  We demonstrate that the well-known 2.6 MeV gamma-ray emission line from
thallium-208 could serve as a real-time indicator of astrophysical heavy
element production, with both rapid (r) and intermediate (i) neutron capture
processes capable of its synthesis. We consider the r process in a Galactic
neutron star merger and show Tl-208 to be detectable from ~12 hours to ~10
days, and again ~1-20 years post-event. Detection of Tl-208 represents the only
identified prospect for a direct signal of lead production (implying gold
synthesis), arguing for the importance of future MeV telescope missions which
aim to detect Galactic events but may also be able to reach some nearby
galaxies in the Local Group.
