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 first balloon flight of the High Energ y Antimatter Telescope (HEAT). Using a magnet spectrometer combined wi th a transition radiation detector, an electromagnetic calorimeter, an d time-of-flight counters we have achieved a high degree of background rejection. Our results do not indicate a major contribution to the po sitron flux from primary sources. In particular, we see no evidence fo r the significant rise in the positron fraction at energies above simi lar to 10 GeV previously reported.