SMILE: A balloon-borne sub-MeV/MeV gamma-ray Compton Camera using an Electron-Tracking gaseous TPC and a scintillation camera

We have been developing an Electron-Tracking Compton Camera (ETCC) as a next generation MeV gammaray detector, consisting of a gaseous electron tracker which measures the three-dimensional track and the energy of the Compton-recoil electron, and pixel scintillation arrays which measure the absorption point and the energy of the scattered gamma ray. As the first step toward a future all-sky survey with a planned sensitivity of one order magnitude higher than COMPTEL, we successfully observed the diffuse cosmic and atmospheric gamma rays at balloon altitudes with a small ETCC in the Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment I (SMILE-I). Building on the success of the SMILE-I ETCC, we are developing a large size ETCC for the next flight to observe celestial objects in the sub-MeV/MeV region. This larger detector is expected to achieve a sensitivity comparable to COMPTEL.