DETECTION OF A COSMIC-RAY WITH MEASURED ENERGY WELL BEYOND THE EXPECTED SPECTRAL CUTOFF DUE TO COSMIC MICROWAVE-RADIATION

We report the detection of a 51 Joule (3.2 +/- 0.9 x 10(20) eV) cosmic ray by the Fly's Eye air shower detector in Utah. This is substantial ly greater than the energy of any previously reported cosmic ray. A Gr eisen-Zatsepin-Kuz'min cutoff of the energy spectrum (due to pion phot oproduction energy losses) should occur below this energy unless the h ighest energy cosmic rays have traveled less than approximately 30 Mpc . The error box for the arrival direction in galactic coordinates is c entered on b = 9.6-degrees, l = 163.4-degrees. The particle cascade re ached a maximum size near a depth of 815 g cm-2 in the atmosphere, a d epth which does not uniquely identify the type of primary particle.