MeV gamma-ray astronomy in an energy range of hundreds of keV to tens of MeV is a unique window for observing nucleosynthesis, however this field has not opened up until recently because of imaging difficulties. Thus, we are developing an electron-tracking Compton camera (ETCC), which consists of a gaseous electron tracker and pixel scintillator arrays, as a next generation MeV gamma-ray telescope. Because the ETCC detects all parameters after Compton scattering, we can determine the momentum of incident gamma-rays with powerful background rejection. This ETCC has confirmed low-noise and high-sensitivity observations at high altitude through Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment I (SMILE-I) in 2006 and SMILE-2+ in 2018. Therefore, we are planning scientific observations using an ETCC with an effective area of ∼10 cm2 for 0.3 MeV, a spatial resolution of ≤10 degrees for 0.5 MeV, and a field of view of 3 sr as the next step (SMILE-3). In this paper, we present the design of the SMILE-3 ETCC and its expected observations.
Y. Nakamura, T. Tanimori, A. Takada, Y. Mizumura, S. Komura, T. Kishimoto, T. Takemura, K. Yoshikawa, T. Tanigushi, K. Onozaka, K. Saito, T. Mizumoto, S. Sonoda, H. Kubo, Y. Matsuoka, Y. Nakamasu, S. Kurosawa, K. Miuchi, T. Sawano
KEYWORDS: Gamma radiation, Cameras, Point spread functions, Scintillators, Data acquisition, Gamma ray imaging, Sensors, Global Positioning System, Compton scattering
To establish imaging spectroscopy of cosmic gamma-rays from a few hundreds of keV to a few tens MeV, we developed an electron-tracking Compton camera (ETCC). The ETCC consists of a time projection chamber (TPC) and pixelated scintillator arrays (PSAs). The ETCC is superior to conventional gamma-ray imaging detectors of this energy band in that the arrival direction of an incident gamma-ray is firmly determined at one point and realizes high noise rejection efficiency. We performed a campaign to demonstrate the gamma-ray imaging performance of the ETCC at balloon altitude via the sub-MeV gamma-ray imaging loaded-on-balloon experiment 2+ (SMILE-2+). The balloon was launched on April 7, 2018, at 6:26 ACST (UTC +9:30) from Alice Springs, Australia. We performed a level flight for 26 hours at an altitude of 39.6 km. The main observation targets were the Galactic Center region and the Crab Nebula and we succeeded in observing them without any critical problems. The configuration of the flight model ETCC and the housekeeping data are described in detail.
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