渡辺 伸

We are developing a semiconductor Compton telescope to explore the universe in the energy band from several tens of keV to a few MeV. A detector material of combined Si strip and CdTe pixel is used to cover the energy range around 60 keV. For energies above several hundred keV, in contrast, the higher detection efficiency of CdTe semiconductor in comparison with Si is expected to play an important role as both an absorber and a scatterer. In order to demonstrate the spectral and imaging capability of a CdTe-based Compton camera, we developed a Compton telescope consisting of a stack of CdTe pixel detectors as a small scale prototype. With this prototype, we succeeded in reconstructing images and spectra by solving the Compton kinematics within the energy band from 122 to 662 keV. The energy resolution (FWHM) of reconstructed spectra is 7.3 keV at 511 keV. The angular resolution obtained at 511 keV is measured to be 12 . 2{ring operator} (FWHM). © 2006.

High-sensitivity wide-band X-ray spectroscopy is the key feature of the Suzaku X-ray observatory, launched on 2005 July 10. This paper summarizes the spacecraft, in-orbit performance, operations, and data processing that are related to observations. The scientific instruments, the high-throughput X-ray telescopes, X-ray CCD cameras, non-imaging hard X-ray detector are also described. © 2007. Astronomical Society of Japan.

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10keV to 600 keV by the combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit background based on a combination of new techniques, including the concept of a well-type active shield counter. With an effective area of 142cm2 at 20keV and 273cm2 at 150keV, the background level at sea level reached ∼ 1 × 10-5 cts s-1 cm-2keV -1 at 30 keV for the PIN diodes, and ∼ 2 × 10-5 cts s-1 cm-2keV-1 at 100keV, and ∼ 7 × 10-6 cts s-1 cm-2keV-1 at 200keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ∼ 4cm thick BGO crystals, have a large effective area for sub-MeV to MeV x-rays. They work as an excellent y-ray burst monitor with limited angular resolution (∼ 5°). The on-board signal-processing system and the data transmitted to the ground are also described. © 2007. Astronomical Society of Japan.

SN 1006 is the milestone of understanding the acceleration mechanism of cosmic rays, and this year is the millennium year for the remnant. We carried out SN 1006 mapping observations with the X-ray Imaging Spectrometers (XIS) onboard Suzaku satellite. Thanks to the excellent spectral response of XIS, K emission lines from highly ionized oxygen were clearly resolved. The intensity maps of these lines have been made additional to the intensity and photon index maps of the nonthermal component. We discovered that regions with strong and hard nonthermal component has weak thermal emission. The north rim of the SNR has the hardest nonthermal component. These facts might have information of efficient cosmic ray acceleration and background plasma.

We report results from a long observation of MCG-6-30-15 with Suzaku. Thanks to its unprecedented sensitivity in the hard band, we could, for the first time, investigate the hard X-ray variability of the source. The variability in the HXD/PIN 14-45 keV band (dominated by a strong reflection hump) is suppressed relative to that at a few keV. This directly demonstrates that the whole reflection spectrum is much less variable than the power-law continuum. The broadband spectral variability can be decomposed into two components - a highly variable power-law and constant reflection - as previously inferred from other observations in the 2-10 keV band. The strong reflection and high iron abundance give rise to a strong broad iron line, which requires the inner disc radius to be at about 2 gravitational radii, confirming that MCG-6-30-15 most likely harbours a rapidly spinning Kerr black hole.

We report on results from Suzaku broadband X-ray observations of the Galactic supernova remnant (SNR) RX J1713.7-3946 with an energy coverage of 0.4-40 keV. With a sensitive hard X-ray measurement from the HXD PIN on board Suzaku, we determine the hard X-ray spectrum in the 12-40 keV range to be described by a power law with photon index Γ = 3.2 ± 0.2, significantly steeper than the soft X-ray index of Γ = 2.4 ± 0.05 measured previously with ASCA and other missions. We find that a simple power law fails to describe the full spectral range of 0.4-40 keV and instead a cutoff power law with hard index Γ = 1.96 ± 0.05 and high energy cutoff εc = 9 ± 1 keV provides an excellent fit over the full bandpass.

Broad line radio galaxies (BLRGs) are a rare type of radio-loud AGN, in which the broad optical permitted emission lines have been detected in addition to the extended jet emission. Here we report on deep (40ksec x4) observations of the bright BLRG 3C~120 using Suzaku. The observations were spaced a week apart, and sample a range of continuum fluxes. An excellent broadband spectrum was obtained over two decades of frequency (0.6 to 50 keV) within each 40 ksec exposure. We clearly resolved the iron K emission line complex, finding that it consists of a narrow K_a core (sigma ~ 110 eV or an EW of 60 eV), a 6.9 keV line, and an underlying broad iron line. Our confirmation of the broad line contrasts with the XMM-Newton observation in 2003, where the broad line was not required. The most natural interpretation of the broad line is iron K line emission from a face-on accretion disk which is truncated at ~10 r_g. Above 10 keV, a relatively weak Compton hump was detected (reflection fraction of R ~ 0.6), superposed on the primary X-ray continuum of Gamma ~ 1.75. Thanks to the good photon statistics and low background of the Suzaku data, we clearly confirm the spectral evolution of 3C120, whereby the variability amplitude decreases with increasing energy. More strikingly, we discovered that the variability is caused by a steep power-law component of Gamma ~2.7, possibly related to the non-thermal jet emission. We discuss our findings in the context of similarities and differences between radio-loud/quiet objects.

