Stopping cross sections for 20 keV positrons $\left(S^{+}\right)$ and electrons $\left(S^{-}\right)$ in a thin (61 $\mu \mathrm{g}/{\mathrm{cm}}^2$) Al target have been measured using an electrostatic analyzer. The results yield a ratio $S^{+}{/S}^{-}=1.15\mathrm{}\pm 0.08$, in good agreement with the theoretically calculated value of 1.097. A marked dependence of the mean energy loss on an emergence angle has been observed for a thin target, and its origin is interpreted with the aid of Monte Carlo simulations.

• Received 28 December 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.3947