The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated
by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the highenergy
universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV.
These instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3–12 keV with
high spectral resolution of ΔE ≦ 7 eV, enabled by a micro-calorimeter array located in the focal plane of
thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5–80 keV, located in the focal plane of
multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4–12 keV,
with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera
type soft gamma-ray detector, sensitive in the 40–600 keV band. The simultaneous broad bandpass, coupled
with high spectral resolution, will enable the pursuit of a wide variety of important science themes.
Following our development of a superconducting transition-edge-sensor (TES) microcalorimeter design that en-
ables reproducible, high performance (routinely better than 3 eV FWHM energy resolution at 6 keV) and is
compatible with high-fill-factor arrays, we have directed our efforts towards demonstrating arrays of identical
pixels using the multiplexed read-out concept needed for instrumenting the Constellation-X X-ray Microcalorime-
ter Spectrometer (XMS) focal plane array. We have used a state-of-the-art, time-division SQUID multiplexer
system to demonstrate 2
×8 multiplexing (16 pixels read out with two signal channels) with an acceptably modest
level of degradation in the energy resolution. The average resolution for the 16 multiplexed pixels was 2.9 eV,
and the distribution of resolution values had a relative standard deviation of 5%. The performance of the array
while multiplexed is well understood. The technical path to realizing multiplexing for the XMS instrument on
the scale of 32 pixels per signal channel includes increasing the system bandwidth by a factor of four and reducing
the non-multiplexed SQUID noise by a factor of two.
In this paper we discuss the characteristics of a uniform 8
×8 array and its performance when read out non-
multiplexed and with various degrees of multiplexing. We present data acquired through the readout chain from
the multiplexer electronics, through the real-time demultiplexer software, to storage for later signal processing.
We also report on a demonstration of real-time data processing. Finally, because the multiplexer provides
unprecedented simultaneous access to the pixels of the array, we were able to measure the array-scale uniformity
of TES calorimeter parameters such as the individual thermal conductances and superconducting transition
temperatures of the pixels. Detector uniformity is essential for optimal operation of a multiplexed array, and
we found that the distributions of thermal conductances, transition temperatures, and transition slopes were
sufficiently tight to avoid significant compromises in the operation of any pixel.
Individual x-ray calorimeters based on superconducting transition-edge sensors (TES) have already demonstrated
the spectral resolution, speed, and quantum efficiency needed for astrophysical x-ray spectroscopy. We are now
beginning to realize this capability on the array scale for the first time. We have developed a new design for the
x-ray absorber that has connections to the TES engineered to allow contact only in regions that do not serve
as the active thermometer. We have further constrained the design so that a low-resistance absorber will not
electrically short the TES, permitting the use of high-conductivity electroplated gold for the x-ray absorber.
With such a well-behaved material for the absorber, we now achieve energy resolution at 6 keV in the range 2.4
- 3.1 eV FWHM in all the pixels of the same design tested in a close-packed array. We have achieved somewhat
higher resolution and faster response by eliminating some of the gold and electroplating bismuth in its place.
These are important steps towards the high-resolution, high-fill-factor, microcalorimeter arrays needed for x-ray
astrophysics observatories such as Constellation-X.
We report on our studies of possible configurations for the focal plane of the Constellation-X mission. Taking
advantage of new developments in both SQUID multiplexing technology and position-sensitive detectors, we
present a viable focal plane intrument design that would greatly enhance the reference Constellation-X configuration
of a 32 × 32 array. An order of magnitude increase in the number of pixels of the focal plane array from
the current 1024-pixel reference design is achievable.