We are developing a Compton telescope based on high-resolution Si and CdTe detectors for astrophysical observations in sub-MeV/MeV gamma-ray region. Recently, we constructed a prototype Compton telescope which consists of six layers of double-sided Si strip detectors (DSSDs) and CdTe pixel detectors to demonstrate the basic performance of this new technology. By irradiating the detector with gamma rays from radio isotope sources, we have succeeded in Compton reconstruction of images and spectra. The obtained angular resolution is 3.9°(FWHM) at 511 keV, and the energy resolution is 14 keV (FWHM) at the same energy. In addition to the conventional Compton reconstruction, i.e., drawing cones in the sky, we also demonstrated a full reconstruction by tracking Compton recoil electrons using the signals detected in successive Si layers. By irradiating 137Cs source, we successfully obtained an image and a spectrum of 662 keV line emission with this method. As a next step, development of larger DSSDs with a size of 4 cm × 4 cm is under way to improve the effective area of the Compton telescope. We are also developing a new low-noise analog ASIC to handle the increasing number of channels. Initial results from these two new technologies are presented in this paper as well. © 2006 Elsevier B.V. All rights reserved.

We report our development of CdTe pixel detectors for Si/CdTe semiconductor Compton cameras. We have constructed a prototype of a Si/CdTe Compton camera, consisting of six layered double-sided silicon strip detectors and three CdTe pixel detectors. By using this prototype, we have demonstrated the concept of the Si/CdTe Compton cameras. We have successfully obtained Compton reconstructed images for 80-662 keV gamma-rays. The achieved angular resolution is about 4 degrees for 511 keV gamma-rays. The energy resolution is 14 keV (FWHM) at 511 keV. In order to improve the performance of the Compton camera, we have evaluated all CdTe pixel detectors we have constructed. We found that the I-V curve is helpful to select good detectors. © 2006 Elsevier B.V. All rights reserved.

We report on a 100 ks Suzaku observation of the bright, nearby (z=0.008486) Seyfert 1.9 galaxy MCG -5-23-16. The broad-band (0.4-100 keV) X-ray spectrum allows us to determine the nature of the high energy emission with little ambiguity. The X-ray continuum consists of a cutoff power-law of photon index $\Gamma=1.9$, absorbed through Compton-thin matter of column density $N_{\rm H}=1.6\times10^{22}$ cm$^{-2}$. A soft excess is observed below 1 keV and is likely a combination of emission from scattered continuum photons and distant photoionized gas. The iron K line profile is complex, showing narrow neutral iron K$\alpha$ and K$\beta$ emission, as well as a broad line which can be modeled by a moderately inclined accretion disk. The line profile shows either the disk is truncated at a few tens of gravitational radii, or the disk emissivity profile is relatively flat. A strong Compton reflection component is detected above 10 keV, which is best modeled by a combination of reflection off distant matter and the accretion disk. The reflection component does not appear to vary. The overall picture is that this Seyfert 1.9 galaxy is viewed at moderate (50 degrees) inclination through Compton-thin matter at the edge of a Compton-thick torus covering $2\pi$ steradians, consistent with unified models.

Suzaku has, for the first time, enabled the hard X-ray variability of the Seyfert 1 galaxy MCG-6-30-15 to be measured. The variability in the 14-45 keV band, which is dominated by a strong reflection hump, is quenched relative to that at a few keV. This directly demonstrates that the whole reflection spectrum is much less variable than the power-law continuum. The broadband spectral variability can be decomposed into two components - a highly variable power-law and constant reflection - as previously inferred from other observations in the 2-10 keV band. The strong reflection and high iron abundance give rise to a strong broad iron line, which requires the inner disc radius to be at about 2 gravitational radii. Our results are consistent with the predictions of the light bending model which invokes the very strong gravitational effects expected very close to a rapidly spinning black hole.

The binary X-ray pulsar A0535 + 262 was observed with the Suzaku X-ray observatory on 2005 September 14 for a net exposure of 22 ks. The source was in the declining phase of a minor outburst, exhibiting 3-50 keV luminosity of ∼3.7 × 1035 ergs s-1 at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mcrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-Ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as that measured when the source is almost 2 orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X0331+53. © 2006. The American Astronomical Society. All rights reserved.

