Detector system of the SENJU single-crystal time-of-flight neutron diffractometer at J-PARC/MLF

The detector system of SENJU—a single-crystal time-of-flight neutron diffractometer—is described. SENJU is the first diffractometer employing a large-area two-dimensional wavelength-shifting fiber-based scintillator detector at the Materials and Life Science Experimental Facility at the Japan Proton Accelerator Research Complex. Thirty-seven detectors were installed in the instrument to cover a scattering angle of 30%. The detector modules exhibited a detector efficiency of 37–47±1% (mean±standard deviation) for thermal neutrons, a 60Co gamma-ray sensitivity of 1.4–5.1±0.1×10−6, a background count rate of 2.1–3.8±1.0×10−4 cps/cm2 and a tolerance to stray magnetic fields up to 120 G. The detector exhibited stable performance under the operating conditions used, such as stable count rates over 400 days and a count temperature coefficient of −0.01±0.02%/°C at around 20 °C. The detector system and its performance metrics are presented together with the specific detector design.