Charge states and energy loss of 300-MeV/u U 731 ions channeled in a silicon crystal

mitted along a ^110& axis of a 120-mm-thick Si crystal. The emerging charge state distribution FC(qout) for well-channeled ions is governed mainly by electron impact ionization ~EII!. The corresponding EII cross sections were obtained by fitting the experimental FC(qout) with Monte Carlo simulations. For M shell ionization, they were found to be twice larger than those given by the binary encounter dipole approximation. The measured energy loss spectra were also compared to Monte Carlo simulations. The mean values and widths of these spectra increase with qout , reflecting the increase of the stopping power S with increasing transverse energy E’ . The measured stopping for channeled ions with frozen charge state 731 and for nonchanneled ions with charge state close to 901 is in good agreement with theoretical estimates. Owing to the very high ion velocity, there is a significant contribution (25%) to the stopping from Si-L shell excitation even for the best channeled ions. The width and the asymmetrical shape ~skewness m) of the energy-loss spectra depend strongly on qout(m.0 for very well-channeled ions, m,0 for poorly channeled ions!. For well-channeled ions, energy-loss spectra were reproduced by Monte Carlo simulations with the S(E’) curve extracted from fitting the mean energy losses. @S1050-2947~99!01604-2#