Efficient production of ${}^{229m,g}$Th via neutron capture in VUV-transparent crystals

Abstract

The low-lying isomeric state of ${}^{229m}$Th, owing to its unique nuclear energy structure, has been widely regarded as one of the most promising candidates for the development of a nuclear clock. However, the limited availability of suitable ${}^{229}$Th sources with sufficiently high activity remains a major challenge for experimental investigations of the ${}^{229m}$Th isomer. We propose a neutron-capture-based approach for the in-situ production of ${}^{229m,g}$Th by doping ${}^{228}$Ra into crystal hosts, where ${}^{229m,g}$Th is generated through neutron-capture reactions followed by a sequence of radioactive decay processes. We systematically investigate the background contributions associated with the three doped crystal hosts, namely CaF${}_2$, SrF${}_2$, and LiF, and evaluate their impact on the detection and identification of ${}^{229m}$Th. Under a neutron flux of $10^{15}n/c{\mathrm{m}}^2/s$ and a ${}^{228}$Ra doping concentration of $10^{19}\mathrm{c}{\mathrm{m}}^{-3}$, the proposed method is capable of producing on the order of $10^{12}$ ${}^{229}$Th and ${}^{229m}$Th nuclei within only 1 s of irradiation, with a signal-to-noise ratios as high as $10^5$. In addition, the influences of detector wavelength resolution and post-irradiation measurement time on the detectability of the ${}^{229m}$Th signal are systematically analyzed, and the corresponding optimal measurement conditions are identified. Furthermore, the spatial distribution of neutron-produced ${}^{229}$Th within the crystal is investigated, providing practical guidance for optimizing crystal geometry and illumination configuration in future continuous-wave VUV absorption spectroscopy experiments. These results suggest that the proposed scheme provides a promising alternative pathway for the production and detection of ${}^{229\mathrm{m},\mathrm{g}}$Th, which may facilitate future studies toward the realization of nuclear-clock-based technologies.