Experimental determination of the ortho-positronium lifetime in vacuum

Moderated positrons from a 22Na source positronium atoms at the surface of a channel plate multiplier in a weak magnetic field. They decay inside a finite volume and the decay quanta are registered by ten plastic scintillation detectors and counted in a fast data acquisition system with zero deadtime. After corrections for the escape of atoms from the observation volume and for prompt annihilation background it is obtained a lifetime of (142.22±0.14) ns. This is in fair agreement with the theoretical value of 142.07 ns but larger than previous experimental determinations. A possible correction from annihilation at the walls of the decay chamber would further increase the lifetime