Reexamination of Cosmic-Ray Composition around 1018 eV from Fly's Eye Data

The cosmic-ray composition around 1018 eV is reexamined using the Chou-Yang model for the simulation of air showers and comparing with Fly's Eye data. This model is quite different from models previously used. Using this model leads to the same conclusion that the primary cosmic-ray composition gets lighter in the region from 3 × 1017 eV to 1019 eV. This conclusion of a changing composition is not sensitive to the choice of model because it results from the measured elongation rate being greater than that predicted for a constant composition by any reasonable model that can account for the measured depths of shower maxima at the Fly's Eye threshold energy. That constraint on elongation rate for a constant composition also implies that the increased rate of energy dissipation that appears in hadron-nucleus interactions near 1014 eV may continue up to the region of 1019 eV.