POSITRONIUM SPIN-FLIP CROSS-SECTION WITH O-2 - COMPARISON WITH MUONIUM SPIN-EXCHANGE

The positron lifetime (PLT) of ortho-Ps produced in silica gels has be en measured as a function of O-2 concentration in order to obtain the spin conversion and spin flip cross sections for Ps + O-2. The observe d thermally averaged spin conversion cross section, [sigma(SC)] = (1.2 +/- 0.1) x 10(-19) cm(2), agrees with [sigma(SC)] previously obtained by the two-photon angular correlation method and with [sigma(SC)] in the gas phase obtained by the PLT technique. This indicates that bulk and surface effects are negligible for Ps + O-2 spin exchange in silic a gels. The conversion cross section [sigma(SC)] obtained in this work has been converted to the thermally averaged spin flip cross section [sigma(SF)] = (4.0 +/- 0.4) x 10(-19) cm(2), by using a prescription b ased on recent theoretical works. This represents the first lime that positronium spin exchange is discussed explicitly in terms of spin fli p cross section, an underlying physical quantity which characterizes t he process, so that Ps + O-2 spin exchange can be compared directly wi th hydrogen and muonium spin exchange, H + O-2 and Mu + O-2, studied b y the electron spin resonance and muon spin relation techniques, respe ctively. The spin flip cross section for Ps + O-2 is found to be three orders of magnitude smaller than the corresponding value for Mu + O-2 in the gas phase, [sigma(SF)] = (5.78 +/- 0.15) x 10(-16) cm(2), obta ined by the muon spin rotation technique with a low transverse magneti c field. (C) 1998 Elsevier Science B.V. All rights reserved.