Hisako Niko

Hard X-ray Detector (HXD) on board Suzaku

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit back ground based on a combination of new techniques, including the concept of well-type active shield counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV, the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV 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 ~4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (~5 degree). The on-board signal-processing system and the data transmitted to the ground are also described.

Development of the HXD-II wide-band all-sky monitor onboard Astro-E2

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 ~2pi 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

A Further Study of the Luminosity-dependent Cyclotron Resonance Energies of the Binary X-Ray Pulsar 4U 0115+63 with the Rossi X-Ray Timing Explorer

We report on the RXTE observations of the binary X-ray pulsar 4U 0115+63, covering an outburst in 1999 March-April with 44 pointings. The 3-30 keV PCA spectra and the 15-50 keV HEXTE spectra were analyzed jointly for cyclotron resonance features. When the 3-50 keV luminosity at an assumed distance of 7 kpc was in the range (5-13) × 1037 ergs s-1, harmonic double cyclotron features were observed in absorption at ~11 and ~22 keV, as was measured previously during typical outbursts. As the luminosity decreased below ~5 × 1037 ergs s-1, the second resonance disappeared, and the fundamental resonance energy gradually increased, up to ~16 keV at 0.16 × 1037 ergs s-1. These results reconfirm the report by Mihara et al. using Ginga, who observed a single absorption at ~16 keV in a minor (~1037 ergs s-1) outburst of this object. The luminosity-dependent cyclotron resonance energy might possibly be understood as a result of a decrease in the accretion column height, in response to a decrease in the mass accretion rate.

Improvements of the astro-E2 hard X-ray detector (HXD-II)

We summarize significant improvements which have been achieved 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 redesigning 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/sup -6/ [cs/sup -1/ keV/sup -1/ cm/sup -2/].

Development and qualification of the HXD-II onboard Astro-E2

The Hard X-ray Detector (HXD-II), one of instruments onboard the Astro-E2 satellite to be launched in February 2005, is in the final stage of its development. The HXD-II probes the universe in the energy range of 10-600 keV with a sensitivity by an order of magnitude better than those of previous missions. The assembly of the HXD-II completed in January 2004, followed by a series of pre-launch qualification tests. As a result, the design goals of the HXD-II have been met. These include; a background level of 5 x 10-6 counts/s/keV/cm2 at 200 keV for GSO and 1 x 10-5 counts/s/keV/cm2 at 30 keV for PIN; energy resolutions of 2.9 keV (PIN diode, at 59.5 keV) and 10% (GSO scintillator, at 662 keV); and low energy thresholds of 10 keV for PIN diodes and 30 keV for GSO scintillators. The measured background predicts a continuum sensitivity of a few x 10-6 photons/s/keV/cm2. Anti-Counter units surrounding the HXD-II provide 50 keV-5 MeV information on gamma-ray bursts and bright X-ray transients.

High resolution fourier synthesis hard X-ray imaging based on CdTe strip detectors

Employing Fourier-synthesis optics and one-dimensional position-sensitive detectors, we are developing a novel hard X-ray imager which can work in the ~10 keV to ~200 keV range either as a telescope or a microscope. As the detection part of our imager, we have developed a strip detector made of Schottky CdTe diode, with its cathode divided into 64 channels of 150 mum pitch. Electrodes of all channels are gold-stud bonded to a fanout board, and connected to low noise analog ASIC. We read out signals from all channels simultaneously. As the grid optics elements, one-dimensional modulation collimator grids of 1 mm thick tungsten have been manufactured, with 10 grid pitches ranging from 0.2 mm to 2 mm with harmonic ratios. Combining the CdTe strip detector and the modulation collimators, we have verified hard X-ray imaging performance of this system. Specifically, by observing an 241Am source, we have successfully obtained an image in the 10-70 keV range

Global-local-grouping multi-grid-type MSGC for neutron applications

We have designed, fabricated and tested a multi-grid-type microstrip gas chamber (M-MSGC) using the global-local-grouping (GLG) method with an active area of 6.4 cm x 6.4 cm. The plate was tested in the 1 atm Ar/CH4 gas and the plate was stably operated at the gas gain of ~1000. The local and the global position detection have been implemented in our ASIC front-end system.

Development of a 2-dimensional multigrid-type MSGC using GLG method for the new generation spallation neutron sources

The neutron detector is one of the key components for the new generation spallation neutron source facilities. We have developed the multigrid-type MSGC(M-MSGC) with He-3 gas as a promising candidate for the new spectrometers. M-MSGC is improved on the ordinal MSGC with putting grids between anodes and cathodes for preventing discharge between electrodes and accumulation of charge on the surface. Moreover, we developed new read out method called as GLG method. Signals are encoded to two groups in this method. One group is named as global, which indicates the coarse position and the other group is local, which shows detail position in a global pattern. A period of the local pattern is corresponding to a global in length. We fabricated a GLG plate, which has 10 times 10 cm2 of active area and 64 readout channels. Then the plate was tested at KEK photon factory as preliminary experiment for neutron detection. In the results, we could demonstrate the usability of the GLG readout method