We have grown 232Th:CaF2 and 229Th:CaF2 single crystals for investigations on the VUV laser-accessible first nuclear excited state of 229Th. 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⋅1019 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, limiting the 229Th concentration to <5⋅1017 cm−3.
@article{arxiv.2211.05445,
title = {Growth and characterization of thorium-doped calcium fluoride single crystals},
author = {Kjeld Beeks and Tomas Sikorsky and Veronika Rosecker and Martin Pressler and Fabian Schaden and David Werban and Niyusha Hosseini and Lukas Rudischer and Felix Schneider and Patrick Berwian and Jochen Friedrich and Dieter Hainz and Jan Welch and Johannes H. Sterba and Georgy Kazakov and Thorsten Schumm},
journal= {arXiv preprint arXiv:2211.05445},
year = {2022}
}