Stopping Cross Sections of Carbon and Hydrocarbon Solids for Low-Energy Protons and Helium Ions

Experimental determinations of stopping cross sections of carbon, polyvinyltoluene, and polypropylene have been made for protons and helium ions in the energy range 30-350 keV. Previous measurements on polyethylene, polystyrene, and a styrene-butadience copolymer (Pliolite S-5A) were refined for protons a bove 160 keV. The data are in reasonable agreement with earlier experimental work, and have standard deviations between 1.7 and 4.3%. Application of the Bragg rule to the hydrocarbon data yields atomic-carbon cross sections about 13% too large at the highest beam energies, demonstrating the failure of addivity. Extrapolation of the proton data to higher energies gives results in excellent agreement with Brandt's theoretical values. A mean excitation energy $I=96\mathrm{}\pm 4$ eV independent of proton energy down to 150 keV was found for carbon. Analysis of available hydrocarbon data indicates that gases have on the average approximately 8% larger proton stopping than solids of similar chemical binding. Furthermore, differences up to 23% in ratios of He to H stopping power (at the same velocities) show that the assumption that this ratio should be independent of the stopping material is untenable. These $\frac{{\epsilon }_{\mathrm{He}}}{{\epsilon }_{\mathrm{H}}}$ ratios, extending from beam velocities of 1.2 to 1.7 (in units of $\frac{e^2}{\hslash }$), are greater for hydrocarbon solids than for gases, in agreement with the finding of Mechbach and Allison for Cd.