“…As of now, only a dozen nuclear lines of astrophysical origin have been observed with HEAO-3, COMPTEL, RHESSI, SPI (and NuSTAR). These include the positron annihilation line from the centre of the Galaxy at 511 keV [e.g., 11,12,13,14,15,16], short-and long-lived ejecta from massive stars and their supernovae such as 44 Ti [e.g., 17,18,19,20,21,22, at 68, 78, 1157 keV], 26 Al [e.g., 23,24,25,26,27, at 1809 keV], and 60 Fe [e.g., 28, 29, 30, at 1173 and 1332 keV], short-lived isotopes powering the early light-curves of type Ia supernovae [e.g., 31, 32, 33, 34, with 56 Ni and 56 Co at 158, 812, and 847, 1238 keV, respectively], as well as nuclear excitation lines from solar flares [e.g., 35, 36, with 511 keV from electron-positron annihilation, 2 H at 2223, 12 C at 4438, and 16 O at 6129 keV, among others]. With a factor of ten improvement in the line sensitivity, the number of detected lines, and therefore the science enabled by this, could increase by the same order of magnitude, eventually finding CR excitation of interstellar medium material, ejecta from classical novae, and multiple supernova lines [e.g., 37].…”