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 Fort Sumn er, New Mexico, in 1994 and yielded results above the geomagnetic cuto ff energy of 4.5 GeV. The second flight, from Lynn Lake, Manitoba, in 1995, permitted measurements over a larger energy interval, from 1 to 50 GeV. 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 Fort Sumner flight. The results confirm that the posi tron fraction does not increase with energy above approximate to 10 Ge V, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger t han that reported from measurements made in the 1960s. This effect cou ld possibly be a consequence of charge dependence in the level of sola r modulation.