The MEGA project: Science goals and hardware development

The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4–50 MeV) and bridge the huge sensitivity gap left after the demise of the COMPTEL and OSSE experiments on the Compton Gamma-Ray Observatory. The scientific goals include compiling a much larger catalog of sources in this energy range, performing far deeper searches for long-lived nuclear lines from supernovae, novae, and supernova remnants, studying prompt decay lines from solar flares and the interstellar medium, better measuring the diffuse galactic continuum and line emissions, identifying the components of the cosmic diffuse gamma-ray emission, searching for nuclear resonance absorption features in bright continuum spectra, and studying the medium-energy properties of black holes, pulsars, and gamma-ray bursts. MEGA detects and images gamma rays by completely tracking Compton and pair creation interactions in a stack of double-sided silicon strip track detectors surrounded by a pixellated CsI calorimeter. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results and describe future plans for the prototype, and describe the proposed satellite mission.