THE COSMIC-RAY ENERGY-SPECTRUM ABOVE 3X10(18) EV MEASURED BY THE AKENO GIANT AIR SHOWER ARRAY

We report the first result on the cosmic ray energy spectrum above 3 x 10(18) eV measured by the Akeno Giant Air Shower Array (AGASA) from July 1990 to February 1994. The analysis method and the energy resolution of the AGASA experiment are described in some detail. The flattening of the spectrum around 10(19) eV (ankle) is observed with a significance of 2.9sigma. If we express the differential energy spectrum of cosmic rays of energy E (in eV) with an ankle energy E(a) as J(E) = kappa(E/E(a))-gamma m-2 s-1 sr-1 eV-1, gamma for 10(18.5) eV less-than-or-equal-to E less-than-or-equal-to E(a) is in good agreement with that from the previous experiment and is 3.2 +/- 0.1. The slope gamma above E(a) depends strongly on the value E(a). For the case E(a) = 10(19) eV, kappa = (2.3(-0.2)+0.1) x 10(-33) and gamma = 2.3(-0.3)+0.5 for 10(19) eV less-than-or-equal-to E less-than-or-equal-to 10(20) eV. If E(a) = 10(18.8) eV, then kappa = (1.0 +/- 0.1) x 10(-32) and gamma = 2.7(-0.4)+0.2 for 10(18.8) eV less-than-or-equal-to E less-than-or-equal-to 10(20) eV, after correcting for both the statistical error and the energy resolution of the present experiment. If we interpret the present results assuming an extragalactic origin for cosmic rays above 10(19) eV, the observed data is consistent with either a homogeneous and isotropic distribution of sources or with localized sources at redshift of greater than approximately 0.1. A (1.7-2.6) x 10(20) eV event was observed on December 3, 1993 from the direction of l = 131-degrees and b = -41-degrees. This shower energy is a factor 3 larger than the second highest energy event.