Study of High-energy Cosmic Gamma Rays and Cosmic Rays in Tibet.

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We study high-energy cosmic gamma rays and cosmic rays with an air shower array (Tibet-II and Tibet-III) in Tibet as a joint research with China. The main research purpose is to study the origin and acceleration mechanism of high-energy cosmic rays by the observation of high-energy primary cosmic-ray energy spectrum and by observation/search of celestial high-energy cosmic gamma-ray point sourcesFrom 1996 to 1999, we set up 80 m^2 emulsion chambers and burst detectors at the center of Tibet-II and studied the energy spectrum of primary cosmic-ray protons in the knee energy region. We obtained the energy spectrum of protons above 10^<15> eV for the first time in the world, by analyzing the emulsion chamber data. The measured energy spectrum showed steepening of the spectral index in the power-law spectrum compared with that below 10^<14> eV, suggesting the acceleration scenario of cosmic rays in the shock wave in a supernova remnant. For this pupose, we developed an automatic analysis device of X-ray films based on a commercial image scanner which enabled us to process X-ray film data. We also did R&D studies of a burst detector array (1000 m^2) for selectively detecting heavy nuclei in the knee energy region, in cooperation with Tibet-III. Four test detectors with a plastic scintillator and optical fibers were setup and in operation in the Tibet-III.We also successfully observed the energy dependence of the apparent anisotropy in multi-TeV galactic cosmic-ray intensitiy due to the terrestrial orbital motion around the Sun for the first time in the world. We obtained a new result on the cosmic-ray anisotropy at the sidereal time frame as well.