研究者業績
基本情報
- 所属
- 国立研究開発法人宇宙航空研究開発機構 宇宙科学研究所 宇宙物理学研究系 准教授総合研究大学院大学 物理科学研究科 宇宙科学専攻 准教授特定国立研究開発法人理化学研究所 光量子工学研究センター 時空間エンジニアリング研究チーム 客員研究員
- 学位
- 博士(理学)(2001年3月 東京大学)
- 研究者番号
- 50334248
- ORCID ID
https://orcid.org/0000-0003-3529-3029- J-GLOBAL ID
- 200901059420620376
- researchmap会員ID
- 1000367787
- 外部リンク
委員歴
1-
2006年11月 - 2007年10月
論文
248-
Advanced Quantum Technologies 4(8) 2170081-2170081 2021年8月
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Advanced Quantum Technologies 2100015-2100015 2021年5月31日
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Journal of Astronomical Telescopes, Instruments, and Systems 6(04) 2020年12月3日 査読有りUnderstanding and reducing in-orbit instrumental backgrounds are essential to achieving high sensitivity in hard x-ray astronomical observations. The observational data of the Hard X-ray Imager (HXI) onboard the Hitomi satellite provide useful information on the background components due to its multilayer configuration with different atomic numbers: The HXI consists of a stack of four layers of Si (Z = 14) detectors and one layer of cadmium telluride (CdTe) (Z = 48, 52) detector surrounded by well-Type Bi4Ge3O12 active shields. Based on the observational data, the backgrounds of the top Si layer, the three underlying Si layers, and the CdTe layer are inferred to be dominated by different components, namely, low-energy electrons, albedo neutrons, and proton-induced radioactivation, respectively. Monte Carlo simulations of the in-orbit background of the HXI reproduce the observed background spectrum of each layer well, thereby quantitatively verifying the above hypothesis. In addition, we suggest the inclusion of an electron shield to reduce the background.
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 71(4) 2019年8月 査読有り
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NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 924 327-331-331 2019年4月21日 査読有り
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Publications of the Astronomical Society of Japan 70(3) 38 2018年6月 査読有りWe present results from the Hitomi X-ray observation of a young composite-type supernova remnant (SNR) G21.5-0.9, whose emission is dominated by the pulsar wind nebula (PWN) contribution. The X-ray spectra in the 0.8-80 keV range obtained with the Soft X-ray Spectrometer (SXS), Soft X-ray Imager, and Hard X-ray Imager (HXI) show a significant break in the continuum as previously found with the NuSTAR observation. After taking into account all known emissions from the SNR other than the PWN itself, we find that the Hitomi spectra can be fitted with a broken power law with photon indices of 1 = 1.74 ± 0.02 and 2 = 2.14 ± 0.01 below and above the break at 7.1 ± 0.3 keV, which is significantly lower than the NuSTAR result (∼9.0 keV). The spectral break cannot be reproduced by time-dependent particle injection one-zone spectral energy distribution models, which strongly indicates that a more complex emission model is needed, as suggested by recent theoretical models. We also search for narrow emission or absorption lines with the SXS, and perform a timing analysis of PSR J1833-1034 with the HXI and the Soft Gamma-ray Detector. No significant pulsation is found from the pulsar. However, unexpectedly, narrow absorption line features are detected in the SXS data at 4.2345 keV and 9.296 keV with a significance of 3.65 s. While the origin of these features is not understood, their mere detection opens up a new field of research and was only possible with the high resolution, sensitivity, and ability to measure extended sources provided by an X-ray microcalorimeter.
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NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 891 92-105 2018年5月 査読有りHard X-ray astronomical observatories in orbit suffer from a significant amount of background due to radioactivation induced by cosmic-ray protons and/or geomagnetically trapped protons. Within the framework of a full Monte Carlo simulation, we present modeling of in-orbit instrumental background which is dominated by radioactivation. To reduce the computation time required by straightforward simulations of delayed emissions from activated isotopes, we insert a semi-analytical calculation that converts production probabilities of radioactive isotopes by interaction of the primary protons into decay rates at measurement time of all secondary isotopes. Therefore, our simulation method is separated into three steps: (1) simulation of isotope production, (2) semi-analytical conversion to decay rates, and (3) simulation of decays of the isotopes at measurement time. This method is verified by a simple setup that has a CdTe semiconductor detector, and shows a 100-fold improvement in efficiency over the straightforward simulation. To demonstrate its experimental performance, the simulation framework was tested against data measured with a CdTe sensor in the Hard X-ray Imager onboard the Hitomi X-ray Astronomy Satellite, which was put into a low Earth orbit with an altitude of 570 km and an inclination of 31 degrees, and thus experienced a large amount of irradiation from geomagnetically trapped protons during its passages through the South Atlantic Anomaly. The simulation is able to treat full histories of the proton irradiation and multiple measurement windows. The simulation results agree very well with the measured data, showing that the measured background is well described by the combination of proton-induced radioactivation of the CdTe detector itself and thick Bi4Ge3O12 scintillator shields, leakage of cosmic X-ray background and albedo gamma-ray radiation, and emissions from naturally contaminated isotopes in the detector system.
