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The quenching processes of the thermalized ortho-positronium(o-Ps) on anoxygen molecule have been studied by the positron annihilation age-momentumcorrelation techinique(AMOC). The Doppler broadening spectrum of the 511 keVgamma-rays from the 2gamma annihilation of o-Ps in O_2 has been measured as afunction of the o-Ps age. The rate of the quenching, consisting of the pickoffand the spin-conversion, is estimated from the positron lifetime spectrum. Theratio of the pickoff quenching rate to the spin-conversion rate is deduced fromthe Doppler broadening of the 511 keV gamma-rays from the annihilation of theo-Ps. The pickoff parameter ^1Z_eff, the effective number of the electrons permolecule which contribute to the pickoff quenching, for O_2 is determined to be0.6 +- 0.4. The cross-section for the elastic spin-conversion quenching isdetermined to be (1.16 +- 0.01) * 10^{-19} cm^2.Comment: 4 pages with 5 eps figures, LaTeX2e(revtex4

Pickoff and spin conversion of orthopositronium in oxygen