Theory of Defect Formation in Alkali Halides by Ionizing Radiation

Abstract

Assuming the creation at low temperatures by ionizing radiation of two halogen atoms at adjacent halide ion sites, we have estimated in a tight-binding approximation the time constants for the electronic and atomic relaxation processes. It is concluded that an appreciable fraction of the atoms may move before the positive holes further separate. The atomic motion is most probably to form two negative halogen molecule ions rather than a neutral halogen molecule. Each halogen molecule ion acquires momentum in the [110] direction from interaction with the asymmetric crystalline field; and by "billiard ball" collisions concurrent with tunnelling of the positive hole, separation of the halide vacancy and the molecule ion may be achieved. A related mechanism of defect formation, but requiring only a single ionization, is considered.