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

📝 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.

💡 Analysis

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.

📄 Content

32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011

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

R.M.MARTIROSOV1, A.P.GARYAKA1, H.S.VARDANYAN1, A.D.ERLYKIN2, N.M.NIKOLSKAYA2, Y.A.GALLANT3, L.W.JONES4, H.A.BABAYAN5
1A. Alikhanyan National Science Laboratory, Yerevan, Armenia 2P N Lebedev Physical Institute, Moscow, Russia 3LUMP, Université Montpellier II, Montpellier, France 4Department of Physics, University of Michigan, USA 5State Engineering University of Armenia, Yerevan, Armenia romenmartirosov@rambler.ru 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. Ara- gats, 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 4σ. 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. Keywords: cosmic ray, knee, primary spectrum, composition

1. Introduction The main goal of the study of the energy spectrum and mass composition of primary cosmic rays is an under- standing of the mechanism of particle acceleration and propagation in the space. To understand the origin of the knee at 3-4 PeV is an important part of this goal. It is predicted that if cosmic rays are confined in super- nova remnants and accelerated at strong shocks through the Fermi process, cosmic rays are enriched by heavy nuclei (like iron) as the primary energies in- crease. Ground-based installations that investigate electromagnetic and muon components of EAS realize this task over a wide energy range. The models [1-4] for the origin of the knee predict changes in the behav- ior of the all-particle spectrum and constituent compo- nent spectra. We need high accuracy experimental data on the EAS components to distinguish the correct model between hosts of them. It is commonly believed that the all-particle flux changes smoothly without any prominent structures. Some of the models predict the appearance of structures in the energy spectra beyond the knee. There are experimental indications that such structures exist [5]. The indirect character of the pri- mary energy and mass determination demands an in- crease in the accuracy of the data treatment methods along with the increasing of the experimental data statistics. We present in this paper the all-particle pri- mary energy spectrum and some EAS characteristic correlations on the basis of extended data from the GAMMA experiment. Special attention was paid for the checking of a possible influence of detector satura- tion on the derived energy spectrum.
2. GAMMA experiment GAMMA is a ground-based EAS array to measure the muon and the electro-magnetic components of the EAS [6,7]. It consists of an array of 33 surface detec- tion stations and underground muon detectors, located at the south side of Mount Aragats in Armenia. The layout of the array is shown in figure 1. Each station contains 3 plastic scintillation detectors with dimen- sions of 1x1x0.05 m3. Each of the central nine stations contains an additional (the 4th) small scintillator with dimensions of 0.3x0.3x0.05 m3 for high particle density (»103 m−2) measurements. In addition recently sepa- R.M.MARTIROSOV ET AL. ENERGY SPECTRUM ABOVE THE KNEE FROM THE GAMMA EXPERIMENT 2 rate 8 detectors were placed to make central part denser. The muon carpet is composed of 150 scintilla- tion detectors which are compactly arranged in the underground hall under the 2.3 kg/cm2 of rock and concrete. The layout of the carpet is shown in figure 2. The scintillator dimensions, casings and photomultipli- ers are the same as in the surface detectors. The ar- rangement of the muon detectors gives the possibility of determining the muon lateral distribution up to 60m from the EAS core at Eµ > 5GeV.

Figure 1. The layout of the GAMMA array. Black squares – detection stations. White circles – single detectors X [m] -10 -5 0 5 10 Y [m] -20 -15 -10 -5 0 5 10 15 20

           Figure 2. The layout of the muon carpet. 

The reconstruction of the EAS size

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