Electron-tracking Compton camera, which is a complete Compton camera with tracking Compton scattering electron by a gas micro time projection chamber, is expected to open up MeV gamma-ray astronomy. The technical challenge for achieving several degrees of the point spread function is the precise determination of the electron-recoil direction and the scattering position from track images. We attempted to reconstruct these parameters using convolutional neural networks. Two network models were designed to predict the recoil direction and the scattering position. These models marked 41$~$degrees of the angular resolution and 2.1$~$mm of the position resolution for 75$~$keV electron simulation data in Argon-based gas at 2$~$atm pressure. In addition, the point spread function of ETCC was improved to 15$~$degrees from 22$~$degrees for experimental data of 662$~$keV gamma-ray source. These performances greatly surpassed that using the traditional analysis.
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.
Driven by deep learning, object recognition has recently made a tremendous leap forward. Nonetheless, its accuracy often still suffers from several sources of variation that can be found in real-world images. Some of the most challenging variations are induced by changing lighting conditions. This paper presents a novel approach for tackling brightness variation in the domain of 2D object detection and 6D object pose estimation. Existing works aiming at improving robustness towards different lighting conditions are often grounded on classical computer vision contrast normalisation techniques or the acquisition of large amounts of annotated data in order to achieve invariance during training. While the former cannot generalise well to a wide range of illumination conditions, the latter is neither practical nor scalable. Hence, We propose the usage of Generative Adversarial Networks in order to learn how to normalise the illumination of an input image. Thereby, the generator is explicitly designed to normalise illumination in images so to enhance the object recognition performance. Extensive evaluations demonstrate that leveraging the generated data can significantly enhance the detection performance, outperforming all other state-of-the-art methods. We further constitute a natural extension focusing on white balance variations and introduce a new dataset for evaluation.
Abstract MeV 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 cm 3 . Gamma-rays from the Crab Nebula were detected with a significance of 4.0 σ 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.
Although 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). 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.
MeV gamma-ray observations provide unique information about nucleosynthesis, diffusion in our galaxy, low-energy cosmic rays, particle acceleration, and other phenomena. However, the detection sensitivity in this band is significantly lower than that in other bands due to a large background contamination. To address this issue, we are developing an electron-tracking Compton camera (ETCC) with powerful background rejection tools based on Compton recoil electron tracks. This will enable future observations to be conducted with greater sensitivity. We have successfully demonstrated the detection technology and performance of the ETCC with two balloon experiments. We are preparing for the next balloon flight, SMILE-3, to observe galactic diffusion gamma rays and some bright celestial objects.
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.
Mega electron volt (MeV) gamma-ray observations are promising diagnostic tools for observing the Universe. However, the sensitivity of MeV gamma-ray telescopes is limited by peculiar backgrounds, restricting the applicability of MeV gamma-ray observations. Thus, background identification is crucial in the design of next-generation telescopes. Here, we assessed the background contributions of the electron-tracking Compton camera (ETCC) onboard SMILE-$2+$ in balloon experiments. This assessment was performed using Monte Carlo simulations. The results revealed that a background below 400 keV existed due to the atmospheric gamma-ray background, cosmic-ray/secondary-particle background, and accidental background. Moreover, an unresolved background component that was not related to direct Compton-scattering events in the ETCC was confirmed above 400 keV. Overall, this study demonstrated that the Compton-kinematics test is a powerful tool for removing backgrounds and principally improves the signal-to-noise ratio at 400 keV by an order of magnitude.
