Diffuse gamma-ray line astronomy with AMEGO

Gamma-ray line astronomy began in the 1970s, but the field is still in its infancy. The signature of positron annihilation at 511 keV was the first gamma-ray line to be detected as originating outside of our solar system. After 40 years of observations, the Galactic sources of positrons are still unconfirmed and this remains one of the pioneering topics in gamma-ray astronomy. The next gamma-ray line to be detected was 1.8 MeV emission from the radioactive decay of Al-26, which was confirmation of active nucleosynthesis in our Galaxy. Nuclear emission lines from isotopes created in massive stars and their supernovae allow for fingerprint-like probes into stellar structure and evolution, a tool which has yet to be fully realized. The All-sky Medium Energy Gamma-ray Observatory (AMEGO), is an Astrophysics Probe concept design that can make significant progress in our understanding of cosmic nucleosynthesis and the source of Galactic positrons. AMEGO will have a wide field-of-view, good spectral resolution, and sensitivity 1-2 orders of magnitude better than previous telescopes. In this presentation we will focus on the topics that can be addressed with long-lived stellar nucleosynthesis products and discuss the intriguing open questions associated with Galactic positrons.