We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (Xmax□), for air shower events collected by the High-Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate dxmax□/d[log□(E)] of 47.9±6.0(stat)±3.2(syst)g/cm2/decade for energies between 1.6 and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at 4×1018eV.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)