The production of point defects in solids by radiation

A number of different aspects of Frenkel defect production are discussed in this thesis. Experiments have been performed to characterise the response of sapphire to radiation and to determine the stability, with respect to temperature and subsequent ionising radiation, of the oxygen vacancies produced. The displacement threshold of the aluminium ion has been determined by irradiating with electrons and comparing measured concentrations of the V centre with theoretical values, and evidence is presented for the stability of the F2+ centre at liquid helium temperatures. The efficiency of point defect production in sapphire by a variety of ion beams has been estimated by monitoring the oxygen vacancy colour centre concentration. Results are presented for H, D, He, B, N, Ne, Ar and Kr bombardment at both room temperature and 77K. Experimentally determined values of the mean number of oxygen vacancies produced per incident ion are compared with theoretical values. The latter have been computed by solving the Lindhard integral equations to determine the partitioning of energy between the oxygen and aluminium sublattices. The stopping powers and cross sections of Ziegler, Biersack and Littmark have been used in these calculations and give significantly improved estimates of the damage energy. Two main points emerge from this study. Firstly, a significant Z1 variation of the damage efficiency is noted for low mass ions. Secondly, in contrast to metals (in particular aluminium) no reduction in efficiency has been found in sapphire with increasing ion mass. This is attributed to differences in the 'thermal spike' behaviour, as a consequence of differing thermodynamic properties, between aluminium and its oxide. The implications of both these results for irradiation characterisation are discussed. In the final chapter we discuss the interaction between conduction electrons and ions in the cooling phase of cascades in metals. Differential equations governing the transfer of heat between the two systems are derived and solved numerically, using physically reasonable parameters for cascades in copper and nickel. Large differences are found in the cooling rates of cascades in these two metals. The degree of cooling due to electronic heat transport is shown to depend on the ratio of a parameter T0, the temperature at which the electron-phonon mean free path reduces to the radius of the Wigner-Seitz sphere, and γe, the coefficient of the electronic heat capacity. The participation of the electronic system in cascade evolution is shown to preclude a universal description of damage efficiency on the basis of simple atomic scaling laws.