroscopy of ²²⁹Th:CaF₂ to investigate the charge state and coordination environment of doped ²²⁹Th. The results indicate that ²²⁹Th displays a 4+ oxidation state at the substitutional site of a Ca²⁺ ion, with charge compensated provided by two F⁻ ions positioned at interstitial sites adjacent to ²²⁹Th.