Cosmic Ray Composition Studies with the High Resolution Fly's Eye

The depth of shower maximum or $X_{max}$ of extensive airshowers is a sensitive probe of the chemical composition of ultra-high energy cosmic rays. Both the evolution of mean $X_{max}$ with energy and the width of the $X_{max}$ distribution can provide clues as to the species of the particles initiating the airshowers. Here, we report the results of composition studies using $X_{max}$ with data collected in stereoscopic mode by the High-Resolution Fly's Eye observatory. The HiRes data are best explained by a predominantly protonic composition above 1.6~EeV.