Dynamics of Radiation Damage in Face-Centered-Cubic Alkali Halides

Computer simulations of ionic motion in 3-dimensional potassium chloride and sodium chloride crystallites have been undertaken to investigate the effect of the regular lattice in influencing the energy spread in the crystal from a primary event such as might be caused by an incident energetic charged particle. Focusing of energy has been found to occur in several low-index crystallographic directions, including those where successive collisions involve oppositely charged ions. The neighboring assisting lines of ions exert considerable influence on the rate of energy loss along the focusing line, on the solid angle in which focusing occurs, and on the threshold energy for permanent ionic displacement from a lattice site, which was found to be a minimum of 25-30 eV in the $〈110〉$ directions of potassium chloride. The feasibility of some mechanisms of $F$-center formation through ionic displacement is discussed in the light of this value of displacement threshold. The computations reveal a new concept of focusing, for which there is a lower energy limit for propagation of a focuson. This limit can be as high as several hundred electron volts for higher-order focusing through asymmetric assisting ionic "lenses." At energies above the lower focusing-energy limit, the momentum transfer of the moving ion to its neighbors between focusing collisions is sufficiently small compared to its forward momentum for its trajectory to be altered only slightly by collision with different parts of the assisting lens. At lower energies, asymmetric collisions with lens ions result in defocusing.