Catching Element Formation In The Act - The Case for a New MeV Gamma-Ray Mission: Radionuclide Astronomy in the 2020s

Gamma-ray astronomyexplores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-massblack holes, nucleosynthesis, the interstellar medium, cosmic raysand relativistic- particle acceleration, and the evolution of galaxies. MeV gamma-raysprovide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-rayphotons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-rayenergies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-rayenergies. This science is enabled by next-generation gamma-rayinstruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-rayinstruments. This transformative capability permits: (a) the accurate identification of the gamma-rayemitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-raymaps of the Milky Wayand other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-rayinstruments to address a wide set of astrophysical questions.