Observation of heavy primary cosmic-ray by CR-39

A test experiment to observe heavy primary cosmic-ray by plastic detectors CR-39 was performed. Five plastic detectors were loaded on an emulsion chamber which made level flight for about 30 hours at the height of 31 km. The etching treatment was applied to the detectors. By scanning with a microscope, 111 tracks were detected. Among these tracks, 24 tracks were those by heavier nuclei than argon. The bulk etch rates were determined, and the excess etch rate ratios were estimated. The calibration of charge, which corresponded to the excess etch rate ratio, was made by tracing the tracks. The dip angle measurement can be done by the comparison of the track positions in emulsion plates and those in CR-39. The charge distribution was determined from the excess etch rate ratio and the dip angle. (Kato, T.)