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Register a new userA performance study of an electron-tracking Compton camera with a compact system for environmental gamma-ray observation
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An electron-tracking Compton camera (ETCC) is a detector that can determine the arrival direction and energy of incident sub-MeV/MeV gamma-ray events on an event-by-event basis. It is a hybrid detector consisting of a gaseous time projection chamber (TPC), that is the Compton-scattering target and the tracker of recoil electrons, and a position-sensitive scintillation camera that absorbs of the scattered gamma rays, to measure gamma rays in the environment from contaminated soil. To measure of environmental gamma rays from soil contaminated with radioactive cesium (Cs), we developed a portable battery-powered ETCC system with a compact readout circuit and data-acquisition system for the SMILE-II experiment. We checked the gamma-ray imaging ability and ETCC performance in the laboratory by using several gamma-ray point sources. The performance test indicates that the field of view (FoV) of the detector is about 1$;$sr and that the detection efficiency and angular resolution for 662$;$keV gamma rays from the center of the FoV is $(9.31 pm 0.95) times 10^{^-5}$ and $5.9^{circ} pm 0.6^{circ}$, respectively. Furthermore, the ETCC can detect 0.15$;murm{Sv/h}$ from a $^{137}$Cs gamma-ray source with a significance of 5$sigma$ in 13 min in the laboratory. In this paper, we report the specifications of the ETCC and the results of the performance tests. Furthermore, we discuss its potential use for environmental gamma-ray measurements.
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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.
A sensitive survey of the MeV gamma-ray sky is needed to understand important astrophysical problems such as gamma-ray bursts in the early universe, progenitors of Type Ia supernovae, and the nature of dark matter. However, the study has not progressed remarkably since the limited survey by COMPTEL onboard CGRO in the 1990s. Tanimori et al. have developed a Compton camera that tracks the trajectory of each recoil electron in addition to the information obtained by the conventional Compton cameras, leading to superior imaging. This Electron Tracking Compton Camera (ETCC) facilitates accurate reconstruction of the incoming direction of each MeV photon from a wide sky at ~degree angular resolution and with minimized particle background using trajectory information. The latest ETCC model, SMILE-2+, made successful astronomical observations during a day balloon flight in 2018 April and detected diffuse continuum and 511 keV annihilation line emission from the Galactic Center region at a high significance in ~2.5 hours. We believe that MeV observations from space with upgraded ETCCs will dramatically improve our knowledge of the MeV universe. We advocate for a space-based all-sky survey mission with multiple ETCCs onboard and detail its expected benefits.
For MeV gamma-ray astronomy, we have developed an electron-tracking Compton camera (ETCC) as a MeV gamma-ray telescope capable of rejecting the radiation background and attaining the high sensitivity of near 1 mCrab in space. Our ETCC comprises a gaseous time-projection chamber (TPC) with a micro pattern gas detector for tracking recoil electrons and a position-sensitive scintillation camera for detecting scattered gamma rays. After the success of a first balloon experiment in 2006 with a small ETCC (using a 10$times$10$times$15 cm$^3$ TPC) for measuring diffuse cosmic and atmospheric sub-MeV gamma rays (Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment I; SMILE-I), a (30 cm)$^{3}$ medium-sized ETCC was developed to measure MeV gamma-ray spectra from celestial sources, such as the Crab Nebula, with single-day balloon flights (SMILE-II). To achieve this goal, a 100-times-larger detection area compared with that of SMILE-I is required without changing the weight or power consumption of the detector system. In addition, the event rate is also expected to dramatically increase during observation. Here, we describe both the concept and the performance of the new data-acquisition system with this (30 cm)$^{3}$ ETCC to manage 100 times more data while satisfying the severe restrictions regarding the weight and power consumption imposed by a balloon-borne observation. In particular, to improve the detection efficiency of the fine tracks in the TPC from $sim$10% to $sim$100%, we introduce a new data-handling algorithm in the TPC. Therefore, for efficient management of such large amounts of data, we developed a data-acquisition system with parallel data flow.
X-ray and gamma-ray polarimetry is a promising tool to study the geometry and the magnetic configuration of various celestial objects, such as binary black holes or gamma-ray bursts (GRBs). However, statistically significant polarizations have been detected in few of the brightest objects. Even though future polarimeters using X-ray telescopes are expected to observe weak persistent sources, there are no effective approaches to survey transient and serendipitous sources with a wide field of view (FoV). Here we present an electron-tracking Compton camera (ETCC) as a highly-sensitive gamma-ray imaging polarimeter. The ETCC provides powerful background rejection and a high modulation factor over a FoV of up to 2$pi$ sr thanks to its excellent imaging based on a well-defined point spread function. Importantly, we demonstrated for the first time the stability of the modulation factor under realistic conditions of off-axis incidence and huge backgrounds using the SPring-8 polarized X-ray beam. The measured modulation factor of the ETCC was 0.65 $pm$ 0.01 at 150 keV for an off-axis incidence with an oblique angle of 30$^circ$ and was not degraded compared to the 0.58 $pm$ 0.02 at 130 keV for on-axis incidence. These measured results are consistent with the simulation results. Consequently, we found that the satellite-ETCC proposed in Tanimori et al. (2015) would provide all-sky surveys of weak persistent sources of 13 mCrab with 10% polarization for a 10$^{7}$ s exposure and over 20 GRBs down to a $6times10^{-6}$ erg cm$^{-2}$ fluence and 10% polarization during a one-year observation.
Full functional and performance tests were performed many times before the Hard X-ray Modulation Telescope (HXMT) launch. During one of the tests, the count rate curves of the 18 High Energy Detectors (HED) have been found increased consistently within an interval of time. A further study on the correlation between the count rate and rainfall was carried out,and the increased net spectrum was also analyzed. The analysis results indicate that the short-lived 222Rn decay products (214Pb and 214Bi) in rainwater were responsible for the transient changes of the background radiation spectra in HEDs. The results show that the HXMT/HEDs have a good detection sensitivity on X/gamma rays, and the detector calibration results are effective.
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