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A phenomenon of abnormally weak absorption of very-high-energy cosmic-ray hadrons in lead is discussed. This phenomenon was first observed in Tien-Shan high-altitude experiments in hadronic cores of extensive air showers and has encouraged to introduce the hypothesis of a so-called long-flying or penetrating cosmic-ray component. A similar effect was detected later with deep uniform lead X-ray emulsion chambers (XREC) at the Pamirs. Experiments, which are sensitive to the production of charmed hadrons in the forward kinematic region (xLab ≳ 0.1), have been carried out at the Tien-Shan and Pamirs with exposing two-storey XRECs with large air gaps to prove explanation of the experimental results with high charm production cross section $${\sigma _{pp \to c\bar c}} \sim 8$$ at xLab ≳ 0.1 at ⟨ELab⟩ ∼ 75 TeV. This $${\sigma _{pp \to c\bar c}}$$ value makes it possible to reproduce the experimental hadron-absorption curve observed with XRECs, including curve’s bending at a depth of ∼70 c.u.

The origin of an excess of hadrons observed in deep homogeneous lead X-ray emulsion chambers (XRECs) at depths of more than 70 radiation length units is analyzed. Preliminary experimental data on the absorption of cosmic-ray hadrons in a two-storied XREC with a large air gap, exposed in the Tien Shan mountains, are presented. The chamber was designed to test the hypothesis that the main source of the excess of dark spots detected on X-ray films deep inside the XREC was substantial growth of the charmed-particle production cross section at energy E lab ∼ 75 TeV. The experimental data obtained using a two-storied REC and in experiments with deep homogeneous XRECs are compared to the results from calculations using the FANSY 1.0 model. The comparison shows qualitative agreement between the experimental and the model data, assuming high values of the charmed-particle production cross section when E lab ∼ 75 TeV in the forward kinematic region with x F ≥ 0.1.

A phenomenon of abnormally weak absorption of very high energy cosmic ray hadrons in thick lead blocks is discussed. This phenomenon was first observed in a Tien Shan high altitude experiment to study hadronic cores of extensive air showers (EAS) with a deep ionization calorimeter and encouraged researchers to introduce the hypothesis of the so-called long-flying or penetrating cosmic ray component. A similar effect was detected later with deep uniform lead X-ray emulsion chambers (XREC) at the Pamirs, which we discuss in detail. To establish the nature of the phenomenon we are have carried out dedicated experiments at the Tien Shan and at the Pamirs by exposing two-storey XRECs with large air gaps. According to detailed simulation of the chamber response, these experiments are very sensitive to the production of charmed hadrons in the forward kinematic region and can prove a hypothesis that the phenomenon under study can be explained on the assumption of very high values of charm particle production cross section at 〈ELab〉 ~ 75 TeV in the forward kinematic region at xLab≳ 0.1 which are near the upper limit of recent results of collider experiments, i.e., σpp→c¯c~ 8 mb. The same factor makes it possible to fit all the features of the experimental hadron absorption curve observed by means of deep uniform lead XRECs, including its bending at a depth of ~ 70 c.u.

Spectra of secondary particles in gamma-families, detected at the Pamirs level in X-ray emulsion chambers, are analyzed. This analysis showed that secondary particle spectra change their slope at the particle energy equal to the summary particle energy at which we select gamma-families. Above this bend the secondary particle slope is sensitive to the PCR spectrum slope and composition. The slopes of secondary particle spectra in the Experiment PAMIR are close to the slopes in the variants of the MC0 model, in which the fraction of primary protons is not less than 25% after the knee.

The response of the X-ray emulsion chamber (XREC) is analyzed. The code GEANT 3.21 has been adopted for simulating the measurement procedure in the PAMIR XREC experiment. The contribution of below threshold gamma-quanta, the mutual influence of neighboring electromagnetic cascades in gamma-families, identification against a background by Raleigh's criterion are taken into account. Our earlier publications overestimated the family energy by about ( 10 ÷ 20 )% and the real family angular distribution is less steeper than the measured one. Consequently, the gamma-family intensity should be reduced by 1.10 ÷ 1.15 times. It means, that the Pamir experiment estimate of the PCR proton fraction in the energy region 1 ÷ 100 PeV slowly decreases from 25% to 20%.

