K-shell cross sections for ions penetrating solids at very low velocities

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Absolute projectile K X-ray yields have been measured as a function of foil thickness for Siq+ ions penetrating carbon at velocities 2.0v0<or=v<or=5.6v0. The data are analysed on the basis of radiation cross sections and fluorescence yields estimated from previously measured charge state distributions. Cross sections for the creation and quenching of projectile K vacancies are extracted at all velocities down to a detection limit, where the K-vacancy fraction falls below 5*10-6. The results may be explained by direct Coulomb ionisation and collision-perturbed spontaneous decay of L-shell electrons. Correspondingly, the measured change in the fluorescence yield of the ion inside or outside the foil is explained solely in terms of the very high collision frequency inside the solid.