Performance improvement of an Electron-Tracking Compton Camera by a new track reconstruction method

For MeV gamma-ray Astronomy, we have developed an Electron Tracking Compton Camera (ETCC) as a next-generation MeV gamma-ray telescope. An ETCC consists of a three-dimensional electron tracker using a gaseous time projection chamber (TPC) and position-sensitive gamma-ray absorbers using pixel scintillator arrays (PSAs). We carried out the balloon borne experiment in 2006 with a small size ETCC and observed successfully the fluxes of the diffuse cosmic and atmospheric gamma rays. As the next flight, we plan to observe bright celestial sources like Crab nebula and have constructed a large size ETCC. To achieve this, an effective area must be larger than 0.5cm² for obtaining a 3 sigma level signal for 3 hours observation. To obtain the required sensitivity, we have improved the electron track reconstruction method by updating the track encoding logic and developing a simple track analysis for the new logic. We performed ground-based experiments in the new method using a test model ETCC and measured the detection efficiency, which is found to be 10 times higher than that in the previous method and consistent with the simulation. In addition, the measured angular resolution is improved remarkably. From these results, we expect that a large size ETCC will have more than 3 times better sensitivity than the original design performance.