Energy Spectrum and Chemical Composition of Cosmic Rays between 0.3 and 10 PeV determined from the Cherenkov-Light and Charged-Particle distributions in Air Showers

Measurements of the lateral distribution of Cherenkov photons with the wide-angle atmospheric Cherenkov light detector array AIROBICC and of the charged particle lateral distribution with the scintillator matrix of the HEGRA air-shower detector complex in air showers are reported. With the atmospheric shower-front sampling technique these detectors measure the electromagnetic component of an extensive air shower via the lateral density distribution of the shower particles and of the Cherenkov photons. The data are compared with events generated with the CORSIKA program package with the QGSJET hadronic-event generator. Consistency checks performed with primary energy-reconstruction methods based on different shower observables indicate satisfactory agreement between these extensive air shower simulations and the experimental data. The energy spectrum features a so called ``knee'' at an energy of E_knee = 3.98 (+4.66) (-0.83) (stat) +- 0.53 (syst) PeV. Power law fits to the differential energy spectrum yield indices of -2.72 (+0.02)(-0.03) (stat) +- 0.07 (syst) below, and -3.22 (+0.47) (-0.59) (stat) +- 0.08 (syst)} above the knee. The best-fit elongation rate for the whole energy range is determined to 78.3 +- 1.0 (stat) +- 6.2 (syst) g/cm^2. At the highest energies it seems to decrease slightly.The best-fit fraction of light nuclei decreases from 37 (+28) (-21) % (combined statistical and systematic) to 8 (+32) (-8) % (combined statistical and systematic) in the energy range discussed here. A detailed study of the systematic errors reveals that a non-changing composition cannot be excluded.