A Multi-Component Measurement of the Cosmic Ray Composition Between 10^{17} eV and 10^{18} eV

The average mass composition of cosmic rays with primary energies between ${10}^{17}$eV and ${10}^{18}$eV has been studied 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, $X_{max}$, and in the change in the muon density at a fixed core location, ${\rho }_{\mu }(600m)$, as a function of energy. The composition has also been evaluated in terms of the combination of $X_{max}$ and ${\rho }_{\mu }(600m)$. The results show that the composition is changing from a heavy to lighter mix as the energy increases.