Systematic Angular-Correlation Studies of Positronium Formation in Silicon Dioxide Powders

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Abstract
Positron-annihilation angular-correlation measurements have been made on compressed silicon dioxide powders having mean particle diameters of 50, 120, and 400 Å. The narrow component in the momentum distribution is much more pronounced in the 50- and 120-Å powders than it is in the 400-Å powder. Measurements made in vacuum reveal a narrow component in the momentum distribution which increases with increasing temperature until a saturation temperature is reached. A positronium diffusion constant has been calculated for five temperatures. Similar measurements made in oxygen show an increased narrow component with a nonsaturating temperature dependence. From these measurements a temperature-dependent oxygen enhancement factor has been determined. Both vacuum bakeout and compressing the powder were found to increase the amount of narrow component.