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The MeV band covers the physics of the lowest energy end of the non-thermal universe and nuclear reactions in metal synthesis and/or matters interacting with cosmic rays. Nevertheless, sensitivity in this band is still limited. Improving MeV sensitivity is one of the most important issues in modern astronomy. The soft gamma-ray detector (SGD) onboard the Hitomi satellite launched in 2016 was an innovative narrow field-of-view semiconductor Compton telescope (SCT), which aims at background reduction through deep active-shielding combined with SCT using Si and CdTe, a Si/CdTe-SCT. Although with a limited operation time, it is the sole SCT proven in orbit to date, and succeeded in detecting ∼ 100 keV polarization with only 5 ks exposure. Based on these achievements, we propose a new approach “narrow field-of-view Si/CdTe-SCT at balloon altitude”. At the altitude of 32-40 km, detectors do not suffer severe proton bomberment, which was one of the major contributor to the SGD background, while able to observe MeV gamma rays from the universe. Atmospheric gamma rays is non-negligible, but as it is stronger near horizon, narrowing the field-of-view and aiming around zenith will cut most of them. The detector loses wide field of view, but can be used as a probe in sub-MeV band. As the first step of this approach, we are planning a balloon experiment, miniSGD, to confirm the concept. It is very compact and not aiming at real observation, but has all the components integrated within the 40 × 40 × 50 cm3 volume, including detectors, electronics and the collimator. The Si/CdTe-SCT was made of a single layer of 0.5 mm thick double-sided Si strip detectors (DSSDs) and newly developed 2 mm thick CdTe double sided strip detectors (CdTe-DSDs), surrounded by nine units of 2-3 cm thick BGO scintillator crystals. The miniSGD experiment is to fly as a piggy-back payload in the 2023 Spring campaign of JAXA Ballooning team at Alice Springs, Australia. Technologies to be verified in miniSGD are also applicable to the future hard X-ray missions, such as the FORCE mission.
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