Growth and characterization of thorium-doped calcium fluoride single crystals

• Published in: Scientific reports Vol. 13; no. 1; p. 3897
• Main Authors: Beeks, Kjeld, Sikorsky, Tomas, Rosecker, Veronika, Pressler, Martin, Schaden, Fabian, Werban, David, Hosseini, Niyusha, Rudischer, Lukas, Schneider, Felix, Berwian, Patrick, Friedrich, Jochen, Hainz, Dieter, Welch, Jan, Sterba, Johannes H., Kazakov, Georgy, Schumm, Thorsten
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 08.03.2023; Nature Publishing Group; Nature Portfolio
• Subjects: 639/301; 639/638; 639/766; Crystals; Energy; Fluorides; Humanities and Social Sciences; Investigations; Laboratories; multidisciplinary; Oxidation; Radioactivity; Science; Science (multidisciplinary); Single crystals; Solids; Spectrum analysis; Thorium
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-023-31045-5

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%) 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, currently limiting the 229 Th concentration to < 5 × 10 17  cm - 3 .