K-Shell Ionization of Carbon by Fast Protons

The cross sections for $K$-shell ionization of carbon have been measured for incident protons with energies from 0.3 to 2.0 MeV. These cross sections were obtained from Auger-electron yields measured for methane, ethane, ethylene, and acetylene gas targets assuming that each $K$-shell vacancy results in the emission of an Auger electron. The molecular surroundings of the carbon atom were found to influence the $K$-Auger-electron yield; e.g., for 1.0-MeV-proton impact, the carbon-$K$-shell ionization cross section was 1.00×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ for acetylene and 1.10×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ for ethane. These differences are larger than the estimated uncertainty in the relative values of the measured cross sections (5%). The estimated uncertainty in the absolute value of the cross sections is 20%. A comparison of our Auger-electron yields with published x-ray production cross sections was used to estimate the fluorescent yield of carbon. The fluorescent yield determined from these data was found to vary with proton energy from 0.0016 at 0.3 MeV to 0.0025 at 2.0 MeV.