A method to detect the VUV photons from cooled 229Th:CaF2 crystals

• DOI: 10.1016/j.nimb.2025.165647
• Publication Date: 2025-02-18T13:38:23Z
• Authors: Ming Guan; Michael Bartokos; Kjeld Beeks; Yuta Fukunaga; Takahiro Hiraki; Takahiko Masuda; Yuki Miyamoto; Ryoichiro Ogake; Koichi Okai; Noboru Sasao; Fabian Schaden; Thorsten Schumm; Kotaro Shimizu; Sayuri Takatori; Akihiro Yoshimi; Koji Yoshimura
• Keywords: Physics - Instrumentation and Detectors; FOS: Physical sciences; Instrumentation and Detectors (physics.ins-det); Nuclear Experiment (nucl-ex); Nuclear Experiment

Thorium-229, with its exceptionally low-energy nuclear excited state, is a key candidate for developing nuclear clocks. $^{229}$Th-doped CaF$_2$ crystals, benefiting from calcium fluoride's wide band gap, show great promise as solid-state nuclear clock materials. These crystals are excited by vacuum ultraviolet (VUV) lasers, which over time cause radiation damage. Cooling the crystals can mitigate this damage but introduces a challenge: photoabsorption. This occurs when residual gas molecules condense on the crystal surface, absorbing VUV photons and deteriorating detection efficiency. To solve this, we developed a cooling technique using a copper shield to surround the crystal, acting as a cold trap. This prevents ice-layer formation, even at temperatures below $-100\,^\circ$C, preserving high VUV photon detection efficiency. Our study detailed the experimental cooling setup and demonstrated the effectiveness of the copper shield in maintaining crystal performance, a critical improvement for future solid-state nuclear clocks operating at cryogenic temperatures.<br/>5 figures, 5 pages<br/>