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, ⁴⁴Ti, 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 clarification of the mechanism for the formation of relativistic jets; provide unique insights into the structure of neutron stars, cosmic gamma ray bursts; and solar flares. A mission to achieve these broad goals should have narrow line sensitivities approaching 10 ^-7 γ/cm²-s for discrete sources. An energy threshold of ~250 keV is required to ensure good coverage of the many important astrophysical gamma ray lines from 511 keV to 2 MeV. Angular resolutions much better than 1 degree are required. Detectors which combine state of the art energy resolution (few keV) and spatial resolution (1 mm) are needed. A major breakthrough in background reduction and sensitivity could be realized if tracking low-energy electrons (~100 keV) can be achieved