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, S. R. 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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202 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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