Observation of the Ankle and Evidence for a High-Energy Break in the Cosmic Ray Spectrum

We have measured the cosmic ray spectrum at energies above ${10}^{17}$ eV using the two air fluorescence detectors of the High Resolution Fly’s Eye experiment operating in monocular mode. We describe the detector, PMT and atmospheric calibrations, and the analysis techniques for the two detectors. We fit the spectrum to models describing galactic and extragalactic sources. Our measured spectrum gives an observation of a feature known as the ``ankle’’ near $3\times 10^{18}$eV,andstrongevidenceforasuppressionnear$6\times 10^{19}$ eV.