Electron capture in close collisions between protons and carbon (CH4)

Abstract

The total probability for electron capture from CH4 by 200-2000 keV protons scattered through 15 or 20 degrees in the laboratory system by the carbon nucleus has been measured. The probability decreases strongly with increasing proton energy, and a clear structure is seen around 400 keV. Complementary differential coincidence measurements of capture by protons from the K shell of carbon (CH4) show that the structure is caused by the K-shell capture component of the total capture probability, which becomes significant in the energy range 200-500 keV. Above 1 MeV, the K-shell capture component dominates the total capture probability. The angular dependence (10-50 degrees ) of the total capture probability was determined at 500 keV, where the K-shell capture component at 20 degrees contributes with (80+or-15)%. An experimental measure of the widths of the impact parameter distributions for K-shell ionisation and capture, respectively, as functions of the collision energy, was obtained on the basis of the pertinent experimental total cross sections and measured probabilities for small impact parameter collisions with nearly straight-line trajectories. The width of the K-shell ionisation distribution increases with increasing projectile kinetic energy, whereas the width of the K-shell capture distribution exhibits a maximum. Particular attention has been paid to the capture probabilities in an energy region around the elastic scattering resonances 12C(p,p)12C near 1.7 MeV.