Study of Ultra-high Energy Cosmic Rays at Tibetan Highlands

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A new air shower experiment, aiming to search for celestial gamma ray point sources, has been started from 1989 with Chinese scientists at Yangbajing (4,300m a. s. l., 90.5^ﾟE and 30.1^ﾟN) in Tibet. The present array consists of 49 scintillation counters for fast timing (FT), each of which has an area of 0.5 m^2 and is placed in a grid pattern with a spacing of 15 m. Other 16 density counters surround the FT counter array to select the showers whose core fall inside the array. The TKO and CAMAC system, controlled by a 32 bit micro-computer, is employed in this experiment. The data are recorded on a 8 mm video tape of about 2 GB capacity. A Rb clock with a stability of <54 mus/month is equipped to receive the arrival time of air showers precisely. The operation, atarted fully from June of 1990, is now successively continued with a high trigger rate of about 20 Hz to search for gammaray point sources in the energy region lower than 100 TeV. For the time being, only our array can detect s … More uch rather low energy gamma-rays effectively, since the observation site is located at high altitude.The performance of the present array was examined by using the experimental data, comparing those with a Monte Carlo simulation. This shows that the arrival direction of air showers can be determined with an angular resolution of about 0.5^ﾟ at 100 TeV. Also the energy resolution is estimated to be about 30% for a 100 TeV gamma ray. The detectable shower energy is estimated to be about 20 TeV, and the non-bias energy corresponds to about 40 TeV for gamma ray showers. Our experiment is therefore considered to have a distinctive feature among a lot of air shower arrays working now in the world, giving a capability of observing gamma ray point sources in the several 10 TeV energy region where both of mirror and array experiment on the ground are inaccessible.We still need to operate this array at least for more than one year to examine whether Cyg. X-3 and other point source candidates really emit high energy gamma rays steadily or not. The first result will be given in this fall. Less