Modelling the 1.8 MeV Sky: Tests for Spiral Structure
  COMPTEL imaging analysis revealed a patchy, asymmetric distribution of diffuse 1.8 MeV emission along the Galactic plane, which is attributed to the decay of radioactive 26Al in the ISM. If massive stars were the major source of Galactic 26Al, the 1.8 MeV emission should be asymmetric and trace the spiral arms of the Galaxy, presumed site of massive star formation. Using model fits, we indeed find weak evidence in the COMPTEL data that the observed 1.8 MeV emission is at least partly confined to spiral arms. We derive a total Galactic 26Al mass of 2.5 Msol from which at least 0.7 Msol can be attributed to massive stars. 
J. Knoedlseder, N. Prantzos, K. Bennett, H. Bloemen, R. Diehl, W.   Hermsen, U. Oberlack, J. Ryan, and V. Schoenfelder