A. Kushino

Study of the X-Ray Background Spectrum and Its Large-Scale Fluctuation with ASCA

We studied the energy spectrum and the large-scale fluctuation of the X-ray background with the ASCA GIS based on the ASCA MSS and LSS observations. A total of 91 fields with Galactic latitude |b|>10 deg were selected with a sky coverage of 50 deg^2 and 4.2 Ms of exposure. For each field, sources brighter than \sim 2\times 10^{-13}\ergs (2-10 keV) were eliminated. Spectral fits with a single power-law model for the individual 0.7-10 keV spectra showed a significant excess below \sim 2 keV, which could be expressed by an additional thermal model with kT\simeq 0.4 keV or a steep power-law model with photon index GS\simeq 6. The 0.5-2 keV intensities of the soft thermal component varied significantly by 1\sigma=52^{+4}_{-5}%, and showed a maximum toward the Galactic Center. This component is considered to be entirely Galactic. As for the hard power-law component, an average photon index of 91 fields was obtained to be GH = 1.412\pm 0.007\pm 0.025 and the average 2-10 keV intensity was calculated as FH = (6.38\pm 0.04\pm 0.64)\times 10^{-8} \ergss. The Galactic component is marginally detected in the hard band. The 2-10 keV intensities shows a 1\sigma deviation of 6.49^{+0.56}_{-0.61}%, while deviation due to the NXB is 3.2%. The observed deviation can be explained by the Poisson noise of source count in the f.o.v. (\sim 0.5 deg^2), even assuming a single \logn relation on the whole sky. Based on the observed fluctuation and the absolute intensity, an acceptable region of the \logn relation was derived, showing a consistent feature with the recent Chandra and XMM-Newton results. Fluctuation of the spectral index was also examined, and it implied a large amount of hard sources and a substantial variation in the intrinsic source spectra (GS\simeq 1.1\pm 1.0).

Erbium-doped yttrium aluminum garnet as a magnetic refrigerant for low temperature x-ray detectors

Garnets doped with rare-earth elements can be used in adiabatic demagnetization refrigerators. We have measured the specific heat and magnetization of a single crystal yttrium aluminum garnet (YAG) doped with 30% Er3+ ion at temperatures between 93 mK and 8 K under magnetic fields up to 8.0 T along the 〈111〉 crystal axis. From the specific heat and magnetization, we derived consistent temperature and magnetic-field dependence of the magnetic entropy. Under zero magnetic field, the magnetic entropy begins to decrease below 2 K and becomes half of R ln 2 at ∼160 mK. This decrease is considered to be due to an antiferromagnetic short-range ordering among Er3+ ions. This behavior of the specific heat in the measured temperature range can be explained by a model in which both the crystalline-electric-field ground state and the first excited state are included. The operating temperature of the Er3+-doped YAG as a magnetic coolant is estimated to extend down to ∼100 mK, which is lower than those with nonsubstitu...

Status of X-ray microcalorimeter development at ISAS

A superconducting phase-transition microcalorimeter is a promising detector for high resolution X-ray spectroscopy. We are developing such a detector for future Japanese X-ray astronomy missions. In our design, a tin absorber is electrodeposited as a mushroom structure to achieve a high covering fraction. We have succeeded in detecting X-rays with the first model of our calorimeter with an electrodeposited absorber, though the energy resolution was limited due to residual resist. We are also developing a phase-transition microcalorimeter with a low transition temperature. We describe the results of these experiments, and discuss the limiting factors of their performance.

Two dimensional imaging by series-connected STJs with sapphire absorber

Abstract The position determination capability of the series-connected superconducting tunnel junctions (STJs) with Nb/Al/AlO x /Al/Nb junctions as a radiation detector is reported. Four groups of series-connected STJs are fabricated on a single crystal of sapphire substrate surrounding 3.5×3.5 mm 2 detection area. The signals detected by two STJs with 1.5 mm separation give a time delay caused by the propagation of phonons, and this enables us to measure the irradiated position. Using α -particles of 5.5 MeV , we clearly distinguished the incident positions with a potential resolution of 0.16±0.07 mm (FWHM). Directional dependence in the phonon propagation in the sapphire substrate is not detected.

