水村 好貴

JAXA operates scientific balloon campaigns, aiming at obtaining scientific results through safe and reliable balloon flights. The development of the prototype of the flight prediction and control system began more than 20 years ago. It has become a mature system through many years of operation and functional enhancement and modification. The main functions of the system are implemented by a database system, which has been used for at least 82 heavy balloon experiments and 102 light balloon experiments since 2007. The applications used in client computers include more than 180 graphical user interface panels. The system is designed to incorporate redundancy for availability during balloon flight operations. Although various constraints face balloon flights, such as scientific requirements, flight safety, and severe high-altitude wind conditions, the flight prediction and control system enable us to construct a detailed flight plan and to control the flight based on predictions. In addition to the report of the system, flight prediction is explained with an example of boomerang flight control planning.

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 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.

将来の大型高精度構造物の実現のために，著者らは高精度変位計測装置の研究開発を行っている．特に，X 線望遠鏡の支持構造など，1 次元的に細長い構造の両端の相対位置の計測に焦点を当てている．変位計測装置は，レーザ光源，ビームスプリッタ，レトロリフレクタ，PSD(Position Sensitive Device)からなる．レーザ光源とレトロリフレクタは，相対位置を計測する基準とターゲットに取り付けられる．開発中の変位計測装置は，実利用に先んじて，大型天文衛星の地上試験に使用され，その有用性が確かめられた．宇宙や成層圏での天文観測において実際に使用するためには，それぞれの環境での適合性を確認する必要がある．そこで，大気球実験における天文観測機器での利用を想定し，成層圏環境での機能実証を試みた．2021 年7 月9 日に，大気球実験：DREAM（DemonstRation Experiment of Alignment Monitor）を実施した．最高高度は29 ㎞，フライト時間は2時間54分であった．気球ゴンドラのサイズ制約から，レーザ光源とレトロリフレクタ間の距離は1mとした．気球実験を通じて，成層圏の気球実験環境下において本変位計測装置が正常に機能することが確かめられた．実験においては，上空で計測対象（レトロリフレクタ）に所定の変位を与えるために，人工的な周期的熱膨張を発生させた．計測された温度から推定された変位と，変位計測装置によって計測された変位の差は0.4μmRMS であった．

<title>Abstract</title> The Electron-Tracking Compton Camera (ETCC), which is a complete Compton camera that tracks Compton scattering electrons with 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 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$^\circ$ of angular resolution and 2.1 mm of position resolution for 75 keV electron simulation data in argon-based gas at 2 atm pressure. In addition, the point-spread function of the ETCC was improved to 15$^\circ$ from 22$^\circ$ for experimental data from a 662 keV gamma-ray source. The performance greatly surpassed that using traditional analysis.

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 an 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 +/- 0.01 at 150 keV for an off-axis incidence with an oblique angle of 30 degrees and was not degraded compared to the 0.58 +/- 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. 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 6 x 10(-6) erg cm(-2) fluence and 10% polarization during a one-year observation.

We have developed an Electron Tracking Compton Camera (ETCC), which provides a well-defined Point Spread Function (PSF) by reconstructing a direction of each gamma as a point and realizes simultaneous measurement of brightness and spectrum of MeV gamma-rays for the first time. Here, we present the results of our on-site pilot gamma-imaging-spectroscopy with ETCC at three contaminated locations in the vicinity of the Fukushima Daiichi Nuclear Power Plants in Japan in 2014. The obtained distribution of brightness (or emissivity) with remote-sensing observations is unambiguously converted into the dose distribution. We confirm that the dose distribution is consistent with the one taken by conventional mapping measurements with a dosimeter physically placed at each grid point. Furthermore, its imaging spectroscopy, boosted by Compton-edge-free spectra, reveals complex radioactive features in a quantitative manner around each individual target point in the background-dominated environment. Notably, we successfully identify a "micro hot spot" of residual caesium contamination even in an already decontaminated area. These results show that the ETCC performs exactly as the geometrical optics predicts, demonstrates its versatility in the field radiation measurement, and reveals potentials for application in many fields, including the nuclear industry, medical field, and astronomy.

