This article presents a detailed analysis of the $K\alpha$ x-ray spectra of Si induced by $11.4$ MeV/u Ca projectiles penetrating a low-density SiO${}_2$ aerogel target measured with high spectral and spatial resolution at the UNILAC accelerator at GSI-Darmstadt. The low-density material used in the experiment was crucial for the space-resolved studies of the Si x-ray radiation (for different energies of stopping Ca ions). The stopping length of the $11$ MeV/u Ca ions reaches up to $10$ mm in the low-density SiO${}_2$ aerogel, whereas in regular quartz solid targets it is about $100$ times shorter. The analysis of the x-ray spectra emitted by the stopping medium has shown a high level of the $L$-shell ionization, especially in the later considered phase ($E_p~5$ MeV/u) of the stopping process. It has been further demonstrated that the population of the highly ionized states produced in the ion-atom collisions can be substantially reduced in the time between the collision and the x-ray emission due to the very intense rearrangement processes occurring in Si situated in the chemical environment of oxygen atoms. Moreover, comparison of the experimental values of the $K\alpha$ $L$-shell satellite energy shifts with the results of the multiconfiguration Dirac-Fock calculations allows us to find that Si valence electron configuration is enriched due to electron transfer from valence-electron-rich oxygen atoms into highly ionized silicon atoms. Our results indicate that the Coulomb explosion in a highly ionized track core is prevented by rapid neutralization in the femtosecond time scale.

• Received 14 April 2010

DOI:https://doi.org/10.1103/PhysRevA.82.012703