High energy resolution GeV gamma ray detector

We design a GeV gamma ray detector based on an electron-positron pair measurement by using the BESS-polar magnet and study it with the GEANT4 simulation code. We consider an additional counter to select an electron-positron pair for which the energy loss by bremsstrahlung is less than 100 MeV against AMS. We obtain a significantly low detector efficiency of 3%. However, an energy resolution better than 1% at 10-100 GeV is achieved by employing a parameter that enables us to obtain a line sensitivity that is almost indetical to that of the GLAST mission. The good energy resolution makes it possible to obtain the three-dimensional map of the neutralino dark matter annihilation line.