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

The multi-physics instrument (MPI) is the first user cooperative instrument at China Spallation Neutron Source (CSNS). It is designed to explore the structures of complex materials at multiple scales based on the neutron total scattering technique. This imposes 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 the momentum transfer. To satisfy these demands, a large-area array of 3He-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 3He LPSDs, covering a range of scattering angles from 3° to 170° with a total area of the detector of about 7 m2. The detector works in the air, which is separated from the vacuum environment to facilitate the installation and maintenance. The characteristics of the MPI detector have been investigated through Monte Carlo (MC) simulations by Geant4 and experimental measurements. The results suggest that the detectors are highly efficient in the range of wavelength of the MPI, and the efficiency over 25% is achievable for above 0.1 Å neutrons. A minimal position resolution of 6.4 mm (FWHM) along the tube length was achieved at a working voltage of 2200 V, and a deviation below 2 mm between the real position and the measured position was attained in the beam experiment. The detector module exhibits good consistency and an excellent counting rate capacity of up to 80 kHz, which satisfies 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 successfully serve the user program.