Vacuum Ultraviolet Search from Thorium-229 Isomer in Crystal Toward Solid-State Nuclear Clock

Thorium-229 (
²²⁹
Th) is a laser-excitable nucleus owing to its extremely low first excitation energy in the order of electron volts. Nuclear clocks, which utilize the resonance frequency of this unique nuclear isomer transition of
²²⁹
Th, are expected to demonstrate relatively higher accuracy compared to conventional atomic clocks. Particularly, solid-state nuclear clocks using
²²⁹
Th-doped crystals are expected to have a variety of applications for both fundamental physics and practical use in society as compact and precision clocks. However, laser excitation of
²²⁹
Th remains challenging due to the energy and lifetime uncertainty of the
²²⁹
Th isomer state. To address this challenge, we developed an original excitation method that does not attempt direct excitation to the first isometric state but instead excites to the second excitation state with transition properties and generates the isomer state known through a de-excitation process from the second excitation state. We are currently applying this method to
²²⁹
Th-doped crystals to search for de-excitation vacuum-ultraviolet (VUV) light from the first isomeric state. This paper presents the current status of our search.