Energy spectrum and mass composition of primary cosmic radiation in the region above the knee from the GAMMA experiment

TL;DR

This study analyzes the energy spectrum and composition of primary cosmic rays above the knee, confirming a spectral flattening and a 'bump' at 70 PeV, indicating an increasing average mass of particles at high energies.

Contribution

It provides expanded data confirming the spectral features and mass composition changes of cosmic rays above 20 PeV, including the 'bump' at 70 PeV.

Findings

Spectral index above the knee is about -3.1.

A flattening of the spectrum is observed beyond 20 PeV.

A 'bump' at about 70 PeV with >4σ significance is confirmed.

Abstract

The energy spectrum of the primary cosmic radiation in the energy range 1 - 100 PeV and the extensive air shower (EAS) characteristics obtained on the basis of the expanded data bank of the GAMMA experiment (Mt. Aragats, Armenia) are presented. With increased statistics we confirm our previous results on the energy spectrum. The spectral index above the knee is about -3.1, but at energies beyond 20 PeV a flattening of the spectrum is observed. The existence of the 'bump' at about 70 PeV is confirmed with a significance of more than 4{\sigma}. In the energy range of 10 - 100 PeV the shower age becomes energy independent and we observe a direct proportionality of the EAS size to the primary energy. This suggests an approximately constant depth of the EAS maximum in this energy range. This is evidence in favour of an increasing average mass of primary particles at energies above 20 PeV. The additional source scenario, which is a possible explanation of the 'bump' in the spectrum, also leads to the conclusion of increasing mass of the primary cosmic rays. A comparison with the data of other experiments is presented.