IMPACT PARAMETER DEPENDENCE OF ENERGY-LOSS AND TARGET-ELECTRON-INDUCED IONIZATION FOR 27 MEV/U XE-35+ INCIDENT IONS TRANSMITTED IN [110] SI CHANNELS

We present original experiments providing information on the impact parameter dependence of stopping power in axial channeling and on electron impact ionization. A beam of 27 MeV/u Xe35+ ions (far from the equilibrium charge state in matter) has been transmitted through a Si crystal, parallel to the [110] axis. The very broad and out-of-equilibrium emerging charge state distribution (35 ≤ Qout ≤ 53) arises nearly only from electron impact ionization. The corresponding cross sections σ(Q → Q + 1) obtained, when fitting the experimental distribution by Monte Carlo simulations. The energy loss spectra measured for each Qout have been analyzed, using the same simulation program. The choice of Qout corresponds to a selection in the transverse energy distribution of the ions and on their accessible transverse space. Detailed information is thus obtained on "local stopping power", i.e. in a given point of the transverse [110] space, and on its relation with the corresponding local electronic density. The respective influence of core and valence electrons is studied. For very well channeled ions, the energy loss, which is only induced by the valence electron gas, depends little on the local electronic density sampled by the particles. It is very close to the .energy loss corresponding to a homogeneous electron gas with density equal to the mean density of valence electrons. © 1990.