Development of high-pressure multi-wire proportional chamber neutron detector for the China Spallation Neutron Source multipurpose reflectometer

Abstract

As a multipurpose reflectometer device, the two-dimensional (2D) position resolution neutron detector with a 200 mm×200 mm effective area is developed for China Spallation Neutron Source (CSNS) in Dongguang, China. Due to the requirements for the specific parameters of the multipurpose reflectometer, it should be designed to have a more than 50% (@2 Å) detection efficiency, better than 2 mm position resolution and 3 times n/γ resolution ability during the whole operation period of 10 years. The high pressure multi-wire proportional chamber (MWPC) neutron detector filling 3He gas is used as a key detector. Some simulation results and the experimental results show that the optimized thickness of the neutron entrance window should be 9 mm with using the 7075 aluminum alloy, the high pressure chamber should be sealed by the aluminous ring and a gas mixture should be filled with 6 bar 3He+2.5 bar C3H8. The assembled detector can achieve a more than 54% (@2 Å) detection efficiency in the normal operation.With the 100 μm wide collimator slit, the position resolution for X-rays is about 0.235 mm. Therefore, the position resolution for neutron is about 1.4 mm when 2.5 bar propane is used as the stopping gas for proton and triton. In the chamber, the water vapor, the oxygen and the organic impurity gases will reduce the gas gain, cause the detector electrodes to break down and the detector to speed up aging. To solve the outgassing effect of the detector components and keep the stable operation, the recycled device is designed to have the purification function for the working gases. It could purify the working gas at a flow rate of 2 L/min to remove the oxygen, the water vapor and the organic impurity gases. The detector gain increases about 27% with the purification function. Finally, the n/γ resolution and 2D imaging ability of the detector are tested with the 252Cf neutron source in Institute of High Energy Physics, Chinese Academy of Sciences, the peak ratio of neutron to gamma is obtained to be above 5 from the energy spectra and the detecor has good 2D imaging ability. The performance of the high-pressure MWPC neutron detector could meet the requirements for the multipurpose reflectometer, and the detector will be mounted in the CSNS in this year.