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.

• Received 26 November 1975

DOI:https://doi.org/10.1103/PhysRevLett.36.395