Equilibrium charge-state distributions for 2-20-MeV argon ions in carbon

Equilibrium charge-state distributions have been measured for argon ions in thin carbon foils between 2 and 20 MeV. Equilibration takes place in foils at least as thin as 5 μg/${\mathrm{cm}}^2$. Our measurements link up smoothly with other work at lower and at higher energies. Several models have been applied to postdict the data. It was found that while the mean charge can be quite accurately determined from empirical models, such as the Gaussian one, or even from extrapolation on $Z$, serious discrepancies arise with respect to the distribution widths. As a result, charge fractions which are off the peaks of the distributions can be in error by factors of 2 or more, rendering their measurement essential for the purposes of the present study. Evidence of ionic $\mathrm{LM}$ shell structure has also been observed in this work.