The EAS size spectrum and the cosmic ray energy spectrum in the region 10(15)-10(16) eV

The cosmic ray energy spectrum in the range E-o = 10(15)-10(16) eV (includi ng 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 ch arged particles at the observation level) are performed as a function of ze nith angle with statistical accuracies of a few percent. The change of slop e of the spectrum is observed in each bin of zenith angle at size values de creasing with increasing atmospheric depth. Its attenuation is compatible w ith the one of shower particles (Lambda(e) = 219 +/- 3 g cm(-2)). This obse rvation provides a consistency check, supporting a normal behaviour of show ers at the break, that make plausible astrophysical interpretations based o n 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 i ntersecting power laws) that represents a constraint with which any interpr etation has to match. The change of slope of the power law index reproducin g the size spectrum is Delta gamma = 0.40 +/- 0.09. The derived all particl e 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 tin units of m(-2) s(-1) sr(-1) TeV-1) S(E-o) = (3.48 +/- 0.06) x 10(-10) (E-o/ 2300)(-2.76+/-0.03) for 900 TeV < E-o < 2300 TeV and S(E-o) = (3.77 +/- 0.0 8) x 10(-11) (E-o/5000)(-3.19+/-0.06) for 5000 TeV < E-o < 10(4) TeV. The s ystematic uncertainties connected to the interaction model and the primary composition are discussed. (C) 1999 Elsevier Science B.V.