Development of an electron-tracking Compton camera based on a gaseous TPC and a scintillation camera for a balloon-borne experiment

Abstract

We have developed an Electron-Tracking Compton Camera (ETCC) based on a gaseous micro Time Projection Chamber (ETCC) based on a gaseous micro Time Projection Chamber (μ-TPC) which measures the direction and the energy of the recoil electron and a GSO(Ce) scintillation camera which surrounds the μ-TPC and measures the Compton scattered gamma ray. If not measuring a direction of a recoil electron, a direction of the incident gamma-ray could only be reconstructed as a circle. Measuring the direction of the recoil electron reduces the Compton cone to a point, and thus reconstructs the incident direction completely for a single photon and realizes the strong background rejection. Using the ETCC with a detection volume of about 10cm×10cm×15cm, we had the balloon-borne experiment supported by ISAS/JAXA in 2006 for the purpose of the observation of diffuse cosmic and atmospheric gamma rays. The ETCC obtained about 200 photons with FOV of 3 str in 3 hours in the energy range from 100 keV to 1 MeV, and the obtained flux was consistent with previous observations. On the basis of the results, we are developing the large size ETCC in order to improve the effective area for the next balloon experiment. The large size ETCC has the detection volume of 23cm ×28cm×30cm which consists of the 23cm×28cm×30cm μ-TPC and the 30cm×30cm×1.3cm scintillation camera. Then we obtained the gamma-ray image and investigated the first performances of the large size ETCC. The Angular Resolution Measure (ARM) and the Scatter Plane Deviation (SPD) are 12.1 degree and 117 degree (FWHM) at 662keV, respectively, and the energy resolution is 16.9%(FWHM) at 662keV.