We present results from quantitative modeling and spectral analysis of the high mass X-ray binary Vela X-1 obtained with the Chandra HETGS. The spectra exhibit emission lines from H-like and He-like ions driven by photoionization, as well as fluorescent emission lines from several elements in lower charge states. In order to interpret and make full use of the high-quality data, we have developed a simulator, which calculates the ionization and thermal structure of a stellar wind photoionized by an X-ray source, and performs Monte Carlo simulations of X-ray photons propagating through the wind. The emergent spectra are then computed as a function of the viewing angle accurately accounting for photon transport in three dimensions including dynamics. From comparisons of the observed spectra with the simulation results, we are able to find the ionization structure and the geometrical distribution of material in Vela X-1 that can reproduce the observed spectral line intensities and continuum shapes at different orbital phases remarkably well. It is found that a large fraction of X-ray emission lines from highly ionized ions are formed in the region between the neutron star and the companion star. We also find that the fluorescent X-ray lines must be produced in at least three distinct regions --(1)the extended stellar wind, (2)reflection off the stellar photosphere, and (3)in a distribution of dense material partially covering and possibly trailing the neutron star, which may be associated with an accretion wake. Finally, from detailed analysis of the emission lines, we demonstrate that the stellar wind is affected by X-ray photoionization.

We have constructed a Monte Carlo simulation base for the radiation environment of the Suzaku X-ray observatory satellite. The system consists of an object-oriented analysis framework used for real-data analysis, the Geant4 toolkit, an interface that allows the users a transparent usage of Geant4 components through the framework and classes that are used to build the simulation by a collaboration among different geometry maintainer and detector developer groups. As a demonstration, the geometry of the Hard X-ray Detector, which is one of the Suzaku detectors, is constructed using the framework and a comparison between the simulation output with the actual data is shown. © 2006 IEEE.

The hard X-ray detector (HXD-II) is one of the scientific payloads onboard Suzaku, the 5th Japanese cosmic Xray satellite. After the launch in July 2005, all the HXD-II components, including the sensors and analog/digital electronics, have been working normally. In order to archive the maximum performance of the HXD-II, especially the GSO/BGO well-type phoswich counters, extensive in-orbit qualification and calibration have been carried out utilizing the data acquired in early operations. Major items of these efforts include; to estimate the circuit dead time, calibrate energy scale, optimize the event selection criteria for background reduction, study the background, and examine the detector response. As a result of these in-orbit calibrations, the HXD-II background in the 10 similar to 600 keV range has been successfully lowered to (0.5 similar to 5.0) x 10(-4) c s(-1)keV(-1)cm(-2). This is the lowest among the background ever achieved in orbit by cosmic hard X-ray detectors.

Suzaku is the fifth Japanese astrophysical satellite, devoted to study high energy phenomena in the X-ray band of 0.5 - 600 keV. It was successfully launched from Uchinoura Space Center in Japan on 10th July 2005. The HXD on board Suzaku covers 10 - 600 keV band with a high sensitivity of about 10(-5) cnt/s/cm(2)/keV level. The sensor consists of 16 identical GSO/BGO well-type-phoswich counters incorporating 2 mm-thick silicon PIN diodes, and 20 surrounding-BGO-shield counters to remove residual non X-ray backgrounds through anti-coincidence rejection. The present paper is particularly focused on timing system of the HXD and in-orbit verifications with pulsars.

Suzaku is the fifth Japanese astrophysical satellite, devoted to study high energy phenomena in the X-ray band of 0.5 - 600 keV. It was successfully launched from Uchinoura Space Center in Japan on 10th July 2005. The HXD on board Suzaku covers 10 - 600 keV band with a high sensitivity of about 10 -5 cnt/s/cm2/keV level. The sensor consists of 16 identical GSO/BGO well-type-phoswich counters incorporating 2 mm-thick silicon PIN diodes, and 20 surrounding-BGO-shield counters to remove residual non X-ray backgrounds through anti-coincidence rejection. The present paper is particularly focused on timing system of the HXD and in-orbit verifications with pulsars. © 2006 IEEE.

This article briefly reviews first the progress of spectroscopy in X-ray astronomy from proportional counters, a major instrument in early phase of X-ray astronomy, to gas scintillation proportional counters, X-ray CCD cameras, transmission and reflection gratings, and finally to X-ray micro-calorimeters. As a typical example of spectral features observed from high mass X-ray binaries (HMXBs), the spectra observed from Vela X-1 with Chandra grating spectrometers are then presented and compared with computer simulations for high mass binary systems. © 2006 COSPAR.

The hard X-ray detector (HXD) onboard Suzaku covers an energy range of 8-700 keV, and thus in combination with the CCD camera (XIS) gives us an opportunity of wide-band X-ray observations of celestial sources with a good sensitivity over the 0.3-700 keV range. All of 64 Si-PIN photo diodes, 16 GSO/BGO phoswich scintillators, and 20 anti-coincidence BGO scintillators in the HXD are working well since the Suzaku launch on July 2005. The rejection of background events is confirmed to be as effective as expected, and accordingly the HXD achieved the lowest background level of the previously or currently operational missions sensitive in the comparable energy range. The energy and angular responses and timing have been continuously calibrated by the data from the Crab nebula, X-ray pulsars, and other sources, and at present several % accuracy is obtained. Even though the HXD does not perform simultaneous background observations, it detected weak sources with a flux as low as ∼0.5 mCrab; stars, X-ray binaries, supernova remnants, active galactic nuclei, and galaxy clusters. Extensive studies of background subtraction enables us to study weaker sources.

