Precision Measurement of the Decay Rate of Orthopositronium in SiO2Powders

Abstract

The decay rate of orthopositronium is found to depend linearly on the mass per free volume in powders of Si${\mathrm{O}}_2$ with grain radii of 35 and 70 Å. Three data points have a decay rate significantly less than both the theoretical free-space decay rate and the results of previous experiments. The value of ${\lambda }_3$ extrapolated to zero density, 7.104±0.006 μ${\sec }^{-1\mathrm{}}$, is (1.9±0.1)% below the theoretical value of the free-space lifetime. Possible interpretations of these results are discussed.