Measurement of the Cosmic Ray Energy Spectrum and Composition from   10^{17} to 10^{18.3} eV Using a Hybrid Fluorescence Technique
  We study the spectrum and average mass composition of cosmic rays with primary energies between 10^{17} eV and 10^{18} eV using a hybrid detector consisting of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon array. Measurements have been made of the change in the depth of shower maximum as a function of energy. A complete Monte Carlo simulation of the detector response and comparisons with shower simulations leads to the conclusion that the cosmic ray intensity is changing f rom a heavier to a lighter composition in this energy range. The spectrum is consistent with earlier Fly's Eye measurements and supports the previously found steepening near 4 \times 10^{17} eV . 
T.Abu-Zayyad, K.Belov, D.J.Bird, J.Boyer, Z.Cao, M.Catanese, G.F.Chen,   R.W.Clay, C.E.Covault, H.Y.Dai, B.R.Dawson, J.W.Elb...