Distribution of exit silicon ions over excited states after penetrating through carbon foils at 2.65, 4.3 and 6.0 MeV/u

Abstract

New experimental data on the charge-state ( q) evolution fractions F q ( D) and equilibrium charge-state fractions F q ∞ are presented when 2.65 MeV/u C 2+, O 3+, Ne 4+ and Si 5+ projectile ions passed through carbon foils with thickness of D = 10–200 μg/cm 2. The equilibrium fractions F q ∞ for Si + C collisions (i.e. for silicon ions colliding with carbon foils) are compared with the data at 4.3 and 6.0 MeV/u previously reported, and the observed significant difference is explained by different energy dependencies of the electron-capture and projectile-ionization cross sections. Detailed information about nl-state distribution functions N q ( nl) of exit silicon ions is obtained for these three energies by solving linear balance equations for N q ( nl) values where n and l are the principal and orbital quantum numbers, respectively, and ∑ nl N q ( nl ) = F q ∞ . The collision cross sections and radiative decay rates as the coefficients of these balance equations are calculated for each ion-beam energy. Based upon the present method, it is found that the fractions of excited Si ions after the carbon foil are about 57%, 33% and 15% at energies of 2.65, 4.3 and 6.0 MeV/u, respectively, clearly indicating the significant reduction of the excited-ion component with the collision energy increasing. The calculated equilibrium charge-state fractions F q ∞ for Si ions are in good agreement with available experimental data.