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

Efficient population of the 229 m, g Th ^229m, g 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 Th. This study proposes a novel approach to populate 229 m, g Th ^229m, g 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 Th, which show 229 m Th ^229m Th -related production cross-sections reaching hundreds of millibarns. Monte Carlo simulations further analyzed the time-wavelength distributions of 229 m Th ^229m Th radiative decay signals and Cherenkov radiation backgrounds in thorium-doped CaF 2 CaF₂, SrF 2 SrF₂, 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 SrF₂ crystals achieved an SNR of 10 6 ^6 under irradiation with bremsstrahlung driven by 1 C electrons at ∼ 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 Th and 230 Th ^230 Th into 229 Th ^229 Th -doped crystals, photonuclear reactions offer a decentralized production route that is independent of the finite 229 Th ^229 Th supply derived from regulated 233 U ^233 U inventories. These findings indicate that the photonuclear population is a promising pathway for efficient 229 m Th ^229m Th production and characterization, effectively mitigating future supply constraints for fundamental research.