We have been developing x-ray microcalorimeters for the Constellation-X mission. Devices based on superconducting transition-edge sensors (TES) have demonstrated the potential to meet the Constellation-X requirements for spectral resolution, speed, and array scale (> 1000 pixels) in a close-packed geometry. In our part of the GSFC/NIST collaboration on this technology development, we have been concentrating on the fabrication of arrays of pixels suitable for the Constellation-X reference configuration. We have fabricated 8x8 arrays with 0.25-mm pixels arranged with 92% fill factor. The pixels are based on Mo/Au TES and Bi/Cu or Au/Bi absorbers. We have achieved a resolution of 4.0 eV FWHM at 6 keV in such devices, which meets the Constellation-X resolution requirement at 6 keV. Studies of the thermal transport in our Bi/Cu absorbers have shown that, while there is room for improvement, for 0.25-mm pixels the standard absorber design is adequate to avoid unacceptable line-broadening from position dependence caused by thermal diffusion. In order to improve reproducibility and to push closer to the 2-eV goal at 6 keV, however, we are refining the design of the TES and the interface to the absorber. Recent efforts to introduce a barrier layer between the Bi and the Mo/Au to avoid variable interface chemistry and thus improve the reproducibility of device characteristics have thus far yielded unsatisfactory results. However, we have developed a new set of absorber designs with contacts to the TES engineered to allow contact only in regions that do not serve as the active thermometer. We have further constrained the design so that a low-resistance absorber will not electrically short the TES. It is with such a design that we have achieved 4.0 eV resolution at 6 keV.
We present our latest results from our development of Position-Sensitive Transition-Edge Sensors (PoSTs). Our devices work as one-dimensional imaging spectrometers. They consist of a long absorber (segmented or solid) with a transition-edge sensor (TES) on each end. When X-rays hit the absorber, the comparison of the signals sensed in the two TESs determine the position of the TES, while the addition of the signals gives the energy of the X-ray. We obtained impedance curves for three different devices and obtained reasonable fits with our theoretical PoST model.
We have investigated the thermal, electrical, and structural properties of Bi and BiCu films that are being developed as X-ray absorbers for transition-edge sensor (TES) microcalorimeter arrays for imaging X-ray spectroscopy. Bi could be an ideal material for an X-ray absorber due to its high X-ray stopping power and low specific heat capacity, but it has a low thermal conductivity, which can result in position dependence of the pulses in the absorber. In order to improve the thermal conductivity, we added Cu layers in between the Bi layers. We measured electrical and thermal conductivities of the films around 0.1 K, the operating temperature of the TES calorimeter, to examine the films and to determine the optimal thickness of the Cu layer. From the electrical conductivity measurements, we found that the Cu is more resistive on the Bi than on a Si substrate. Together with a SEM picture of the Bi surface, we concluded that the rough surface of the Bi film makes the Cu layer resistive when the Cu layer is not thick enough to fill in the roughness. From the thermal conductivity measurements, we determined the thermal diffusion constant to be 2 x 10<sup>3</sup> μm<sup>2</sup>μs<sup>-1</sup> in a film that consists of 2.25 μm of Bi and 0.1 μm of Cu. We measured the position dependence in the film and found that its thermal diffusion constant is too low to get good energy resolution, because of the resistive Cu layer and/or possibly a very high heat capacity of our Bi films. We show plans to improve the thermal diffusion constant in our BiCu absorber.
We are developing a superconducting transition-edge sensor (TES) calorimeter for future Japanese X-ray astronomy missions (e.g. NeXT mission). The performance of our single pixel TES calorimeter is presented. We fabricated a Ti/Au (40 nm/110 nm) bilayer TES
on a thin silicon-nitride membrane, which is adjusted to have a transition temperature of about 100 mK. The size of the TES is 500μm × 500μm, and 300μm × 300μm gold with a thickness of 300 nm is deposited with sputtering as an X-ray absorber. The TES calorimeter was installed in a dilution refrigerator operated at about 40 mK, with a combination of 400-series SQUID array as an ammeter. Collimated 5.9 keV X-rays (200 um in diameter) from 55Fe isotope were irradiated and X-ray pulses were obtained. Simultaneously with a fast falling time constant of 74.2 us,
the energy resolution of 6.6+-0.4 eV was attained, while the baseline noise was 6.4 eV. The contents of the energy resolution are
5.1 eV of the excess noise, 3.3 eV of the readout noise, 1.6 eV of the pulse by pulse variation, and 1.9 eV of the intrinsic noise.
