1.7 Production of atomic defects in metals by irradiation

This chapter discusses production of atomic defects in metals by irradiation. When an energetic particle interacts with the nucleus of a lattice atom, a series of processes is initiated. Their sequence in time is given in a table. While the first three steps, energy transfer from particle to nucleus, interatomic collisions, dissipation of epithermal energy and thermal migration, are largely athermal, the last step contributes only when point defects are mobile. Due to the high mobility of interstitials in metals, the production of Frenkel pairs can only be studied in experiments near liquid helium temperature. The production of Frenkel pairs in metals by irradiation can be separated into two distinct processes. The first is the interaction of the irradiation particle with a lattice atom (the primary knock-on atom: PKA), resulting in the transfer of an energy to the PKA. The second process is the distribution of this energy to the surrounding atoms resulting in the production of Frenkel pairs (displacement process). Experiments as well as computer simulation indicate that the transport of energy and of matter, i.e. the separation of interstitial and vacancy, most easily proceeds along close packed lattice directions. The energy transferred to the PKA must exceed the displacement energy in order to produce stable atomic defects. Stability is achieved when the separation between interstitial and vacancy exceeds a certain value, the so-called instability radius. According to computer calculations, this instability radius is anisotropic, having maxima along the close packed directions.