Precision measurement of orthopositronium decay rate using SiO2 powder

The intrinsic decay rate of orthopositronium formed in SiO2 powder is measured using the direct 2gamma correction method such that the time dependence of the pick-off annihilation rate is precisely determined using high energy-resolution germanium detectors. As a systematic test, two different types of SiO2 powder are used with consistent findings. The intrinsic decay rate of orthopositronium is found to be 7.0396 +/- 0.0012(stat) +/- 0.0011 (sys) mus(-1), which is consistent with previous measurements using SiO2 powder with about twice the accuracy. Results agree well with a recent O(alpha(2)) QED prediction, varying 3.8-5.6 experimental standard deviations from other measurements. (C) 2003 Elsevier B.V. All rights reserved.