The mass composition of the primary cosmic rays (PCR) is deduced from the intensity and energy spectrum of γ -families recorded in X-ray emulsion chambers (XREC) at the Pamirs. The analysis is based on comparison of the experimental data with model calculation. The best fit is attained assuming that the proton fraction in the PCR gradually decreases from 25% at PCR energy E 0 = 10 15 eV to 15% at E 0 = 10 17 eV . The unusual phenomena (alignment effect and penetrating hadrons) are also discussed. Further experiments with XREC at the Tien Shan is in the progress.

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 E 0 = 10 15 eV to 20% at E 0 = 10 17 eV . To bring model calculations in agreement with experimental data at PCR energy E 0 = 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 .

A new complex installation ATHLET is now being built at Tien Shan near Almaty. Some problems of the planned cosmic ray studies are discussed.

The origin of the cosmic ray hadron excess observed in a deep uniform lead X-ray emulsion chamber (XREC) at depths larger than 70 radiation lengths is analyzed. We present preliminary experimental data on the absorption of cosmic ray hadrons in the two-storey XREC with a large air gap exposed at the Tien Shan mountains. The design of the chamber was especially invented to prove the hypothesis on a substantial increase of the charm particle production cross section with energy at ELab ∼ 75 TeV as the main source of the darkness spot excess observed on X-ray films. Experimental data obtained with both a 2-storey XREC and a deep uniform XREC are compared with simulation results calculated with the FANSY 1.0 model. The comparison reveals a qualitative agreement between experimental and simulated data under the assumption of high values of charm particle production cross section at ELab ∼ 75 TeV in the forward kinematic region at xLab > 0.1.

Adjusted data on the fraction of protons in the mass composition of primary cosmic rays (PCRs) in the energy range of 1015–1017 eV are presented. Adjustments are made according to detailed calculations of the response of the X-ray emulsion chamber in the PAMIR experiment. It is demonstrated that the fraction of protons in a PCR is 16–18% for E0 ≈ 1015–1016 eV and does not change within the error for E0 ≈ 1016–1017 eV.

Spectra of secondary particles (γ-rays) in γ-families detected in the X-ray chambers in the Pamirs (H = 600 g cm−2) have been analyzed. These γ-ray spectra show a bend at the energy E*γ ≈ (ΣEγ)min, where (ΣEγ)min is the lowest total energy of γ-rays in the families above which γ-families were selected. The bend is not related to the knee in the spectrum of primary particles; it is due to the use of the ΣEγ selection criterion. The Eγ spectrum slope is sensitive to the spectrum of the primary cosmic rays in the region Eγ ≥ (ΣEγ)min.

γ Families with halos detected in the “Pamir” experiment have been analyzed. Comparison of the experimental data with the results of calculation within the quark-gluon string model (MC0 code) made it possible to determine the efficiency of halo formation by protons, α particles, and heavy nuclei, as well as the fraction of protons in the mass composition of primary cosmic rays at an energy of 1016 eV. It is shown that halos are formed predominantly by protons. The fraction of protons in the mass composition of primary cosmic rays at an energy of 1016 eV is 20%.

