A method to detect the VUV photons from cooled $^{229}$Th:CaF$_2$ crystals

TL;DR

This paper presents a cooling method with a copper shield to prevent ice formation on $^{229}$Th:CaF$_2$ crystals, enhancing VUV photon detection efficiency crucial for nuclear clock development.

Contribution

A novel cooling technique using a copper shield effectively prevents ice buildup on thorium-doped crystals at cryogenic temperatures, improving VUV photon detection for nuclear clock applications.

Findings

Copper shield prevents ice formation below -100°C.

Cooling preserves high VUV detection efficiency.

Method enhances stability of solid-state nuclear clocks.

Abstract

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.