Growth and characterization of thorium-doped calcium fluoride single crystals

We have grown ^232 Th:CaF _2 and ^229 Th:CaF _2 single crystals for investigations on the VUV laser-accessible first nuclear excited state of ^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 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 :ThF _4 :PbF _2 powder in order to grow single crystals. Concentrations of 4· 10^19 cm ^-3 have been realized with ^232 Th with good (> 10