A sealed neutron beam monitor based on boron-lined multi-wire proportion chamber

Purpose The purpose of this study is to develop a high transmittance, sealed neutron beam monitor, which can be used in vacuum environment such as the Small-Angle Neutron Scattering spectrometer at China Spallation Neutron Source (CSNS).
Methods Boron-lined multi-wire proportion chamber was adopted to measure the neutron counting rate. A small anode spacing of 4 mm was chosen to realize high counting rate ability of the monitor. GEANT4 simulation was first carried out to optimize the cell structure of the detector. The relationship between the neutron detection and the boron layer thickness, the scattering rate of the detector body to the neutron beam, etc. were obtained. Then the monitor was manufactured and tested with Am-241 source. Finally, neutron beam tests were conducted at the 20th beamline (BL20) of the CSNS, and the response of the monitor to the neutron beam was obtained.
Results and conclusion A monitor with detection efficiency of 0.15%@1.8 Å and sensitive area of 100 × 100 mm² was constructed. The monitor maintained good performances for over 6 months and can obtain neutron beam parameters including neutron counting rate and neutron time-of-flight spectrum, etc. Then the neutron beam intensity, neutron wavelength spectrum, etc. can be provided.