The cosmic ray energy spectrum above 4*10¹⁷eV as measured by the Haverah Park array

The authors report a final assessment of the energy spectrum above 4*10 17 eV which has been produced from measurements made at the Haverah Park extensive air shower array between 1968 and 1987. The methods of analysis are described in some detail and they confirm that the slope of the energy spectrum from 4*10 17 eV to 4*10 18 eV is steeper than the value measured at lower energies, and that above 10 19 eV the spectral slope flattens again. The integral intensity at 10 18 eV is (1.8+or-0.1)*10 -12 m -2 s -1 sr -1 , or about 60 events km -2 sr -1 year -1 . They have demonstrated that there is no evidence for any cut-off in the spectrum at least to 10 22 eV, close to the highest energy observed. At 10 20 eV their estimate of the integral intensity (based on only eight events) is (3+or-2)*10 -16 m -2 s -1 sr -1 , or about one event per km 2 per century per steradian. Thus to explore this energy range and the decade beyond in greater detail requires exposure rates of at least 1000 km 2 per year. They have compared their measurements with data from Akeno, Fly's Eye and Yakutsk.