Long counters are widely used as neutron fluence monitors by utilizing their flat-response feature. A bulky neutron moderator is required however to achieve the flat-response feature, which can be a disadvantage from the viewpoint of portability as well as an undesirable source of scattered neutrons. Just downsizing the moderator deteriorates the flat response especially for high-energy neutrons. We proposed a new method to downsize the moderator with keeping the flat-response feature and developed a compact flat-response neutron detector in the present study. The detector consists of two small spherical ~3He proportional counters embedded in appropriate positions in a compact cylindrical polyethylene moderator. The sensitivity ratio between the two counters was adjusted to provide a flat response in the total outputs from the two counters. It was demonstrated by MCNPX calculations and response measurements for Cf and Am-Be neutrons and monoenergetic neutrons that the present detector had a good flat response in the wide energy range up to 20 MeV while being downsized by one fifth compared with long counters. The positions of the effective center were determined as a function of the neutron energy. We also proposed a method to evaluate the neutron energy using the detector, which will allow various new applications by taking advantage of measuring the neutron fluence and energy at the same time.
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