Coincident Rutherford Backscattering Spectrometry: A novel technique for measuring charge state distributions in violent ion–atom collisions

Charge-state distributions in violent ion–atom collisions were investigated using a novel combination of traditional Rutherford backscattering spectrometry (RBS), time-of-flight (TOF) coincidence, and position-imaging techniques. The combination is termed Coincident Rutherford Backscattering Spectrometry (CRBS). A special apparatus was built in which the backscattered and recoil ions are time and charge state correlated. CRBS measurements for 0.5 and 0.6 MeV He+–Ar collisions are presented. From the recoil ion-projectile ion coincidence measurements of the charge state distributions, it was observed that backscattered projectile ions of the same charge state correlate with different recoil ion charge states and vice versa, indicating that any particular charge state may result from different reaction channels. Moreover, the Ar recoil-ion and He projectile-ion correlation exhibits a strong dependence on the projectile beam energy. An energy deposition model was attempted to account for some of the recoil ion charge state distributions. The model qualitatively accounts for the distributions and confirms that energy loss of a backscattered projectile due to its interaction with the target electrons is very small compared to that due to its interaction with the target nucleus.