A study of the composition of ultra-high-energy cosmic rays using the High-Resolution Fly's Eye

R. U. Abbasi, T. Abu-Zayyad, G. Archbold, R. Atkins, J. Bellido, K. Belov, J. W. Belz, S. Benzvi, D. R. Bergman, J. Boyer, G. W. Burt, Z. Cao, R. Clay, B. M. Connolly, B. Dawson, W. Deng, Y. Fedorova, J. Findlay, C. B. Finley, W. F. HanlonG. A. Hughes, P. Huntemeyer, C. C.H. Jui, K. Kim, M. A. Kirn, B. Knapp, E. C. Loh, M. M. Maetas, K. Martens, G. Martin, N. Manago, E. J. Mannel, J. A.J. Matthews, J. N. Matthews, A. O'Neill, L. Perera, K. Reil, R. Riehle, M. D. Roberts, M. Sasaki, M. Seman, Stephen Schnetzer, K. Simpson, J. D. Smith, R. Snow, P. Sokolsky, C. Song, R. W. Springer, B. T. Stokes, J. R. Thomas, S. B. Thomas, G. B. Thomson, S. Westerhoff, L. R. Wiencke, A. Zech

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193 Scopus citations


The composition of ultra-high-energy cosmic rays is measured with the High Resolution Fly's Eye cosmic-ray observatory data using the Xmax technique. Data were collected in stereo between 1999 November and 2001 September. The data are reconstructed with well-determined geometry. Measurements of the atmospheric transmission are incorporated in the reconstruction. The detector resolution is found to be 30 g cm-2 in Xmax and 13% in energy. The Xmax elongation rate between 1018.0 and 1019.4 eV is measured to be 54.5 ± 6.5(stat) ± 4.5(sys) g cm-2 per decade. This is compared with predictions using the QGSJet01 and SIBYLL 2.1 hadronic interaction models for both protons and iron nuclei. CORSIKA-generated extensive air showers are incorporated directly into a detailed detector Monte Carlo program. The elongation rate and the Xmax distribution widths are consistent with a constant or slowly changing and predominantly light composition. A simple model containing only protons and iron nuclei is compared with QGSJet and SIBYLL. The best agreement between the model and the data is for 80% protons for QGSJet and 60% protons for SIBYLL.

Original languageEnglish (US)
Pages (from-to)910-926
Number of pages17
JournalAstrophysical Journal
Issue number2 I
StatePublished - Apr 1 2005

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Acceleration of particles
  • Cosmic rays
  • Large-scale structure of universe

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