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

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 similar to 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 greater than or equal to 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 similar to 0.5 x 10(15)eV for oxygen and similar to 2 x 10(15)eV for iron nuclei with good statistical accuracy. (C) 1997 COSPAR.