Charge state dependence of the stopping power of 1MeV/A58Ni-Ions in thin carbon foils

Energy loss and energy straggling of 60 MeV 58Ni ions in ultra thin carbon foils have been measured with a Q3D magnetic spectrograph in dependence on the incident and analysed charge state of the ions. The foils have been prepared by e--beam evaporation of graphite with lensodel as parting agent to obtain good foil homogeneity. The minimum thickness of the self-supporting foils was determined to be 0.8 μg/cm2 ± 10%. The qin = 8+ and qin = 18+ charge states were selected for the incident ions. The stopping power in the first few atomic layers deviates significantly from the value in larger depths and can be scaled with the square of the incident charge state q. For larger depths the stopping power coincides with the tabulated value for both cases. Therefore one can calculate an average charge state q = 15.4 for the Ni-ions in the bulk consistently for both incident charge states. With the average charge state behind carbon foils being bar q = 17.6 the ion has to lose two electrons after having passed the foils, e.g. by Auger processes. Calculating the stopping power in linear response approximation an effective charge state qeff = 16.7 is obtained when starting from q = 15.4 inside the bulk. This is significantly higher than the value obtained from q2-scaling of the stopping power of protons with the same velocity which results in qeff = 13.3. This discrepancy is suggested to be an effect of the simultaneous scattering of many electrons by the heavy ion in the solid due to the high scattering cross section which results in an additional dynamic screening and therefore in a reduced stopping power. The energy stragling of the Ni-ions can be approximated by the known formula of Bohr. Nevertheless the systematics of the dependence on the incident and analysed charge state indicates that charge changing events contribute a significant amount to the energy straggling