Identification of Cosmic-ray Positrons with the Transition Radiation Detector of the Ams Experiment on the International Space Station

Cosmic-ray positrons have been proposed as important probes for dark matter annihilation, as well as for the presence of cosmic accelerators in the vicinity of the Solar System. The measurement of the positron fraction has therefore been the first target for the AMS experiment, installed on the International Space Station since May 2011. Because protons are more abundant than positrons by a factor of i04, AMS needs very good capabilities for the suppression of the proton background. The transition radiation detector (TRD) of AMS is a powerful instrument for this purpose, in addition to the electromagnetic calorimeter. We present the performance of the TRD during the first two years of the AMS flight, in particular the proton rejection and the corresponding selection efficiency for positrons, to show that the proton background has indeed been suppressed to a negligible level in the measurement of the positron fraction by AMS.