The Cosmic-Ray Energy Spectrum Observed by the Fly's Eye

We report on the cosmic-ray energy spectrum above 10¹⁷ eV as observed by the Fly's Eye detector. The detector has been operated in monocular mode (Fly's Eye I only) since 1981 and in stereo mode (both Fly's Eye I and Fly's Eye II) since 1986. This paper includes data through 1992 July. The cosmic-ray primary energy is measured by integrating over the atmospheric development curve of the extensive air shower produced by the primary particle. The differential monocular energy spectrum above 10¹⁷ eV is J(E) = 10^-29.55 x (E/10¹⁸ eV)^{-3.07 + or -0.01}/sq m/s/sr/eV. We observe a dip in the energy spectrum near 10^18.5 using the higher resolution stereo data. A 3 x 10²⁰ eV shower was detected by the monocular Fly's Eye on 1991 October 15.