The Radiation Damage Event

Abstract

The radiation damage event is defined as the transfer of energy from an incident projectile to the solid and the resulting distribution of target atoms after completion of the event. The radiation damage event is actually composed of several distinct processes. These processes and their order of occurrence are as follows: 1. The interaction of an energetic incident particle with a lattice atom. 2. The transfer of kinetic energy to the lattice atom giving birth to a primary knock-on atom (PKA). 3. The displacement of the atom from its lattice site. 4. The passage of the displaced atom through the lattice and the accompanying creation of additional knock-on atoms. 5. The production of a displacement cascade (collection of point defects created by the PKA). 6. The termination of the PKA as an interstitial. The radiation damage event is concluded when the PKA comes to rest in the lattice as an interstitial. The result of a radiation damage event is the creation of a collection of point defects (vacancies and interstitials) and clusters of these defects in the crystal lattice. It is worth noting that this entire chain of events consumes only about 10 −11 s (see Table 1.1). Subsequent events involving the migration of the point defects and defect clusters and additional clustering or dissolution of the clusters are classified as radiation damage effects. What we first need to know in order to understand and quantify radiation damage is how to describe the interaction between a particle and a solid that produces displacements, and later on how to quantify this process. The most simple model is one that approximates the event as colliding hard spheres with displacement occurring when the transferred energy is high enough to knock the struck atom off its lattice site. In addition to energy transfer by hard sphere collisions, the moving atom loses energy by interactions with electrons, the Coulomb field of nearby atoms, the periodicity of the crystalline lattice, etc. The problem is reduced to the following. If we can describe the energy-dependent flux of the incident particle and the energy transfer cross sections (probabilities) for collisions between atoms, then we can quantify the PKA production in a differential energy range and utilize this to determine the number of displaced atoms.