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

Efficient population of the 229 m , g Th $^{229m,g}{\rm Th}$ 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 Th $^{229}{\rm Th}$ . This study proposes a novel approach to populate 229 m , g Th $^{229m,g}{\rm Th}$ via photonuclear reactions in thorium-doped crystals. Theoretical calculations were performed to evaluate the photonuclear reaction cross-sections of four thorium isotopes 229 - 232 Th $^{229-232}{\rm Th}$ , which show 229 m Th $^{229m}{\rm Th}$ -related production cross-sections reaching hundreds of millibarns. Monte Carlo simulations further analyzed the time-wavelength distributions of 229 m Th $^{229m}{\rm Th}$ radiative decay signals and Cherenkov radiation backgrounds in thorium-doped CaF 2 ${\rm CaF}_{2}$ , SrF 2 ${\rm SrF}_{2}$ , and LiF crystals under bremsstrahlung irradiation, revealing the correlation between the signal-to-noise ratio (SNR) and incident electron parameter. The results show that SrF 2 ${\rm SrF}_{2}$ crystals achieved an SNR of 10 6 $^{6}$ 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 Th $^{232}{\rm Th}$ and 230 Th $^{230}{\rm Th}$ into 229 Th $^{229}{\rm Th}$ -doped crystals, photonuclear reactions offer a decentralized production route that is independent of the finite 229 Th $^{229}{\rm Th}$ supply derived from regulated 233 U $^{233}{\rm U}$ inventories. These findings indicate that the photonuclear population is a promising pathway for efficient 229 m Th $^{229m}{\rm Th}$ production and characterization, effectively mitigating future supply constraints for fundamental research.