Compton scatter imaging in astrophysics

Compton telescopes have been an important tool in gamma-ray astronomy, most recently with the COMPTEL instrument on NASA's COMPTON Gamma Ray Observatory (CGRO). Scientific objectives in low/medium energy gamma-ray astronomy place highest priority on significant improvement in sensitivity relative to CGRO and ESA's planned INTEGRAL mission. The observational requirements include both discrete and extended sources, and narrow, broad, and continuum spectral features. A high spectral and spatial resolution Compton telescope is the preferred instrument for a future mission. The broad range of scientific objectives that such a mission will address include: mapping radioactivity across the Galaxy (26Al, 44i, 60Fe), detection of several Type Ia supernovae per year (56Ni, 56Co), study nuclear burning in novae (22Na, 7Be, 511 keV), study the accretion of matter onto galactic and massive extra-galactic black holes and the associated formation of relativistic jets, and provide unique insights into neutron stars, cosmic gamma-ray bursts, and solar flares. A mission to achieve these goals should have discrete source, narrow line sensitivities approaching [10.sup.-7] [Gamma]/[cm.sup.2]-s for the important astrophysical gamma-ray lines from 511 keV to 2 MeV. Detectors which combine state-of-the-art energy resolution (few keV) and spatial resolution (1 mm) are required.