Astronomy in the medium-energy gamma-ray band (0.1 – 100 MeV) holds a rich promise for elucidating many fundamental questions concerning, e.g., nucleosynthesis in stellar explosions (novae and supernovae), the origin of cosmic rays, the laws of physics around neutron stars and black holes, the active sun, or the origin of terrestrial gamma-ray flashes. In addition, this field of astronomy also contributes to the advancement of cosmology and fundamental physics.

The 0.1 – 100 MeV energy domain covers the emission decay of neutral pions, whose photon spectrum peaks at about 70 MeV, as well as the emission of nuclear gamma-ray lines between 0.1 and 10 MeV. In contrast, spectral features in much of the common domains of astronomical bands arise from atomic-shell transitions. The latter depend on the temperature and ionization state of the atoms in the observed gas, while nuclear transitions have intrinsically different origins – radioactivity, nuclear collisions, neutron capture, antimatter annihilation – and are independent of the often uncertain thermodynamic state of the gas.

Listed below are a number of scientific topics that can be studied in the medium-energy gamma-ray domain. For each topic, the main scientific objectives are briefly outlined and the required instrumental performances to address these objectives are summarized. These fact sheets are the result of a workshop held in APC (Paris) on January 15 – 16, 2013.

Theme 1: Radioactivity and antimatter