Measurement of the Positron-Electron Ratio in the Primary Cosmic Rays from 5 to 50 GeV

We report the first measurement of primary-cosmic-ray positrons above 5 GeV. Both $e^{+}$ and $e^{-}$ fluxes were measured, using a balloon-borne magnetic spectrometer. Background was suppressed using a new technique which combines both selective trigger and identification of bremsstrahlung photons created by $e^{\pm }$ upon entering the instrument. If no positrons originate at cosmic-ray sources, our observed ratio $\frac{e^{+}}{(e^{+}+e^{-})}=0.08\mathrm{}\pm 0.02$ requires source protons to traverse 3.5±1.5 g/${\mathrm{cm}}^2$ of interstellar material.