CLOSE ENCOUNTER PROCESSES IN MONTE-CARLO SIMULATIONS OF ION CHANNELING

The backscattering yield is analyzed in terms of the depth-dependent distribution of ion trajectories, characterized by instantaneous values of such parameters as the ion energy, the scattering angle, the angle between the ion direction and the surface normal, and the impact parameter with respect to the equilibrium site of the nearest lattice atom. The normalized nuclear encounter probability (NEP) is reformulated as the expected value of the normalized NEP for a single trajectory, and the definition is extended for directions tilted by a large angle with respect to a major axis. It is shown that the normalized NEP cannot be defined to correspond to a physically measurable quantity if the energy and angular spreads of the beam in the crystal are taken into account. The scattering yield measured in the energy spectrum - to be used in comparisons of simulations with experiment - is also expressed as the expected value of the respective yield associated with a single trajectory. Examples of the distribution of trajectories are given for 1.5 MeV He-4 ions traversing a Si(100) single crystal.