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A fast neutron detector system that can figure out neutron source direction is developed. The system consists of two rod-shaped fast neutron detectors and one polyethylene moderator. Pulse shape discrimination plastic scintillators are applied for reduced gamma-ray background and improved system sensitivity for fast neutron detection. The neutron detectors are placed on the back and front sides of the moderator facing a neutron source. Experiments are conducted for a single rod-shaped detector and the system changing the PuBe neutron source direction. Simulation-based studies are also performed using the Monte Carlo simulation code, PHITS (Particle and Heavy ion Transport code System). Taking the ratio of counts of the front and back system detectors, linear change with an increased detector angle is observed, except for near 0° and 90°. This enables us to deduce neutron source direction from several measurements. The method is probably useful for a quick and efficient survey of neutron sources or special nuclear materials. [ABSTRACT FROM AUTHOR]

Abstract: We measured the energy spectra and spatial distributions of the neutron beam of Accurate Neutron-Nucleus Reaction Measurement Instrument (ANNRI) at the Japan Proton Accelerator Research Complex/Materials and Life Science Experimental Facility (J-PARC/MLF). Our research team designed and built ANNRI to measure nuclear data with high precision. The measurements of the neutron beam were performed on three types of beams provided by ANNRI in the neutron energy range from 1.5meV to 10keV. The energy spectra show a typical feature of para-hydrogen moderator, and the absolute intensities almost agree with predictions based on both a simulation calculation of the Japan Spallation Neutron Source (JSNS) and a neutron transmission calculation of the beamline. The available neutron intensities at 21.5m are 7.5×105, 1.6×104, and 1.1×105 n/cm2/s in the energy ranges 1.5–25meV, 0.9–1.1eV, and 0.9–1.1keV, respectively, under the 17.5kW JSNS operation. The measured spatial distributions of the beams formed by three different collimators are consistent with those expected from the collimator-system design of the beamline. The beam sizes in FWHM are about 29, 14, and 11mm for the three different beam collimators. The edges of the spatial distributions are relatively sharp, enabling us to measure the nuclear data successfully. [ABSTRACT FROM AUTHOR]