Mass composition of primary cosmic rays at energies of 1-1000 PeV according to data of experiment ”Pamir” * V.S.Puchkov, A.S.Borisov, Z.M.Guseva, V.G.Denisova, E.A.Kanevskaya, V.M.Maximenko, S.E.Pyatovsky, S.A.Slavatinsky P.N.Lebedev Physical Institute of RAS, Leninsky Prospect, 53, Moscow, 119991, Russia 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 10 15 - 10 17 eV becomes slowly enriched with heavy nuclei, so that the proton fraction decreases from 33% at E0 = 10 15 eV to 20% at E0 = 10 17 eV. To bring model calculations in agreement with experimental data at PCR energy E0 = 10 17 - 10 18 eV 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. 1. Introduction Recently some authors argued that the knee in the energy spectrum of primary cosmic rays (PCR) may be explained in the framework of the diffusion model of PCR propagation [1–3]. The important point of these works is the conclusion that in the PCR energy range E 0 = 10 15 - 10 17 eV the fraction of protons and α-particles be- comes vanishingly small. This conclusion is de- rived from comparison of experimental data on EAS with simulations on the basis of the QGS Jet model by KASKADE [1] or similar Cosmos model by the Tibet group [2]. But this conclusion is in contradiction with results of the Experiment ”Pamir” obtained using X-ray emulsion chambers (XREC). Our previous analysis [4,5] of gamma- families with total energy release in the XREC ΣE γ ≥ 500 TeV with halo accompaniment gave evidence for a comparatively large fraction of pro- tons and α-particles (no less than 30% at E 0 = 10 16 eV). Here we continue the analysis based on comparison of experimental data with simulations by different versions of quark-gluon-string model which satisfactorily reproduce the main features of gamma-families with ΣE γ = 100 - 400 TeV. * This work is supported by Russian Foundation for Basic Research, project 03-02-17465. Calculations by MC0 and QGSJet codes show that ∼ 90% of halo events are generated by pro- tons and α-particles with E 0 ≥ 10 16 . Calcula- tions under the assumption that protons and α- particles in the PCR composition at E 0 ≥ 10 16 eV are absent and substituted by CNO nuclei result in a decrease of flux of halo events by a factor of 3- 5. But this is inconsistent with the experimental data. Analysis of the average EAS age in Ref. [6] lead to the conclusion that at E 0 = 10 15 -10 16 eV the proton fraction at least does not decrease. Ac- cording to data from the RUNJOB collaboration [7], the proton fraction in the PCR at E 0 =4·10 14 eV is also substantial and constitutes (30-40)% while exponents of energy spectra of protons and all PCR particles are close at E 0 = 10 14 - 10 15 eV. PCR particles with E 0 ≥ 10 16 eV are not only responsible for the production of gamma-families with ΣE γ ≥ 500 TeV but as well contribute ap- preciably to the production of gamma-families with ΣE γ ≥ 100 TeV. The disappearence of pro- tons and α-particles in the PCR composition with growth of energy should result in a decrease of the number of events with large values of ΣE γ and, consequently, a steeper ΣE γ spectrum of gamma- families. In this work the differential ΣE γ spectrum for Nuclear Physics B (Proc. Suppl.) 151 (2006) 236–239 0920-5632/$ – see front matter © 2005 Published by Elsevier B.V. www.elsevierphysics.com doi:10.1016/j.nuclphysbps.2005.07.066