Surfaces of SiO2-Bi2O3 glasses studied by a slow positron beam

A slow positron beam with a Doppler broadening technique have been applied to study near-to-surface properties of the glasses used in channel electron multipliers. Measurements in SiO2-Bi2O3 glasses as-obtained show a monotonic fall of the S parameter from 0.515 at the surface to 0.49 in the bulk. In samples reduced in H-2 atmosphere, showing good surface electric conductivity, a sharp maximum of the S-parameter within the first 100 nm is observed, followed by a higher "bulk" S-parameter. Atomic force microscopy, secondary electron emission, differential calorimetry and electric conductivity measurements were also performed on these samples. Existence of Bi-metallic phase on the surface and Bi-clusters in the bulk is deduced.