The EAS size spectrum and the cosmic ray energy spectrum in the region 10 15–10 16 eV

Abstract

The cosmic ray energy spectrum in the range E 0 = 10 15–10 16eV (including the region of the steepening, “knee”) is studied by means of the EAS-TOP array (Campo Imperatore, Gran Sasso Laboratories, atmospheric depth 820 g cm −2). Measurements of the electromagnetic size ( N e = total number of charged particles at the observation level) are performed as a function of zenith angle with statistical accuracies of a few percent. The change of slope of the spectrum is observed in each bin of zenith angle at size values decreasing with increasing atmospheric depth. Its attenuation is compatible with the one of shower particles ( Λ e = 219 ± 3 g cm −2). This observation provides a consistency check, supporting a normal behaviour of showers at the break, that make plausible astrophysical interpretations based on an effect on primaries occurring at a given primary energy. The break has a “sharp” shape (i.e., within experimental errors is compatible with two intersecting power laws) that represents a constraint with which any interpretation has to match. The change of slope of the power law index reproducing the size spectrum is Δγ = 0.40 ± 0.09. The derived all particle energy spectrum is in good agreement with the extrapolation of the direct measurements at low energies and with other EAS data at and above the knee. Power laws fits to the energy spectrum below and above the knee give (in units of m −2 s −1 sr −1 TeV −1) S( E 0) = (3.48 ± 0.06) × 10 −10( E 0/2300) −2.76±0.03 for 900 TeV < E 0 < 2300 TeV and S( E 0) = (3.77 ± 0.08) × 10 −11( E 0/5000) −3.19±0.06 for 5000 TeV < E 0 < 10 4TeV. The systematic uncertainties connected to the interaction model and the primary composition are discussed.