Model for Fission-Fragment Damage in Thin Polycrystalline Metal Films

Abstract

Fission fragments produce tracks in thin films made up of small crystallites and do not produce tracks in continuous films. This has been difficult to understand on the basis of the spike models where one considers only the energy transmitted directly to the lattice and neglects the energy acquired by the electrons. The energy to evaporate the crystallites must come from the electron gas because it has acquired almost all the energy initially lost by the fission fragment; consequently, in order to explain the evaporation of particles one must understand the subsequent electron energy‐transfer processes. Examining the various relaxation times, we find that the energy is transferred first to other electrons via electron—electron collisions. If the electron gas is confined to a small region, as in a film of small crystallites, this immediately leads to a size effect. The largest crystallite that can be evaporated is one in which the total energy of the electron gas just equals the energy required for evaporation. This size estimate is an upper limit because the electrons may simultaneously lose energy via thermionic emission. A comparison of the two rate processes for an isolated particle of a typical metal suggests that electron—phonon energy transfer is the more important process, and that a negligible amount of energy is lost to the environment via escaping electrons. Consequently, almost all of the fission‐fragment energy ultimately appears in the lattice. Estimates of the critical size for Au yield a diameter of about 170 Å for a spherical crystallite, in reasonable agreement with experiment. We conclude that thin metal films will exhibit evaporation fission tracks provided that the film is made up of crystallites in poor electrical contact with one another and that the crystallites are smaller than the largest that can be evaporated.