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Journal of Astronomical Telescopes, Instruments, and Systems 4(2) 2018年4月The Hard X-ray Imager (HXI) onboard Hitomi (ASTRO-H) is an imaging spectrometer covering hard x-ray energies of 5 to 80 keV. Combined with the Hard X-ray Telescope, it enables imaging spectroscopy with an angular resolution of 10.7 half-power diameter, in a field of view of 9' × 9'. The main imager is composed of four layers of Si detectors and one layer of CdTe detector, stacked to cover a wide energy band up to 80 keV, surrounded by an active shield made of Bi4Ge3O12 scintillator to reduce the background. The HXI started observations 12 days before the Hitomi loss and successfully obtained data from G21.5-0.9, Crab, and blank sky. Utilizing these data, we calibrate the detector response and study properties of in-orbit background. The observed Crab spectra agree well with a powerlaw model convolved with the detector response, within 5% accuracy. We find that albedo electrons in specified orbit strongly affect the background of the Si top layer and establish a screening method to reduce it. The background level over the full field of view after all the processing and screening is as low as the preflight requirement of 1 - 3 × 10-4 counts s-1 cm-2 keV-1.
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JOURNAL OF ASTRONOMICAL TELESCOPES INSTRUMENTS AND SYSTEMS 4(2) 1-1 2018年4月 査読有り
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Journal of Astronomical Telescopes, Instruments, and Systems 4(2) 021402-1-021402-13 2018年4月 査読有り
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Journal of Astronomical Telescopes, Instruments, and Systems 4(1) 2018年1月1日 査読有り© The Authors 2018. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. The soft x-ray spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch) is a microcalorimeter-type spectrometer, installed in a dewar to be cooled at 50 mK. The energy resolution of the SXS engineering model suffered from microvibration from cryocoolers mounted on the dewar. This is mitigated for the flight model (FM) by introducing vibration isolation systems between the cryocoolers and the dewar. The detector performance of the FM was verified before launch of the spacecraft in both ambient condition and thermal-vacuum condition, showing no detectable degradation in energy resolution. The in-orbit detector spectral performance and cryocooler cooling performance were also consistent with that on ground, indicating that the cryocoolers were not damaged by launch environment. The design and performance of the vibration isolation system along with the mechanism of how the microvibration could degrade the cryogenic detector is shown. Lessons learned from the development to mitigate unexpected issues are also described.
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Journal of Astronomical Telescopes, Instruments, and Systems 4(1) 2018年1月1日 査読有りFast timing capability in x-ray observation of astrophysical objects is one of the key properties for the ASTRO-H (Hitomi) mission. Absolute timing accuracies of 350 or 35 μs are required to achieve nominal scientific goals or to study fast variabilities of specific sources. The satellite carries a GPS receiver to obtain accurate time information, which is distributed from the central onboard computer through the large and complex SpaceWire network. The details of the time system on the hardware and software design are described. In the distribution of the time information, the propagation delays and jitters affect the timing accuracy. Six other items identified within the timing system will also contribute to absolute time error. These error items have been measured and checked on ground to ensure the time error budgets meet the mission requirements. The overall timing performance in combination with hardware performance, software algorithm, and the orbital determination accuracies, etc. under nominal conditions satisfies the mission requirements of 35 μs. This work demonstrates key points for space-use instruments in hardware and software designs and calibration measurements for fine timing accuracy on the order of microseconds for midsized satellites using the SpaceWire (IEEE1355) network.
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Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 912 199-204 2018年 査読有りThis work presents a long-term operation of two stacked CdTe double sided strip detectors that are comparable with the CdTe detectors onboard Hitomi's HXI. The goal of this test is to study the evolution of the spectroscopic performance of the detectors during a one year operation cycle which resembles the in-orbit operation cycle of Hitomi HXI. Crystal defects inside CdTe cause a degradation of the spectroscopic performance (polarization effect) of the crystal which is becoming worse during detector operation. In order to prevent crystal polarization, the detectors are reset (switch-off of the depletion voltage) once a day. Our main investigation was to study if a long-term degradation can occur as a result of incomplete depolarization during the reset. We present the hardware setup and the analytical steps that were used to investigate the detector stability during each day and over the whole testing period. For the anode signals our results show at 60 keV: a daily line drift of (−2.8±0.7) eV/ks while the long-term drift is (−1.5±1.2) eV/day. The degradation of the energy resolution is measured to be (+2.4±0.3) eV/ks FWHM and the loss of efficiency is (−0.29±0.02) %/ks.
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Journal of Spacecraft and Rockets 55(1) 77-84 2018年 査読有りCopyright © 2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. This study presents the evaluation results of the thermal performance for Japan Aerospace Exploration Agency's X-ray astronomy satellite (Hitomi) ASTRO-H mission. Hitomi was successfully launched on 17 February 2016. The thermal design of the spacecraft and scientific instruments was challenging because the high-energy resolution and the sensitivity needed to achieve the desired scientific objectives required small thermal distortion and a strict temperature range, in spite of large heat dissipation. Hitomi housed four telescopes and six detectors. Each detector had its own radiator and heat pipes for heat dissipation. The most essential mission instrument, the soft X-ray spectrometer, had four loop heat pipes to transfer heat from two sets of compressors and the cold heads of two-stage Stirling coolers. Six fans were mounted close to six cryocoolers for ground cooling in the fairing until just before launch. The heat pipes were operated properly in orbit. Two loop heat pipes for the cryocooler compressors continued operating from the time of launch, thereby maintaining cryocooler temperatures within the allowable temperature range. Comparing the thermal analysis results and flight data, the difference between the predicted and measured temperatures was less than 4°C for more than 95% of all measured temperatures. The thermal control system functioned properly in both launch operation and in orbit, and the thermal mathematical model simulated the actual thermal design of Hitomi.