Since the discovery of nuclear gamma-rays, its imaging has been limited to pseudo imaging, such as Compton Camera (CC) and coded mask. Pseudo imaging does not keep physical information (intensity, or brightness in Optics) along a ray, and thus is capable of no more than qualitative imaging of bright objects. To attain quantitative imaging, cameras that realize geometrical optics is essential, which would be, for nuclear MeV gammas, only possible via complete reconstruction of the Compton process. Recently we have revealed that "Electron Tracking Compton Camera" (ETCC) provides a well-defined Point Spread Function (PSF). The information of an incoming gamma is kept along a ray with the PSF and that is equivalent to geometrical optics. Here we present an imaging-spectroscopic measurement with the ETCC. Our results highlight the intrinsic difficulty with CCs in performing accurate imaging, and show that the ETCC surmounts this problem. The imaging capability also helps the ETCC suppress the noise level dramatically by - 3 orders of magnitude without a shielding structure. Furthermore, full reconstruction of Compton process with the ETCC provides spectra free of Compton edges. These results mark the first proper imaging of nuclear gammas based on the genuine geometrical optics.

The measurement of the direction of WIMP-induced nuclear recoils is a compelling but technologically challenging strategy to provide an unambiguous signature of the detection of Galactic dark matter. Most directional detectors aim to reconstruct the dark-matter induced nuclear recoil tracks, either in gas or solid targets. The main challenge with directional detection is the need for high spatial resolution over large volumes, which puts strong requirements on the readout technologies. In this paper we review the various detector readout technologies used by directional detectors. In particular, we summarize the challenges, advantages and drawbacks of each approach, and discuss future prospects for these technologies. (C) 2016 Elsevier B.V. All rights reserved.

The field of MeV gamma-ray astronomy has not opened up until recently owing to imaging difficulties. Compton telescopes and coded-aperture imaging cameras are used as conventional MeV gamma-ray telescopes; however their observations are obstructed by huge background, leading to uncertainty of the point spread function (PSF). Conventional MeV gamma-ray telescopes imaging utilize optimizing algorithms such as the ML-EM method, making it difficult to define the correct PSF, which is the uncertainty of a gamma-ray image on the celestial sphere. Recently, we have defined and evaluated the PSF of an electron-tracking Compton camera (ETCC) and a conventional Compton telescope, and thereby obtained an important result: The PSF strongly depends on the precision of the recoil direction of electron (scatter plane deviation, SPD) and is not equal to the angular resolution measure (ARM). Now, we are constructing a 30 cm-cubic ETCC for a second balloon experiment, Sub-MeV gamma ray Imaging Loaded-on-balloon Experiment: SMILE-II. The current ETCC has an effective area of similar to 1 cm(2) at 300 keV, a PSF of similar to 10 degrees at FWHM for 662 keV, and a large field of view of similar to 3 sr. We will upgrade this ETCC to have an effective area of several cm(2) and a PSF of similar to 5 degrees using a CF4-based gas. Using the upgraded ETCC, our observation plan for SMILE-II is to map of the electron-positron annihilation line and the 1.8 MeV line from Al-26. In this paper, we will report on the current performance of the ETCC and on our observation plan.

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 x 10 x 15 crn(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 arid 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 frorn similar to 10% to similar to 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. (C) 2015 Elsevier B.V. All rights reserved.

Photon imaging for MeV gammas has serious difficulties due to huge backgrounds and unclearness in images, which originate from incompleteness in determining the physical parameters of Compton scattering in detection, e.g., lack of the directional information of the recoil electrons. The recent major mission/instrument in the MeV band, Compton Gamma Ray Observatory/COMPTEL, which was Compton Camera (CC), detected a mere similar to 30 persistent sources. It is in stark contrast with the similar to 2000 sources in the GeV band. Here we report the performance of an Electron-Tracking Compton Camera (ETCC), and prove that it has a good potential to break through this stagnation in MeV gamma-ray astronomy. The ETCC provides all the parameters of Compton-scattering by measuring 3D recoil electron tracks; then the Scatter Plane Deviation (SPD) lost in CCs is recovered. The energy loss rate (dE/dx), which CCs cannot measure, is also obtained, and is found to be helpful to reduce the background under conditions similar to those in space. Accordingly, the significance in gamma detection is improved severalfold. On the other hand, SPD is essential to determine the point-spread function (PSF) quantitatively. The SPD resolution is improved close to the theoretical limit for multiple scattering of recoil electrons. With such a well-determined PSF, we demonstrate for the first time that it is possible to provide reliable sensitivity in Compton imaging without utilizing an optimization algorithm. As such, this study highlights the fundamental weak-points of CCs. In contrast we demonstrate the possibility of ETCC reaching the sensitivity below 1 x 10(-12) erg cm(-2) s(-1) at 1 MeV.

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 [1, 2]. 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 +/- 0.95) x 10(-5) and 5.9 degrees +/- 0.6 degrees, respectively. Furthermore, the ETCC can detect 0.15 mu Sv/h from a Cs-137 gamma-ray source with a significance of 5 sigma in 13min 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.

