Characterization of the thorium-229 defect structure in CaF crystals

Takatori, Sayuri*; Pimon, M.*; Pollitt, S.*; Bartokos, M.*; Beeks, K.*; Grneis, A.*; Hiraki, Takahiro*; Homma, Tetsuo*; Hosseini, N.*; Leitner, A.*; Masuda, Takehiko*; Morawetz, I.*; Nitta, Kiyofumi*; Okai, Koichi*; Riebner, T.*; Schaden, F.*; Schumm, T.*; Sekizawa, Oki*; Sikorsky, T.*; Takahashi, Yoshio*; Toscani De Col, L.*; Yamamoto, Ryogo*; Yomogida, Takumi   ; Yoshimi, Akihiro*; Yoshimura, Koji*

Recent reports on laser excitation of the low-energy thorium-229 (Th) nuclear isomeric state in calcium fluoride single crystals render this system a promising candidate for a solid-state nuclear clock. However, experimental characterization of the microscopic ion arrangement around the doped Th and its electronic charge state, crucial for the precise control of the clock transition and assessing the solid-state clock's performance, remains an unresolved task. This study uses X-ray absorption fine structure spectroscopy of Th:CaF to investigate the charge state and coordination environment of doped Th. The results indicate that Th is doped with a 4+ valence at the substitutional site of the Ca ion, with charge compensated provided by two F ions located at interstitial sites adjacent to Th.