Highly sensitive nuclear emulsion films and high-speed imaging technology have been developed for recording tracks of minimum ionization particles such as cosmic-ray muons and electrons. The ionization energy of the heavy ions in cosmic rays is too high to identify the individual nuclear charges because the signal of pulse-height volume for tracks is saturated on this system. We have developed desensitized nuclear emulsion films by adding a rhodium compound (Na3RhCl6∙5H2O). This modification reduces the film\u27s sensitivity to heavy ions by 37%, resulting in better linearity between ionization energy and the signal of the tracks. The signals due to heavy ions were compared with the signals recorded on a CR-39 nuclear track-detector. The desensitized nuclear emulsion films successfully distinguished adjacent nuclear charges of heavy ions
