Average mass of primary cosmic rays in the knee energy region inferred from Tibet experiment

Study of the chemical composition of cosmic rays in the knee region has been made by the Tibet AS_γ Collaboration using the Tibet-III air shower array and an air-shower-core detector. Based on the data of all-particle spectrum, proton, and helium spectra obtained by Tibet hybrid experiment, upper and lower limits of the average mass number of primary cosmic rays were estimated in the energy interval between 10¹⁵eV and 10¹⁶eV assuming unmeasured components (all − proton − helium) are any mixture of nuclei between carbon and iron. The lower limit of hln Ai with carbon model is approximately 2 and the upper limit with iron model is approximately 3.5 with weak energy dependences. The systematic errors involved in estimating hln Ai due to the primary energy determination or the interaction model dependence in deriving the flux of each nuclear element are discussed and found to be small enough to set the boundary for hln Ai. A comparison of our result with recent Icecube data suggests that the primary mass composition is dominated by carbon at 10¹⁵ eV and it tends to be dominated by iron at 10¹⁶ eV.