Characteristics of Formation of Nanometer-Size Tracks and Application to Dosimetry of High-LET Radiation

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The solid state nuclear track detector is one of most promising elements for personal neutron dosimetry, which is very important for radiation protection in space activities, aircraft travel, operations around high-intensity proton accelerators such as J-PARC, and emergent situations such as an accident of criticality. It is the purpose of this study to clarify the characteristics of nanometer-size etched track and to establish a technique for effective measurement of high-energy neutrons. The results are summarized as follows :1.Characteristics of nanometer-size tracksAfter irradiation of O,Ne,Si,Ar,Fe,I,Xe,Au ions to CR-39 track detectors and succeeding chemical etching of short duration, very small etched tracks were observed with an atomic force microscope. The growth curve for each ion was measured for early stage up to several tens of nm, from which the radii of track core could be obtained. It is also found that the track core were formed in the region where the local dose exceeded about 100 kGy.2.Application of high-energy neutron measurementIt was found from the first experiment that a polyethylene, a most popular radiator material, has insufficient efficiency for neutrons with an energy of a few tens of MeV or higher. As a result of theoretical consideration was proposed a new technique of two-layer structure consisting of polyethylene and a deuterized hydrocarbon. The latter material should play both roles of a radiator of deuterons and a degrader for energetic protons produced in outer polyethylene layer. Its performance was confirmed from the experiments of 45 and 70 MeV neutron irradiation at TIARA, Takasaki site, JAEA.