NEWAGE is a direction-sensitive dark matter search experiment using a micro-time-projection chamber filled with CF4 gas. Following our first underground measurement at Kamioka in 2008, we developed a new detector with improved sensitivity, NEWAGE-0.3b'. NEWAGE-0.3b' has twice the target volume of the previous detector, a lower energy threshold, and an improved data acquisition system. In 2013, a dark matter search was undertaken by NEWAGE-0.3b' in Kamioka underground laboratory. The exposure of 0.327 kg . days achieved a new 90% confidence level direction-sensitive spin-dependent cross-section limit of 557 pb for a 200 GeV/c(2) weakly interacting massive particle. Relative to our first underground measurements, the new direction-sensitive limits are improved by a factor of similar to 10, and are the best achieved to date.

We have developed an electron-tracking Compton camera (ETCC) for use in next-generation MeV gamma ray telescopes. An ETCC consists of a gaseous time projection chamber (TPC) and pixel scintillator arrays (PSAs). Since the TPC measures the three dimensional tracks of Compton-recoil electrons, the ETCC can completely reconstruct the incident gamma rays. Moreover, the ETCC demonstrates efficient background rejection power in Compton-kinematics tests, identifies particle from the energy deposit rate (dE/dX) registered in the TPC, and provides high quality imaging by completely reconstructing the Compton scattering process. We are planning the "Sub-MeV gamma ray Imaging Loaded-on-balloon Experiment" (SMILE) for our proposed all-sky survey satellite. Performance tests of a mid-sized (30 cm) 3 ETCC, constructed for observing the Crab nebula, are ongoing. However, observations at balloon altitudes or satellite orbits are obstructed by radiation background from the atmosphere and the detector itself [1]. The background rejection power was checked using proton accelerator experiments conducted at the Research Center for Nuclear Physics, Osaka University. To create the intense radiation fields encountered in space, which comprise gamma rays, neutrons, protons, and other energetic entities, we irradiated a water target with a 140MeV proton beam and placed a SMILE-II ETCC near the target. In this situation, the counting rate was five times than that expected at the balloon altitude. Nonetheless, the ETCC stably operated and identified particles sufficiently to obtain a clear gamma ray image of the checking source. Here, we report the performance of our detector and demonstrate its effective background rejection based in electron tracking experiments.

Despite the scientific importance of MeV gamma-ray studies, sufficient observations have not been performed due to the large radiation backgrounds and the unclearness of MeV gamma-ray imaging. To advance the MeV gamma-ray astronomy, we have developed an Electron-Tracking Compton Camera (ETCC) with a gaseous electron tracker. By measuring three dimensional tracks of Compton-recoil electrons, our ETCC has attained the high-quality imaging and powerful background rejection. In order to verify such performance of an ETCC, we have carried out the balloon-borne experiments, Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment (SMILE) since 2006. The performance of current ETCC has already been surpassed the requirements to detect the Crab Nebula for 5 sigma level with several hours balloon observations. In addition, the ability of the polarization measurements has been revealed. The modulation factor is estimated to be 0.6 for the energy region below 200 keV by the Monte Carlo simulation. We have measured the polarization ability using the polarized X-ray beam-line at SPring-8, and then modulation factor of 0.6 is obtained at 130 keV, which is consistent with the results of the simulation and shows that the ETCC has an excellent performance as a sub-MeV gamma-ray polarimeter. By using the pressured CF4 based gas at 3 atm, the detection efficiency of the ETCC will be increased one order. Therefore we have a plan of the long duration observation for deep sky survey with polarization measurements of bright sources including Gamma-Ray Bursts. Here we present the concept of ETCC and the future prospects based on the performance of the current ETCC.

The Electron Tracking Compton Camera (ETCC) has the wide energy range (0.1-2 MeV). We upgrade the recoil electron readout circuit, and the tracking algorithm. Due to this improvement, the data acquisition rate is faster than before, and the imaging quality is increased. The performance of ETCC is checked by using the test-tube with F-18 water solution. We have developed the ETCC for new medical imaging device and succeeded in imaging the some imaging reagents. Thus, this detector has the possibility of new medical imaging.

