TGRS Measurements of the Positron Annihilation Spectrum from the   Galactic Center
  We have obtained spectra of the Galactic center at energies 400-600 keV from high-resolution data acquired by the TGRS Ge spectrometer on board the WIND mission during 1995-1997. The data were obtained using an on-board occulter, and are relatively free from systematics and backgrounds. Analysis of the spectra reveals a well-resolved electron-positron annihilation line at 511 keV and the associated continuum due to annihilation via positronium formation. Measurements of the line width and the line-to-continuum ratio allow some constraints to be placed on the interstellar sites where annihilation occurs. 
M. J. Harris, B. J. Teegarden, T. L. Cline, N. Gehrels, D. M. Palmer,   R. Ramaty, H. Seifert