γ-ray lines from cosmic-ray interactions with interstellar dust grains

As pointed out by Lingenfelter and Ramaty [ApJ 211 (1977) L19], the shapes of some gamma-ray lines produced by cosmic-ray interactions with the interstellar medium potentially contain valuable information on the physical properties of dust grains, including their compositions and size distributions. The most promising of such lines are at 847, 1369, 1779 and 6129 keV, from Fe-56*, Mg-24, Si-28* and O-16*, respectively. We performed detailed calculations of their profiles using, in particular, available laboratory measurements combined with optical model calculations to evaluate the energy distributions of the recoiling excited nuclei. We show that the line shapes are mainly sensitive to relatively large interstellar grains, with radii greater than or similar to0.25 mum. Line fluxes from the inner Galaxy are then predicted. (C) 2003 Elsevier B.V. All rights reserved.