To explore the sub-MeV/MeV gamma-ray window for astronomy, we have developed the Electron-Tracking Compton Camera (ETCC), and carried out the first performance test in laboratory conditions using several gamma-ray sources in the sub-MeV energy band. Using a simple track analysis for a quick first test of the performance, the gamma-ray imaging capability was demonstrated with clear images and 5.3 degrees of angular resolution measure (ARM) measured at 662 keV. As the greatest impact of this work, a gamma-ray detection efficiency on the order of 10(-4) was achieved at the sub-MeV gamma-ray band, which is one order of magnitude higher than our previous experiment. This angular resolution and detection efficiency enables us to detect the Crab Nebula at the 5 sigma level with several hours observation at balloon altitude in middle latitude. Furthermore, good consistency of efficiencies between this performance test and simulation including only physical processes is very important; it means we achieve nearly 100% detection of Compton recoil electrons and means that our predictions of performance enhancement resulting from future upgrades are more realistic. We are planning to confirm the imaging capability of the ETCC by observation of celestial objects in the SMILE-II (Sub-MeV gamma ray Imaging Loaded-on-balloon Experiment II). The SMILE-II and following SMILE-III project will be an important key of sub-MeV/MeV gamma-ray astronomy.

As a next generation MeV gamma-ray telescope, we develop an electron-tracking Compton camera (ETCC) that consists of a gaseous electron tracker surrounded by pixel scintillator arrays. The tracks of the Compton-recoil electron measured by the tracker restrict the incident gamma-ray direction to an arc region on the sky and reject background by using the energy loss rate dE/dx and a Compton-kinematics test. In 2013, we constructed, for a balloon experiment, a 30-cm-cubic ETCC with an effective area of similar to 1 cm(2) for detecting sub-MeV gamma rays (5 sigma detection of the Crab Nebula for 4 h). In future work, we will extend this ETCC to an effective area of similar to 10 cm(2). In the present paper, we report the performance of the current ETCC.

A micro pixel chamber (μ-PIC), the development of which started in 2000 as a type of a micro pattern gas detector, has a high gas gain greater than 6000 in stable operation, a large detection area of 900 cm2, and a fine position resolution of about 120 μm. However, for its development, simulation verification has not been very useful, because conventional simulations explain only part of the experimental data. On the other hand, some μ-PIC applications require precise understanding of the fluctuation of the gas avalanche and signal waveform for their improvement; therefore, there is a need to update the μ-PIC simulation. Hence, we adopted Garfield++, which is developed for simulating a microscopic avalanche in an effort to explain experimental data. The simulated avalanche size was well consistent with the experimental gas gain. Moreover, we calculated a signal waveform and successfully explained the pulse height and time-over-threshold. These results clearly indicate that the simulation of μ-PIC applications will improve and that Garfield++ simulation will easily facilitate the μ-PIC development.© 2013 IOP Publishing Ltd and Sissa Medialab srl.

The Electron Tracking Compton Camera (ETCC) has the wide energy range. We upgrade the recoil electron tracking algorithm to improve the detection efficiency. The performance of ETCC is checked by using F-18 (FDG) which is used for PET imaging. We have developed the ETCC for new medical imaging device and succeeded in imaging the seine imaging reagents. Thus, this detector has the possibility of new medical imaging.

An electron tracking Compton camera (ETCC) has been developed for the application of an image mapping of gamma ray from cesium in environment for the purpose of decontamination work in Fukushima. A first prototype ETCC was built based on established technologies in the MeV/sub-MeV gamma cosmic-ray search in astronomy. After the performance test using a Cs-137 radiation source, the field test was performed at Shirakawa, Fukushima. In this test, fine images of contaminated soil bags were obtained with ETCC. By comparing energy spectra with a background spectrum, we can naively estimate radiation doses from each test sample. These results suggest that we can succeed in imaging of gamma ray from low contaminated soils around 0.02 mu Sv/hr, even if the radiation dose is lower than a background ambient dose of approximately 0.05 mu Sv/hr. After improving ETCC detection efficiency and developing associated software and hardware systems, we expect the ETCC yields to an imaging and dose monitor device to measure contaminated soils in less than 15 min in a radiation dose of approximately 0.2 mu Sv/hr in environment.

For MeV gamma-ray Astronomy, we have developed an Electron Tracking Compton Camera (ETCC) as a next-generation MeV gamma-ray telescope. An ETCC consists of a three-dimensional electron tracker using a gaseous time projection chamber (TPC) and position-sensitive gamma-ray absorbers using pixel scintillator arrays (PSAs). We carried out the balloon borne experiment in 2006 with a small size ETCC and observed successfully the fluxes of the diffuse cosmic and atmospheric gamma rays. As the next flight, we plan to observe bright celestial sources like Crab nebula and have constructed a large size ETCC. To achieve this, an effective area must be larger than 0.5cm(2) for obtaining a 3 sigma level signal for 3 hours observation. To obtain the required sensitivity, we have improved the electron track reconstruction method by updating the track encoding logic and developing a simple track analysis for the new logic. We performed ground-based experiments in the new method using a test model ETCC and measured the detection efficiency, which is found to be 10 times higher than that in the previous method and consistent with the simulation. In addition, the measured angular resolution is improved remarkably. From these results, we expect that a large size ETCC will have more than 3 times better sensitivity than the original design performance.

