Nuclear emulsion with excellent long-term stability developed for cosmic-ray imaging

Nuclear emulsion is a film-type charged particle track detector that uses the technology of silver halide photosensitive material. Because nuclear emulsion is compact and does not require an electric power source, it is suitable as a detector for cosmic-ray imaging (also called cosmic-ray muon radiography or muography) that requires outdoor observation. However, problems with the long-term stability and temperature tolerance of nuclear emulsion limit the observation periods, seasons and locations where it might be used. To enable long-term observation and outdoor use throughout the year, we investigated factors that affect the long-term characteristics of nuclear emulsion. The results of our experiments clearly showed that gelatin, additive chemicals, and packing material affect the long-term stability. Based on these results, we have developed a nuclear emulsion with excellent long-term stability. To confirm the properties of this newly developed nuclear emulsion, we examined the temperature dependence, and created an Arrhenius plot of the reaction rate. The prediction based on Arrhenius law showed that the Grain Density stayed at 25/100μm or more for more than one year at 10 °C and 20 °C, and for 260 days at 30 °C; and the Fog Density stayed at 5/1000μm3 or less for more than one year at 10 °C and 20 °C, and for 270 days at 30 °C. This represents a new nuclear emulsion that is not limited by the observation period, season, and location. In fact, the developed stable nuclear emulsion is being used for cosmic-ray imaging observations in the pyramid of Khufu and is expected to be used for future cosmic-ray imaging observations and other elementary particle research.