Density Effect in Equilibrium Charge-State Distributions

Cross sections for single and multiple electron capture and loss have been measured for iodine ions in hydrogen at 4.50 MeV. Values were determined from the initial behavior of charge-fraction growth curves under thin-target conditions. Equilibrium charge-state fractions $F_i$ were then calculated from the measured cross sections, and these values were compared with the fractions measured directly in a dense target. It is concluded that when multiple collisions occur, excited states in the projectile influence the effective charge-changing cross sections. Consequently measured values of the "equilibrium" charge-state fractions depend in a complex way upon the excited-state lifetimes and the target density. In this experiment, the mean charge calculated from cross-section values was 1.98, whereas, the measured mean charge was 2.33.