Upper limits on the isotropic gamma ray / cosmic ray ratio from the Grapes III experiment at Ooty

We have attempted to determine the isotropic gamma-ray / cosmic ray ratio using the very large area (560m) and packed muon detector operating with the EAS array at Ooty in southern India. The showers with their cores incident within the area of the EAS array but with no associated penetrating track in the muon detectors, classified as muon-poor showers, have been considered as primary gamma-ray initiated shower candidates. We have obtained upper limits on the flux of isotropic gamma-rays over the energy region from 20 TeV to 500 TeV. A conservative upper limit on the ratio, Igamma/ICR of 1*10 -3 at 90% C.L., over the energy region, 100 TeV to 500 TeV, has been placed.