Indications of proton-dominated cosmic-ray composition above 1.6 EeV.

@article{Abbasi2010IndicationsOP,
  title={Indications of proton-dominated cosmic-ray composition above 1.6 EeV.},
  author={R. U. Abbasi and T. Abu-Zayyad and Mohammed A. Al-Seady and Monica Allen and J. F. Amman and R. Anderson and G. Archbold and Konstantin V. Belov and J. W. Belz and D. R. Bergman and Stephen Blake and O. A. Brusova and G. W. Burt and C. Cannon and Zhen Cao and W. Deng and Yulia Fedorova and Chad Finley and Richard Carl Gray and W. Hanlon and Cyrus M. Hoffman and M. H. Holzscheiter and D. Ivanov and Gareth Hughes and P. H{\"u}ntemeyer and B. F. Jones and Charles C. H. Jui and K. Kim and Malina Kirn and Eugene C. Loh and J. Liu and J. P. Lundquist and M. M. Maestas and Naohiro Manago and L. J. Marek and Kalen Martens and J. A. Matthews and J. N. Matthews and S. A. Moore and A. C. O'Neill and C. A. Painter and Lalith Padmasiri Perera and Kevin Alexander Reil and Robert Riehle and Michael D. Roberts and Diego Rodriguez and Naruo Sasaki and Steve Schnetzer and Lauren M. Scott and G. Sinnis and J. D. Smith and P. Sokolsky and C. Song and R. W. Springer and Bradford T. Stokes and Sean R. Stratton and S. B. Thomas and J. R. Jr. Thomas and Gordon Bennett Thomson and Dale Tupa and Andreas Zech and X. Z. Zhang},
  journal={Physical review letters},
  year={2010},
  volume={104 16},
  pages={
          161101
        }
}
We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (X(max)), for air shower events collected by the High-Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d/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… 

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