G. G. Struchkov

The spectrum features of UHECRs below and surrounding GZK

The energy spectrum of ultra-high energy cosmic rays (UHECRs) is discussed on the basis of the Yakutsk array database analysis. In the region E 0 = 10 17 to 3 × 10 19 eV the showers are detected under trigger-500, while at energies above 3 × 10 19 eV the whole acceptance area for trigger-1000 is used in order to utilize all the data available in the region of Greisen-Zatsepin-Kuzmin (GZK) cutoff. Updated relations are adapted to estimate the primary energy having detected densities of charged particles at the distance 300/600 m from the shower axis.

MUONS WITH Eth ≥ 1 GEV AND MASS COMPOSITION IN THE ENERGY RANGE 1018 - 1020EV OBSERVED BY YAKUTSK EAS ARRAY

The ratio of the muon flux density to charged particle flux density at distances of 300 and 600 m from the shower axis (ρμ(300)/ρs(300) and ρμ(600)/ρs(600)) is measured. In addition, the energy dependence of ρμ(1000) is analysed for showers with energies above 1018 eV. A comparison between the experimental data and calculations performed with the QGSJET model is given for the cases of primary proton, iron nucleus and gamma-ray. We conclude that the showers with E0 ≥ 3 × 1018eV can be formed by light nuclei with a pronounced fraction of protons and helium nuclei. It is not excluded however that a small part of showers with energies above 1019 eV could be initiated by primary gamma-rays.

FLUCTUATIONS OF Xmax AND PRIMARY PARTICLE MASS COMPOSITION IN THE RANGE OF ENERGY 5 × 1017 - 3 × 1019EV BY YAKUTSK DATA

The experimental distributions of Xmax obtained with the Yakutsk EAS array at fixed energies of 5 × 1017, 1 × 1018 and 5 × 1018eV are analysed. A recent version of the QGSJET model is used as a tool of our analysis. In the framework of this model, the most adequate mass composition of primary particles satisfying the experimental data on Xmax is selected.

SPECTRUM OF COSMIC RAYS WITH ENERGY ABOVE 1017 EV

The energy spectrum of primary cosmic rays with ultra-high energies based on the Yakutsk EAS Array data is presented. For the largest events values of S600 and axis coordinates have been obtained using revised lateral distribution function. The effect of the arrival time distribution at several axis distance on estimated density for Yakutsk and AGASA is considered.

INFLUENCE OF PRIMARY COSMIC RADIATION MASS COMPOSITION ON THE ESTIMATION OF EAS ENERGY

Fraction of energy, Eem/E0, transferred to the electron-photon component of EAS at E0 = 1015 ± 1019eV was estimated by using the Cherenkov radiation data and the data on charged particles obtained at the Yakutsk EAS array. The results are compared with predictions of different models for energy dissipation into the EAS electron-photon component and with calculations performed with different primary nuclei content. In the energy ranges 1015 ± 1016eV and 1018 ± 1019eV, the ratio Eem/E0 is equal to (77 ± 2)% and (88 ± 2)%, respectively, that does not contradict to a mixed composition of primary particles in the first energy interval and purely proton composition in the second one.

Characteristic features of EAS in the energy range of 50–500 PeV by the Yakutsk data

Data for the electron-photon, the muon and Cerenkov components of EAS, registered with the Yakutsk array during long data collection periods, are presented. The data appear of specific interest as control samples for the development of advanced hadronic interaction models and for checks of differences of various types of models (scaling procedures — QCD inspired models). Preliminary results of the analysis exhibit a satisfactory description of the experimental observations above 10 18 eV by scaling violation, and they require slightly larger multiplicities of the secondary particle production in the energy range of 50 – 500 PeV. The results appear also to be in agreement with a mixed composition in this energy range, approximately like that known for primaries of 10 12 eV. This conclusion is also consistant with the frequency of muons (≥ 1 GeV), observed at 300 m distance from the core of 50 – 500 PeV EAS.