Transition Radiation Detectors for Cosmic Rays near the Knee

Precise observations of the energy spectra and relative abundances of cosmic-ray nuclei require instruments that exhibit individual charge resolution and a calibrated energy response. If energies up to ∼ 1015 eV are to be covered, the low intensity of the heavier nuclei (Z ² 3) also mandates detector areas of several square meters. X-ray transition radiation detectors (TRDs) appear to provide the only practical means of fulfilling all of these requirements for balloon or space-b orne instruments. However, for measurements up to the cosmic-ray "knee", care must be taken that the energy response of the TRD does not saturate for Lorentz factors less than ∼ 105 . We have designed detectors to meet this goal, and have successfully tested prototypes at an accelerator beam at CERN. We shall present and discuss the results of these measurements.