Growth and characterization of thorium-doped calcium fluoride single crystals

We have grown Th-232:CaF (2) and Th-229:CaF (2) single crystals for investigations on the VUV laser-accessible first nuclear excited state of Th-229, with the aim of building a solid-state nuclear clock. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of Th-229, 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):ThF 4:PbF (2) powder in order to grow single crystals. Concentrations of 4 center dot 1019 cm (- 3) have been realized with Th-232 with good (> 10%) VUV transmission. However, the intrinsic radioactivity of Th-229 drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, currently limiting the (229) Th concentration to < 5 x 10(17) cm (- 3).