Physical design of fast fission chamber for neutron spectrum and flux measurement at the CSNS backstreaming white neutron source

Abstract Background: The backstreaming white neutron source (Back-n) of China spallation neutron source (CSNS) will provide pulsed white neutron beam with a high fluence rate for nuclear data measurement. The accurate measurement of the neutron spectrum and fluence rate after the Back-n's completion will be the basis of the following physical experiments. Purpose: This study aims to design a fast fission chamber for the CSNS Back-n flux measuring and monitoring system. Methods: The detector performance including time resolution, particle distinction and detection efficiency was studied by experiments. Based on the study, the parameters of the fission chamber were determined, and its detection efficiency calculated by the ²³⁵U sample quantity was compared with that measured by concomitant particle method. Results: Considering the particle distinction, the distance between the anode and the target of the fission chamber was determined as 10 mm, the time resolution of the fission chamber was measured to be about 15 ns. The deviation of calculated detection efficiency from the measurement was in the range of 5%. Conclusion: According to the detection efficiency and the demand of the neutron flux monitoring, the quantity of the uranium for the fast fission chamber can be determined. With appropriate parameters, the fast fission chamber can be used in CSNS Back-n flux measuring and monitoring. (authors)