The hard X-ray imager (HXI) is the primary detector of the NeXT mission, proposed to explore high-energy non-thermal phenomena in the universe. Combined with a novel hard X-ray mirror optics, the HXI is designed to provide better than arc-minutes imaging capability with 1 keV level spectroscopy, and more than 30 times higher sensitivity compared with any existing hard X-ray instruments. The base-line design of the HXI is improving to secure high sensitivity. The key is to reduce the detector background as far as possible. Based on the experience of the Suzaku satellite launched in July 2005, the current design has a well-type tight active shield and multi layered, multi material imaging detector made of Si and CdTe. Technology has been under development for a few years so that we have reached the level where a basic detector performance is satisfied. Design tuning to further improve the sensitivity and reliability is on-going.

The hard X-ray detector (HXD-II) is one of the three scientific instruments onboard Japanese X-ray astronomy satellite Astro-E2 scheduled to be launched in 2005. This mission is very unique in a point of having a lower background than any other past missions in the 10-600 keV range. In the HXD-II, the large and thick BGO crystals are used as active shields for particle and gamma-ray background to the main detector. They have a wide field of view of ∼2π and a large effective area of 400 cm2 even at 1 MeV. Hence, the BGO shields have been developed as a wide-band all-sky monitor (WAM) with a broadband coverage of 50-5000 keV. In this paper, overall design and performance of the HXD-II/WAM based on the results of preflight calibration tests carried out in June 2004 are described. By irradiating various radio isotopes with the WAM flight model, we verified that it had comparable capabilities with other gamma-ray burst detectors. © 2005 IEEE.

The soft gamma-ray detector (SGD) onboard the Japanese future high energy astrophysics mission (NeXT) is a Compton telescope with narrow field of view, which utilizes Compton kinematics to enhance its background rejection capabilities. It is realized as a hybrid semiconductor gamma-ray detector which consists of silicon and cadmium telluride (CdTe) detectors. It can detect photons in a wide energy band (0.05-1 MeV) at a background level of 5 × 10-7 counts/s/cm2/keV; the silicon layers are required to improve the performance at a lower energy band (<0.3 MeV). Excellent energy resolution is the key feature of the SGD, allowing it to achieve both high angular resolution and good background rejection capability. An additional capability of the SGD, its ability to measure gamma-ray polarization, opens up a new window to study properties of astronomical objects. We will present the development of key technologies to realize the SGD: high quality CdTe, low noise front-end application-specific integrated circuit, and bump bonding technology. Energy resolutions of 1.7 keV (full-width at half-maximum) for CdTe pixel detectors and 1.1 keV for Si strip detectors have been measured. We also present the validation of Monte Carlo simulation used to evaluate the performance of the SGD. © 2005 IEEE.

We are developing a Compton camera based on Si and CdTe semiconductor imaging devices with high energy resolution. In this paper, results from the most recent prototype are reported. The Compton camera consists of six layered double-sided Si Strip detectors and CdTe pixel detectors, which are read out with low noise analog ASICs, VA32TAs. We obtained Compton reconstructed images and spectra of line gamma-rays from 122 keV to 662 keV. The energy resolution is 9.1 keV and 14 keV at 356 keV and 511 keV, respectively. © 2005 IEEE.

We have been developing a semiconductor Compton telescope to explore the universe in the energy band from several tens of keV to a few MeV. We use a Si strip and CdTe pixel detector for the Compton telescope to cover an energy range from 60 keV. For energies above several hundred keV, the higher efficiency of CdTe semiconductor in comparison with Si is expected to play an important role as an absorber and a scatterer. In order to demonstrate the spectral and imaging capability of a CdTe-based Compton Telescope, we have developed a Compton telescope consisting of a stack of CdTe pixel detectors as a small scale prototype. With this prototype, we succeeded in reconstructing images and spectra by solving the Compton equation from 122 keV to 662 keV. The energy resolution (FWHM) of reconstructed spectra is 7.3 keV at 511 keV and 3.1 keV at 122 keV, respectively. The angular resolution obtained at 511 keV is measured to be 12.2 degree (FWHM).

The Hard X-ray Detector (HXD-II) is one of the scientific payloads on board the fifth Japanese cosmic X-ray satellite Astro-E2, scheduled for launch in 2005. The HXD-II is designed to cover a wide energy range of 10-600 keV with a high sensitivity of about 10-5 cnt/s/cm2 /keV. In order to derive the energy response of the sensor and to estimate the background, a Monte Carlo simulator based on the Geant4 toolkit is currently being developed. This paper describes the design concept of the HXD-II software package, including the analysis tools and the Monte Carlo simulator, and its verification through a comparison with actual data taken by pre-flight radio-isotope irradiation experiments, together with calculated outputs that can demonstrate the in-orbit performance of the HXD-II. © 2005 IEEE.

