MEASUREMENTS OF COSMIC-RAY AIR SHOWER DEVELOPMENT AT ENERGIES ABOVE 1017 EV

We present measurements of the depth of maximum for extensive air showers (EAS) produced by cosmic-ray nuclei with energies above 1017 eV. The air showers were observed using the University of Utah's Fly's Eye detectors operating in stereo mode. Measurements of the atmospheric nitrogen fluorescence light generated by EAS have been used to obtain longitudinal development curves of individual showers and, in particular, the depth of maximum of each shower. The data imply an elongation rate of 69.4 ± 5.0 g cm-2 per decade above 1017 eV without correction for triggering and resolution effects. These effects contribute approximately + 5 g cm-2 per decade to the apparent elongation rate. The distribution of depths of shower maximum has a mean value of 690 ± 3 g cm-2 (with an estimated systematic uncertainty of ±20 g cm-2) and a width (standard deviation) of 85 ± 2 g cm-2 for showers with energies above 3 × 1017 eV. The tail of the distribution has a logarithmic decrement of λ= 70 ± 14 g cm-2. Comparison with predictions for a pure proton and a pure iron flux indicates that the data are not consistent with either and are instead consistent with a mixture. This will be further discussed in an upcoming publication by Gaisser et al.