Abbasi, R.Abu-Zayyad, T.Archbold, G.Atkins, R.Bellido Caceres, J.Belov, K.Belz, J.BenZvi, S.Bergman, D.Boyer, J.Burt, G.Cao, Z.Clay, R.Connolly, B.Dawson, B.Deng, W.Fedorova, Y.Findlay, J.Finley, C.Hanlon, W.et al.2006-12-042006-12-042005The Astrophysical Journal (ApJ), 2005; 622(2):910-9260004-637X1538-4357http://hdl.handle.net/2440/17873© 2005. The American Astronomical SocietyThe 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.enacceleration of particlescosmic rayslarge-scale structure of universeJournal article002005021710.1086/4279310002280934000132-s2.0-1994436184255212Bellido Caceres, J. [0000-0002-0833-9194]Clay, R. [0000-0002-9040-9648]Dawson, B. [0000-0002-4271-3055]