Improved measurement of the neutron flux of CSNS Back-n ES#2 in low energy region from 0.3 eV to 150 keV

The Back-streaming neutron beamline (Back-n) at China Spallation Neutron Source (CSNS) is a newly built time-of-flight facility providing white neutrons from 0.3 eV to 300 MeV. The neutron flux of Back-n is of crucial importance for the feasibility studies of planned measurements and data analysis of conducted experiments. The initial neutron flux measurement of Back-n End Station 2 (ES#2) was conducted by a fission chamber. The measured flux was found to lack accuracy below 150 keV because the fission cross-section of ²³⁵U in this region is not a neutron cross-section standard. In this paper, we report an improved measurement of the energy-dependent neutron flux at ES#2 in low energy region based on ${}^6\mathrm{Li}(n,t)$ reaction. The neutron flux was measured by a ${}^6$Li-Si monitor from 0.3 eV to 150 keV, achieving an accuracy of 3.4%–6.7%. The flux shapes of ES#1 and ES#2 with the same beam configuration were compared as well. It was found that the difference in shape is energy-dependent and the maximum difference can reach up to approximately 20%, even though they were on the same beamline and used same beam configuration.