Motion of Energetic Particles in Crystals

Abstract

During the past five years, studies of directional effects on the trajec­ tories, ranges, and energy losses of energetic particles in solids have done much to elucidate the nature of the processes involved in stopping. The information gained from these studies has also pointed to new methods which can be used to study a variety of crystal properties. These include the determination of stopping-power distributions and interatomic potentials in crystals, and the direct demonstration of certain crystallographic features, such as lattice defects and impurities. That directional effects should exist can easily be visualized from the lattice model pictured in Figure 1. If the crystal is viewed along a low-index direction, the lattice appears to be highly trans­ parent. For the face-centered cubic lattice of Figure I, the most transparent direction is (101), the face diagonal on the front of Figure lb. The second best direction is (010). Figure la, a perspective view of the crystal along this axis, shows the (010) atomic rows. Four neighboring rows form an axial (010) channel. Rotating the crystal of Figure la about the (100) axis leads to configurations of the type shown in Figure lc. The transparency does not disappear, because open planes are maintained between the densely packed sheets of atoms. The degree of transparency depends on the radii of the spheres repre­ senting the lattice atoms which in turn depend on the physical effect under investigation. The interaction between an energetic particle and each lattice atom can be represented by a repulsive potential, and the hard-sphere radius may be defined by the distance of closest approach for a head-on