Development of Measurement Technique of Fast Neutron Flux with Nuclear Emulsion in the Underground Laboratory

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Nuclear emulsion is high sensitive photographic film used for detection of three-dimensional trajectory of charged particles. The principle of the measurement of fast neutron with nuclear emulsion is the detection of proton recoiled by neutron. However, electron tracks scattered by gamma rays are also recorded in the nuclear emulsion. These electron tracks are major background for fast neutron measurement. We developed new nuclear emulsion by adding a tiny amount of the rhodium into the silver bromide crystals in order to control the sensitivity and achieved high performance of electron background rejection. We can use this new emulsion to measure fast neutron flux in the underground experiment.