The Extensive Air Shower Experiment Kascade - First Results

The extensive air shower (EAS) experiment KASCADE has started its operation at the laboratory site of the Forschungszentrum Karlsruhe, at 49 N, 8 E, 110 m above sea level. It consists of an extended (200 200m 2) array of electron detectors and muon detectors as well as of a compact and complex central detector system. The latter contains a dense scintillator matrix, muon tracking detectors, a scintillator trigger layer, and as its main part a large (16 20m 2) hadron calorimeter. The principal aims of the KASCADE project are the determination of the energy spectrum and of the chemical composition in the ultra high energy (UHE) range around the knee in the primary cosmic ray spectrum. The main advantage of the new installation lies in the simultaneous measurement of a large number of observables for each individual event. This is achieved by the combination of various detection techniques for electrons, muons, and hadrons in extensive air showers and is supplemented by detailed simulation work on air shower properties and detector response. In multiparameter analyses the eeects of intrinsic uctuations of shower parameters can be considerably reduced. Both the event reconstruction and especially the interpretation of the results depend on model predictions. Air shower event simulations (e.g. with the CORSIKA code) in the UHE range have to rely on extrapolations of the hadronic interactions to higher energies and into the forward angle region not covered in collider experiments. Diierent interaction models give quite diierent observables at ground level. Therefore, it seems very important to test the results of these calculations experimentally. Especially the central detector system of KASCADE enables a variety of such model tests. Data taking with a large part of the experiment has started in 1996. First (preliminary) results on air shower properties like lateral particle distributions, on details of the hadronic shower core, on electron and muon size spectra, and on the energy dependence of the primary composition have been deduced from a subset of data. INTRODUCTION Since several decades one central topic of UHE cosmic ray studies has been the investigation of the knee in the primary spectrum. This distinct change in spectral index of the charged particle ux at observation level is generally believed to reeect a change in the energy spectrum