Radiative electron capture by oxygen ions in single-crystal channels: Experiment and theory

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Abstract
The phenomenon of radiative electron capture (REC) has been studied for 17-40-MeV O ions channeled through thin single crystals of Ag and Si. As a consequence of the unique constraints imposed on the interactions of well-channeled ions, it is possible to study the REC phenomenon for fully stripped oxygen ions interacting primarily with the free or weakly bound electrons in single-crystal channels. Radiation is identified which results from electrons captured directly into the lowest-energy state of the moving oxygen ions as well as that arising from a number of bremsstrahlung processes. The measured cross sections and widths of the REC radiation and the bremsstrahlung processes are compared to calculations made specifically for the channeling situation. The calculations incorporate standard radiation theory and a statistical model of the electron states in the single crystal. The space-varying electron density is averaged over impact parameters appropriate for the channeled ions. The measured REC cross sections show a marked deviation from theory which is not understood at this time. The shapes of the REC lines and the x-ray background present in the measured spectra are in reasonable agreement with the calculations, but the widths of the REC lines as a function of ion energy show only fair agreement. Results of these studies are discussed and possible explanations offered for some of the discrepancies.