Study on improvement of sensitivity and spatial resolution in fast neutron imaging

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In the present study, we have investigated the method to improve the sensitivity and resolution in fast neutron profile measurement using 1) the combination of Imaging Plate (IP) and a polyethylene converter and 2) two-dimensional position sensitive counter, and achieved the following :1) By combining IP with a polyethylene converter with appropriate thickness, it became possible to measure fast neutron profile with good quality for 5-15 MeV mono-energetic neutrons produced by an accelerator within a few hours. It is also possible to measure a profile of neutrons with energy lower than a hundreds of keV using a ィイD16ィエD1LiF converter.2) We found the relation between the sensitivity vs energy deposit, converter thickness, spatial resolution, fading property, and the optimum irradiation condition for fast neutron profile measurements experimentally and theoretically. The optimum thickness of converters proved to be around the range of particles.3) We observed that the effect of scattered neutrons is serious for fast neutrons and separation of objects from IP is effective to reduce the effect, and that it is important to employ collimators made from nuclides which emit smaller number of gamma-rays.4) We designed and fabricated a two-dimensional position-sensitive counter using drift time and charge-division scheme to reduce the effects of gamma-ray backgrounds and scattered neutrons., and confirmed the possibility of the method.5) To enhance the portability and sensitivity of the counter, we slatted design of counters using a position -sensitive photo-multipliers.