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The 8−eV first nuclear excited state in Th229 is a candidate for implementing a nuclear clock. Doping Th229 into ionic crystals such as CaF2 is expected to suppress nonradiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of Th229 prohibits the growth of high-quality single crystals with high Th229 concentration; radiolysis causes fluoride loss, increasing absorption at 8eV. These radioactively doped crystals are thus a unique material for which a deeper analysis of the physical effects of radioactivity on growth, crystal structure, and electronic properties is presented. Following the analysis, we overcome the increase in absorption at 8eV by annealing Th229-doped CaF2 at 1250∘C in CF4. This technique allows to adjust the fluoride content without crystal melting, preserving its single-crystal structure. Superionic state annealing ensures rapid fluoride distribution, creating fully transparent and radiation-hard crystals. This approach enables control over the charge state of dopants, which can be used in deep-UV optics, laser crystals, scintillators, and nuclear clocks. Published by the American Physical Society 2024
Beeks et al. (Mon,) studied this question.