Long-Range Channeling Effects in Irradiated Crystals

Abstract

If an energetic atom, a primary, slows down in a lattice, it has the possibility of traveling long distances without essential interaction along almost force-free channels bordered by close-packed atomic chains. This effect has not been considered until recently, because, for the sake of simplicity, the lattice has been replaced mostly by a corresponding random arrangement. This ``channeling'' behavior has been found in machine calculations by Robinson, Holmes, and Oen which take the lattice structure into account. In this paper, the behavior of a primary moving along a channel bordered by the most densely packed directions is investigated analytically. Numerical results are given for a Cu primary moving in a Cu crystal, but the results can easily be extended to other cases. Two potentials are used: an exponentially screened Coulomb potential after Bohr, used also in the machine calculations and thought to give an adequate description for relatively high energies and small interatomic distances; and a purely exponential potential after Born—Mayer, better suited for relatively low energies and large atomic distances. The maximum ranges are very large, for 10 keV in the order of 103 lattice parameters for the Born—Mayer potential and up to 107 for the Bohr potential. Presumably, the Born—Mayer potential is a better description for these events. The investigation is confined to motions near the channel axis. Therefore, we only obtain the range distribution near the maximum range. From an adequate analysis of experimental data on long ranges of primaries, shot onto crystals in low indexed directions, one can obtain information about the potential at distances of about half the lattice spacing.