Browsing by Author "Takemura, Taito"
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Item First observation of MeV gamma-ray universe with true imaging spectroscopy using the Electron-Tracking Compton Telescope aboard SMILE-2+(2021-07-01) Takada, Atsushi; Takemura, Taito; Yoshikawa, Kei; Mizumura, Yoshitaka; Ikeda, Tomonori; Nakamura, Yuta; Onozaka, Ken; Abe, Mitsuru; Hamaguchi, Kenji; Kubo, Hidetoshi; Kurosawa, Shunsuke; Miuchi, Kentaro; Saito, Kaname; Sawano, Tatsuya; Tanimori, ToruMeV gamma-ray is a unique window for the direct measurement of line emissions from radioisotopes, but there is no significant progress in the observation after COMPTEL/{\it CGRO}. Hence, for observing celestial objects in this band, we are developing an electron-tracking Compton camera (ETCC), which enables us to perform true imaging spectroscopy similar to X-ray or GeV telescopes. Therefore, we can obtain the energy spectrum of the observation target by a simple ON-OFF method using the correctly defined a proper point-spread function. For validating the performance of celestial object observation using an ETCC, the second balloon SMILE-2+, which had an ETCC based on a gaseous electron tracker with a volume of 30×30×30~cm³, was launched at Alice Springs, Australia on April 7, 2018. SMILE-2+ observed the southern sky including the Crab nebula with a live time of 5.1 h at the zenith angle of ∼50 degrees and detected gamma-rays from the Crab nebula with a significance of 4.0σ at the energy range of 0.15--2.1~MeV. Additionally, an enhancement of gamma-ray events due to the Galactic center region was clearly observed in the light curve. The realized detection sensitivity agrees well with the sensitivity estimated before launching based on the total background of extragalactic diffuse, atmospheric gamma-rays, and a small number of instrumental gamma-rays suppressed to one-third of the total background. We have succeeded to overcome the most difficult and serious problem of huge background for the stagnation of MeV gamma-ray astronomy for the first time in the world, and thus demonstrate that an ETCC can pioneer a deeper survey than COMPTEL in MeV gamma-ray astronomy.Item First Observation of the MeV Gamma-Ray Universe with Bijective Imaging Spectroscopy Using the Electron-tracking Compton Telescope on Board SMILE-2+(AAS, 2022-04-28) Takada, Atsushi; Takemura, Taito; Yoshikawa, Kei; Mizumura, Yoshitaka; Ikeda, Tomonori; Nakamura, Yuta; Onozaka, Ken; Abe, Mitsuru; Hamaguchi, Kenji; Kubo, Hidetoshi; Kurosawa, Shunsuke; Miuchi, Kentaro; Saito, Kaname; Sawano, Tatsuya; Tanimori, ToruMeV gamma-rays provide a unique window for the direct measurement of line emissions from radioisotopes, but observations have made little significant progress since COMPTEL on board the Compton Gamma-ray Observatory (CGRO). To observe celestial objects in this band, we are developing an electron-tracking Compton camera (ETCC) that realizes both bijective imaging spectroscopy and efficient background reduction gleaned from the recoil-electron track information. The energy spectrum of the observation target can then be obtained by a simple ON–OFF method using a correctly defined point-spread function on the celestial sphere. The performance of celestial object observations was validated on the second balloon SMILE-2+ , on which an ETCC with a gaseous electron tracker was installed that had a volume of 30 × 30 × 30 cm3. Gamma-rays from the Crab Nebula were detected with a significance of 4.0σ in the energy range 0.15–2.1 MeV with a live time of 5.1 hr, as expected before launch. Additionally, the light curve clarified an enhancement of gamma-ray events generated in the Galactic center region, indicating that a significant proportion of the final remaining events are cosmic gamma-rays. Independently, the observed intensity and time variation were consistent with the prelaunch estimates except in the Galactic center region. The estimates were based on the total background of extragalactic diffuse, atmospheric, and instrumental gamma-rays after accounting for the variations in the atmospheric depth and rigidity during the level flight. The Crab results and light curve strongly support our understanding of both the detection sensitivity and the background in real observations. This work promises significant advances in MeV gamma-ray astronomy.Item High-energy extension of the gamma-ray band observable with an electron-tracking Compton camera(Elsevier, 2024-06-01) Oka, Tomohiko; Ogio, Shingo; Abe, Mitsuru; Hamaguchi, Kenji; Ikeda, Tomonori; Kubo, Hidetoshi; Kurosawa, Shunsuke; Miuchi, Kentaro; Mizumura, Yoshitaka; Nakamura, Yuta; Sawano, Tatsuya; Takada, Atsushi; Takemura, Taito; Tanimori, Toru; Yoshikawa, KeiAlthough the MeV gamma-ray band is a promising energy-band window in astrophysics, the current situation of MeV gamma-ray astronomy significantly lags behind those of the other energy bands in angular resolution and sensitivity. An electron-tracking Compton camera (ETCC), a next-generation MeV detector, is expected to revolutionize the situation. An ETCC tracks each Compton-recoil electron with a gaseous electron tracker and determines the incoming direction of each gamma-ray photon; thus, it has a strong background rejection power and yields a better angular resolution than classical Compton cameras. Here, we study ETCC events in which the Compton-recoil electrons do not deposit all energies to the electron tracker but escape and hit the surrounding pixel scintillator array (PSA). The PSA provides additional information on the electron-recoil direction, which enables us to improve significantly the angular resolution. We developed an analysis method for this untapped class of events and applied it to laboratory and simulation data. We found that the energy spectrum obtained from the simulation agreed with that of the actual data within a factor of 1.2. We then evaluated the detector performance using the simulation data. The angular resolution for the new-class events was found to be twice as good as in the previous study at the energy range 1.0–2.0 MeV, where both analyses overlap. We also found that the total effective area is dominated by the contribution of the double-hit events above an energy of 1.5 MeV. Notably, applying this new method extends the sensitive energy range with the ETCC from 0.2–2.1 MeV in the previous studies to up to 3.5 MeV. Adjusting the PSA dynamic range should improve the sensitivity in even higher energy gamma-rays. The development of this new analysis method would pave the way for future observations by ETCC to fill the MeV-band sensitivity gap in astronomy.