Tamotsu Sato

The Si/CdTe semiconductor Compton camera of the ASTRO-H Soft Gamma-ray Detector (SGD)

Abstract The Soft Gamma-ray Detector (SGD) is one of the instrument payloads onboard ASTRO-H, and will cover a wide energy band (60–600xa0keV) at a background level 10 times better than instruments currently in orbit. The SGD achieves low background by combining a Compton camera scheme with a narrow field-of-view active shield. The Compton camera in the SGD is realized as a hybrid semiconductor detector system which consists of silicon and cadmium telluride (CdTe) sensors. The design of the SGD Compton camera has been finalized and the final prototype, which has the same configuration as the flight model, has been fabricated for performance evaluation. The Compton camera has overall dimensions of 12xa0cm×12xa0cm×12xa0cm, consisting of 32 layers of Si pixel sensors and 8 layers of CdTe pixel sensors surrounded by 2 layers of CdTe pixel sensors. The detection efficiency of the Compton camera reaches about 15% and 3% for 100xa0keV and 511xa0keV gamma rays, respectively. The pixel pitch of the Si and CdTe sensors is 3.2xa0mm, and the signals from all 13,312 pixels are processed by 208 ASICs developed for the SGD. Good energy resolution is afforded by semiconductor sensors and low noise ASICs, and the obtained energy resolutions with the prototype Si and CdTe pixel sensors are 1.0–2.0xa0keV (FWHM) at 60xa0keV and 1.6–2.5xa0keV (FWHM) at 122xa0keV, respectively. This results in good background rejection capability due to better constraints on Compton kinematics. Compton camera energy resolutions achieved with the final prototype are 6.3xa0keV (FWHM) at 356xa0keV and 10.5xa0keV (FWHM) at 662xa0keV, which satisfy the instrument requirements for the SGD Compton camera (better than 2%). Moreover, a low intrinsic background has been confirmed by the background measurement with the final prototype.

Discovery of recombining plasma in the supernova remnant 3C 391

Recent X-ray study of middle-aged supernova remnants (SNRs) reveals strong radiative recombination continua (RRCs) associated with overionized plasmas, of which the origin still remains uncertain. We report our discovery of an RRC in the middle-aged SNR 3C 391. If the X-ray spectrum is fitted with a two-temperature plasma model in collisional ionization equilibrium (CIE), residuals of Si XIV Ly alpha line at 2.006 keV, S XVI Ly alpha line at 2.623 keV and the edge of RRC of Si XIII at 2.666 keV are found. The X-ray spectrum is better described by a composite model consisting of a CIE plasma and a recombining plasma (RP). The abundance pattern suggests that the RP is associated to the ejecta from a core-collapse supernova with a progenitor star of 15 solar mass. There is no significant difference of the recombining plasma parameters between the southeast region and the northwest region surrounded by dense molecular clouds. We also find a hint of Fe I K alpha line at 6.4 keV (~2.4 sigma detection) from the southeast region of the SNR.

Imaging and spectral performance of CdTe double-sided strip detectors for the Hard X-ray Imager onboard ASTRO-H

The imaging and spectral performance of CdTe double-sided strip detectors (CdTe-DSDs) was evaluated for the ASTRO-H mission. The charcterized CdTe-DSDs have a strip pitch of 0.25 mm, an imaging area of 3.2 cm × 3.2 cm and a thickness of 0.75 mm. The detector was successfully operated at a temperature of -20°C and with an applied bias voltage of 250 V. By using two-strip events as well as one-strip events for the event reconstruction, a good energy resolution of 2.0 keV at 59.5 keV and a sub-strip spatial resolution was achieved. The hard X-ray and gamma-ray response of CdTe-DSDs is complex due to the properties of CdTe and the small pixel effect. Therefore, one of the issues to investigate is the response of the CdTe-DSD. In order to investigate the spatial dependence of the detector response, we performed fine beam scan experiments at SPring-8, a synchrotron radiation facility. From these experiments, the depth structure of the electric field was determined as well as properties of carriers in the detector and successfully reproduced the experimental data with simulated spectra.

Suzaku spectra of a Type-II supernova remnant, Kes 79

This paper reports results of a Suzaku observation of the supernova remnant (SNR) Kes 79 (G33.6+0.1). The X-ray spectrum is best fitted by a two-temperature model: a non-equilibrium ionization (NEI) plasma and a collisional ionization equilibrium (CIE) plasma. The NEI plasma is spatially confined within the inner radio shell with kT~0.8 keV, while the CIE plasma is found in more spatially extended regions associated with the outer radio shell with kT~0.2 keV and solar abundance. Therefore, the NEI plasma is attributable to the SN ejecta and the CIE plasma is forward shocked interstellar medium. In the NEI plasma, we discovered K-shell line of Al, Ar and Ca for the first time. The abundance pattern and estimated mass of the ejecta are consistent with the core-collapse supernova explosion of a ~30-40 solar mass progenitor star. An Fe line with center energy of ~6.4 keV is also found in the southeast (SE) portion of the SNR, a close peripheral region around dense molecular clouds. One possibility is that the line is associated with the ejecta. However, the centroid energy of ~6.4 keV and the spatial distribution of enhancement near the SE peripheral do not favor this scenario. Since the ~6.4 keV emitting region coincides to the molecular clouds, we propose another possibility that the Fe line is due to K-shell ionization of neutral Fe by the interaction of locally accelerated protons (LECRp) with the surrounding molecular cloud. Both these possibilities, heated ejecta or LECRp origin, are discussed based on the observational facts.