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NATURE 551(7681) 478-+ 2017年11月 査読有りThe metal abundance of the hot plasma that permeates galaxy clusters represents the accumulation of heavy elements produced by billions of supernovae(1). Therefore, X-ray spectroscopy of the intracluster medium provides an opportunity to investigate the nature of supernova explosions integrated over cosmic time. In particular, the abundance of the iron-peak elements (chromium, manganese, iron and nickel) is key to understanding how the progenitors of typical type Ia supernovae evolve and explode(2-6). Recent X-ray studies of the intracluster medium found that the abundance ratios of these elements differ substantially from those seen in the Sun(7-11), suggesting differences between the nature of type Ia supernovae in the clusters and in the Milky Way. However, because the K-shell transition lines of chromium and manganese are weak and those of iron and nickel are very close in photon energy, highresolution spectroscopy is required for an accurate determination of the abundances of these elements. Here we report observations of the Perseus cluster, with statistically significant detections of the resonance emission from chromium, manganese and nickel. Our measurements, combined with the latest atomic models, reveal that these elements have near-solar abundance ratios with respect to iron, in contrast to previous claims. Comparison between our results and modern nucleosynthesis calculations(12-14) disfavours the hypothesis that type Ia supernova progenitors are exclusively white dwarfs with masses well below the Chandrasekhar limit (about 1.4 times the mass of the Sun). The observed abundance pattern of the iron-peak elements can be explained by taking into account a combination of near-and sub-Chandrasekhar-mass type Ia supernova systems, adding to the mounting evidence that both progenitor types make a substantial contribution to cosmic chemical enrichment(5,15,16).
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 69(3) 2017年6月 査読有りWe will review results for gamma-ray bursts (GRBs) and soft gamma repeaters (SGRs), obtained from the Suzaku Wide-band All-sky Monitor (WAM) which operated for about 10 years from 2005 to 2015. The WAM is a BGO (bismuth germanate: Bi4Ge3O12) lateral shield for the Hard X-ray Detector (HXD),used mainly for rejecting its detector background, but it also works as an all-sky monitor for soft gamma-ray transients in the 50-5000 keV range thanks to its large effective area (similar to 600 cm(2) at 1MeV for one detector) and wide field of view (about half of the entire sky). The WAM actually detected more than 1400 GRBs and 300 bursts from SGRs, and this detection number is comparable to that of other GRB-specific instruments. Based on the 10 years of operation, we describe timing and spectral performance for short GRBs, weak GRBs with high redshifts, and time-resolved pulses with good statistics.
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ASTROPHYSICAL JOURNAL LETTERS 837(1) 2017年3月 査読有りX-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified E approximate to 3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the XMM measurement uncertainties for this region, the inconsistency with Hitomi is at a 99% significance for a broad dark matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-n transitions of S XVI (E similar or equal to 3.44 keV rest-frame)-a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment.
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2017 IEEE AEROSPACE CONFERENCE 9905 2017年 査読有りInherently nonlinear vibration isolation systems offer compelling analytical performance advantages over their linear alternates. Verification testing followed by post-test correlation is essential to convincing otherwise conservative project management teams to adopt these technologies for their mission. This paper details a comprehensive test verification program developed and executed for a novel vibration isolation system developed for the Soft X-ray Spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch). More specifically, the dual-stage / thermally conductive / nonlinear vibration isolation system developed for on-orbit amelioration of cryocooler-induced mechanical vibration, needed to undergo ground testing to demonstrate both on-orbit performance and launch induced response level rejection to protect sensitive payload temperature control components. The system, referred to as the VIS, realizes both small signal transmissibility at high frequency and small acceleration and displacement against vibration during launch and ground testing.
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NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 831 235-241 2016年9月 査読有りThe Hard X-ray Imager (HXI) is one of the instruments onboard the ASTRO-H mission [14 to be launched in early 2016. The HXI is the focal plane detector of the hard X-ray reflecting telescope that covers an energy range from 5 to 80 keV. It will execute observations of astronomical objects with a sensitivity for point sources as faint as 1/100,000 of the Crab nebula at > 10 keV. The HXI camera - the imaging part of the HXI - is realized by a hybrid semiconductor detector system that consists of silicon (Si) and cadmium telluride (CdTe) semiconductor detectors. Here, we present the final design of the HXI camera and report on the development of the flight model. The camera is composed of four layers of Double-sided Silicon Strip Detectors (DSSDs) and one layer of CdTe Double -sided Strip Detector (CdTeDSD), each with an imaging area of 32 mm x 32 mm. The strip pitch of the Si and CdTe sensors is 250 pm, and the signals from all 1280 strips are processed by 40 Application Specified Integrated Circuits (ASICs) developed for the HXI. The five layers of sensors are vertically stacked with a 4 mm spacing to increase the detection efficiency. The thickness of the sensors is 0.5 mm for the Si, and 0.75 mm for the CdTe. In this configuration, soft X-ray photons will be absorbed in the Si part, while hard X-ray photons will go through the Si part and will be detected in the CdTe part. The design of the sensor trays, peripheral circuits, power connections, and readout schemes are also described. The flight models of the HXI camera have been manufactured, tested and installed in the HXI instrument and then on the satellite. (C) 2016 Elsevier B.V. All rights reserved.
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NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 831 410-414 2016年9月 査読有りThe hard X-ray Imager and Soft Gamma-ray Detector onboard ASTRO-H demonstrate high sensitivity to hard X-ray (5-80 keV) and soft gamma-rays (60-600 keV), respectively. To reduce the background, both instruments are actively shielded by large, thick Bismuth Germanate scintillators. We have developed the signal processing system of the avalanche photodiode in the BGO active shields and have demonstrated its effectiveness after assembly in the flight model of the HXI/SGD sensor and after integration into the satellite. The energy threshold achieved is about 150 keV and anti-coincidence efficiency for cosmic -ray events is almost 100%. Installed in the BGO active shield, the developed signal processing system successfully reduces the room background level of the main detector. (C) 2016 Elsevier B.V. All rights reserved.
