Revised data on gamma-families observed in X-ray emulsion chambers of the Experiment PAMIR

Recently essential efforts were made to improve measurement routine with X-ray films exposed in the X-ray emulsion chambers at the Pamirs. Analysis of X-ray emulsion response upon recorded events show that gamma-family energy and intensity in early publications were over estimated. The main physical results of the new analysis are presented.


Introduction
Over many years the collaboration of the Experiment PAMIR [1] investigated high-energy nuclear interactions in cosmic rays with Χ-ray emulsion chambers (ΧREC) exposed at the Pamirs at an altitude of 4370 m above sea level (600 g/cm 2 ).Various components of nuclearelectromagnetic cascades (NEC) induced by protons and nuclei of primary cosmic rays (PCR) at an energy E 0 ≥10 15 eV are recorded.The investigations are concerned mainly with «families», i.e. bundles of the most energetic secondary particles (threshold for particle detection in the XREC is 1-2 TeV ) in the core of just the same NEC.The secondaries are hadrons and particles of electromagnetic nature (γ-rays and electrons ) which, for brevity, hereafter are referred to as γ-rays.
In this paper we consider families recorded in a thin XREC with lead absorber (Г-block); its structure schematically is as follows: 4 cm Pb + X-ray film + 1 cm Pb + X-ray film + 1 cm Pb + X-ray film.The XREC of this type were exposed either as a single Γ-block or as an upper part of a compound experimental set-up with hadron blocks.The main fraction of particles recorded in Γ-block are γ-rays, and the corresponding families are called γ-families.
The characteristics of γ-family are sensitive to a composition and energy spectra of the PCR.The analysis is based on comparison of experimental data with simulations.
The proper improvement of measurement procedure with Χ-ray films exposed in the XREC was made in order to increase an accuracy of physical results.

Experimental data
The present analysis is based on γ-families recorded during a total XREC exposure ST=2635 m 2. year and selected by following criteria: a) total energy of γ-rays in the family ΣE γ ≥100 TeV; b) energy of γ-rays and their incidence angle (with respect to vertical) E γ ≥4 TeV and θ γ ≤45 ⁰ , respectively; c) deviation of separate γ-rays from the energy-weighted center of a family in the target diagram plane at the observation level R γ ≤15 cm; d) number of γ-rays in the family satisfying above criteria n γ ≥ 3. The total number of selected and analyzed γfamilies is N fam =1003.
Among γ-families with total energy ΣE γ ≥500 TeV there are events, in which closely related electromagnetic cascades in the central part of γ-family overlap and make up a large diffusive dark spot with high optical density (halo).There are γ-families with single-center and multicenter halo (fig. 1 The 61 γ-families with halo were recorded over an exposure ST=3000 m 2 year and also used in our analysis.Model calculations show that single-center halo produced ISVHECRI 2012 Symposium on Very High Energy Cosmic Ray Interactions 10-15 August 2012, Berlin mainly by protons, while multicenter halo are produced by heavy nuclei. The criteria for selection of γ-families with halo are as follows: a) ΣE γ ≥500 TeV; b) the area of halo S D=0.5 bounded by isodense with optical density D=0.5.S D=0.5 ≥4 mm 2 -for single-center halo, and ΣS i D=0.5 ≥4 mm 2 , if S i D=0.5 ≥1 mm 2 -for multi-center halo.

Model calculations
Artificial events of γ-families were sampled by code MC0 of quark-gluon-string model [2], which was elaborated for Experiment PAMIR and satisfactorily reproduced main characteristics of γ-families with ΣE γ >100 TeV.Spectrum of PCR at an energy E 0 =2•10 14 -3•10 18 eV was taken from experimental data of KASKADE and Tibet [3].Mass composition in MC0 model is presented in Table 1.Calculations revealed that about 80% of all events are produced by primary protons, 10% by helium nuclei and no more than 10% by heavier nuclei.This conclusion is almost independent of the models and thus provides a possibility to estimate the fraction of protons in PCR in the range of E 0 =10 15 -10 17 eV.Detailed analysis of a response of the XREC to events recorded Χ-ray films is based on simulation of measurement procedure by code GEANT3.21.The contribution of under threshold electromagnetic cascades, the mutual influence of neighboring cascades in γ-family, identification of cascades against background by Raleigh criterion are taken into account.The analysis show that in our early publication there was overestimation of family energy approximately by 20%.
To prove the suggestion that at energy E 0 >10 16 eV most events with halo are generated by primary protons we compare the fraction of γ-families with multi-center halo in the experiment and calculations (Table 2).It is evident that recorded γ-families with halo almost entirely are generated by primary protons with possible little addition of He.

Dependence of g-family flux on the PCR mass composition
The intensities of the γ-families in MC0 model and Pamir experiment are presented in Table 3.To match this experimental value of γ-families intensity in the interval of E 0 =10 15 -10 16 eV and calculations of code MC0 fraction of protons in the model should be reduced from 33% to (18±2)% if the ejected part of protons is substituted by nuclei of iron group or to (16±2)%if the ejected protons are substituted by helium nuclei.The similar conclusion with the same quantitive estimation of the proton fraction in PCR was made for the energy E 0 >10 16 eV by analysis of γ-families with halo.

Conclusions
1) γ-families in the Experiment PAMIR at an primary energy E 0 =10 15 -10 16 eV are produced by primary protons with a small amount (∼10%) of helium; 2) the fraction of protons in the PCR composition at an energy E 0 =10 15 -10 16 eV is about 15-20% and do not change appreciably up to E 0 =10 17 eV.
). a) b) Fig.1.a) Example of γ-family on the X-ray film; b) scanner image of the halo event called «FIANIT».

Table 1 .
Mass composition in MC0 model.

Table 2 .
Fraction of γ-families with multi-center halo generated by different nuclei.