Atsushi Harayama

The Hard X-ray Imager (HXI) for the ASTRO-H mission

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.

Soft gamma-ray detector (SGD) onboard the ASTRO-H mission

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.

High-z gamma-ray bursts for unraveling the dark ages mission HiZ-GUNDAM

We are now investigating and studying a small satellite mission HiZ-GUNDAM for future observation of gamma-ray bursts (GRBs). The mission concept is to probe “the end of dark ages and the dawn of formation of astronomical objects”, i.e. the physical condition of early universe beyond the redshift z > 7. We will consider two kinds of mission payloads, (1) wide field X-ray imaging detectors for GRB discovery, and (2) a near infrared telescope with 30 cm in diameter to select the high-z GRB candidates effectively. In this paper, we explain some requirements to promote the GRB cosmology based on the past observations, and also introduce the mission concept of HiZ-GUNDAM and basic development of X-ray imaging detectors.

X-ray Observation of AM Herculis in a Very Low State with Suzaku

The X-ray observation of AM Herculis (AM Her) in a very low state was performed with Suzaku in 2008 October. One flare event with a timescale of ~3700 s was detected at the X-ray luminosity of 6.0 × 1029 erg s–1 in the 0.5-10 keV band at the assumed distance of 91 pc. The X-ray spectrum is represented by a thermal plasma emission model with a temperature of 8.67+1.31 –1.14 keV. During the quiescence in the flare interval, Suzaku also detected significant X-rays at a luminosity of 1.7 × 1029 erg s–1 in the 0.5-10 keV band, showing a clear spin modulation at a period of 0.1289273(2) days at BJD 2454771.581. The X-ray spectra in the quiescence were represented by a MEKAL + Power Law model or a single CEMEKL model, which are also supported by phase-resolved analyses. A correlation between the temperature and the volume emission measure was found together with historical X-ray measurements of AM Her in various states. In order to account for a possible non-thermal emission from AM Her, particle acceleration mechanisms in the AM Her system are also discussed, including the new proposal of a shock acceleration process on the top of the accretion column.The X-ray observation of AM Her in a very low state was performed with Suzaku in October 2008. One flare event with a time scale of ∼ 3700 sec was detected at the X-ray luminosity of 6.0 × 10 erg sec in the 0.5 – 10 keV band assuming at a distance of 91 pc. The X-ray spectrum is represented by a thermal plasma emission model with a temperature of 8.67 −1.14 keV. During the quiescence out of the flare interval, Suzaku also detected significant X-rays at a luminosity of 1.7 × 10 erg sec in the 0.5 – 10 keV band, showing a clear spin modulation at a period of 0.1289273(2) days at BJD 2454771.581. The X-ray spectra in the quiescence were represented by a MEKAL + Power Law (PL) model or a single CEMEKL model, which are also supported by phase-resolved analyses. A correlation between the temperature and the volume emission measure was found together with historical X-ray measurements of AM Her in various states. In order to account for a possible non-thermal emission from AM Her, particle acceleration mechanisms in the AM Her system are also discussed, including a new proposal of a shock acceleration process on the top of the accretion column. Subject headings: novae, cataclysmic variables – stars: individual (AM Herculis) – plasmas – acceleration of particles

Gain properties of gas electron multipliers (GEMs) for space applications

Fine-pitch and thick-foil GEMs have been produced using a laser etching technique for photoelectric X-ray polarimeters onboard future missions. The finest hole pitch of the thick-foil GEM is 80 μm with a hole diameter of 40 μm, and a thickness of the insulator is 100 µm. The maximum effective gain in a 70%-30% mixture of argon and carbon dioxide reaches 3×104 at voltage of 750 V between GEM electrodes. No significant gain increase or decrease was observed during 24 hours test in which applied high voltage was ramped up and down frequently. The measured gain stability was less than 4%. An accelerated test of the high voltage ramp up and down for two years LEO operations were carried out. During the 6500 times voltage ramp up and down, the GEM kept its gain within 4% variation and no unexpected behavior was observed.

