Limits on the isotropic diffuse flux of ultrahigh energy gamma radiation

Diffuse ultrahigh energy g radiation can arise from a variety of astrophysical sources, including the interaction of 1020 eV cosmic rays with the 2.7 K microwave background radiation or the collapse of topological defects created in the early Universe. We describe a sensitive search for diffuse g rays at ultrahigh energies using the Chicago Air Shower Array-Michigan Muon Array experiment. An isotropic flux of radiation is not detected, and we place stringent upper limits on the fraction of the g-ray component relative to cosmic rays (,1024) at energies from 5.7 3 1014 to 5.5 3 1016 eV. This result represents the first comprehensive constraint on the g-ray flux at these energies.