The Burst Alert Telescope (BAT) onboard the Swift gamma-ray burst explorer has a coded aperture mask and a detector array of 32 K CdZnTe semiconductor devices. Due to the small mobility and short lifetime of carriers, the electron-hole pairs generated by gamma-ray irradiation cannot be fully collected. Hence the shape of the measured spectra has a broad low-energy tail. We have developed a method to model the spectral response by taking into account the charge transport properties which depend on the depth of the photon interaction [1]. The mobility-lifetime products for detectors derived from our method vary by more than one order of magnitude among detectors. In this paper, we focus on the nonuniformity of the mobility at the millimeter scale by employing a scanning experiment for a single detector. We reveal almost an order of magnitude variance in the mobility-lifetime product of holes within a single detector, while those of electrons remains fairly uniform. © 2005 IEEE.

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and γ-ray detection. The high-atomic number of the materials (ZCd=48,ZTe=52) gives a high quantum efficiency in comparison with Si. The large band-gap energy (Eg=1.5eV) allows to operate the detector at room temperature. Based on recent achievements in high-resolution CdTe detectors, in the technology of ASICs and in bump-bonding, we have proposed the novel hard X-ray and γ-ray detectors for the NeXT mission in Japan. The high-energy response of the super mirror onboard NeXT will enable us to perform the first sensitive imaging observations up to 80 keV. The focal plane detector, which combines a fully depleted X-ray CCD and a pixellated CdTe detector, will provide spectra and images in the wide energy range from 0.5 to 80 keV. In the soft γ-ray band up to ∼1MeV, a narrow field-of-view Compton γ-ray telescope utilizing several tens of layers of thin Si or CdTe detector will provide precise spectra with much higher sensitivity than present instruments. The continuum sensitivity will reach several ×10-8photons-1keV-1cm-2 in the hard X-ray region and a few ×10-7photons-1keV-1cm-2 in the soft γ-ray region. © 2005 Elsevier B.V. All rights reserved.

The properties of 32K CdZnTe (4×4mm2 large, 2 mm thick) detectors have been studied in the pre-flight calibration of the Burst Alert Telescope (BAT) on-board the Swift Gamma-ray Burst Explorer (scheduled for launch in November 2004). In order to understand the energy response of the BAT CdZnTe array, we first quantify the mobility-lifetime (μτ) products of carriers in individual CdZnTe detectors, which produce a position dependency in the charge induction efficiency and results in a low-energy tail in the energy spectrum. Based on a new method utilizing 57Co spectra obtained at different bias voltages, the μτ for electrons ranges from 5.0×10-4 to 1.0×10-2cm2V-1 while the μτ for holes ranges from 1.3×10-5 to 1.8×10-4cm2V-1. We find that this wide distribution of μτ products explains the large diversity in spectral shapes between CdZnTe detectors well. We also find that the variation of μτ products can be attributed to the difference of crystal ingots or manufacturing harness. We utilize the 32K sets of extracted μτ products to develop a spectral model of the detector. In combination with Monte Carlo simulations, we can construct a spectral model for any photon energy or any incident angle. © 2005 Elsevier B.V. All rights reserved.

The Hard X-ray Detector (HXD-II) is one of the three instruments onboard the Astro-E2 satellite scheduled for launch in 2005. The HXD-II consists of 16 main counters (Well units), surrounded by 20 active shield counters (Anti units). The Anti units have a large geometrical area of ∼ 800 cm2 with an uncollimated field of view covering ∼ 2π steradian. Utilizing 2.6 cm thick BGO crystals, they realize a large effective area of 400 cm 2 for 1 MeV photons. In the energy range of 300-5000 keV, the expected effective area is significantly larger than those of other gamma-ray burst instruments, such as CGRO/BATSE, HETE-2/FREGATE, and GLAST/GBM. Therefore, the Anti units act as a Wideband All-sky Monitor (WAM) for gamma-ray bursts in the energy range of 50-5000 keV. © Società Italiana di Fisica.

We summarize significant improvements which have been achived in the development of Astro-E2 Hard X-ray Detector (HXD-II). An expanded energy range and better energy resolution have been achieved from progresses in device materials and re-designing of the front-end electronics. An improved estimation for the detector background in orbit has also been conducted based upon results from our proton irradiation experiment. The sensitivity of HXD-II can be expected to reach an order of 10-6 [c s-1 keV-1 cm-2].

