An all-particle primary energy spectrum in the 3 200 PeV energy range

We present an all-particle primary cosmic-ray energy spectrum in the 3 × 10⁶-2 × 10⁸ GeV energy range obtained by a multi-parametric event-by-event evaluation of the primary energy. The results are obtained on the basis of an expanded EAS data set detected at mountain level (700 g cm^-2) by the GAMMA experiment. The energy evaluation method has been developed using the EAS simulation with the SIBYLL interaction model taking into account the response of GAMMA detectors and reconstruction uncertainties of EAS parameters. Nearly unbiased (<5%) energy estimations regardless of a primary nuclear mass with an accuracy of about 15-10% in the 3 × 10⁶-2 × 10⁸ GeV energy range respectively are attained. An irregularity ('bump') in the spectrum is observed at primary energies of ~7.4 × 10⁷ GeV. This bump exceeds a smooth power-law fit to the data by about 4 standard deviations. By not rejecting the stochastic nature of the bump completely, we examined the systematic uncertainties of our methods and conclude that they cannot be responsible for the observed feature.