The cosmic-ray positron-to-electron ratio in the energy range 0.85 to 14 GeV RID G-6769-2011

We report on the positron to electron ratio in the cosmic radiation over more than one decade in energy from 0.85 to 14 GeV, using the NMSU-WiZard/CAPRICE balloon borne magnet spectrometer. The spectrometer uses a solid radiator RICH detector and a silicon-tungsten calorimeter for particle identification. The proton rejection factor of the two instruments combined is better than 2 x 10(-6) between 0.6 and 3 GV/c dropping to 6 x 10(-5) at 5 GV/c and 10(-4) at 10 GV/c. The data was collected during 23 hours at a mean residual atmosphere of 4.0 g/cm(2). From a total of 2756 well identified e(-) and 498 e(+) we are able to construct the positron fraction R = e(+) /(e(+) + e(-)) as a function of energy with small errors from 0.85 GeV to 14 GeV for the first time. We observe a decreasing ratio in this energy region. This energy dependent behaviour is consistent with the simple leaky box model. The positron fraction in the upper energy bins are in agreement with the latest high energy measurements.