The observation of diffuse cosmic and atmospheric gamma rays with an electron-tracking compton camera loaded on a balloon

As next-generation MeV gamma-ray telescope, we have developed an electron-tracking Compton camera (ETCC) that consists of a gaseous tracker and an absorber. The ETCC obtains the energy and direction of both a scattered gamma ray and a recoil electron, and determines both the energy and direction of an incident gamma ray, photon by photon. Furthermore, this camera features powerful background rejection based on Compton-scattering kinematics. To prepare for future spacecraft loading, we planned the balloon experiment, "Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment" (SMILE). As the first step in SMILE, we demonstrated gamma-ray detection using our camera by observing diffuse cosmic gamma rays and secondary gamma rays generated by collisions between high-energy cosmic rays and nuclei in the air. On September 1, 2006, our detector was loaded on a balloon and launched from Sanriku Balloon Center, ISAS/JAXA. The balloon realized level flight at an altitude of about 35 km during a live time of 3.0 h. We succeeded in obtaining about 420 downward gamma-ray events in the 0.1-1 MeV band in the field of view of 3 str, and derived the fluxes of diffuse cosmic and atmospheric gamma rays.