Growth and characterization of thorium-doped calcium fluoride single crystals

Abstract

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. 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 $4\cdot 10^{19}$ cm${}^{-3}$ have been realized with ${}^{232}$Th with good ($>$10%) VUV transmission. However, the intrinsic radioactivity of ${}^{229}$Th drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, limiting the ${}^{229}$Th concentration to $<5\cdot 10^{17}$ cm${}^{-3}$.

12 pages, 18 figures