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The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory (CGRO) has detected significant 511 keV positron annihilation line emission from the Galactic center region (Purcell et al.\/ 1993, Ap.~J. 413, L85). The observed line emission is found to be concentrated in the direction of the Galactic center, with an additional component suggesting emission from the Galactic disk. Significant positronium continuum emission has also been observed. In preliminary analyses, the positronium fraction found for a typical Galactic center observation, $0.95 \pm 0.04$, is generally consistent with previous measurements. Measurements of the positronium fraction provide important information about the physical characteristics of the annihilation medium. Preliminary results of an analysis of the OSSE observations of the positronium continuum in the Galactic plane and Galactic center region will be presented. The spatial distribution of the positronium fraction will also be considered.
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