Towards the knee: Direct measurements of the cosmic ray composition with electronic detectors

Abstract Over a wide range of energies, the cosmic ray composition can be described with a uniform source spectrum for all components, and by a propagation pathlength that continually decreases with energy. However, there are indications that this description may no longer be valid above ∼ 10 13 eV/particle, where observational data become scarce. We discuss how progress can be made in new direct measurements above the atmosphere over the region 10 13 –10 15 eV/particle. Most important is the availability of well calibrated detectors with known charge and energy resolution and very large sensitive area. For the heavier nuclei (Z ≥ 3), these specifications can be met with a new generation of light weight transition radiation detectors. In balloon flights of a few days duration these will reach maximum energies of ∼ 0.5 × 10 15 eV for oxygen and ∼ 2 × 10 15 eV for iron nuclei with good statistical accuracy.