Study of Ultra-High Energy Cosmic Rays at Tibetan highlands

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The Tibet Air shower array at Yangbajing (4,300m, 90.5ﾟ E and 30.1ﾟ N) in Tibet has been operated continuously since January 1990, with Chinese scientists. The present array consists of 65 scintillation detectors which are placed in a lattice of 15m spacing. The mode energy of observed showers is estimated to be about 7 TeV for gamma rays. The angular resolution was well examined by observing the shadowing of cosmic rays by the moon. The moon shadow was observed with the significance of 7.1sigma and the angular resolution of the array is estimated to be 0.88ﾟ for showers with its mode energy 7 TeV and 0.52ﾟ for those with its mode energy 35 TeV.A small deflection of the shadow to the west by about 0.16ﾟ is comparable with that expected from the geomagnetic field effect.On the other hand, the sun shadow was observed in the direction away from the sun by 0.70ﾟ to the west and 0.40ﾟ to the south. This deflection is considered to be caused by the combined effect of the magnetic fields betw … More een the sun and the earth. Note that these data were taken when the sun was still in an active phase between 1990 and 1993. Such effect on the cosmic ray shadow is the first observation and is expected to bring us new information on the large scale structure of the solar and interplanetary magnetic field and its time variation.A search for steady emission of 10 TeV gamma rays has been made for various point sources including the Crab nebula, Cyg X-3 and Her X-1. For Cyg X-3, flux limits obtained at 10 TeV and 30 TeV for Cyg X-3 are far below the levels obtained by extrapolation of the Kiel flux. For the Crab nebula, the result obtained with no excess is inconsistent with the possitive results from the THEMISTOCLE experiment, while still being consistent with the Whipple new data. We also searched for gamma-ray emission from active galactic nuclei (AGNs) which have been reported to be poweful gamma-ray sources by EGRET on board the Compton GRO.In particular, the result for the nearest AGN,Markarian 421 at 10 TeV is interesting, and the further observation on this source will provide new information on the strength of infrared photon field in the universe. The present air shower array will be extended up to an effective area of 3.7 *10^4 m^2 in 1994. This array will consist of 221 scintilation detectors which are placed in a lattice of 15 m spacing. The trigger rate is taken to be about 250 Hz with a dead time less than 30 %. The mode energy for proton showers is estmated to be about 5 TeV.This new array will provide important information on the interplanetary magnetic field as well as acceleration of cosmic rays in the very near future. Less