Effectiveness and performance of a full ray-tracing sub-MeV Compton imager

As the next generation of sub-MeV/MeV gamma-ray explorer for astronomy, we have developed an electron-tracking Compton camera (ETCC) which consists of combination of a gaseous ray-tracing chamber as a Compton-scattering target and position sensitive scintillator arrays as a scattered gamma-ray absorber. Our detector has full ray-tracing capability for Compton scattering reconstruction, the capability has two significant benefits for Compton gamma-ray imager, one is efficient background rejection without reduction of effective area for gamma rays, and another one is clear imaging. We had launched a balloon-borne experiment of a small size ETCC in 2006, and succeeded for observation of the spectra of diffuse cosmic and atmospheric gamma rays. However, obtaining the proof of imaging capability by observation of a bright celestial object such as the Crab nebula is strongly required to propose an all-sky survey satellite. Aiming to the proof of clear imaging, we are planning a next balloon flight and we have constructed a 30 cm-cube size ETCC which has about 100 times higher sensitivity than the previous flight detector. This sensitivity enables us to detect the Crab nebula with 5σ level for several hours observation. We report imaging demonstrations at the ground and performance tests of gamma-ray detection efficiency, energy resolution, and imaging resolution in range of 150 keV to 1 MeV gamma-ray band.