Measurements of the Cosmic-Ray Positron Fraction From 1 to 50 GeV
Two measurements of the cosmic-ray positron fraction as a function of energy have been made using the High Energy Antimatter Telescope (HEAT) balloon-borne instrument. The first flight took place from Ft. Sumner, New Mexico in 1994, and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The second flight from Lynn Lake, Manitoba in 1995 permitted measurements over a larger energy interval, from 1 GeV to 50 GeV. In this letter we present results on the positron fraction based on data from the Lynn Lake flight, and compare these with the previously published results from the Ft. Sumner flight. The results confirm that the positron fraction does not increase with energy above ~10 GeV, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger than that reported from measurements made in the 1960's. This effect could possibly be a consequence of charge dependence in the level of solar modulation.