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NATURE 535(7610) 117-+ 2016年7月 査読有りClusters of galaxies are the most massive gravitationally bound objects in the Universe and are still forming. They are thus important probes(1) of cosmological parameters and many astrophysical processes. However, knowledge of the dynamics of the pervasive hot gas, the mass of which is much larger than the combined mass of all the stars in the cluster, is lacking. Such knowledge would enable insights into the injection of mechanical energy by the central supermassive black hole and the use of hydrostatic equilibrium for determining cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50-million-kelvin diffuse hot plasma filling its gravitational potential well. The active galactic nucleus of the central galaxy NGC 1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These bubbles probably induce motions in the intracluster medium and heat the inner gas, preventing runaway radiative cooling-a process known as active galactic nucleus feedback(2-6). Here we report X-ray observations of the core of the Perseus cluster, which reveal a remarkably quiescent atmosphere in which the gas has a line-of-sight velocity dispersion of 164 +/- 10 kilometres per second in the region 30-60 kiloparsecs from the central nucleus. A gradient in the line-of-sight velocity of 150 +/- 70 kilometres per second is found across the 60-kiloparsec image of the cluster core. Turbulent pressure support in the gas is four per cent of the thermodynamic pressure, with large-scale shear at most doubling this estimate. We infer that a total cluster mass determined from hydrostatic equilibrium in a central region would require little correction for turbulent pressure.
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 68 S301-S311 2016年6月 査読有りWe report on the T-90 and T-50 duration distributions and their relations with spectral hardness using 1464 gamma-ray bursts (GRBs), which were observed by the Suzaku Wide-band All-sky Monitor (WAM) from 2005 August 4 to 2010 December 29. The duration distribution is clearly bimodal in three energy ranges (50-120, 120-250, and 250-550 keV), but is unclear in the 550-5000 keV range, probably because of the limited sample size. The WAM durations decrease with energy according to a power-law index of -0.058(-0.034, +0.033). The hardness-duration relation reveals the presence of short-hard and long-soft GRBs. The short: long event ratio tends to be higher with increasing energy. We compared the WAM distribution with ones measured by eight other GRB instruments. The WAM T-90 distribution is very similar to those of INTEGRAL/SPI-ACS and Granat/PHEBUS, and least likely to match the Swift/BAT distribution. The WAM short: long event ratio (0.25:0.75) is much different from Swift/BAT (0.08:0.92), but is almost the same as CGRO/BATSE (0.25:0.75). To explain this difference for BAT, we examined three effects: BAT trigger types, energy dependence of the duration, and detection sensitivity differences between BAT and WAM. As a result, we found that the ratio difference could be explained mainly by energy dependence including soft extended emissions for short GRBs and much better sensitivity for BAT which can detect weak/long GRBs. The reason for the same short: long event ratio for BATSE and WAM was confirmed by calculation using the trigger efficiency curve.
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PHYSICS IN MEDICINE AND BIOLOGY 61(9) 3638-3644 2016年5月 査読有り
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PHYSICAL REVIEW E 93(2) 2016年2月 査読有りThunderclouds can produce bremsstrahlung gamma-ray emission, and sometimes even positrons. At 00:27:00 (UT) on 13 January 2012, an intense burst of gamma rays from a thundercloud was detected by the GROWTH experiment, located in Japan, facing the Sea of Japan. The event started with a sharp gamma-ray flash with a duration of <300 ms coincident with an intracloud discharge, followed by a decaying longer gamma-ray emission lasting for similar to 60 s. The spectrum of this prolonged emission reached similar to 10 MeV, and contained a distinct line emission at 508 +/- 3(stat.) +/- 5(sys.) keV, to be identified with an electron-positron annihilation line. The line was narrow within the instrumental energy resolution (similar to 80 keV), and contained 520 +/- 50 photons which amounted to similar to 10% of the total signal photons of 5340 +/- 190 detected over 0.1-10 MeV. As a result, the line equivalent width reached 280 +/- 40 keV, which implies a nontrivial result. The result suggests that a downward positron beam produced both the continuum and the line photons.
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Proceedings of SPIE - The International Society for Optical Engineering 9905 2016年 査読有り© 2016 SPIE. The Hitomi (ASTRO-H) mission is the sixth Japanese X-ray astronomy satellite developed by a large international collaboration, including Japan, USA, Canada, and Europe. The mission aimed to provide the highest energy resolution ever achieved at E > 2 keV, using a microcalorimeter instrument, and to cover a wide energy range spanning four decades in energy from soft X-rays to gamma-rays. After a successful launch on 2016 February 17, the spacecraft lost its function on 2016 March 26, but the commissioning phase for about a month provided valuable information on the on-board instruments and the spacecraft system, including astrophysical results obtained from first light observations. The paper describes the Hitomi (ASTRO-H) mission, its capabilities, the initial operation, and the instruments/spacecraft performances confirmed during the commissioning operations for about a month.
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SPACE TELESCOPES AND INSTRUMENTATION 2016: ULTRAVIOLET TO GAMMA RAY 9905 2016年 査読有りThe Soft Gamma-ray Detector (SGD) is one of science instruments onboard ASTRO-H (Hitomi) and features a wide energy band of 60-600 keV with low backgrounds. SGD is an instrument with a novel concept of "Narrow field-of-view" Compton camera where Compton kinematics is utilized to reject backgrounds which are inconsistent with the field-of-view defined by the active shield. After several years of developments, the flight hardware was fabricated and subjected to subsystem tests and satellite system tests. After a successful ASTRO-H (Hitomi) launch on February 17, 2016 and a critical phase operation of satellite and SGD in-orbit commissioning, the SGD operation was moved to the nominal observation mode on March 24, 2016. The Compton cameras and BGO-APD shields of SGD worked properly as designed. On March 25, 2016, the Crab nebula observation was performed, and, the observation data was successfully obtained.
