Evidence for a High-Energy Cosmic-Ray Spectrum Cutoff

Abstract

We report a measurement of the ultrahigh-energy cosmic-ray spectrum using an atmospheric fluorescence technique for extensive-air-shower detection. The differential spectrum between 0.1 and 10 EeV (1 EeV = ${10}^{18}$ eV) is well fitted by a power law with slope 2.94\ifmmode\pm\else\textpm\fi{}0.02. Above 10 EeV evidence is presented for the development of a spectral "bump" followed by a cutoff at 70 EeV.