The long-standing discrepancy [G. S. Adkins, R. N. Fell, and J. Sapirstein, Ann. Phys. (N.Y.) 295, 136 (2002)] between the theoretical calculations of the orthopositronium (o-Ps) annihilation decay rate (λT=1/lifetime) and some of the experimental measurements has been resolved. A focused beam of positrons incident on a special nanoporous silica film produces near-thermal energy o-Ps in vacuum that is slow enough to be virtually free of perturbing interactions. The fitted decay rate requires only a 500 ppm correction for nonthermal o-Ps effects. The new value of λT=7.0404(10)(8) μs^-1 is in excellent agreement with theory.
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Vallery et al. (2003) studied this question.
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