Charge state dependence of [formula omitted] for ions in very thin targets

Abstract

Relative stopping powers of 3 MeV/u He, Li and C ions have been measured in very thin carbon targets (2–26 μg/cm 2). Energy losses were measured using a high resolution position sensitive detector (PSD) in the focal plane of a magnetic spectrometer, so that only one charge state (the incident charge state) was detected. As expected for fully stripped light projectiles in C targets, the stopping power data for He and Li ions scale as Z 1 2 to within our ~1% experimental precision. However, for C ions, charge-changing effects are found to influence the stopping considerably. The evolution of the charge-state distribution as a function of foil thickness for 3 MeV/u C ions in C foils has been determined in prior experiments. A two-component charge state model is used to predict the corresponding evolution of stopping power for both C 6+ and C 5+ ions, using these charge state data. Comparison of the model with our observations of dE dx show that charge-changing events themselves contribute significantly to stopping in thicker foils, even in the present high velocity regime. From our data we extract the stopping cross sections S 00 and S 11 for the fully stripped and one-electron ions in the absence of charge-changing events, and the energy loss U = (2.3 ±0.2) keV undergone in the charge-change cycle C 6+ C 5+. A higher order Z 1 effect of ∼ −2.8% is observed in the total stopping cross-section S o for the fully stripped ion. This is comparable with the usual Bloch term plus Lindhard's polarisation term, which together yield a −2.0% effect. However, the presence of charge-changing energy losses means that the polarisation theory gives only a very sketchy estimate of the appropriate higher order correction for heavy ions.