The baseline noise are dominated by an unknown excess noise,
which increases roughly in proportion to the inverse of the TES resistance. The pulse height is sensitive to the operating conditions,
and the superconducting shield appears to have improved it
by a factor of about 2. The calorimeter works fine over six months
surviving five thermal cycles, even though it is kept in air.
We are developing a superconducting Transition-Edge Sensor (TES) calorimeter array. We adopt calorimeter multiplex in frequency domain to read signals from the calorimeter array with a small number of front-end electronics and wirings. We further utilize Calorimeter Bridge Biased by an AC Generator (CABBAGE) approach to eliminate the AC carrier in the output. We tested the method using a TES calorimeter, which has a transition temperature of 390 mK. Because of the high operating temperature, energy resolution of the calorimeter is limited to 200 eV at 5.9 keV even when it is biased with a DC current. We operated the calorimeter in CABBAGE circuit with 30 kHz sinusoidal bias and obtained an energy resolution of 250 eV. We found that there remains a small-amplitude residual in the output even at the bridge balance point. The residual contains not only 30 kHz component but also odd-order harmonics. We consider that this is due to the variation of the TES resistance with bias current. The 50 eV degradation of the energy resolution from DC to AC biases can be explained by the fact that some of signal power is carried in the odd-order harmonics, which we did not utilize in the data reduction process. We also succeeded to operate the CABBAGE by 100 kHz, although the energy resolution was degraded to 380 eV probably due to low response of the signal readout circuit at the frequency.
An X-ray microcalorimeter that consists of an x-ray absorber to transfer the incident photon energy to the temperature rise, a temperature sensor to detect the temperature change and suspending beams for thermal isolation from the substrate have been fabricated. Titanium/Gold thin film transition edge sensor (TES) is used as the temperature sensor. We fabricated and tested the first prototype in the previous study and obtained the transition temperature of 0.52 K, energy resolution of 550 eV (FWHM) for 6 keV radiation. These values were smaller than that of expected. We applied a Sn absorber and redesigned the microstructure of the x-ray microcalorimeter. Consequently, we have obtained 158 eV at 5.9 keV radiation of the energy resolution, which is about 4 times higher than that of the first prototype. This value is nearly equal to the conventional X-ray CCD. The highest energy resolution of the x-ray microcalorimeter of our design is estimated to approximately 5 eV at the operating point of 0.2 K. To realize such a good energy resolution calorimeter array, we are going to improve the sensitivity of the TES by optimizing the process condition. A Sn absorber formed by electroplating is also under evaluating simultaneously. It is necessary to fabricate uniform array structures.
The Hard X-ray Detector (HXD) is one of the three instruments on the fifth Japanese cosmic X-ray satellite ASTRO-E, scheduled for launch in January 2000. The HXD covers a wide energy range of 10-600 keV, using 16 identical GSO/BGO phoswich-counter modules, of which the low-energy efficiency is greatly improved by adding 2 m-thick silicon PIN diodes. Production of the HXD has been completed and pre-flight calibration is now in progress. The design concept of the HXD sensor, detail of the production process, and a brief summary of the measured performance is reported.
The hard x-ray detector (HXD) is one of the three experiments of the Astro-E mission, the fifth Japanese X-ray Satellite devoted to studies of high energy phenomena in the universe in the x-ray to soft gamma-ray region. Prepared for launch at the beginning of 200 via the newly developed M-V launch vehicle of the Institute of Space and Astronomical Science, the Astro-E is to be thrown in to a near-circular orbit of 550 km altitude, with an inclination of 31 degrees. The flight model has been finished assembled this year, and we carried out various tests to verify the performance. We acquired the background spectrum at sea level, and confirmed that our system is operating effectively in reducing the background level. The HXD will observe photons in the energy range of 10-600 keV, and the calculations based on the preflight calibration suggest that the HXD will have the highest sensitivity ever achieved in this energy range. We also verified that our electronic system will maintain its performance against charged particle events expected in orbit.
