“…Presently, the reported research attempts on solid-state nuclear clock materials mainly include Th:LiF, Th:CaF 2 , Th:LiCaAlF 6 /LiSrAlF 6 , and Th:MgF 2 , − among which, Th:CaF 2 single crystal can easily achieve high concentration doping, because its cation radius and electronegativity are very close to Th ions. In addition, it has good VUV transmittance and low background luminescence in the nuclear transition band, making it a very promising solid nuclear clock material. − However, according to the latest report of Beeks et al, the VUV transmittance of their highly doped 232 Th:CaF 2 crystals with doping concentrations of 4.0 × 10 19 and 2.6 × 10 20 cm –3 are ∼35% and ∼5% at 150 nm, respectively. The VUV transmittance exhibits a strong negative doping concentration dependence, sharply decreasing with increasing doping concentration.…”