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SPACE TELESCOPES AND INSTRUMENTATION 2016: ULTRAVIOLET TO GAMMA RAY 9905 2016年 査読有りHitomi X-ray observatory launched in 17 February 2016 had a hard X-ray imaging spectroscopy system made of two hard X-ray imagers (HXIs) coupled with two hard X-ray telescopes (HXTs). With 12 m focal length, they provide fine (2' half-power diameter; HPD) imaging spectroscopy at 5 to 80 keV. The HXI main imagers are made of 4 layers of Si and a CdTe semiconductor double-sided strip detectors, stacked to enhance detection efficiency as well as to enable photon interaction-depth sensing. Active shield made of 9 BGO scintillators surrounds the imager to provide with low background. Following the deployment of the Extensible Optical Bench (EOB) on 28 February, the HXI was gradually turned on. Two imagers successfully started observation on 14 March, and was operational till the incident lead to Hitomo loss, on 26 March. All detector channels, 1280 ch of imager and 11 channel of active shields and others each, worked well and showed performance consistent with those seen on ground. From the first light observation of G21.5-0.9 and the following Crab observations, 5 80 keV energy coverage and good detection efficiency were confirmed. With blank sky observations, we checked our background level. In some geomagnetic region, strong background continuum, presumably caused by trapped electron with energy similar to 100 keV, is seen. But by cutting the high-background time-intervals, the background became significantly lower, typically with 1-3x10(-4) counts s(-1) keV(-1) cm(-2) (here cm(2) is shown with detector geometrical area). Above 30 keV, line and continuum emission originating from activation of CdTe was significantly seen, though the level of 1-4x10(-4) counts s(-1) keV(-1) cm(-2) is still comparable to those seen in NuSTAR. By comparing the effective area and background rate, preliminary analysis shows that the HXI had a statistical sensitivity similar to NuSTAR for point sources, and more than twice better for largely extended sources.
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Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XVIII 9968 2016年 査読有りWe studied a surface effect of Double-sided Si Strip Detectors (DSSDs) in order to apply it for imaging spectroscopy of X-ray photons down to 5 keV for the first time. The Japanese cosmic X-ray satellite Hitomi, launched in February 2016, is equipped with the Hard X-ray Imager (HXI), which employs the DSSDs in 5-80 keV. In such a low energy band, the surface effect is non-negligible. When interstrip regions of p-side are irradiated, the DSSD sometimes show signals with negative pulse heights, presumably caused by positive surface charges between Si and Si0(2) layers.(1-5) The effect modifies the X-ray response of the HXI towards its low-energy end, below 10 keV. By irradiating the DSSD with uncollimated mono-energetic X-rays of different energies, we measured the fraction of the negative events to be 2% at 26.4 keV and 30% at 6.0 keV. Using an 8 keV collimated X-ray beam, we directly verified that the negative events originated from the interstrip gaps on the p-side where the Si02 layers exist. The measured energy-and position-dependences can be modeled by assuming that the negative events are produced in approximately 25 pm deep and 120 pm wide interstrip regions. When the bias voltage are halved (from 350 V to 180 V), fraction of the negative events increased by a factor of similar to 1.7, qualitatively consistent with this picture.
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Vibration isolation system for cryocoolers of Soft X-ray Spectrometer (SXS) onboard ASTRO-H (Hitomi)SPACE TELESCOPES AND INSTRUMENTATION 2016: ULTRAVIOLET TO GAMMA RAY 9905 2016年 査読有りSoft X-ray Spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch) is a microcalorimeter-type spectrometer, installed in a dewar to be cooled at 50 mK. The energy resolution of the SXS engineering model suffered from micro-vibration from cryocoolers mounted on the dewar. This is mitigated for the flight model by introducing vibration isolation systems between the cryocoolers and the dewar. The detector performance of the flight model was verified before launch of the spacecraft in both ambient condition and thermal-vac condition, showing no detectable degradation in energy resolution. The in-orbit performance was also consistent with that on ground, indicating that the cryocoolers were not damaged by launch environment. The design and performance of the vibration isolation system along with the mechanism of how the micro-vibration could degrade the cryogenic detector is shown.
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Transactions of Japanese Society for Medical and Biological Engineering 52 496-496 2014年8月17日 査読有りWe are currently developing a Compton camera apparatus for use as a novel imaging modality of nuclear medicine. It is based on the advanced technology used in Si/CdTe semiconductor Compton cameras developed by the Japan Aerospace Exploration Agency. In this study, the influence of scattered gamma rays by body trunk on image quality was evaluated using the developed Compton camera and a NEMA IEC body phantom. Na-22, Co-57 or Ba-133 gamma point source, which emit 511 keV, 122 keV or 356 keV respectively, was placed at the center of the body phantom. We compared the Compton images taken with the empty phantom and taken with the phantom filled with water. The image quality was degraded notably although the position resolution was not decreased. This is thought to be aftereffects of the reduction of the number of Compton events, broadening of the energy spectral region and the increase of the background.
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SPACE TELESCOPES AND INSTRUMENTATION 2014: ULTRAVIOLET TO GAMMA RAY 9144 2014年 査読有りThe 6th Japanese X-ray satellite, ASTRO-H, is scheduled for launch in 2015. The hard X-ray focusing imaging system will observe astronomical objects with the sensitivity for detecting point sources with a brightness of 1/100,000 times fainter than the Crab nebula at > 10 keV. The Hard X-ray Imager (HXI) is a focal plane detector 12 m below the hard X-ray telescope (HXT) covering the energy range from 5 to 80 keV. The HXI is composed of a stacked Si/CdTe semiconductor detector module and surrounding BGO scintillators. The latter work as active shields for efficient reduction of background events caused by cosmic-ray particles, cosmic X-ray background, and in-orbit radiation activation. In this paper, we describe the detector system, and present current status of flight model development, and performance of HXI using an engineering model of HXI.
