SEARCH FOR ISOTROPIC GAMMA-RADIATION IN THE COSMOLOGICAL WINDOW BETWEEN 65-TEV AND 200-TEV

Electromagnetic energy injected into the universe above a few hundred TeV is expected to pile up as gamma radiation in a relatively narrow energy interval below 100 TeV due to its interaction with the 2.7 degrees K background radiation. We present an upper limit (90% C.L.) on the ratio of primary gamma to charged cosmic rays in the energy interval 65-160 TeV (80-200 TeV) of 10.3 . 10(-3) (7.8 . 10(-3)). Data from the HEGRA cosmic-ray detector complex consisting of a wide angle Cerenkov array (AIROBICC) measuring the lateral distribution of air Cerenkov light and a scintillator array, were used with a novel method to discriminate gamma-ray and hadron induced air showers. If the presently unmeasured universal far infrared background radiation is not too intense, the result rules out a topological-defect origin of ultrahigh energy cosmic says for masses of the X particle released by the defects equal to or larger than about 10(16) GeV.