High-Energy Cosmic-Ray Electrons: A New Measurement Using Transition-Radiation Detectors

A new detector for cosmic-ray electrons, consisting of a combination of a transitionradiation detector and a shower detector, has been constructed, calibrated at accelerator beams, and exposed in a balloon flight under 5 g/${\mathrm{cm}}^2$ of atmosphere. The design of this instrument and the methods of data analysis are described. Preliminary results in the energy range 9-300 GeV are presented. The energy spectrum of electrons is found to be significantly steeper than that of protons, consistent with a long escape lifetime of cosmic rays in the galaxy.