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PROCEEDINGS OF THE 2014 6TH INTERNATIONAL SPACEWIRE CONFERENCE (SPACEWIRE) 2014年 査読有りConventional protocols have been integrated with SpaceWire through service oriented approach with reference to SPACECRAFT ONBOARD INTERFACE SERVICES (SOIS). The design framework is based on the definition of determinism provided by SpaceWire-D draft standard in order to keep established services inherited from previous satellite projects. The implementation result is under evaluation in order to establish the consistency with the draft standard of SpaceWire - Plug-and-play protocol. This paper describes the integration approach and the evaluation of implementation experience.
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SPACE TELESCOPES AND INSTRUMENTATION 2014: ULTRAVIOLET TO GAMMA RAY 9144 2014年 査読有りThe 6th Japanese X-ray satellite, ASTRO-H, is scheduled for launch in 2015. The hard X-ray focusing imaging system will observe astronomical objects with the sensitivity for detecting point sources with a brightness of 1/100,000 times fainter than the Crab nebula at > 10 keV. The Hard X-ray Imager (HXI) is a focal plane detector 12 m below the hard X-ray telescope (HXT) covering the energy range from 5 to 80 keV. The HXI is composed of a stacked Si/CdTe semiconductor detector module and surrounding BGO scintillators. The latter work as active shields for efficient reduction of background events caused by cosmic-ray particles, cosmic X-ray background, and in-orbit radiation activation. In this paper, we describe the detector system, and present current status of flight model development, and performance of HXI using an engineering model of HXI.
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SPACE TELESCOPES AND INSTRUMENTATION 2014: ULTRAVIOLET TO GAMMA RAY 9144 2014年 査読有りThe Hard X-ray Imager and the Soft Gamma-ray Detector, onboard the 6th Japanese X-ray satellite ASTRO-H, aim at unprecedentedly-sensitive observations in the 5-80 keV and 40-600 keV bands, respectively. Because their main sensors are composed of a number of semi-conductor devices, which need to be operated in a temperature of -20 to -15 degrees C, heat generated in the sensors must be efficiently transported outwards by thermal conduction. For this purpose, we performed thermal design, with the following three steps. First, we additionally included thermally-conductive parts, copper poles and graphite sheets. Second, constructing a thermal mathematical model of the sensors, we estimated temperature distributions in thermal equilibria. Since the model had rather large uncertainties in contact thermal conductions, an accurate thermal dummy was constructed as our final step. Vacuum measurement with the dummy successfully reduced the conductance uncertainties. With these steps, we confirmed that our thermal design of the main sensors satisfies the temperature requirement.
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SPACE TELESCOPES AND INSTRUMENTATION 2014: ULTRAVIOLET TO GAMMA RAY 9144 2014年 査読有りRecent progress in wide field of view or all-sky observations such as Swift/BAT hard X-ray monitor and Fermi GeV gamma-ray observatory has opened up a new era of time-domain high energy astro-physics addressing new insight in, e.g., particle acceleration in the universe. MeV coverage with comparable sensitivity, i.e. 1 similar to 10 mCrab is missing and a new MeV all-sky observatory is needed. These new MeV mission tend to be large, power-consuming and hence expensive, and its realization is yet to come. A compact sub-MeV (0.2-2 MeV) all-sky mission is proposed as a path finder for such mission. It is based on a Si/CdTe semiconductor Compton telescope technology employed in the soft gamma-ray detector onboard ASTRO-H, to be launched in to orbit on late 2015. The mission is kept as small as 0.5 x 0.5 x 0.4 m(3), 150 kg in weight and 200 W in power in place of the band coverage above a few MeV, in favor of early realization as a sub-payload to other large platforms, such as the international space station.
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Proceedings of SPIE - The International Society for Optical Engineering 9144 2014年 査読有り© 2014 SPIE. The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions developed by the Institute of Space and Astronautical Science (ISAS), with a planned launch in 2015. The ASTRO-H mission is equipped with a suite of sensitive instruments with the highest energy resolution ever achieved at E > 3 keV and a wide energy range spanning four decades in energy from soft X-rays to gamma-rays. The simultaneous broad band pass, coupled with the high spectral resolution of ΔE ≤ 7 eV of the micro-calorimeter, will enable a wide variety of important science themes to be pursued. ASTRO-H is expected to provide breakthrough results in scientific areas as diverse as the large-scale structure of the Universe and its evolution, the behavior of matter in the gravitational strong field regime, the physical conditions in sites of cosmic-ray acceleration, and the distribution of dark matter in galaxy clusters at different redshifts.
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SPACE TELESCOPES AND INSTRUMENTATION 2014: ULTRAVIOLET TO GAMMA RAY 9144 2014年 査読有りThe Soft Gamma-ray Detector (SGD) is one of observational instruments onboard the ASTRO-H, and will provide 10 times better sensitivity in 60-600 keV than the past and current observatories. The SGD utilizes similar technologies to the Hard X-ray Imager (HXI) onboard the ASTRO-H. The SGD achieves low background by constraining gamma-ray events within a narrow field-of-view by Compton kinematics, in addition to the BGO active shield. In this paper, we will present the results of various tests using engineering models and also report the flight model production and evaluations.