For MeV gamma ray astronomy, we have developed an Electron Tracking Compton Camera (ETCC) as a MeV gamma ray telescope in the next generation. Our detector consists of a gaseous Time Projection Chamber (TPC), which uses mu-PIC as the two-dimensional readout detector, and a position sensitive scintillation camera. We launched a small size ETCC with a 10 cm x 10 cm x 15 cm TPC loaded on a balloon in 2006, and obtained the fluxes of diffuse cosmic and atmospheric gamma rays (SMILE-I). As the next step of SMILE-I, we have a plan of the next measurement of MeV gamma ray celestial sources like Crab Nebula with a middle size (30 cm)(3) ETCC (SMILE-II) for the test of its imaging property. For SMILE-II, we developed the new Data AcQuisition (DAQ) system of an ETCC to reduce the dead time and power consumption, including the new data acquisition algorithm of electron tracking. The detection efficiency obtained using the new algorithm is about 10 times larger than the one based on the SMILE-I's algorithm. In this record, we report the new SMILE-II ETCC DAQ system and its performances.

For MeV gamma-ray Astronomy, we have developed an Electron Tracking Compton Camera (ETCC) as a next-generation MeV gamma-ray telescope. An ETCC consists of a three-dimensional electron tracker using a gaseous time projection chamber (TPC) and position-sensitive gamma-ray absorbers using pixel scintillator arrays (PSAs). We carried out the balloon borne experiment in 2006 with a small size ETCC and observed successfully the fluxes of the diffuse cosmic and atmospheric gamma rays. As the next flight, we plan to observe bright celestial sources like Crab nebula and have constructed a large size ETCC. To achieve this, an effective area must be larger than 0.5cm 2 for obtaining a 3 sigma level signal for 3 hours observation. To obtain the required sensitivity, we have improved the electron track reconstruction method by updating the track encoding logic and developing a simple track analysis for the new logic. We performed ground-based experiments in the new method using a test model ETCC and measured the detection efficiency, which is found to be 10 times higher than that in the previous method and consistent with the simulation. In addition, the measured angular resolution is improved remarkably. From these results, we expect that a large size ETCC will have more than 3 times better sensitivity than the original design performance. © 2013 IEEE.

An electron tracking Compton camera (ETCC) has been developed for the application of an image mapping of gamma ray from cesium in environment for the purpose of decontamination work in Fukushima. A first prototype ETCC was built based on established technologies in the MeV/sub-MeV gamma cosmic-ray search in astronomy. After the performance test using a 137Cs radiation source, the field test was performed at Shirakawa, Fukushima. In this test, fine images of contaminated soil bags were obtained with ETCC. By comparing energy spectra with a background spectrum, we can naively estimate radiation doses from each test sample. These results suggest that we can succeed in imaging of gamma ray from low contaminated soils around 0.02 μSv/hr, even if the radiation dose is lower than a background ambient dose of approximately 0.05 μSv/hr. After improving ETCC detection efficiency and developing associated software and hardware systems, we expect the ETCC yields to an imaging and dose monitor device to measure contaminated soils in less than 15 min in a radiation dose of approximately 0.2 μSv/hr in environment. © 2013 IEEE.

A silicon photomultiplier (SiPM) is expected to serve as one of the alternatives to PMT in some fields, such as radiology, high energy physics, astroparticle physics, and so on. We studied the basic properties of some different samples of MPPCs for the application to the focal plane camera of the future IACTs. We confirmed that the properties of 3mm× 3 mmMPPCs are consistent with those expected from 1 mm× 1 mm ones which we previously measured. And we verified that the differences of over voltage and temperature dependences of the gain among the different samples, such as different pixel size or different package materials, are little enough, and also that the difference of those characteristics among different channels on the monolithic array is less than 2%.

Observation by the CANGAROO-III stereoscopic system of the Imaging Cherenkov Telescope has detected extended emission of TeV gamma rays in the vicinity of the pulsar PSR B1706-44. The strength of the signal observed as gamma-ray-like events varies when we apply different ways of emulating background events. The reason for such uncertainties is argued in relevance to gamma rays embedded in the "OFF-source data," that is, unknown sources and diffuse emission in the Galactic plane, namely, the existence of a complex structure of TeV gamma-ray emission around PSR B1706-44.