I. V. AMURINA,1 V. P. ANTONOVA,2 G. M. AUTOVA,3 M. K. BABAEV,4 A. S. BAYGUBEKOV,4 P. F. BEYL’,1 R. U. BEISEMBAEV,1 A. E. BODUNOV-SKVORTSOV,5 A. S. BORISOV,5 K. V. CHERDYNTSEVA,5 A. P. CHUBENKO,5 P. A. CHUBENKO,5 V. G. DENISOVA,5 V. I. DROBZHEV,2 E. N. GUDKOVA,5 A. V. GUREVICH,5 Z. M. GUSEVA,5 E. A. KANEVSKAYA,5 J. KEMPA,6 M. G. KOGAN,5 E. V. KOSTIN,5 O. N. KRYAKUNOVA,2 A. V. KRUGLOV,1 S. V. KRYUKOV,2 M. I. KRYLOWA,1 A. I. KUPCHISHIN,7 A. K. MANAGADZE,8 I. S. MART’YANOV,4 V. M. MAXIMENKO,5 K. M. MUKASHEV,3 R. A. MUKHAMEDSHIN,9 R. A. NAM,1 N. M. NESTEROVA,5 KH. NURITDINOV,10 V. V. OSKOMOV,7 V. P. PAVLYUCHENKO,5 V. V. PISCAL’,1 R. YU. POLYAKOV,1 V. S. PUCHKOV,5 S. E. PYATOVSKY,5 I. V. RAKOBOL’SKAYA,8 T. M. ROGANOVA,8 N. N. ROINISHVILI,11 V. A. RYABOV,5 N. O. SADUEV,7 T. KH. SADYKOV,4 S. B. SHAULOV,5 A. L. SHCHEPETOV,5 S. A. SLAVATINSKY,5 A. V. STEPANOV,1 L. G. SVESHNIKOVA,8 G. T. SHOIYNBAEVA,3 ZH. T. TOLEGEN,7 YU. N. VAVILOV,5 A. V. VARGASOV,5 N. G. VIL’DANOV,1 L. I. VIL’DANOVA,1 V. I. YAKOVLEV,5 T. S. YULDASHBAEV,10 N. N. ZASTROZHNOVA,4 G. T. ZATSEPIN9 and V. V. ZHUKOV1 1Tien Shan Mountain Scientific Station of LPI, Almaty, 480037 Kazakhstan 2Institute of Ionosphere, Ministry of Education and Science, Almaty, 480020 Kazakhstan 3Abai Almaty State University, Almaty, 480100 Kazakhstan 4Physical-Technical Institute of Academy of Sciences of Uzbekistan, Almaty, 700084 Kazakhstan 5P. N. Lebedev Physical Institute, Moscow, 119991 Russia 6Warsaw University of Technology, Warszawska Politechnika, Off-Campus, Plock, 09400 Poland 7Al-Faraby Kazakh National University, Ministry of Education and Science, Almaty, 480012 Kazakhstan 8D. V. Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119899 Russia 9Institute for Nuclear Research, Russian Academy of Sciences, Moscow, 117312 Russia 10Physical-Technical Institute of Academy of Sciences of Uzbekistan, Tashkent, 700084 Uzbekistan 11Institute of Physics Academy of Sciences of the Georgian Republic, Tbilisi, 380077 Georgia

The experimental spectrum of halo areas for gamma-families with Σ E γ ≥ 500 TeV is obtained and compared with simulations in quark-gluon string model. The comparison suggests a ∼ 2 times decrease of the proton fraction in primary cosmic ray (PCR) composition in the energy region 10 16 ÷ 10 17 eV. However at an energy above 10 17 eV, the experimental flux of halo events with area exceeding 300 ÷ 400 mm 2 is 5 ÷ 10 times larger than the simulated one. Such a difference may be explained by the assumption that in this region the fraction of protons increases as much as 3 times, while the slope of the spectrum decreases (γ = 2.6 ÷ 2.7).

Parameters R ¯ > , ER ¯ > of gamma-hadron families with a visible energy Σ E γ ≥ 100 TeV are analysed with the aim to prove their sensitivity to the mass composition of primary cosmic rays (PCR). The experimental values of these parameters are in agreement with the assumption that PCR become enriched with CNO nuclei at an energy ≥ 10 16 PeV.

Spectrum of “halo” areas in gamma-hadron families with ∑ Eγ ≥ 500 TeV is analyzed. The comparison of experimental data and simulations by quark-gluon string model suggests an increasing contribution of nuclei heavier than protons and α-particles to the primary cosmic radiation (PCR) at energy E0 > 1016 eV. Such a model is consistent both with the accelerator data and those from X-ray emulsion chamber experiments under assumption that PCR mass composition is normal for E0

The attenuation length of the γ-families with Σ E γ ≥ 100 TeV ( N fam = 1003), λ att = (77 ± 3) g/cm 2 and their vertical intensity I vert = 0.63 ± 0.04 m −2 ·year −1 ·sr −1 are determined from the angular distribution. The experimental data are compared with model simulations.