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

The evolution of the cosmic ray primary composition in the energy range 10(6)–10(7) 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 (Nl) 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 EHe k = (3,5 +/0,3) 10(6) 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 circa 10(6) 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 (gamma(p;He )> 3,1)), (ii) a spectrum of the intermediate mass group harder than the one of the light component (gamma(CNO) circa 2,75,possibly bending at ECNO k =(6–7) 10(6 )GeV), (iii) a constant slope for the spectrum of the heavy primaries (gamma(Fe) circa 2,3–2,7) consistent with the direct measurements. In the investigated energy range,the average primary mass increases from <ln A>= 1,6–1,9 at E0 circa 1,5 10(6) GeV to <ln A> =2,8–3,1 at E0 circa 1,5 10(7) 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.