Considerations for the Next Compton Telescope Mission

A high resolution Compton telescope has been identified by the Gamma Ray Astronomy Program Working Group (GRAPWG) as the highest priority major mission in gamma ray astrophysics following GLAST. This mission should provide 25-100 times improved sensitivity, relative to CGRO and INTEGRAL, for MeV gamma ray lines. It must have good performance for narrow and broad lines and for discrete and diffuse emissions. Several instrumental approaches are being pursued to achieve these goals. We discuss issues relating to this mission including alternative detector concepts, instrumental configurations, and background reduction techniques. We have pursued the development of position-sensitive solidstate detectors (Ge, Si) for a high spectral resolution Compton telescope mission. A ∼1 m germanium Compton telescope of position-sensitive germanium detectors was the basis for one of the GRAPWG concepts. Preliminary Monte Carlo estimates for the sensitivities of this instrument are encouraging. However, there are technical challenges of cooling large volumes of Ge and providing the large number of spectroscopy channels. We also show that with only two Compton scatter interactions followed by a third interaction, the incident gamma ray energy and direction cone can be precisely determined in detectors with excellent energy and position resolution. Full energy deposition is not required. We present a promising concept for a high efficiency Compton instrument for which thick silicon strip detectors might be preferred.