CHAPTER 4 - High Energy Density Collision Cascades and Spike Effects

Abstract

This chapter discusses the high energy density collision cascades and spike effects. Widespread experimental evidence exists showing that significant spike effects occur whenever the deposited energy density within the collision cascade approaches the eV/atom level. The quenching time tq is usually far too rapid for the electron-phonon equilibrium to be established. Hence, the choice of an appropriate thermal diffusivity value and even the validity of Maxwell-Boltzmann statistics will present severe problems in attempting to develop a quantitative treatment of such spike effects. In this chapter, the theoretical basis for estimating the mean deposited energy density Θ within an individual cascade is described briefly in terms of various experimental parameters. The experimental evidence for spike effects in three widely differing systems examined are as follows: (1) radiation damage in heavy ion bombardment of Si and Ge, (2) sputtering of heavy metals (Au, Pt, Ag) by heavy ions, and (3) sputtering of ice and frozen gases by MeV helium ions. The relevant thermal spike parameters are considered for each system in order to provide a basis for assessing the extent to which thermal arguments may be appropriate.