Cosmic ray positrons at high energies: A new measurement.

We present a new measurement of the cosmic-ray positron fraction e + =(e + + e) obtained from the rst balloon ight of the High Energy Antimatter Telescope (HEAT). Using a magnet spectrometer combined with a transition radiation detector, an electromagnetic calorimeter, and time-of-ight counters we have achieved a high degree of background rejection. Our results do not indicate a major contribution to the positron ux from primary sources. In particular, we see no evidence for the signiicant rise in the positron