Mass composition of primary cosmic rays at energies of 1–1000 PeV according to data of experiment “Pamir” ⁎

Experimental data obtained with X-ray emulsion chambers by the "Pamir" collaboration are compared with calculations using the code MC0 of the quark-gluon-string model. The best fit to the experimental data is attained assuming that the primary cosmic ray (PCR) mass composition in the PCR energy range 1015–1017 eV becomes slowly enriched with heavy nuclei, so that the proton fraction decreases from 33% at to 20% at . To bring model calculations in agreement with experimental data at PCR energy it is necessary to assume a rapid increase of the proton fraction in PCR composition and a possible change of slope exponent of the PCR energy integral spectrum from γ=2.05 to γ=1.65.