Experimental Evidence for the Concept of the Displacement Spike

THE mechanisms by which fast, heavy, and charged particles lose energy to crystalline matter are generally explained in terms of either thermal 1 or displacement 2 spikes. The mere fact, however, that a majority of the energy is dissipated in the excitation of orbital electrons and the creation of a thermal spike does not necessarily imply that the resulting radiation damage is so formed. In uranium, for example, the existence of a continuous band of energy-levels above the Fermi surface permits a temporary excitation of the valence electrons, since these may fall back to their original levels and so deliver their energy to the lattice as heat. For such cases, the bulk of the remanent radiation damage may well derive from the Coulomb interactions which the particle makes with lattice atoms in the crystal. Brinkman 3 has shown, by use of the atomic interaction potential: which approximates to a Coulomb repulsion for close interactions and a Born-Mayer exponential repulsion for large separations, that, for a light uranium fission fragment travelling through uranium, atomic displacements are produced every 40 Å on the average. Occasionally the displacement produces a highly energetic (∼ 10 5 eV) primary knock-on which proceeds to lose energy by hard-sphere collisions. In such cases a displacement spike can be formed. Atoms are driven into the surrounding lattice to occupy interstitial positions on a closed shell which surrounds a core of vacant lattice sites.