Simulation study for the higher sensitivity of an Electron-Tracking Compton Camera at over 1 MeV

We have developed an Electron-Tracking Compton Camera (ETCC) as a next-generation MeV gamma-ray telescope. Our detector consists of a gaseous electron tracker as a Compton-scattering target and a position sensitive scintillation camera as a scattered gamma ray absorber. We launched a small size (10 cm cubic) ETCC loaded on a balloon in 2006, and obtained the fluxes of diffuse cosmic and atmospheric gamma rays in the energy range between 125 keV and 1:25 MeV. However, for MeV gamma-ray astronomy, we need to detect the photons at higher energy, thus we must improve the sensitivity of the ETCC in the higher energy range above 1 MeV. For this purpose, we investigate an electron absorber, which is placed between the electron tracker and the photo-absorber. Using Geant4 simulation, we confirmed that the detection energy range is shifted to the range of 0:35-5:0 MeV for the new configuration of the ETCC.