The dependence of calcium ion subshell populations on the target density during the ion stopping process was analyzed using a five charge-state collisional-radiative model. The model, which consists of the ground and three excited states for every ion charge, was successfully compared with the experiment. The gas-solid difference of calcium ion charge state distribution has been numerically demonstrated. For Ca projectiles with energies of 4-11 MeV/u, the increase of the mean ion charge in solid target is explained by the increase of the total ionization rate and by suppression of the bound electron capture process at high densities of the stopping medium.
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