0420021

We propose to continue and complete our program for observing the sky at intermediate Galactic latitudes above and below the Galactic bulge with the goal of establishing the large-scale spatial distribution of positron annihilation radiation and in particular of proving the existence of a halo component. The detection of an annihilation halo would constitute a significant break-through for positron astrophysics with far-reaching implications for origin diffusion and annihilation of positrons in our Galaxy.We have published the first sky maps in the 511 keV line (Knoedlseder et al. 2005) and in the positronium continuum (Weidenspointner et al. 2006) which clearly showed that the positron annihilation radiation from our Galaxy is brightest around the Galactic center. The observed emission could not be unambiguously decomposed into bulge halo and disk components. But recent analyses suggest more and more strongly the existence of a very extended halo component. We propose to address this issue by observing at intermediate latitudes south of the Galactic center (l=0 deg b=-25 deg) for 2 Ms complementing a similar Cycle 3 observation north of the Galactic center (l=0 deg b=+25 deg). Any 511 keV line emission from these regions - for which there is increasing evidence - is dominated by the halo component. The proposed observations are therefore ideally suited to establish the existence of or to set stringent limits on a Galactic halo component. Possible in situ halo positron sources include members of the old stellar population (such as Type Ia supernovae novae or low-mass X-ray binaries) or light dark matter. [truncated! Please see actual data for full text]