The mass composition of primary cosmic rays of energy 6×1015-2×1018eV

Abstract
Measurements of the mean and fluctuation in the depth of cascade maximum of large cosmic-ray showers are reported. These are compared with computer simulation predictions for iron nucleus and proton primaries incorporating a range of models of the hadronic interaction. The authors conclude that the primary composition comprises predominantly iron-like nuclei at 1016 eV with an increasing proportion of lighter nuclei at higher energies.