Investigation of heavy ion produced defect structures in insulators by small angle scattering

Abstract

Fast heavy ions produce stable defects in most dielectrica. As examples mica, Polyethylenterephtalat and Polystyrol were irradiated with Ar, Ni, Kr, Xe and U ions in an energy range from 0.5 up to 20 MeV/u. The resulting defects were investigated by neutron and x-ray small-angle scattering. The ion beam supplied by the UNILAC accelerator at GSI Darmstadt is characterized by its small emittance, the well defined mass, charge and energy of the ions and their stochastical distribution in the phase space. In scattering experiments the system of scattering centers created by these ions causes a scattered intensity distribution which strongly depends on the orientation of the sample with respect to the unscattered neutron or x-ray beam. This dependence is investigated and explained.

By a mathematical model — describing form, size, and density of the average ion track — the measured intensity distribution is simulated. Based on the model, computer procedures are written, simulating the scattering experiment by varying the most important experimental and instrumental parameters and calculating the expected theoretical intensity distribution on the detector. The parameter values of this model — the maximum density difference in the track, length of the defect, and radial dimension — are determined by least square fits to the measured data. A simple description of the dependence of these parameters on the ion energy can be given in relation to the energy loss of the primary ion. It is not only possible now to predict an expected track, to calculate its volume and the number of missing atoms, but moreover to check theories of the track formation.