Growth and characterization of thorium-doped calcium fluoride single crystals

Abstract

Abstract We have grown $$^{232}$$ 232 Th:CaF $$_2$$ 2 and $$^{229}$$ 229 Th:CaF $$_2$$ 2 single crystals for investigations on the VUV laser-accessible first nuclear excited state of $$^{229}$$ 229 Th, with the aim of building a solid-state nuclear clock. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of $$^{229}$$ 229 Th, we have scaled down the crystal volume by a factor 100 compared to established commercial or scientific growth processes. We use the vertical gradient freeze method on 3.2 mm diameter seed single crystals with a 2 mm drilled pocket, filled with a co-precipitated CaF $$_2$$ 2 :ThF $$_4$$ 4 :PbF $$_2$$ 2 powder in order to grow single crystals. Concentrations of $$4\cdot 10^{19}$$ 4 · 10 19 cm $$^{-3}$$ - 3 have been realized with $$^{232}$$ 232 Th with good (> 10%) VUV transmission. However, the intrinsic radioactivity of $$^{229}$$ 229 Th drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, currently limiting the $$^{229}$$ 229 Th concentration to $$<5\times 10^{17}$$ < 5 × 10 17 cm $$^{-3}$$ - 3 .