Positron lifetimes in alkali borate glasses

Positron lifetimes have been studied in alkali borate glasses of types x Na2O, (1 − x) B2O3 and 0.17 Na2O, 0.83 B2O3 + m Al2O3 for various values of x and m, respectively. It is known that the coordination and free volume in these glasses show a marked variation with the molar fractions x and m. Present measurements show that the orthopositronium lifetimes τ2 and their intensities I2 are significantly affected by the changes in the coordination and the free volume in these glasses. Although the behavior of I2 is in qualitative agreement with the trends predicted by the free volume model, the lifetime τ2 of the orthopositronium shows an anomalous behavior. Four‐coordinated aluminum has been found to have more influence in determining τ2 and I2 than four‐coordinated boron in these glasses.