28th International Cosmic Ray Conference 111 Proton Fraction in PCR Mass Composition at Energies of 10 15 - 10 17 eV (Experiment “Pamir”) A.S.Borisov, 1 Z.M.Guseva, 1 V.G.Denisova, 1 E.A.Kanevskaya, 1 J.Kempa, 2 V.M.Maximenko, 1 R.A.Mukhamedshin, 3 S.E.Pyatovsky, 1 V.S.Puchkov, 1 and S.A.Slavatinsky 1 (1)P.N.Lebedev Physical Institute of RAS, Moscow, Russia (2)Warsaw Polytechnical Institute, Dep. of Mat. and Phys., Plock, Poland (3)Institute for Nuclear Research, Moscow, Russia Abstract The fraction of halo events among gamma-hadron families with ∑ E γ > 500 TeV is analyzed. The comparision of experimentsl data and Monte-Carlo simulations by MC0 code of quark-gluon string model under various assumptions on the PCR mass composition suggests a slowly increasing contribution of nuclei havier than protons and α-particles to PCR mass composition. Effective atomic number of PCR particles changes in the energy interval 10 15 - 10 17 eV so that lnA grows from 2 to 2.5. 1. Introduction This paper is devoted to the investigation of gamma-families with total energy ∑ E γ ≥ 500 TeV (“superfamilies”), recorded in X-ray emulsion chambers (XREC) of Experiment “Pamir” at an altitude of 4370 m a.s.l. (600 g/cm 2 ) [1]. Experimental data are compared with model sampling by code MC0 of the quark- gluon model [2] which satisfactorily describes the main features of gamma-families with ∑ E γ = 100 - 400 TeV (primary energies ≤ 10 PeV) under assumption of normal PCR mass composition (fraction of protons and α-particles at an energy E 0 = 1 PeV constitutes 53%). With increasing of PCR energy and ∑ E γ , the cascades induced by high-energy gamma-rays begin to overlap each other and, as a result, in the central part of superfamilies, recorded in the X-ray chamber, a diffuse region of high optical density, (so called halo) arise, its area S amounts up to several centimetres squared. The calculations suggest that halo is induced, as a rule, by a narrow beam of high-energy particles, which incident upon the XREC from the atmosphere and provides the necessary density of energy flow (≥ 20 TeV ·mm -2 ). The beam may be produced by development both nuclear or electromagnetic cascade in the atmosphere. Thus a halo area S reflects the density of energy flow in the central part of EAS core. Fraction of halo events increases with growth of family energy and at ∑ E γ ≥ 1000 TeV amounts up to pp. 111–114 c 2003 by Universal Academy Press, Inc.