The stopping power for antiprotons has been measured for the first time. The antiproton stopping power of silicon is found to be 3%–19% lower than for equivelocity protons over the energy range 3.01 to 0.538 MeV. The ‘‘$Z_1^3$ contribution’’ to the stopping power (the Barkas effect) is deduced by comparing the stopping power for protons and antiprotons.

• Received 19 January 1989

DOI:https://doi.org/10.1103/PhysRevLett.62.1731