No abstract

The energy spectrum of UHECRs is discussed on the basis of the Yakutsk array database analysis. In the region E=0.1 to 30 EeV the showers are detected under trigger-500, while at energies above 30 EeV the whole acceptance area for trigger-1000 is used in order to utilize all the data available in the region of GZK cutoff.Comment: Invited talk at CRIS2004: GZK and surroundings, Catania, Italy, 31.05.04. To appear in Nucl. Phys. B Proc. Supp

Abstract. One of several working groups established for this workshop was charged with examining results and methods associated with the UHECR energy spectrum. We summarize the results of our discussions, which include a better understanding of the analysis choices made by groups and their motivation. We find that the energy spectra determined by the larger experiments are consistent in normalization and shape after energy scaling factors are applied. Those scaling factors are within systematic uncertainties in the energy scale, and we discuss future work aimed at reducing these systematics.

We present new data on Cherenkov light observations obtained during the period 1994-2009, after a modernization of the Yakutsk EAS array. A complex analysis of xmax and its fluctuations σ(xmax) was performed over a wide energy range. With the new data, according to QGSJet II model, an estimation was made of the cosmic ray mass composition for E 0 ∼ 10 17 − 3 × 10 19 eV. The result points towards a mixed composition with a large portion of heavy nuclei at E 0 ∼ 10 17 eV and the dominance of light nuclei at E 0 ∼ 10 19 eV. The analysis of σ(xmax) energy dependence for the same energies qualitatively confirms this result. The shape of the xmax distribution at fixed energy 10 18 eV is analysed to make more precise conclusions on cosmic ray mass composition.

Responses of surface and underground scintillation detectors of Yakutsk array are calculated for showers initiated by primary particles with energy E 0 ≥ 10 17 eV within the frameworks of QGSJet01d, QGSJet-II-04, SIBYLL-2.1 and EPOS-LHC hadron interaction models. A new estimation of E 0 is obtained with the use of various methods. The resulting energy is lower compared to the obtained with earlier method by factor ∼ 1.33.