Characterization of Thorium-229 Crystal towards Vacuum Ultraviolet Search from Nuclear Clock isomer

Thorium-229 (²²⁹Th) is a nucleus that is excitable by laser light owing to its extremely low first excitation level of approximately 8 eV. It is expected to enable the realization of a nuclear clock using the unique nuclear transition of ²²⁹Th isomeric state. In particular, 'solid-state nuclear clocks' using ²²⁹Th-doped crystals enable the excitation of a large number of nuclei simultaneously and are expected to be applied in multiple fields such as fundamental physics and practical applications as stable clocks.

However direct excitation of ²²⁹Th from ground state has not been successfully achieved and the properties of the ²²⁹Th isomer in solids have not been experimentally clarified yet. Instead of attempting direct excitation to the ²²⁹Th isomer state, we excite ²²⁹Th by synchrotron X-ray irradiation to the second excited state with known transition properties and produce the isomeric state of ²²⁹Th through a de-excitation process.

In this talk, we would introduce our ongoing characterization experiments using X-ray absorption fine structure spectroscopy (XAFS) methods and the status of our experiments aimed at searching for de-excitation light of ²²⁹Th-doped target crystals by scanning with the irradiated X-ray energy at SPring-8, Japan.