Precise low-energy electron tracking using a gaseous Time Projection Chamber for the balloon-borne gamma ray compton telescope

For MeV gamma ray astronomy, we have developed an Electron Tracking Compton Camera (ETCC) as a MeV gamma ray telescope in the next generation. Our detector consists of a gaseous Time Projection Chamber (TPC), which uses μ-PIC as the two-dimensional readout detector, and a position sensitive scintillation camera. We launched a small size ETCC with a 10 cm × 10 cm × 15 cm TPC loaded on a balloon in 2006, and obtained the fluxes of diffuse cosmic and atmospheric gamma rays (SMILE-I). As the next step of SMILE-I, we have a plan of the next measurement of MeV gamma ray celestial sources like Crab Nebula with a middle size (30 cm)³ ETCC (SMILE-II) for the test of its imaging property. For SMILE-II, we developed the new Data AcQuisition (DAQ) system of an ETCC to reduce the dead time and power consumption, including the new data acquisition algorithm of electron tracking. The detection efficiency obtained using the new algorithm is about 10 times larger than the one based on the SMILE-I´s algorithm. In this record, we report the new SMILE-II ETCC DAQ system and its performances.