The Hard x-ray Detector (HXD) is one of three instruments on the fifth Japanese x-ray astronomy satellite, Astro-E, scheduled for launch in 2000. The sensitivity of the Astro-E HXD will be higher by more than one order of magnitude than that of nay previous instrument between 10 keV and several 100 keV. The electronic system is designed to handle many independent data channels from the HXD within the limitation of size and power consumption required in Astro-E. In this paper, we will present the design and the preliminary performance of the processing electronic system.
ASTRO-E is the next Japanese x-ray satellite to be launched in the year 2000. It carries three high-energy astrophysical experiments, including the hard x-ray detector (HXD) which is unique in covering the wide energy band from 10 keV to 700 keV with an extremely low background. The HXD is a compound-eye detector, employing 16 GSO/BGO well-type phoswich scintillation counters together with 64 silicon PIN detectors. The scintillation counters cover an energy range of 40 - 700 keV, while the PIN diodes fill the intermediate energy range from 10 keV to 70 keV with an energy resolution about 3 keV. In this paper, we report on the developments of the large area, thick silicon PIN diodes. In order to achieve a high quantum efficiency up to 70 keV with a high energy resolution, we utilize a double stack of silicon PIN diodes, each 20 by 20 mm<SUP>2</SUP> in size and 2 mm thick. Signals from the two diodes are summed into a single output. Four of these stacks (or eight diodes) are placed inside the deep BGO active-shield well of a phoswich counter, to achieve an extremely low background environment. Thus, the HXD utilizes 64 stacked silicon PIN detectors, achieving a total geometrical collecting area of 256 cm<SUP>2</SUP>. We have developed the 2 mm thick silicon PIN diodes which have low leakage current, a low capacitance, and a high breakdown voltage to meet the requirements of our goal. Through various trials in fabricating PIN diodes with different structures, we have found optimal design parameters, such as mask design of the surface p<SUP>+</SUP> layer and the implantation process.
Astro-E is the x-ray satellite to be launched in the year 2000 by Inst. of Space & Astronautical Science. This report deals with the design and expected performance of the hard x-ray detector (HXD), one of the 3 experiments aboard Astro- E. The HXD is a combination of GSO/BGO well-type phoswich counters and silicon PIN diodes: the two combined will cover a wide energy band of 10 - 700 keV. The detector is characterized by its low background of approximately 10<SUP>-5</SUP>/s/cm<SUP>2</SUP>/keV and its sensitivity higher than any past missions between a few 10 keV and several 100 keV. Combined with the other 2 experiments, a micro-calorimeter array (XRS) and 4 CCD arrays (XIS), both with x-ray mirrors, the mission will cover the soft and hard x-ray range at a highest sensitivity.
The fourth Japanese x-ray astronomy satellite, ASCA, carries two imaging gas scintillation proportional counters (GIS) on its focal plane. Extensive ground calibration has established its position resolution to be 0.5 mm and FWHM energy resolution to be 8.0% both at 6 keV. When combined with the x-ray telescope, a sensitivity range becomes 0.7 - 10 keV. These properties have been confirmed through in-orbit calibrations. The in-orbit background of the GIS has been confirmed to be as low as (5 - 7) X 10<SUP>-4</SUP> c s<SUP>-1</SUP>cm<SUP>-2</SUP>keV<SUP>-1</SUP> over the 1 - 10 keV range. The long-term detector gain is stable within a few % for two years. Gain dependence on the position and temperature has been calibrated down to 1%. The overall energy response is calibrated very accurately. Thus the GIS is working as an all-round cosmic x-ray detector.