Recent results from the Schottky CdTe diode detectors employing a guard-ring (GR) electrode are reported. A cathode electrode, made of platinum, was separated into an active electrode(s) and a surrounding GR. Typical leakage current of a device with active area of 2 × 2 mm2 and 0.5 mm thickness surrounded by a GR, is 7 and 20 pA at a bias of 100 and 500 V, respectively, operated at 20°C. Spectral resolution of this device is 0.93 and 1.2 keV (FWHM) at 59.5 and 122 keV, respectively, operated at 20°C with a bias of 800 V. Detailed study of the characteristics of these devices working as a gamma-ray detector is presented.

We describe the recent progress on the use of Schottky Cadmium telluride (CdTe) diode detectors for γ-ray spectroscopy. The extremely low leakage current of the newly developed CdTe diode allows us to apply a much higher bias voltage in comparison with the previous CdTe detectors. Both the improved charge-collection efficiency and the low leakage current lead to a good energy resolution even at room temperature. Large-area CdTe diode detectors with dimensions of 21.5 × 21.5 mm2 are now available. By stacking 40 layers of these large-area devices, we have achieved a good energy resolution of several keV (FWHM) for MeV γ-rays. © 2003 Elsevier B.V. All rights reserved.

The Hard X-ray Detector (HXD-II) is (one of the scientific payloads on board the fifth Japanese cosmic X-ray satellite Astro-E2, scheduled for launch in 2005. The HXD-II is designed to cover a wide energy range of 10 - 600 keV with a high sensitivity of similar to 10(-5) cnt/S/cm(2)/keV, using 16 identical GSO and BGO phoswich counters combined with 2 mm-thick silicon PIN diodes. In order to investigate the in-orbit performance of HXD-II in cosmic radiation environment, a Monte Carlo simulator based (in the Geant4 toolkit is currently developed. There are two main goals of this simulator, which is directly connected to the detector's performance. One is to derive energy response to photons within the acceptance energy range, with 5% accuracy, after several types of standard event-selection of the HXD-II. The other is to estimate detector background with 10% accuracy. In addition to the background caused directly by the primary and secondary cosmic-rays, of particular importance is the radio-activation background induced by MeV protons trapped in the South Atlantic Anomaly. The simulator is also used in the pre-launch verifications of the HXD-II hardware. This paper describes the design concept of the Monte Carlo simulator, and its verification through comparison with the actual data of pre-flight radio-isotope irradiation experiments, together with calculated outputs that can demonstrate the in-orbit performance of the HXD-II.

The Hard X-ray Detector (HXD) is one of the three instruments onboard Japanese X-ray astronomy satellite Astro-E2 scheduled for launch in 2005. This mission is very unique in a point of having a lower detector background than any other past missions in the 10-600 keV range. In the HXD, the large and thick BGO crystal are used as active shields for reducing the particle and gamma-ray background to the main detector. These anticoincidence shields are called as "Anti counters", which have a large geometrical area similar to 800 cm(2) and an uncollimated field of view of similar to 2 pi. Furthemore, they also have a larger effective area, corresponding to 400 cm(2) at even 1 MeV due to their thick high-Z materials. This feature enables us to observe the high energy radiation of Gamma-ray bursts with a higher sensitivity than previous all-sky monitors. Hence, the Anti counters have been developed as all-sky monitors with a broadband coverage of 50-5000 keV. In this paper, we will describe overall design of the HXD Anti counters, then report on the results of the pre-flight calibration test on June 2004 using the flight model. By irradiating various radio isotopes with Anti counters, we confirmed that they have capability as all-sky monitors. It is striking that the low energy threshold has been archived about 30 keV in spite of large volume of BGO scintillators.

We are developing a Compton camera based on Si and CdTe semiconductor imaging devices with high energy resolution. In this paper, results from the most recent prototype are reported. The Compton camera consists of six stacked double-sided Si Strip detectors and CdTe pixel detectors, which are read out with low noise analog ASICs, VA32TAs. We obtained Compton reconstructed images and spectra of line gamma-rays from 80 keV to 662 keV. The energy resolution (FWHM) is 10 keV and 16 keV at 356 keV and 511 keV, respectively.

We constructed a Monte-Carlo simulator framework for the radiation environment of Astro-E2, the Japanese 5th X-ray astronomy satellite. We used Geant4 as the simulator engine, and embedded it into the analysis framework derived from what has been used for other Astro-E2 software development. The entire architecture is designed to make the learning cost of the framework programming as low as possible for not only end users but developers, and we implemented some software mechanisms to register Geant4 components such as physics processes and geometry builder into the framework without using the Geant4's run manager explicitly.

