Gamma-ray astronomy above 50 TeV with muon-poor showers

In this paper we make a quantitative evaluation of the rejection power for proton-induced showers available in principle with air-shower arrays. From a sample of more than 50 000 Monte Carlo showers we conclude that diffuse $\gamma$ rays of PeV energy can be observed to a level approaching ${10}^{-5\mathrm{}}$ of the background cosmic-ray flux by detectors sensitive to the muon component of the cascades. This may be sufficient to see the galactic disk in ultrahigh-energy photons.