RECENT measurements of cosmic ray chemical composition between 1 and 100 GeV nucleon−1 (refs 1–3) show a decrease of the abundance ratios of secondary to parent nuclei above ≃10 GeV/N. This suggests that above this energy the mean path length of cosmic rays decreases with increasing energy3,4. Indeed, the variations of the abundance ratios illustration and illustration imply a decrease of the mean path length from several g cm−2 at E≃2 GeV/N to a fraction of a gram at E≃100 GeV/N. Open image in new window Open image in new window