Speaker
Description
The Back-streaming neutron beamline (Back-n) at the China Spallation Neutron Source (CSNS) is a newly built time-of-flight facility that provides white neutrons. The neutron flux of Back-n is of crucial importance for the feasibility studies and data analysis of experiments conducted utilizing this resource. In particular, the accurate flux is a prerequisite for conducting high-precision nuclear data measurement. The energy region of Back-n is exceptionally broad, spanning 10 orders of magnitude, and includes two endstations (ES#1 and ES#2) with various beamline configurations to accommodate different beam requirements. Consequently, systematic and comprehensive research is essential.
The flux from 0.3 eV to 150 keV was measured using a Li-Si monitor, while the flux from 150 keV to 300 MeV was measured using a fission ionization chamber. Experiments revealed and confirmed spectral differences between ES#1 and ES#2, with discrepancies reaching up to 20% in the 0.3 eV to 150 keV range, thereby clarifying longstanding inconsistencies in neutron capture cross-section measurements. Additionally, it was observed that the shape of the energy spectrum remained unchanged despite the increase in CSNS accelerator power from 20 kW to 125 kW between 2018 and 2022. However, with the power expected to reach 170 kW in 2024, adjustments in the beam window structure have led to changes in the energy spectrum shape. Furthermore, it was found that the shape of the energy spectrum varies with the beam profile; the large beam spot (60-60-60) in ES#2, exhibits a consistent spectrum shape with the combined beam spot (60-30-30), yet it significantly differs from the small beam spot (30-30-30).
