Target-thickness dependence of K-shell vacancy fractions in foil-excited ion beams

K-shell vacancy fractions have been measured for 2.7- and 0.4-MeV nitrogen ions exiting thin carbon foils as a function of foil thickness. The vacancy fractions per projectile ion were deduced from measuring the yields of K Auger electrons emitted by the foil-excited nitrogen ions. It is found that the vacancy fractions for these ions reach equilibrium only for foils thicker than approx.20 μg/cm2. The variation of vacancy fractions in foils with thicknesses below 20 μg/cm2 is a strong function of the projectile energy