The Burst Alert Telescope (BAT) onboard the Swift Gamma-ray Burst Explorer (scheduled for launch in May of 2004) has a coded aperture mask and a detector array of 32,768 Cd0.9Zn0.1Te1.0 (4x4mm(2) large, 2mm thick) semiconductor devices. Due to small mobility and short lifetime of carriers, the electron-hole pairs generated by irradiation of gamma-rays cannot be completely collected. Since the shape of the measured spectra has the broad low-energy tail, it is very useful for us to estimate the obtained spectra to fit the model which considers the charge transport properties depended on the depth of the photon interaction (G.Sato, 2002) [1]. The energy calibration of the BAT array and coded mask experiments were carried out at NASA/Goddard Space Flight Center between December 2002 and March 2003. We applied the model fitting to the calibration spectra, to yield the mobility-lifetime products for each detector and these values differ by over 2 orders of magnitude among detectors. Also using the mobility-lifetime parameters, we can identified the detector energy response as a function of the temperature and illumination angle. But we figure out a difference between the model and the obtained data. To determine the difference between the model and the measured data, we conducted the detailed check experiment for a single CdZnTe, to show that the cause of the excess is due to the areal nonuniformity of the mobility-lifetime parameter.

The Hard X-ray Detector (HXD-II) is one of the scientific payloads on board the fifth Japanese cosmic X-ray satellite Astro-E2, scheduled for launch in 2005. The HXD-II is designed to cover a wide energy range of 10 - 600 keV with a high sensitivity of ∼ 10-5 cnt/s/cm2/keV, using 16 identical GSO and BGO phoswich counters combined with 2 mm-thick silicon PIN diodes. In order to investigate the in-orbit performance of HXD-II in cosmic radiation environment, a Monte Carlo simulator based on the Geant4 toolkit is currently developed. There are two main goals of this simulator, which is directly connected to the detector's performance. One is to derive energy response to photons within the acceptance energy range, with 5% accuracy, after several types of standard event-selection of the HXD-II. The other is to estimate detector background with 10% accuracy. In addition to the background caused directly by the primary and secondary cosmic-rays, of particular importance is the radio-activation background induced by MeV protons trapped in the South Atlantic Anomaly. The simulator is also used in the pre-launch verifications of the HXD-II hardware. This paper describes the design concept of the Monte Carlo simulator, and its verification through comparison with the actual data of pre-flight radio-isotope irradiation experiments, together with calculated outputs that can demonstrate the in-orbit performance of the HXD-II. © 2004 IEEE.

The current status of Schottky CdTe diode detectors, especially in view of their space application for hard X-ray and gamma-ray astronomy, are reported. For practical use in space science, a large-area CdTe diode with a size of 21.5 × 21.5 mm2 and a thickness of 0.5 mm was developed. A good energy resolution, 2.8 keV (FWHM) at -20°C, and high homogeneity to within 0.2% over the detector were achieved for the spectral performance. This device has successfully passed a series of tests required for its use in space, in view of utilizing Japanese M-V rockets. The tests include the mechanical environment test, vacuum test, long run for weeks and proton-beam radiation. Initial results from a 2 × 2 segmented electrode large-area device with a guard-ring are also presented. © 2003 Elsevier B.V. All rights reserved.

We report the detection of a fully-resolved, Compton-scattered emission line in the X-ray spectrum of the massive binary GX 301-2 obtained with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory. The iron K-alpha fluorescence line complex observed in this system consists of an intense narrow component centered at an energy of E = 6.40 keV and a redward shoulder that extends down to ~6.24 keV, which corresponds to an energy shift of a Compton back-scattered iron K-alpha photon. From detailed Monte Carlo simulations and comparisons with the observed spectra, we are able to directly constrain the physical properties of the scattering medium, including the electron temperature and column density, as well as an estimate for the metal abundance.

Cadmium Telluride (CdTe), with its high photon absorption efficiency, has been regarded as a promising semiconductor material for the next generation X/γ-ray detectors. In order to apply this device to astrophysics, it is essential to investigate the radiation hardness and background properties induced by cosmic-ray protons in orbit. We irradiated Schottky CdTe diodes and a CdTe block with a beam of mono-energetic (150 MeV) protons. The induced activation in CdTe was measured externally with a germanium detector, and internally with the irradiated CdTe diode itself. We successfully identified most of radioactive isotopes induced mainly via (p, xn) reactions, and confirmed that the activation background level of CdTe diode is sufficiently low in orbit. We compared energy resolution and leakage current before and after the irradiation and also monitored the signals from a calibration source during the irradiation. There have been no significant degradation. CdTe diodes are tolerant enough to radioactivity in low earth orbit.

We are developing a large array detector composed of 1024 individual CdTe diodes. Each detector has the dimensions of 1.2 mm × 5.0 mm and a thickness of 1.2 mm. An edge-on geometry is used for the injection of γ-rays, to obtain a cross-section thickness of 5 mm. With this geometry, the distance between the two electrodes can be kept small, and we can therefore apply the high electric field which is necessary to achieve a high energy resolution (by reducing the low energy tail) and also to sustain the long-term stability of the CdTe diode. Signals from each detector element are fed into newly developed low noise ASICs. We use 32 chips for the readout of 1024 elements. In this paper, we report the basic characteristics of the individual detectors and the overall performance of the gamma-camera. Design of the readout electronics system is also described.

