Measurement of the 209Bi(γ,n)208Bi photoneutron cross section in the giant dipole resonance region γ rays at SLEGS

摘要: The photoneutron cross sections for the $^{209}$Bi$(\gamma,n)^{208}$Bi reaction have been measured using quasi-monoenergetic $\gamma$-ray beams from 7.5 to 25.0 MeV at the Shanghai Laser Electron Gamma Source (SLEGS). As a heavy magic nucleus with $N=126$, $^{209}$Bi serves as a critical benchmark for the collective excitation modes of spherical nuclei. Our measurement aims to provide a definitive resolution to the long-standing 20\% discrepancy between the experimental results from the Lawrence Livermore National Laboratory (LLNL) and Saclay. A high-efficiency flat-efficiency neutron detector (FED) was employed to minimize systematic uncertainties related to neutron energy dependence. The extracted giant dipole resonance (GDR) parameters were analyzed within the framework of the TALYS statistical model code. The results demonstrate that our data are consistent with the Saclay trend and IAEA evaluations, suggesting an underestimation in the earlier LLNL data. These findings provide essential constraints for the $\gamma$-ray strength functions ($\gamma$SF) and contribute to a more accurate understanding of the $p$-process nucleosynthesis and lead-bismuth eutectic coolant systems in nuclear reactors.