Fundamental study of fast neutron radiography

Fast neutron radiography (FNR) has been expected to be one of useful imaging techniques for an object which is difficult to get image with thermal neutron radiography (TNR), for instance a hydrogen rich material thicker than a few centimeters. However the R and D of the FNR is far behind that of the thermal neutron radiography (TNR). Imaging technique of the FNR is more difficult than the TNR. The FNR has been studied with use of the fast neutron source reactor YAYOI of the University of Tokyo. Three kinds of imaging techniques have been applied to the FNR. A CR39 nuclear track detector has been used successfully in the first FNR experiments at the YAYOI. This is called as a track-etch method with the CR39. The etched out images on the CR39 sheet usually have a low optical density. In order to perform an imaging technique with high resolving power and high contrast, a thin type of luminescent converter and X-ray film method (film method) has been developed for the FNR. The converter is made of mixture of PMMA and ZnS(Ag), the thickness of which is 40 μm. The system could observe the gap of 100 μm. In the procedure with use of the CR39, it took long time, about 24 hours, to get images. And it took about 3 hours to get imaging results by the film method. To make required time of the images shorter, a television system of fast neutron radiography (FNR-TV) was proposed. In the system, the same technique as a current thermal neutron television system was applied while different luminescent converter was used. The prototype converter was made of mixture of polypropylene resin and ZnS(Ag). (author)