Evidence for Changing of Cosmic Ray Composition between1017and1018eV from Multicomponent Measurements

The average mass composition of cosmic rays with primary energies between ${10}^{17}$ and ${10}^{18}\mathrm{eV}$ 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 and the muon density as a function of energy. The results show that the composition is changing from a heavy to lighter mix as the energy increases.