The energy spectrum of cosmic rays around the knee measured by Tibet air-shower experiment is summarized and its characteristic features are discussed under two possible scenarios. The result of Tibet experiment measured over wide range of 1014-1017 eV with high statistics provided details of the knee at the energy around 4×1015 eV. The study of the chemical composition based on measurements of proton and helium spectra obtained from air-shower core detection indicates the dominance of heavy nuclei around the knee. Such feature can be explained either by contribution of nearby sources with source composition dominated by heavy nuclei or by nonlinear effect in diffusive shock acceleration mechanism.
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