Growth and characterization of thorium-doped calcium fluoride single crystals

We have grown 232Th:CaF2 and 229Th:CaF2 single crystals for investigations on the VUV laser-accessible first nuclear excited state of 229Th, with the aim of building a solid-state nuclear clock. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of 229Th, 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 CaF2:ThF4:PbF2 powder in order to grow single crystals. Concentrations of 4 · 10 19 cm-3 have been realized with 232Th with good (> 10%) VUV transmission. However, the intrinsic radioactivity of 229Th drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, currently limiting the 229Th concentration to < 5 × 10 17 cm-3.