Measurement of photo-neutron cross-sections in 208Pb and 209Bi with 50-70 MeV bremsstrahlung

The photo-neutron cross-sections of 208Pb and 209Bi induced by 50-70 MeV bremsstrahlung have been measured by using the off-line γ-ray spectrometric technique in the electron linac at the Pohang Accelerator Laboratory. The experimental 208Pb(γ,xn) and the 209Bi(γ,xn) reactions cross-sections at the bremsstrahlung energy region of 50-70 MeV, which are determined for the first time, are in general good agreement with the theoretical values based on the TALYS 1.0 code. We observed that the photo-neutron cross-sections for the (γ,xn) reactions of 209Bi and 208Pb increase with increasing of bremsstrahlung energy from 50 to 70 MeV, which indicates the role of excitation energy. It was also observed that the (γ,xn) reaction cross-sections of the doubly magic shell nuclei 82Pb126 are always higher than those of the singly magic shell nuclei 83Bi126 in the bremsstrahlung energy of 50-70 MeV. This may be due to the fact that either the shell effect are washed out at the higher excitation energy or due to the lower fission cross-section of 208Pb compared to that of 209Bi. This observation indicates that there is a competition reaction between fission and neutron emission.