A limit to the rate of ultra high energy gamma-rays in the primary cosmic radiation

An upper limit to the flux of Ultra High Energy (UHE) gamma-rays in the primary cosmic radiation is obtained through the data of the electromagnetic and the muon detectors of the EAS-TOP Extensive Air Shower array (Campo Imperatore, National Gran Sasso Laboratories, atmospheric depth 810 g cm(-2)). The search is performed by selecting Extensive Air Showers (EAS) with low muon content, For EAS electron sizes N-e > 6.3 . 10(5), no showers are observed with the core located inside a fiducial area and no muons recorded in the 140 m(2) muon detector, during a live time of 8440 h. The 90% c.l. upper limit to the relative intensity of gamma-ray with respect to cosmic ray (c.r.) primaries is I-y/I-c.r. < 7.3 . 10(-5), at primary energy E(0) greater than or equal to 10(15) eV: this limit is lower than reported in previous measurements.