Measurement of the neutron flux of CSNS Back-n ES#1 under small collimators from 0.5 eV to 300 MeV

The newly built back-streaming white neutron beam line (Back-n) at China spallation Neutron Source (CSNS) makes a breakthrough for China’s domestic nuclear data measurement, which is pushing nuclear data measurement in China towards higher precision and wider energy range. The neutron flux of the facility was measured at the first beginning since it is an extremely important characterization for the experiment proposal and data analysis. The neutron fluxes under big collimators in end station 1 (ES#1) and end station 2 (ES#2) had been characterized by a fission chamber. In this paper, the first measurement of the neutron flux of Back-n ES#1 with small collimators is reported. The neutron flux from 0.5 eV to 300 MeV was measured by three detectors: a silicon monitor based on a 6LiF sample for the energy range below 200 keV, a fast multi-layer fission ionization chamber for the range from 10 keV to 300 MeV, and a proton recoil telescope for the high energy region from 10 to 70 MeV. The flux and its uncertainty with 50 bins per decade were finally evaluated by combining the data of the three detectors. The integrated flux in the full energy range at CSNS beam power of 100 kW is 1.642 × 106 n/cm2/s, which is consistent with the FLUKA simulation within a difference of about 4