Abstract
Using data from more than ten-years of observations with the Akeno Giant Air
Shower Array (AGASA), we published a result that the energy spectrum of
ultra-high energy cosmic rays extends beyond the cutoff energy predicted by
Greisen, and Zatsepin and Kuzmin. In this paper, we reevaluate the energy
determination method used for AGASA events with respect to the lateral
distribution of shower particles, their attenuation with zenith angle, shower
front structure, delayed particles observed far from the core and other
factors. The currently assigned energies of AGASA events have an accuracy of
$\pm$25% in event-reconstruction resolution and $\pm$18% in systematic errors
around 10$^20$eV. This systematic uncertainty is independent of primary
energy above 10$^19$eV. Based on the energy spectrum from 10$^14.5$eV to a
few times 10$^20$eV determined at Akeno, there are surely events above
10$^20$eV and the energy spectrum extends up to a few times 10$^20$eV
without a GZK-cutoff.
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