The Stopping Power for Protons in Several Metals

The rate of energy loss of low velocity (i.e., from 50 to 400 kev) protons has been measured in five different stopping materials: Be, Al, Cu, Ag, and Au. Agreement with previous measurements at higher velocities (greater than 400 kev) is good where such measurements have been published. The stopping measurements were made on foils prepared by an evaporation technique and are believed to be more accurate than measurements made on the usual (hammered or rolled) foils; the results give a value for stopping about 15 percent higher (in Au) and about 7 percent lower (in Al) than those published by Wilcox. A comparison to existing theory at very low and at high velocities (the critical velocity being defined as $\frac{e^2}{\hslash }$ was made with good agreement shown at the low end. The high end theory is of doubtful validity and gives a stopping power about 10 percent higher than observed. It is argued that this discrepancy cannot be accounted for by a proton charge less than unity due to the electron capture-loss process.