Photonuclear Population of ^229m,gTh in Th-Doped Crystals Toward Nuclear Clock Development

Efficient population of the ${}^{229m,g}\mathrm{Th}$ is essential for advancing nuclear clock technology, whose progress faces strategic challenges related to the finite legacy supply and competing high-value medical applications of ${}^{229}\mathrm{Th}$ . This study proposes a novel approach to populate ${}^{229m,g}\mathrm{Th}$ via photonuclear reactions in thorium-doped crystals. Theoretical calculations were performed to evaluate the photonuclear reaction cross-sections of four thorium isotopes ${}^{229-232}\mathrm{Th}$ , which show ${}^{229m}\mathrm{Th}$ -related production cross-sections reaching hundreds of millibarns. Monte Carlo simulations further analyzed the time-wavelength distributions of ${}^{229m}\mathrm{Th}$ radiative decay signals and Cherenkov radiation backgrounds in thorium-doped ${\mathrm{CaF}}_2$ , ${\mathrm{SrF}}_2$ , and LiF crystals under bremsstrahlung irradiation, revealing the correlation between the signal-to-noise ratio (SNR) and incident electron parameter. The results show that ${\mathrm{SrF}}_2$ crystals achieved an SNR of 10 ${}^6$ under irradiation with bremsstrahlung driven by 1 C electrons at $\sim$ 40 MeV, outperforming other materials. Compared to VUV laser direct excitation and X-ray resonant scattering pumping, this method exhibits unique advantages, including target material flexibility, broad light source compatibility, and higher excitation rates. Moreover, by enabling the conversion of abundant ${}^{232}\mathrm{Th}$ and ${}^{230}\mathrm{Th}$ into ${}^{229}\mathrm{Th}$ -doped crystals, photonuclear reactions offer a decentralized production route that is independent of the finite ${}^{229}\mathrm{Th}$ supply derived from regulated ${}^{233}U$ inventories. These findings indicate that the photonuclear population is a promising pathway for efficient ${}^{229m}\mathrm{Th}$ production and characterization, effectively mitigating future supply constraints for fundamental research.