Energy spectrum and chemical composition of cosmic rays between 0.3 and 10PeV determined from the Cherenkov-light and charged-particle distributionsin air showers

Measurements of the lateral distribution of Cherenkov photons with the wide -angle atmospheric Cherenkov light detector array AIROBICC and of the charg ed particle lateral distribution with the scintillator matrix of the HEGRA air-shower detector complex in air showers are reported. They are used in c onjunction to determine the energy spectrum and coarse chemical composition of charged cosmic rays in the energy interval from 0.3 PeV to 10 PeV. With the atmospheric shower-front sampling technique these detectors measure th e electromagnetic component of an extensive air shower via the lateral dens ity distribution of the shower particles and of the Cherenkov photons. The data are compared with events generated with the CORSIKA program package wi th the QGSJET hadronic-event generator. Consistency checks performed with p rimary energy-reconstruction methods based on different shower observables indicate satisfactory agreement between these extensive air shower simulati ons and the experimental data. This permits to derive results concerning th e energy spectrum and composition of charged cosmic rays. The energy spectrum features a so called "knee" at an energy of E-knee=3.98 (-0.83)(+4.66)(stat) +/- 0.53(syst) PeV. Power law fits to the differential energy spectrum yield indices of -2.72(-0.03)(+0.02)(stat) +/- 0.07(syst) below and -3.22(-0.59)(+0.47)(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(-21)(+28)% (combined statistical and systematic) to 8(-8)(+32)% (combine d statistical and systematic) in the energy range discussed here. A detaile d study of the systematic errors reveals that a non-changing composition ca nnot be excluded.