Study of High Energy Cosmic Rays in the Tibetan Highlands

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In 1995, our air shower array, which has been operating at Yangbajing (4300 above sea level in Tibet as a collaboration experiment between Japan and China, was enlarged (the number of detectors increased from 65 to 221), and we are searching for 10 TeV celestial gamma-ray point sources and also studying the primary cosmic rays in ultra-high energy region. This is a unique air shower array capable of detecting 10 TeV cosmic rays with sufficient accuracy and we have already obtained a lot of important scientific results. Using the data obtained from this experiment, we obtained the primary energy spectrum in the energy region from 4x10^{14} to 2x10^{16} eV,showing a gradual steepening at around 2x10^{15} eV.This result will give a strong constraint to the mechanism of high energy cosmic ray acceleration. We also examined a yearly time variation of the Sun's shadow by cosmic rays in the period during 1990 through 1993 (corresponding to near maximum or declining phase of the solar activity cycle, and found that there exists a strong correlation between the movement of the sun's shadow and the phase of the solar activity. Eventually, our observation will obtain new information about the solar and interplanetary magnetic fields.In 1996, we also constructed a high density air shower array (detector separation is 7.5m) and a detector complex of emulsion chambers and burst counters with the total area of 80 m^2 near the center of the old array. The former is to search for supra TeV gammma ray sources and the latter is to observe the primary proton spectrum around the knee region and will be continued until summer of 1999. A preliminary results will be obtained around the end of this year since the data are now under analysis.