0633000

For about 30 years we have known that positrons are being born and annihilated in the central region of our Galaxy. The origin of these positrons however has remained a mystery. Based on the first 4 years of observations with INTEGRAL/SPI a very recent publication reported a distinct and totally unexpected asymmetry in the 511 keV line emission emanating from the inner Galactic disk. This asymmetry resembles one in the distribution of hard LMXBs as observed by INTEGRAL/IBIS indicating that these systems may be the dominant origin of the positrons. If this association is confirmed it will be the first identification of a specific origin for the positrons (apart from the small fraction which must come from the decay of 26Al). Either way the asymmetry in the 511 keV line emission signifies a unique characteristic of our Galaxyand gives important clues towards solving a long-standing mystery. INTEGRAL has capabilities uniquely matched to pursuing this problem - capabilities that will not be equaled or surpassed for a very long time. It is crucial to use these to verify the asymmetry in the 511 keVline. To this end we propose a key programme(which could be spread over multiple observing cycles) that requires in total 4.1 Ms of observation time. This programme is designed to determine theflux difference between two symmetric fields in the inner disk at (l b) = (+- 25deg0deg) in a robust and largely model independent way by using a special ?beam switching? (chopping) pointing strategy. The two fields are in important regions of the inner Galactic plane that include spiral arm tangents and the 4 kpc dust torus. Our observations can therefore be used via the general observing programme for studies of a large variety and number of steady and transient point sources. Other important INTEGRAL science topics that will benefit from our programme are studies of Galactic disk diffuse line (notably at 18 [truncated! Please see actual data for full text]