Positronium spin flip cross section with O 2: Comparison with muonium spin exchange

Abstract

The positron lifetime (PLT) of ortho-Ps produced in silica gels has been 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 observed thermally averaged spin conversion cross section, 〈 σ SC〉=(1.2±0.1)×10 −19 cm 2, agrees with 〈 σ SC〉 previously obtained by the two-photon angular correlation method and with 〈 σ 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 silica gels. The conversion cross section 〈 σ SC〉 obtained in this work has been converted to the thermally averaged spin flip cross section 〈 σ SF〉=(4.0±0.4)×10 −19 cm 2, by using a prescription based on recent theoretical works. This represents the first time that positronium spin exchange is discussed explicitly in terms of spin flip cross section, an underlying physical quantity which characterizes the process, so that Ps + O 2 spin exchange can be compared directly with hydrogen and muonium spin exchange, H + O 2 and Mu + O 2, studied by the electron spin resonance and muon spin rotation techniques, respectively. 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, 〈 σ SF〉=(5.78±0.15)×10 −16 cm 2, obtained by the muon spin rotation technique with a low transverse magnetic field.