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The interactions of muonium with silica surfaces Harshman, Dale Richard


The behavior of muonium on the surface of fine (35 Å mean radius) SiO₂ powders has been studied using the techniques of muon spin rotation (μSR). Results indicate diffusion and trapping behavior of the muonium atoms on the silica surface, which is strongly influenced by the concentration of surface hydroxyl groups. Specifically, the presence of the surface hydroxyl groups has been shown to inhibit the motion of muonium on the silica surface. These studies have also provided information regarding the origin of the relaxation of the muon spin polarization for muonium on the silica surface. Specifically, a random anisotropic distortion of the muonium hyperfine interaction, induced by the local surface environment of the muonium atom, has been shown to be a principal contributor to the relaxation of the muon ensemble spin polarization, whereas the random local magnetic fields due to the neighboring hydroxyl protons were found to play only a minor role. From this result, the observed strong dependence of the relaxation on the surface hydroxyl concentration has been attributed to an associated hyperfine distortion, induced by the neighboring hydroxyls. A new spin relaxation theory, for the case of random anisotropic hyperfine distortions, has also been developed to explain the data. Gas adsorption isotherm studies were also performed, with ⁴He at 6 K, which show the muonium asymmetry to be strongly influenced by the fractional surface coverage. These results clearly indicate that the muonium formation probability decreases with increasing surface coverage, suggesting that the charge exchange cross section at the silica surface is significant. The implication of these results with regard to the origins of muonium formation (i.e., surface or bulk formation) is as yet unclear, however, since the precise role played by the adsorbed helium atoms is not known. These investigations have also been extended to platinum loaded silica, where the first surface reaction of muonium has been observed; the reaction rate of muonium with the surface of oxygen-covered platinum microcrystals was found to be 3.5 ± 0.15 μs⁻¹.

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