Reduction of the Electron Capture Cross-Section for Protons in Hydrocarbon Cases

The total electron capture cross section of 0.8 - 3.0 MeV protons in various hydrocarbon gases (CHn - C4H , n = 2 - 8) has been measured. We find that the electron capture cross section per carbon atom decreases as the number of carbon atoms per molecule is increased. This reduction amounts to ~7% for 3 MeV protons and is ~ 25% for 0.8 MeV protons. We find no significant change in the electron capture cross section for carbon as the number of hydrogen atoms per molecule is increased. The observed variations in the cross section can be reproduced by assuming that post-collision-interactions destroy the required velocity match between projectile and electron.