Using channeling properties for studying the impact parameter dependence of electron capture by 20 MeV/u uranium ions in a silicon crystal

The impact parameter dependence of electron capture by 20 MeV/u U$^{91+}$ ions has been studied by means of channeling in a 11 $\mu$m thick silicon crystal. Such ions are far from their equilibrium charge state in matter, and channeling offers a unique opportunity to study electron capture in conditions going from the extreme case of a single capture event (for the best channeled ions) to the case of multiple charge exchange events leading to the charge state equilibrium (for unchanneled ions). For each incident ion, the charge state at emergence, energy loss, electron emission and X-ray yields are measured. The correlations between these quantities are studied. The data are reproduced by simulations based on the ion flux distribution. We show that the Mechanical Electron Capture (MEC) dominates at impact parameters smaller than 0.5 Angstrom, whereas Radiative Electron Capture (REC) is the only process occurring beyond. Specific features associated to highly charged heavy ions at intermediate velocities are discussed, in particular ionization following capture into highly excited states, and local electron density enhancement due to the electron gas polarization. The measured impact parameter dependence of capture probabilities is compared to CDW-EIS (continuum distorted waves – eikonal initial state) calculations, extrapolated to n>5 final states.