The Energy Spectrum and the Chemical Composition of Primary Cosmic Rays with Energies from 10¹⁴ to 10¹⁶ eV

We have measured extensive air showers with primary energies above 6 TeV at Mount Chacaltaya in Bolivia. The data were collected by an air shower array called the Minimum Air Shower (MAS) array starting in 2000 March. We applied an equi-intensity analysis method to the extensive air showers extended over the region of their maximum development. We varied the mixture of protons and iron in our simulations and compared these to the data to determine the mixing ratio of protons as a function of the primary energy. Using this, we derived the primary energy spectrum from 10¹⁴ to 5 × 10¹⁶ eV. Consequently, we conclude that the power-law index of the spectrum changes gradually around 10^15.5 eV and that the obtained proton ratio decreases with increasing energy. We directly measured the longitudinal development of air showers generated by primaries with energies around the knee. We found that the average mass number of primary cosmic rays shows a steady increase with energy above 10^14.5 eV and that the dominant component around the knee is not protons.