Experiment

Abstract. Isotropic gamma ray flux is expected as a result of interactions of ultra high energy cosmic rays (E> 1019 eV) with the cosmic microwave background radiation and/or the annihilation/decay of topological defects. On the other hand, diffuse gamma ray flux from the galactic plane is expected as a result of interactions of cosmic rays with the interstellar matter. Thus these gamma rays are one of the possible probes to study cosmic rays and their source distribution. We search for diffuse gamma rays with the GRAPES-3 air shower array. It is a high-density array of 1 m2 area plastic scintillation detectors and the large area (560 m2) shielded muon detectors, and it is located at Ooty in the southern India. High statistics with such large area muon detectors help us to distinguish gamma ray showers with hadron showers. Based on the observations from March 2000 to July 2004 we obtained a new upper limit of the ratio of isotropic gamma rays to cosmic rays in the energy range of 20 – 400 TeV, and also we obtained an upper limit of galactic gamma ray flux (|b | < 2◦)