Stopping Cross Sections of Some Hydrocarbon Gases for 40-250 keV Protons and Helium Ions

The stopping cross sections of air, He, C${\mathrm{H}}_4$, ${\mathrm{C}}_2$ ${\mathrm{H}}_2$, ${\mathrm{C}}_2$ ${\mathrm{H}}_4$, ${\mathrm{C}}_3$ ${\mathrm{H}}_8$, ${\left(\mathrm{C}{\mathrm{H}}_2\right)}_3$, and ${\mathrm{C}}_3$ ${\mathrm{H}}_6$ have been measured for protons in the energy range 40 to 250 keV. The stopping cross sections of ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ and ${\mathrm{C}}_3$ ${\mathrm{H}}_8$ were also measured for helium ions. The data are in good agreement with previous experimental work, and have standard deviations between 1.5 and 4%. They clearly demonstrate the failure of the Bragg rule of additivity for proton energies less than 150 keV. The helium-ion data indicate that the effect of chemical binding on stopping cross sections is unlikely to be any greater for incident helium ions than for incident protons at the same energy.