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SPACE TELESCOPES AND INSTRUMENTATION 2014: ULTRAVIOLET TO GAMMA RAY 9144 2014年 査読有りThe hard X-ray imager (HXI) and soft gamma-ray detector (SGD) onboard Astro-H observe astronomical objects with high sensitivity in the hard X-ray (5-80 keV) and soft gamma-ray (40-600 keV) energy bands. To achieve this high sensitivity, background rejection is essential, and these detectors are surrounded by large and thick bismuth germinate scintillators as an active shield. We have developed adequate trigger logic for both the HXI and SGD to process signals from main detector and BGO shield simultaneously and then we optimized the trigger delay and width, with consideration of the trigger latch efficiency. The shield detector system performs well, even after it is assembled as the HXI sensor. The energy threshold maintains the same level as that observed during the prototype development phase, and the experimental room background level of the main detector is successfully reduced by our optimized trigger timing.
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2014 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE (NSS/MIC) 2014年 査読有り
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The 7th KOREA-JAPAN Joint Meeting on Medical Physics 63 2014年 査読有り
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Proceedings of 33rd International Cosmic Ray Conference 2013-October 2013年7月12日
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Physical Review Letters 111(1) 2013年7月1日 査読有り
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Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 699 225-229 2013年1月21日 査読有りHard X-ray Imager (HXI) and Soft Gamma-ray Detector (SGD) onboard the 6th Japanese X-ray satellite, ASTRO-H, utilize double-sided silicon strip detectors (DSSD) and pixel array-type silicon sensors (Si-pad), respectively. The DSSD with a 3.4 cm×3.4 cm area has an imaging capability in the lower energy band for the HXI covering 5-80 keV. The Si-pad consists of 16×16 pixels with a 5.4 cm×5.4 cm area and measures a photon direction with the Compton kinematics in 10-600 keV. Since the ASTRO-H will be operated in a low earth orbit, these detectors will be damaged by irradiation of cosmic-ray protons mainly in the South Atlantic Anomaly. In order to evaluate damage effects of the sensors, we have carried out irradiation tests with 150 MeV proton beams and 60Co gamma-rays with a total dose of 10-20 years irradiation level. In both experiments, the leakage current has increased by ∼0.2 - 1.1nA/ cm2 under an expected operation temperature at -15 °C, which resulted in the noise level within a tolerance of 20 years. In this report, we present a summary of the basic performance of silicon detectors, and radiation effects on them by the irradiation tests. © 2012 Elsevier B.V.
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NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 699 112-115 2013年1月 査読有りAstro-H is the sixth Japanese X-ray space observatory which will be launched in 2014. Two of onboard instruments of Astro-H, Hard X-ray Imager and Soft Gamma-ray Detector are surrounded by many number of large Bismuth Germanate (Bi4Ge3O12; BGO) scintillators. Optimum readout system of scintillation lights from these BGOs are essential to reduce the background signals and achieve high performance for main detectors because most of gamma-rays from out of field-of-view of main detectors or radio-isotopes produced inside them due to activation can be eliminated by anti-coincidence technique using BGO signals. We apply Avalanche Photo Diode (APD) for light sensor of these BGO detectors since their compactness and high quantum efficiency make it easy to design such large number of BGO detector system. For signal processing from APDs, digital filter and other trigger logics on the Field-Programmable Gate Array (FPGA) is used instead of discrete analog circuits due to limitation of circuit implementation area on spacecraft. For efficient observations, we have to achieve as low threshold of anti-coincidence signal as possible by utilizing the digital filtering. In addition, such anti-coincident signals should be sent to the main detector within 5 mu s is to make it in time to veto the AD conversion. Considering this requirement and constraint from logic size of FPGA, we adopt two types of filter, 8 delay taps filter with only 2 bit precision coefficient and 16 delay taps filter with 8 bit precision coefficient. The data after former simple filter provides anti-coincidence signal quickly in orbit, and the latter filter is used for detail analysis after the data is down-linked. (C) 2012 Elsevier B.V. All rights reserved.
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2013 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE (NSS/MIC) 2013年 査読有りWe studied a new method to monitor a beam range in heavy-ion radiation therapy by measuring low energy photons emitted from a track of the ion beam. A 290 MeV/u carbon beam was injected into a cylindrical water phantom. A CdTe semiconductor detector with a lead slit having a width of 2mm was placed at a side of the phantom. In order to measure the position dependence of the low energy photon count, the beam range was varied by changing the injection energy using a binary energy degrader placed about 100 cm upstream of the beam focal point. The measured photon count decreased when the detector got closer to the end point of the beam range and the derivative of the photon count clearly changed in front of the range position. This was explained by our theoretical study assuming the photons were secondary electron bremsstrahlung. These results indicate that this new method could estimate the range position from the observation of bremsstrahlung with an accuracy of a few mm.
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NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 699 230-234 2013年1月 査読有りHard X-ray Imager and Soft Gamma-ray Detector are being developed as onboard instruments for the Astro-H mission, which is scheduled for launch in 2014. In both detectors, EGO scintillators play key roles in achieving high sensitivity in low Earth orbit (LEO), by generating active veto signals to reject cosmic-ray events and gamma-ray backgrounds from radio-activated detector materials. In order to maximize background rejection power, it is also important to minimize the energy threshold of this shield. As a readout sensor of weak scintillation light from a number of EGO crystals in a complicated detector system, high performance, reverse-type Avalanche Photodiodes (APDs), with an effective area of 10 x 10 mm(2) are being employed, instead of bulky photomultiplier tubes (PMTs). Another advantage of using APDs is their low power consumption, although the relatively low gain of APDs (compared to conventional PMTs) requires dedicated analog circuits for noise suppression. In this paper, we report on the development and performance of APD detectors specifically designed for the Astro-H mission. In addition to APD performance, various environmental tests, including radiation hardness and qualification thermal cycling, will be described in detail. Moreover, a dedicated charge sensitive amplifier and analog filters are newly developed and tested here to optimize the performance of APDs to activate fast veto signals within a few is from the EGO trigger. We will also report on overall performance testing of a prototype EGO detector system that mimics the data acquisition system onboard Astro-H. (C) 2012 Elsevier B.V. All rights reserved.
