Development of the balloon-borne sub-MeV gamma-ray Compton camera using an electron-tracking gaseous TPC and a scintillation camera

We have developed an Electron-Tracking Compton Camera (ETCC) for use onboard a balloon to observe sub-MeV/MeV gamma rays from celestial objects. The ETCC is constructed with a three dimensional gaseous tracker for recoil electrons from Compton scattering, and GSO:Ce pixel scintillator arrays as absorber of the Compton-scattered gamma-ray. By using the ETCC, we can reconstruct the energy and direction of individual gamma rays. We have developed a prototype ETCC with a (30 cm)³ TPC, and tested its performance in the range of 356 - 835 keV in the laboratory. As the result, we succeeded in taking images of gamma ray sources and determined a detection efficiency of 9.0 × 10⁻⁶ and an effective area of 8.0 × 10⁻³ cm² at 662 keV for the prototype ETCC. Furthermore, we developed a new power saving readout circuit for the scintillators that achieves the electric power consumption of 0.41 W/channel, an energy dynamic range of 81 - 1333 keV, and an energy resolution of 10.3% at full width at half maximum at 662 keV.