Development of wide-field low-energy X-ray imaging detectors for HiZ-GUNDAM

We are planning a future gamma-ray burst (GRB) mission HiZ-GUNDAM to probe the early universe beyond the redshift of z > 7. Now we are developing a small prototype model of wide-field low-energy X-ray imaging detectors to observe high-z GRBs, which cover the energy range of 1 – 20 keV. In this paper, we report overview of its prototype system and performance, especially focusing on the characteristics and radiation tolerance of high gain analog ASIC specifically designed to read out small charge signals.

A compact imaging system with a CdTe double-sided strip detector for non-destructive analysis using negative muonic X-rays

Abstract A CdTe double-sided strip detector (CdTe-DSD) is an ideal device for imaging and spectroscopic measurements in the hard X-ray range above 10 keV. Recent development enables us to realize an imager with a detection area of ∼ 10 cm 2 . An energy resolution of 1–2 keV (FWHM) and a position resolution of a few hundred μ m are available from the detector. This type of imager has been long awaited for non-destructive elemental analysis, especially by using negative muons, because energies of characteristic X-rays from muonic atoms are about 200 time higher than those from normal atoms. With the method that uses negative muons, hard X-ray information gives the spatial distribution of elements in samples at a certain depth defined by the initial momentum of the muon beam. In order to study three-dimensional imaging capability of the method, we have developed a compact imaging system based on CdTe-DSD and a ϕ 3 mm pinhole collimator as the first prototype. We conducted experiments with samples which consist of layers of Al, BN and LiF irradiated by negative muon beams in MUSE/J-PARC and successfully reconstruct hard X-ray images of muonic X-rays from B, N and F at various depths.

Compact hard x-ray imaging system with a large FOV

We have developed a compact hard X-ray imaging system composed of a cadmium telluride double-sided strip detector (CdTe-DSD) and a coded mask. We investigate the imaging performance using two different coded masks with different sizes and patterns. In our system, a CdTe-DSD of pitch 250μm is used in conjunction with a coded mask is placed 70-100 mm above the detector to form a compact imaging system. We obtained an angular resolution of up to 11.8 arc min, as measured from gamma-ray lines of point-like radioactive isotope sources. This is consistent with that expected from the geometry. The energy resolution is 1.7 keV (FWHM) at 60 keV and the energy range of imaging is from 5 keV to 122 keV. These results agree very well with Monte Carlo simulations of the detector.

Residual Radiation Measurements at J-PARC MR Using the ASTROCAM 7000HS Newly Developed Radioactive Substance Visualization Camera

Mitsubishi heavy Industries, Ltd. (MHI) released the ASTROCAM 7000HS, a radioactive substance visualization camera. The ASTROCAM 7000HS incorporates the technologies for the gamma-ray detector used for the ASTRO-H satellite, which MHI has been developing under entrustment from and together with scientists at the Institute of Space and Astronautical Science (ISAS) at the Japan Aerospace Exploration Agency (JAXA), and the design was modified for use on land to commercialize the product [1]. MHI and Mitsubishi Heavy Industries Mechatronics Systems, Ltd. (MHI-MS) performed on-site residual radiation measurements at the 50 GeV Main Ring (MR) of the Japan Proton Accelerator Research Complex (J-PARC) under collaboration with the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Agency (JAEA) and succeeded visualization of radiation hot spots of the accelerator components. The outline of the ASTROCAM 7000HS, the measurement principle and the first measurement results at the JPARC MR are described.

Systematic surveys of the non thermal emission from white dwarfs with Suzaku and INTEGRAL

Continuous searches for other possible white dwarf (WD) pulsars like AE Aquarii[12] with Suzaku and INTEGRAL have been performed. After picking up WDs with known magnetic field strengths and spin periods from catalogs of CVs and isolated WDs, objects whose induced electric potentials exceed 1012 volts and dipole radiations over 1029 erg s−1 are selected; AM Her, EUVE J0317–85, PG1031+234, LHS1734, PG1015+014 etc. Their X‐rays were studied with INTEGRAL archive data and/or Suzaku follow‐up observations. A promising non‐thermal emission from an object, AM Her in a very low state, has been found with Suzaku at the X‐ray luminosity of 6.6×1029 erg s−1.