Estimates of the cosmic gamma-ray flux at PeV to EeV energies from the   EAS-MSU experiment data
  Archival EAS-MSU data are searched for anomalous muonless events which may be caused by primary gamma rays with energies between 10^15 eV and 10^18 eV. We consider a refined sample of high-quality data and confirm the previously reported detection of a non-zero gamma-ray flux at ~5x10^16 eV with a similar flux value but at somewhat lower statistical significance, corresponding to the depletion of the sample. We present upper limits on the flux below and above these energies, including the first constraints in the range (10^17-10^18) eV never studied by any other experiment. 
Yu.A. Fomin, N.N. Kalmykov, G.V. Kulikov, V.P. Sulakov and S.V.   Troitsky
astro-ph.HE