Fabrication of an X-ray microcalorimeter with an electrodeposited X-ray microabsorber

For the next generation of astronomical X-ray imaging detectors, arrays with large numbers of microstructures (∼1000 pixels) will be required. To meet this requirement, a tin absorber for an X-ray microcalorimeter, which has a so-called “mushroom” shape, is fabricated by electrodeposition and polishing. This method enables the fabrication of a large number of arrayed microcalorimeters. Details of the fabrication process, the characteristics of the absorber and the fabricated microcalorimeters are reported.

Measurements of the linearity of an STJ and position resolution of series-connected STJs

Abstract Superconducting tunnel junctions (STJ) have been developed mainly at high-resolution spectrometers for use in X-ray astronomy. A FWHM energy resolution of 112 eV at 5.9 keV is obtained using an STJ developed at Nippon Steel Corporation connected with a cooled FET (∼100 K). The pulse height of the signal is represented by a logarithmic function of energy based on consideration of the recombination of the quasi-particles in the junction. Experiments using series-connected STJs for an imaging radiation detector are performed. Both the pulse height and the rise time of signals from 241 Am α-particles indicate good position sensitivity with resolution less than 0.5 mm.

Entropy behavior of Er-doped YAG for application to ADRs

Garnets doped with rare-earth element are proposed for the coolant of adiabatic demagnetization refrigerators (ADR). We measured the specific heat and magnetization of a single crystal YAG (yttrium aluminum garnet) doped with 30% Er3+ ions for a temperature range between 93 mK and 8 K under a magnetic field up to 8.0 T applied along the 〈111〉 crystal axis. After correcting for the phonon and nuclear contributions, we derived the temperature and magnetic-field dependence of the magnetic entropy. Under a zero magnetic field, the entropy starts to decrease below 2 K and becomes half of the R ln 2 level at ∼160 mK, which implies that the YAG refrigerator can be used for operating temperatures down to 100 mK.

Development of superconducting tunnel junction as an imaging radiation detector

Abstract Superconducting tunnel junctions (STJs) as X-ray detectors have been developed mainly aiming at high resolution spectrometers. We archived an energy resolution of 106 eV at 5.9 keV (FWHM) using an STJ developed at Nippon Steel Corporation with a cooled (∼ 100K) FET. Furthermore, series-connected STJs as an imaging radiation detector are developed. Both the pulse hight and the rise time of signals from 241 Am α-particles irradiated on a series-connected STJ give a good position sensitivity, indicating the intrinsic position resolution less than 0.5 mm.

Development of STJ as a New X-Ray Detector

STJs are promising X-ray detectors as high energy resolution spectrometers due to the small excitation energy to break the Cooper pairs to product detectable electrons. The expected energy resolution is about 5 eV for a 6 keV incident X-rays (see review by Kraus et al. and Esposito et al.). We have developed a large area (178 x 178μm 2) Nb/Al/AlOx/Al/Nb STJs (Kurakado et al. 1993) and series-connected STJs with a position resolution of 35μm for α particles (Kurakado 1997) at Nippon Steel Corporation. As a focal plane detector in future X-ray missions, we are developing STJs whose targert characteristics are ; an energy resolution of 20 eV at 6keV, an effective area of 1 cm2, and position resolution of 100μm.

Mapping the Virgo Cluster of Galaxies with ASCA

In the Virgo cluster, we can perform a close study of the gas injection mechanism from galaxies into the cluster space and the interaction between the injected gas and the sorrouding cluster medium. In 1996 to 1997, we carried out mapping observations of a 2.∘5 x 2.∘5 area in the north-west region of the cluster. There are 16 pointings in total in this region, and the observed results are briefly reported here.