N. M. Nikolskaya

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 × 10 6 – 2 × 10 8 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 )b y 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 × 10 6 –2 × 10 8 GeV energy range respectively are attained. An irregularity (‘bump’) in the spectrum is observed at primary energies of ∼7.4 × 10 7 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.

Investigation of the main characteristics of the superhigh energy primary cosmic radiation in the GAMMA experiment (Mt. Aragats, Armenia)

The all-particle energy spectrum of the Primary Cosmic Radiation obtained from the data of GAMMA array (3200 m a.s.l., 700 g/cm2 of atmospheric depth) is presented. The results are obtained by the event-by-event method of the E0 -energy estimation which is developed using the EAS simulation with use of the software CORSICA and the SIBYLL interaction model with subsequent reconstruction of shower parameters taking into account the response of GAMMA detectors, reconstruction errors, as well as fluctuations of EAS development. By using this method an essential “bump”-shaped irregularity is observed in the all-particle spectrum at primary energies of ∼7.4×107 GeV; the bump exceeds a smooth power-law fit to the data by about 4 standard deviations. From the results of studies of the systematic uncertainties of our methods conclusion is made that the observed irregularity has a physical nature rather than is caused by methodical errors. A model explaining this phenomenon is proposed.

On absorption of the hadron component of EAS cores in a large lead calorimeter at 'knee'-range energies

The absorption of the core particles of extensive air showers (EAS) in the large 36 m2 lead ionization calorimeter (at the Tien-Shan mountain station) is analyzed in comparison with full Monte Carlo simulations. The EAS development in the atmosphere is simulated in the framework of CORSIKA+QGSJET code whereas the passage of hadrons and muons through the calorimeter has been simulated using the FLUKA transport code. It is shown that in EAS with energies of a few PeV the value of absorption length, Λ, of core hadron energy increases with energy much faster than that expected from simulations. This effect may be connected with the appearance of a small (a few per cent) excess of abnormal EAS cores with large ionization deposited in the lower layers of the calorimeter. It is shown that the shape of ionization curves in the lead calorimeter observed in abnormal EAS resembles that of high-energy muon groups. This effect cannot be explained by the increase of heavy nuclei in the PCR in the knee region. The hypothesis of prompt muons originated in the interaction of particles with atomic nuclei in the atmosphere also cannot explain the excess of abnormal absorption even at large values of charmed particle production cross sections.