Large area He-3 tube array detector with modular design for multi-physics instrument at CSNS

The multi-physics instrument (MPI) is the first user cooperative instrument at the China Spallation Neutron Source (CSNS). It was designed to explore the structures of complex materials at multiple scales based on the neutron total scattering technique. This imposes the requirements for the detector, including a high detection efficiency to reduce the measurement time and a large solid angle coverage to cover a wide range of momentum transfers. To satisfy these demands, a large-area array of He-3-filled linear position-sensitive detectors (LPSDs) was constructed, each with a diameter of 1 inch and pressure of 20 atm. It uses an orbicular layout of the detector and an eight-pack module design for the arrangement of He-3 LPSDs, covering a range of scattering angles from 3 degrees to 170 degrees with a total detector area of approximately 7 m(2). The detector works in air, which is separated from the vacuum environment to facilitate installation and maintenance. The characteristics of the MPI detector were investigated through Monte Carlo (MC) simulations using Geant4 and experimental measurements. The results suggest that the detectors are highly efficient in the wavelength range of the MPI, and an efficiency over 25% is achievable for above 0.1 angstrom; neutrons. A minimal position resolution of 6.4 mm full width at half maximum (FWHM) along the tube length was achieved at a working voltage of 2200 V, and a deviation below 2 mm between the real and measured positions was attained in the beam experiment. The detector module exhibited good consistency and an excellent counting rate capacity of up to 80 kHz, which satisfied the requirements of experiments with a high event rate. Observations of its operation over the past year have shown that the detector works steadily in sample experiments, which allows the MPI to serve the user program successfully.