The cosmic ray primary composition in the ``knee'' region through the EAS electromagnetic and muon measurements at EAS-TOP

Details The cosmic ray primary composition in the ``knee'' region through the EAS electromagnetic and muon measurements at EAS-TOP The cosmic ray primary composition in the ``knee'' region through the EAS electromagnetic and muon measurements at EAS-TOP

Sep 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aph....21..583a&link_type=abstract

Astroparticle Physics, Volume 21, Issue 6, p. 583-596.

Physics

42

Scientific paper

The evolution of the cosmic ray primary composition in the energy range 106-107 GeV (i.e. the ``knee'' region) is studied by means of the e.m. and muon data of the Extensive Air Shower EAS-TOP array (Campo Imperatore, National Gran Sasso Laboratories). The measurement is performed through: (a) the correlated muon number (Nμ) and shower size (Ne) spectra, and (b) the evolution of the average muon numbers and their distributions as a function of the shower size. From analysis (a) the dominance of helium primaries at the knee, and therefore the possibility that the knee itself is due to a break in their energy spectrum (at EkHe=(3.5+/-0.3)×106 GeV) are deduced. Concerning analysis (b), the measurement accuracies allow the classification in terms of three mass groups: light (p,He), intermediate (CNO), and heavy (Fe). At primary energies E0~106 GeV the results are consistent with the extrapolations of the data from direct experiments. In the knee region the obtained evolution of the energy spectra leads to: (i) an average steep spectrum of the light mass group (γp,He>3.1), (ii) a spectrum of the intermediate mass group harder than the one of the light component (γCNO~=2.75, possibly bending at EkCNO~(6-7)×106 GeV), (iii) a constant slope for the spectrum of the heavy primaries (γFe~=2.3-2.7) consistent with the direct measurements. In the investigated energy range, the average primary mass increases from =1.6-1.9 at E0~=1.5×106 GeV to =2.8-3.1 at E0~=1.5×107 GeV. The result supports the standard acceleration and propagation models of galactic cosmic rays that predict rigidity dependent cut-offs for the primary spectra of the different nuclei. The uncertainties connected to the hadronic interaction model (QGSJET in CORSIKA) used for the interpretation are discussed.

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