Growth and characterization of thorium-doped calcium fluoride single crystals

Abstract

We have grown $$^{232}$$ 232 Th:CaF$$_2$$ 2 and $$^{229}$$ 229 Th:CaF$$_2$$ 2 single crystals for investigations on the VUV laser-accessible first nuclear excited state of $$^{229}$$ 229 Th, with the aim of building a solid-state nuclear clock. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of $$^{229}$$ 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$$_2$$ 2 :ThF$$_4$$ 4 :PbF$$_2$$ 2 powder in order to grow single crystals. Concentrations of $$4\cdot 10^{19}$$ 4 · 10 19 cm$$^{-3}$$ - 3 have been realized with $$^{232}$$ 232 Th with good (> 10%) VUV transmission. However, the intrinsic radioactivity of $$^{229}$$ 229 Th drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, currently limiting the $$^{229}$$ 229 Th concentration to $$<5\times 10^{17}$$ < 5 × 10 17 cm$$^{-3}$$ - 3 .

Structures and Properties of High-Concentration Doped Th:CaF2 Single Crystals for Solid-State Nuclear Clock Materials

“…We consider that the weak negative doping-concentration-dependent VUV transmittance of our samples is mainly attributed to the increase in the types and numbers of defects induced by the high-level doping. However, the factors of doping concentration should not result in such a dramatic decrease in VUV transmittance as observed in the literature. We contend that the dramatic decrease may be attributed to the system factors, such as the choice of growth methodology, processing of raw materials, miniaturized growth processes, samples’ polishing quality and size, and vacuum degree of VUV spectrometer.…”

“…This perspective can be confirmed in samples with low doping concentrations, whose vacuum ultraviolet (VUV) transmittance is only slightly lower than that of pure CaF 2 crystals grown under similar conditions. However, according to the report by Beeks et al, the VUV transmittance performance of Th:CaF 2 crystals grown via vertical gradient freeze method exhibits a significant negative dependence on doping concentration, which seriously deteriorates with increasing doping concentration. Their highest doped sample (∼2.6 × 10 20 cm –3 ) is almost opaque ( T ≈ 5% at 150 nm).…”

“…Presently, the reported research attempts on solid-state nuclear clock materials mainly include Th:LiF, Th:CaF 2 , Th:LiCaAlF 6 /LiSrAlF 6 , and Th:MgF 2 , − among which, Th:CaF 2 single crystal can easily achieve high concentration doping, because its cation radius and electronegativity are very close to Th ions. In addition, it has good VUV transmittance and low background luminescence in the nuclear transition band, making it a very promising solid nuclear clock material. − However, according to the latest report of Beeks et al, the VUV transmittance of their highly doped 232 Th:CaF 2 crystals with doping concentrations of 4.0 × 10 19 and 2.6 × 10 20 cm –3 are ∼35% and ∼5% at 150 nm, respectively. The VUV transmittance exhibits a strong negative doping concentration dependence, sharply decreasing with increasing doping concentration.…”

Vacuum-ultraviolet laser source for spectroscopy of trapped thorium ions

“…The spectroscopy of Th-doped crystals seems to be more promising for an initial VUV laser excitation of the isomer transition. The number of thorium nuclei in Th doped crystals is many orders of magnitude higher (up to 10 18 ) than in the trapped ion approach [24,25]. Therefore we have implemented an additional VUV optical port in the vacuum system behind the ion trap vacuum chamber to combine both experiments, allowing us to perform spectroscopy experiments with trapped ions and Th-doped crystals at the same time.…”

Controlling 229Th isomeric state population in a VUV transparent crystal

“…The development of a solid-state nuclear clock requires a superior-grade 229 Th-doped crystal, characterized by an adequate concentration, facilitating isomer excitation and detection of the isomeric decay while suppressing non-radiative decay processes in the crystal. Theoretical and experimental studies on thorium-doped crystals such as Th:CaF 2 suggest that the thorium substitutes a Ca 2+ ion in charge state Th 4+ in the crystal 13 , 14 , realizing [Rn] electronic configuration. The absence of energetically near-by electronic states should suppress the internal conversion of 229m Th and make direct radiative vacuum ultraviolet (VUV) photon emission the dominating decay process.…”