Charge states and energy loss of 300-MeV/u U73+ ions channeled in a silicon crystal

We have studied the emerging charge states q(our),,,, and energy loss of 30 0-MeV/u U73+ incident ions transmitted along a [110] axis of a 120-mu m-thi ck Si crystal. The emerging charge state distribution Fc(q(out)) for well-c hanneled ions is governed mainly by electron impact ionization (EII). The c orresponding EII cross sections were obtained by fitting the experimental F -c(q(out)) with Monte Carlo simulations. For M shell ionization, they were found to be twice larger than those given by the binary encounter dipole ap proximation. The measured energy loss spectra were also compared to Monte C arlo simulations. The mean values and widths of these spectra increase with q(out), reflecting the increase of the stopping power S with increasing tr ansverse energy E-perpendicular to. The measured stopping for channeled ion s with frozen charge state 73+ and for nonchanneled ions with charge state close to 90+ is in good agreement with theoretical estimates. Owing to the very high ion velocity, there is a significant contribution (25%) to the st opping from Si-L shell excitation even for the best channeled ions. The wid th and the asymmetrical shape (skewness mu) of the energy-loss spectra depe nd strongly on q(out)(mu > 0 for very well-channeled ions, mu < 0 for poorl y channeled ions). For well-channeled ions, energy-loss spectra were reprod uced by, Monte Carlo simulations with the S(E-perpendicular to,) curve extr acted from fitting the mean energy losses. [S1050-2947(99)01604-2].