Projectile charge dependence of heavy ion stopping

The dependence of the electronic stopping power upon projectile charge has been investigated by measuring the energy loss of 3 MeV/u He, Li, C and O ions in carbon foils. The stopping powers for He and Li ions are found to scale approximately as the square of the projectile charge Z12, which is consistent with current theories which predict a near cancellation of the Z13 and Z14 corrections to Bethe stopping theory. For the heavier ions all charge states show departures from Z12 scaling, considerably in excess of the predictions of current theory. The energy-loss data for the various charge states of individual ions converge as the foil thickness increases. To interpret the heavy-ion energy-loss data, account has been taken of the processes of charge exchange that the projectile undergoes as it passes through the target. This required the measurement of the cross sections for charge exchange of 3 MeV/u O ions in carbon. The energy loss results indicate that a significant part of the heavy ion total stopping arises from charge-changing events. The magnitude of this component is comparable to the polarization and Bloch correction terms. A procedure for the incorporation of charge changing energy loss in stopping theory is discussed. (author)