Optical Transmission Enhancement of Ionic Crystals via Superionic Fluoride Transfer: Growing VUV-Transparent Radioactive Crystals

The 8 eV first nuclear excited state in ${}^{229}$Th is a candidate for implementing an nuclear clock. Doping ${}^{229}$Th into ionic crystals such as CaF${}_2$ is expected to suppress non-radiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of ${}^{229}$Th prohibits the growth of high-quality single crystals with high ${}^{229}$Th concentration; radiolysis causes fluoride loss, increasing absorption at 8 eV. We overcome this roadblock by annealing ${}^{229}$Th doped CaF${}_2$ at 1250$\text{unicode}x2103$ in CF${}_4$. The technique presented here 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.