ENERGY-SPECTRUM OF PRIMARY COSMIC-RAYS ABOVE 1017.0-EV DETERMINED FROM EXTENSIVE AIR SHOWER EXPERIMENTS AT AKENO

The energy spectrum of primary cosmic rays above 10(17.0) eV has been updated from data collected with two extensive air shower arrays operating at Akeno, one with area 1 km2 and the other with area 20 km2. Along with our previous results in the lower energy region, the energy spectrum has been determined over about five decades of energy from 10(14.5) eV to 10(19.8) eV. A change in the index of the power-law energy spectrum is observed around 10(17.8) eV, as well as the usual features, namely the knee around 10(15.7) eV and the ankle around 10(19.0) eV. The indices of the differential power-law energy spectrum are: (2.62 +/- 0.12) below 10(15.7) eV, (3.02 +/- 0.03) for 10(15.7) approximately 10(17.8) eV and (3.16 +/- 0.08) for 10(17.8) approximately 10(18.8) eV. There is an indication of a flattening of the spectrum above approximately 10(18.8) eV with an index of (2.8 +/- 0.3). The flux above 10(18) eV is (1.5 approximately 2.4) x 10(-12) m-2 s-1 sr-1 and is in good agreement with other experiments. The number of showers above 10(19.5) eV is seven for an exposure of 80 km2 yr sr and further investigation by a new Akeno giant air shower array (AGASA), whose operation has started, is necessary to determine a cutoff energy in the spectrum, if any.