Direct Measurement of the Parapositronium Lifetime inα−SiO2

The mean lifetime of delocalized parapositronium in $\alpha \mathrm{\text{−}}{\mathrm{S}\mathrm{i}\mathrm{O}}_2$ has been determined directly for the first time using a newly developed positron lifetime spectrometer based on ${\mathrm{B}\mathrm{a}\mathrm{F}}_2$ scintillators and a fast digital oscilloscope. The lifetime is found to be $156\pm 4\mathrm{p}\mathrm{s}$, which is much longer than its intrinsic lifetime of 125 ps. This indicates clearly that the primary many-body effect on positronium in $\alpha \mathrm{\text{−}}{\mathrm{S}\mathrm{i}\mathrm{O}}_2$ is the screening of the Coulomb interaction between the constituent particles by electrons of the medium and that the average distance between the electron and positron in positronium becomes larger than its vacuum value.