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IEEE Nuclear Science Symposium Conference Record 2013年 査読有りImaging technique of RI tracer for physiological function analysis is substantially useful and popular as positron emission tomography (PET), positron-emitting tracer imaging system (PETIS) or single photon emission computed tomography (SPECT). Recently, Compton camera is thought to become a promising imaging apparatus in several field, for example, medical, biological application and security inspection, because of its simultaneous imaging ability against the wide energy range gamma-rays from a few hundred keV to a few MeV. In this work, three-dimensional imaging ability of our Compton camera system developed for human body imaging has been studied by use of Monte Carlo simulation. The imaging system consists of plural Compton-camera head-modules having the diameter of 20 cm and height of 20 cm. Each of head modules has a 32 mm wide Si double-side strip detector (DSD) and four CdTe-DSDs stacked at intervals of 4 mm. The head modules are settled on XYZ-stages and can be moved by XYZ-stage around the subject imaged during data acquisition. The simulation study was performed by use of a Geant4-based Compton-camera simulator developed by JAXA/ISAS. Three sphere-shaped gamma-sources, having diameters of 10, 13 and 17 mm, of the NEMA NU2-2007 standard body-phantom was assumed to be placed around the center of simulation space. The RI density is uniform and the energy of gamma ray 511 keV. Six camera-placements around the back and a side of the body phantom. Three-dimensional image was reconstructed by use of the List-Mode Expectation-Maximizing Maximum-Likelihood method. The positions of each sphere were clearly identified at the correct positions. The RI distributions of the imaging result are not asymmetrical between X- and Z-direction, which is found to be owing to the asymmetrical placement of the detector stacks. The RI intensities deduced from the image tend to reflect the real RI intensities although improvement is still required. © 2013 IEEE.
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IEEE Nuclear Science Symposium Conference Record 2013年 査読有りWe studied a new method to monitor a beam range in heavy-ion radiation therapy by measuring low energy photons emitted from a track of the ion beam. A 290 MeV/u carbon beam was injected into a cylindrical water phantom. A CdTe semiconductor detector with a lead slit having a width of 2mm was placed at a side of the phantom. In order to measure the position dependence of the low energy photon count, the beam range was varied by changing the injection energy using a binary energy degrader placed about 100 cm upstream of the beam focal point. The measured photon count decreased when the detector got closer to the end point of the beam range and the derivative of the photon count clearly changed in front of the range position. This was explained by our theoretical study assuming the photons were secondary electron bremsstrahlung. These results indicate that this new method could estimate the range position from the observation of bremsstrahlung with an accuracy of a few mm. © 2013 IEEE.
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NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 695 179-183 2012年12月 査読有りWe have developed a new Compton camera based on silicon (Si) and cadmium telluride (CdTe) semiconductor double-sided strip detectors (DSDs). The camera consists of a 500-mu m-thick Si-DSD and four layers of 750-mu m-thick CdTe-DSDs all of which have common electrode configuration segmented into 128 strips on each side with pitches of 250 mu m. In order to realize high angular resolution and to reduce size of the detector system, a stack of DSDs with short stack pitches of 4 mm is utilized to make the camera. Taking advantage of the excellent energy and position resolutions of the semiconductor devices, the camera achieves high angular resolutions of 4.5 degrees at 356 key and 3.5 degrees at 662 keV. To obtain such high resolutions together with an acceptable detection efficiency, we demonstrate data reduction methods including energy calibration using Compton scattering continuum and depth sensing in the CdTe-DSD. We also discuss imaging capability of the camera and show simultaneous multi-energy imaging. (C) 2012 Elsevier B.V. All rights reserved.
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JOURNAL OF INSTRUMENTATION 7(6) 2012年6月 査読有りScheduled for launch in 2014, Astro-H is the sixth Japanese X-ray astronomy satellite mission. More than 60 silicon avalanche photodiodes (Si-APDs; hereafter APDs) will be used to read out BGO scintillators, which are implemented to generate a veto signal to reduce background contamination for the hard X-ray imager (HXI) and a soft gamma-ray detector (SGD). To date, however, APDs have rarely been used in space experiments. Moreover, strict environmental tests are necessary to guarantee APD performance for missions expected to extend beyond five years. The radiation hardness of APDs, as for most semiconductors, is particularly crucial, since radiation in the space environment is severe. In this paper, we present the results of radiation tests conducted on reverse-type APDs (provided by Hamamatsu Photonics) irradiated by gamma rays (Co-60) and 150 MeV protons. We show that, even under the same 100 Gy dose, high energy protons can cause displacement (bulk) damage in the depletion region and possibly change the activation energy, whereas gamma-ray irradiation is less prone to cause damage, because ionization damage dominates only the surface region. We also present quantitative guidance on how to estimate APD noise deterioration over a range of temperatures and radiation doses. As a practical example, we discuss the expected degradation of the BGO energy threshold for the generation of veto signals, following several years of Astro-H operation in Low Earth Orbit (LEO), and directly compare it to experimental results obtained using a small BGO crystal.
MISC
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JOURNAL OF ASTRONOMICAL TELESCOPES INSTRUMENTS AND SYSTEMS 4(2) 2018年4月 査読有り
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日本物理学会講演概要集(CD-ROM) 73(1) ROMBUNNO.25pK307‐5-492 2018年3月23日
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