[en] Nuclear emulsion and cellulose-nitrate plastic detectors have been combined in a detector system for mass determination of galactic cosmic ray iron particles. The detector has been exposed and 17 iron nuclei have been indentified with an accuracy of 0.3-0.4 AMU. The energy interval was 500-750 MeV/nucleon at the top of the atmosphere. The mass analysis is based on measurements of cone-lengths in Kodak plastics and residual ranges in plastics and emulsion. Significant sensitivity variations between the plastic sheets were discovered. The high mass resolution was obtained by applying corrections to overcome the inhomogeneous detector response. (author)