OVERVIEW ON DIRECT AND INDIRECT MEASUREMENTS OF COSMIC RAYS Some thoughts on galactic cosmic rays and the knee

Cosmic rays (CRs) in the energy range from several GeV up to about 100 PeV are assumed to be mostly of galactic origin. At energies up to several 100 MeV individual isotopes can be identified, e.g. with the ACE/CRIS experiment1, a satellite borne silicon detector telescope. At higher energies, CRs are identified by their charge and the energy measurement becomes an experimental challenge. Various techniques are utilized, like the determination of the particles momenta in magnet spectrometers (e.g. BESS2), the (partial) absorption of nuclei in calorimeters (e.g. ATIC3), or the measurement of transition radiation emitted by relativistic particles (e.g. TRACER4). Circumpolar long duration balloon flights offer the possibility of a long exposure (≥ 14 d) combined with low atmospheric overburden (typically < 5 g/cm) as recently demonstrated by the ATIC5, TIGER6, and TRACER7 experiments. At energies above 1 PeV the steeply falling spectrum requires large detection areas (exceeding several 10 m) and exposure times of several years, which presently can be realized only in ground based installations. They measure the secondary products generated by the CR particles in the atmosphere – the extensive air showers. The challenge of these investigations is to reveal the properties of the primary particle behind an absorber – the atmosphere – with a total thickness, corresponding to 11 hadronic interaction lengths or 30 radiation lengths. Consequently, these