Balloon-Borne Sub-MeV Gamma-ray Imager Using Electron Tracking Gaseous TPC and Scintillation Camera

A balloon-borne sub-MeV gamma-ray imaging detector for astrophysics has been developed. It measures a three dimensional track of a Compton-recoil electron and determines the incident direction of each photon in a reduced arc projected on the sky. Our developed detector consists of two parts: One is a gaseous time projection chamber (mu-TPC) with a size of 10 times 10 times 14 cm³ with a micro pixel chamber (mu-PIC) readout, which measures both the three dimensional track and the energy of the Compton recoil electron. The other is a position-sensitive scintillation camera enclosing the mu-TPC, which consists of arrays of pixellated GSO(Ce) scintillators coupled to multi-anodes PMTs and measures both the direction and the energy of the scattered gamma ray. By the full reconstruction of incident gamma rays from these measurements, we have successfully obtained gamma-ray images in the 0.1-0.9 MeV range. The angular resolutions (FWHM) were 8.5deg and 120deg for single photon with 662 keV for an angular resolution measure and a scatter plane deviation, respectively. The detector had no collimator, and the field of view was 1 sr. In order to measure both the cosmic and the atmospheric gamma-ray background in the sub-MeV region, a balloon equipped with this detector was launched from JAXA/ISAS Sanriku Balloon Center in Japan on September 1, 2006, and flew for 7 hours with the highest altitude of 35 km. Based on the detector, a larger detector with the mu-TPC with a size of 30 times 30 times 30 cm³ is being developed in order to observe celestial objects in the sub-MeV region.