Stacking several ten layers of thin CdTe devices is a new concept of a gamma-ray detector, featuring both good energy resolution and high efficiency. We have developed prototype models of the CdTe stacked detector which consists of 10 layers and 40 layers based on a newly-developed large CdTe diode. The energy resolution of the detector is measured to be 1-2% (FWHM) from 500 keV to 6 MeV. In addition to the gamma-ray spectroscopy, we propose to use the stack detector as a new type of range finder for gamma-ray sources below a few hundred keV. We demonstrate that the distance of a 57Co radio active source, which is located 5 cm from the detector surface, is measured within the accuracy of 1-2 mm. © 2003 Elsevier Science B.V. All rights reserved.

The Burst Alert Telescope (BAT) onboard the Swift Gamma-ray Burst Explorer (scheduled for launch in May of 2004) has a coded aperture mask and a detector array of 32,768 Cd 0.9Zn 0.1Te 1.0 (4×4mm 2 large, 2mm thick) semiconductor devices. Due to small mobility and short lifetime of carriers, the electron-hole pairs generated by irradiation of gamma-rays cannot be completely collected. Since the shape of the measured spectra has the broad low-energy tail, it is very useful for us to estimate the obtained spectra to fit the model which considers the charge transport properties depended on the depth of the photon interaction (G.Sato, 2002). The energy calibration of the BAT array and coded mask experiments were carried out at NASA/Goddard Space Flight Center between December 2002 and March 2003. We applied the model fitting to the calibration spectra, to yield the mobility-lifetime products for each detector and these values differ by over 2 orders of magnitude among detectors. Also using the mobility-lifetime parameters, we can identified the detector energy response as a fucntion of the temperature and illumination angle. But we figure out a difference between the model and the obtained data. To determine the difference between the model and the measured data, we conducted the detailed check experiment for a single CdZnTe, to show that the cause of the excess is due to the areal nonuniformity of the mobility-lifetime parameter.

Recent results from the CdTe Schottky diode detectors employing a guard-ring electrode are reported. Cathode electrode, made of Pt, was separated into an active electrode(s) and a surrounding guard-ring. Typical leakage current of a device with an active area of 2 × 2 mm2 and 0.5 mm thickness surrounded by a guard-ring, is 7 pA and 20 pA at a bias of 100 V and 500 V, respectively, operated at 20°C. Spectral resolution of this device is 0.93 keV and 1.2 keV (FWHM) at 59.5 keV and 122 keV, respectively, operated at 20°C with a bias of 800 V. Detailed study of the characteristics of these devices working as a gamma-ray detector is presented.

We present the first results of the Chandra and optical follow-up observations of hard X-ray sources detected in the ASCA Medium Sensitivity Survey (AMSS). Optical identifications are made for five objects. Three of them show either weak or absent optical narrow emission lines and are at low redshift <z>~0.06. One of them is a broad line object at z=0.910 and one is a z=0.460 object with only narrow lines. All the narrow line objects show strong evidence for absorption in their X-ray spectra. Their line ratios are consistent with a Seyfert II/LINER identification as are the line widths. The three low redshift objects have the colors of normal galaxies and apparently the light is dominated by stars. This could be due to the extinction of the underlying nuclear continuum by the same matter that absorbs X-rays and/or due to the dilution of the central source by starlight. These results suggest that X-ray sources that appear as ``normal'' galaxies in optical and near-IR bands significantly contribute to the hard X-ray background. This population of objects has a high space density and probably dominates the entire population of active galaxies.

This paper summarizes the design and performance of the hard X-ray detector constructed for the ASTRO-E satellite. The detector utilizes the GSO/BGO well-type phoswich counters in a compound-eye configuration to achieve an extremely low background level of a few × 10-5counts s-1 cm-2keV-1 [1]. The GSO scintillators installed in the BGO active shield wells are sensitive to 30-600 kev photons, while the 2-mm-thick silicon PIN diodes, placed in front of each GSO crystal, cover the 10-60 kev energy band with a spectral resolution of ∼3.5-keV full-width at half-maximum. The design goals, of both low background and high energy resolution, in the hard X-ray bands were verified through the preflight calibration experiments.

We measured timing properties of CdTe and CdZnTe semiconductor detectors with planar configuration. We developed a new method to evaluate their performance in timing resolution utilizing an 241Am-doped plastic scintillator. We confirmed that the low mobility and short lifetime of holes are major obstacles to their timing resolution. However, their timing properties can be very much improved, either by applying a high electric field that increases the carrier speed or by selecting those events which are dominated by the electron signal. We demonstrated the latter through a pulse-shape discrimination technique using two different integration time constants. In conjunction with a newly developed CdTe diode, we obtained a superior timing resolution of 5.8 ns. We also discussed the application of CdTe to positron emission tomography (PET), employing the standard 511 keV gamma-gamma coincidence method. We confirmed that a geometrical configuration in which the electrodes are parallel to the incident γ-rays gives about three times better timing response than a geometry when the electrodes are perpendicular to the γ-ray beam.