Bismuth and lead doped silicon and germanium-oxide glasses are used, among others, for electron continous-dynode, electron (and photon) amplifiers. To make them conducting, reduction procedures in hydrogen atmosphere are applied. The depth of the reduced layer must be thoroughly chosen, in order to assure a high secondary-electrons yield, and to assure a low surface conductivity, on the other hand.
We show, how a beam of low-energy positons can be used to monitor in a detailed way the dynamics of metal precipitates in near-to-surface layers. S-parameter defining annihilation with low momentum and W-parameter - with high momentum electrons are monitored. In particular for silica glass samples, lead and bismuth-doped one notes that long duction times (91 h at 300C and 21 h at 400C, respectively) cause "over reduction" of the samples - it means that reduced layer thickness is bigger than apparatus capabilities (about 2 micrometers). Samples of the second series, were annealed in slightly higher temperature (340C) but for shorter times (up to 5 h), and therefore was possible to observe the whole reduced layer, extending down to 700 nm. For bismuth doped samples, positron annihilation shows two reduced layers - one for the first few tens of nm, and a deeeper, extending down to 500 nm, which merge for high reduction times. Existence of the near-to-surface reduced layer is less clear in germanium-based glasses. Additionally, the fact that W/S ratio for glasses with different proportions are at the same straight lines shows that annihilation occurs in homogenous "glass phase", independently of the reduction rate, and not in separatate, two separated oxide phases.

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