Study on chemical composition of primary cosmic rays with energies above 10^<16> eV

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The purpose of this research is to get information on the chemical composition of primary cosmic rays with energies above 10 TeV.For this we have observed air cherenkov light at MAS array on Mt. Chacaltaya in Bolivia since August 2000. In 1999 we examined to get the most suitable parameter for use of determination of the chemical composition and to estimate the accuracy in the determination. In this procedure, we used CORSIKA code to simulate air showers. As a result, we confirmed to be able to determine the composition by measuring both the lateral distributions and the arrival time distributions of air Cherenkov light. Then we started to construct detectors up to the end of 1999. In April 2000, we ship these equipments to Bolivia and installed these in MAS array at Mt. Chacaltaya. The operation was started in August 2000 and the observed data was transferred to Japan. The analysis has been made simultaneously in Japan and in Bolivia. The preliminary result shows that observed chemical composition is much consistent with those obtained with direct measurements. This is a crucially important result because this consistency guarantee the rightness to apply our measuring method in more high energy range. We are going to make the observation for more three years and will get the final result.