Chemical composition of cosmic rays around the knee observed by the Tibet air-shower-core detector

Abstract

Recent results and future prospects of the Tibet air-shower experiment are reported on the all-particle energy spectrum and the chemical composition of cosmic rays around 1015eV energy range. The knee of the all-particle energy spectrum has been clearly observed at the energy around 4x1015eV by Tibet III air-shower array located at Yangbajing, Tibet, China (atmospheric depth 606 g/cm2, area 37,000 m2). The energy spectrum of light components (protons and helium) measured by air-shower-core detector exhibits steeper power index than that of direct measurements below 1014eV, leading to decreasing fraction of the light component toward the knee. Hence, it is strongly suggested that the origin of the knee is related with the change of the chemical composition of cosmic rays. A new air-shower-core detector called 'YAC' is under the preparation for explicit measurement of the heavy component. © 2009 The Physical Society of Japan.