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As a fast heavy-ion passes through a material medium, it undergoes multiple collisions in which electrons are continuously lost and recaptured by the ion, and as a result, an equilibrium distribution of excited states is quickly established. The high density of electrons in a solid state environment enables these collisional excitation and decay processes to compete with x-ray and Auger decay. As a result, the widths of the x-ray lines exhibit a dependence on electron density due to collisional broadening. In addition, the potential of the ion is modified by the polarization of loosely bound electrons of the medium. In effect, these electrons act to screen the ion potential, thereby causing the core electron binding energies of the ion to decrease. As a result of this "dynamic screening" effect, the x-ray transition energies inside the medium differ from those in vacuum.