POSITRON DYNAMICS IN SURFACE-CHARGED SOLID ARGON

Studies have been made of the reemission of positrons incident at low energies upon solid argon to which electric fields were applied by charging an overlayer of molecular oxygen. An enhancement in positron reemission was observed which reached a maximum for an applied field of around 7 kV mm(-1) At this field strength the same yield was observed for implantation energies ranging from 1 to 10 keV, consistent with enhancement due to field-induced positron drift to the exit surface. At higher electric fields, the observed gradual decrease in enhancement was attributed to the heating of the positron energy distribution above the positronium formation threshold. Quantitative agreement with our experimental results has been obtained using a Monte Carlo simulation from which estimates for the positron diffusion length and mobility of 1.7(+2.0, -0.4) mu m and 4.7(+2.9, -0.4) X 10(-3) m(2)V(-1)s(-1), respectively, have been derived. This model was also able to successfully reproduce previous results obtained using surface-charged argon beta(+) moderators. An abrupt and almost complete reduction in positron reemission was observed for applied surface potentials above a value which showed a weak dependence on film thickness.