A ground calibration of the engineering model of the SXT onboard ASTRO-H using the ISAS 30m pencil beam facility

The Japanese ASTRO-H mission, planned to be launched in 2014, will carry several instruments for covering a wide energy range from a few keV to 600 keV. Among them there are four thin-foil-nested Wolter-I X-ray telescopes. Two of them are Soft X-ray Telescopes (SXTs) covering up to ~12 keV. Each of them focuses an image on the focal plane detectors of the CCD camera (SXI) and the calorimeter (SXS-XCS), respectively. In 2011, we performed a ground calibration of a quadrant engineering model (EM) of SXT that was fabricated at MASAs Goddard Space Flight Center (GSFC). The ground calibration was made with a combination of the measurements at the GSFC and Institute of Space and Astronautical Science (ISAS) facilities. In this paper we report the results of the calibration at the ISAS 30m beamline facility. We used a raster san method with a pencil beam at the baseline length of 30m. An effective area and angular resolution of the EM quadrant were measured. The effective area is 147 cm2 at 1.49 keV and 116 cm2 at 4.51 keV, respectively, while the angular longer by ~20mm from nominal length. We also measured imaging performance in separate parts of nested mirrors. The angular resolution of parts at outer radius is larger than those at inner radius, and the quadrant have different focal lengths in radius.

Modeling of proton-induced radioactivation background in hard X-ray telescopes: Geant4-based simulation and its demonstration by Hitomi ’s measurement in a low Earth orbit

Abstract 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 ° , 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 Bi 4 Ge 3 O 12 scintillator shields, leakage of cosmic X-ray background and albedo gamma-ray radiation, and emissions from naturally contaminated isotopes in the detector system.

A Monte Carlo simulation framework to study ASTRO-H in-orbit radiation and detector responses based on Geant4 toolkit

We are developing an ASTRO-H data analysis framework with the Geant4-based Monte Carlo simulation core, and numerical models of the on-orbit environmental radiation and full-satellite mass structure. The framework uses not only Geant4 but also a traditional X-ray mirror ray-tracing simulator, and a file format compatible with the SimX simulator for input and output of celestial body information. The data exchange between the framework and the external software is based on FITS files, which makes it easy to record and trace the internal steps of the whole simulation framework.

Development and calibration of fine collimators for the ASTRO-H Soft Gamma-ray Detector

The Soft Gamma-ray Detector (SGD) is a Si/CdTe Compton telescope surrounded by a thick BGO active shield and is scheduled to be onboard the ASTRO-H satellite when it is launched in 2015. The SGD covers the energy range from 40 to 600 keV with high sensitivity, which allows us to study nonthermal phenomena in the universe. The SGD uses a Compton camera with the narrow field-of-view (FOV) concept to reduce the non-Xray background (NXB) and improve the sensitivity. Since the SGD is essentially a nonimaging instrument, it also has to cope with the cosmic X-ray background (CXB) within the FOV. The SGD adopts passive shields called “fine collimators” (FCs) to restrict the FOV to ≤ 0.6° for low-energy photons (≤ 100 keV), which reduces contamination from CXB to less than what is expected due to NXB. Although the FC concept was already adopted by the Hard X-ray Detector onboard Suzaku, FCs for the SGD are about four times larger in size and are technically more difficult to operate. We developed FCs for the SGD and confirmed that the prototypes function as required by subjecting them to an X-ray test and environmental tests, such as vibration tests. We also developed an autocollimator system, which uses visible light to determine the transmittance and the optical axis, and calibrated it against data from the X-ray test. The acceptance tests of flight models started in December 2013: five out of six FCs were deemed acceptable, and one more unit is currently being produced. The activation properties were studied based on a proton-beam test and the results were used to estimate the in-orbit NXB.

In-orbit activation study of ASTRO-H X-ray observatory using Geant4

We are developing an integrated simulation framework based on Geant4 to estimate in-orbit performance of instruments onboard ASTRO-H, the international X-ray observatory to be launched in 2014. One of the most important purposes of our simulation is to estimate radiation background of the Hard X-ray Imager (HXI) and the Soft Gamma-ray Detector (SGD), both of which are composed of CdTe and silicon. We developed a new code based on Geant4 to handle the generation and the decay of radioactive nuclei and to estimate the activation of CdTe detector. In addition, to verify simulation results, we performed beam irradiation experiments at NIRS IDMAC using 150 MeV protons, which is typical energy of protons reaching to the CdTe detectors placed in the shielding materials, and measured the activation background spectra for several months. Our simulation results show good agreements with the experimental data.

Sear ch for a correlation between giant radio pulses and hard X-ray emissions in the Crab pulsar

We present the results of the search for a correlation between giant radio pulses (GRPs) at 1.4 GHz and hard X-rays at 15-75 keV from the Crab pulsar. We made simultaneous ground and satellite observations of the Crab pulsar over 12 hours in three occasions in April 2010, March and September 2011, and got a sample of 1.3*10^4 main-pulse phase GRPs. From these samples we have found statistically marginal enhancement (21.5%, 2.70 sigma) of hard X-ray flux within +/- 1.5 degree phase angle of the synchronous peak of main-pulse phase GRPs. This enhancement, if confirmed, implicates that GRPs may accompany plasma density increases in the pulsar magnetosphere.