Measurement of the Cosmic Ray Composition at the Knee with the SPASE-2/AMANDA-B10 Detectors

Measuring the mass composition of cosmic rays at energies above 1015 eV, the region of the "knee", can provide crucial information for the understanding of the origin of cosmic rays. The SPASE-2 surface air shower array and the AMANDA-B10 neutrino telescope at the South Pole are used in coincidence to measure air showers at these energies. Information from the electron component at SPASE and the high-energy muon component at AMANDA are used together to determine the change of the cosmic ray mass composition in the energy range from 400 TeV to 6 PeV. Mass composition was calibrated to existing data from direct measurements at low energies. Our data show an increase of the mean log atomic mass ln A within the energy range studied. We also discus the robustness of this technique against various systematic effects. SPASE and AMANDA: Detectors and Reconstruction Multicomp onent measurements are needed to obtain primary energy and mass from an air shower. The South Pole Air Shower experiment (SPASE-2) [1] reconstructs the shower direction from the arrival times in its scintillators. This track is required to pass within AMANDA's geometric volume, otherwise the event is discarded. The shower core position and shower size are reconstructed by fitting the lateral distribution of particle density to the Nishimura-KamataGreisen (NKG) function [2] and then evaluating the fit at a fixed distance (30 m) from the shower core. The density of charged particles is measured by this parameter, S (30). Showers with an S (30) less than 5 m-2 or with a core located outside the SPASE array are discarded. The high-energy (> 300 GeV) muon component of the shower can penetrate to AMANDA depth as a muon bundle. Reconstruction of a coincidence event in AMANDA [3] requires two steps. First, the combined detectors (separated by 1750 m center-to-center) are used to get the bundle's position and direction, by fixing the track position at the shower core in SPASE and reconstructing the direction with AMANDA, allowing only the angles (θ , φ) to vary. Second, the