Cosmic ray data and their interpretation: about the Tibet Shower Array: Chemical Composition of Cosmic Rays at the Knee Viewed from the Tibet Air Shower Experiment

A brief summary is given about present status of composition measurement of cosmic rays by direct observations and indirect ones which provides key information on the origin of the cosmic rays. The result of Tibet hybrid experiment measuring proton and helium spectra at the knee is presented, in which the decreasing fractions of both of the proton and helium component to the all particle spectrum with increasing energy are observed suggesting that the knee is dominated by heavy nuclei. It is planed to explicitly measure the iron spectrum at the knee by detecting the core structure of the air showers in the next phase of Tibet experiment ‘YAC’ (Yangbajing Air shower Core array).