Energy spectrum of primary cosmic rays above 1017.0eV 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 km 2 and the other with area 20 km 2 . Along with the authors 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+or-0.12) below 10 15.7 eV, (3.02+or-0.03) for 10 15.7 approximately 10 17.8 eV and (3.16+or-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+or-0.3). The flux above 10 18 eV is (1.5 approximately 2.4)*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 km 2 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.