Experimental and theoretical absorptance profiles of tracks of fast heaoy ions in nuclear emulsion. M. Jensen, L. Larsson, 0.Transverse absorptance profiles of 27 tracks of cosmic ray nuclei recorded in Ilford G5 nuclear emulsion have been measured by a nuclear track photometer. Included in the study were nuclei with atomic numbers 14, 16, 20, 24 and 26 with velocities relative to that of light within the interval 0.3 GpSO.8. 'The experimental absorptance profiles have been compared with theoretical profiles calculated from the &ray theory of track formation developed by Katz and coworkers. Taking into account the rescattering of light into the cone of acceptance of the photometer, good agreement has been found between experimental and theoretical profiles within the entire charge and velocity interval. The semi-empirical formula for high aperture photometry is discussed in some detail, as well as the theory of &ray production and the transport of electrons and their energy dissipation in nuclear emulsion. ~~ ' Present address: