Future X-ray Astronomy Missions Research Articles

Grinding and chemical mechanical polishing process for micropore x-ray optics fabricated with deep reactive ion etching.

Silicon micropore optics using deep reactive ion etching of silicon wafers has been being developed for future x-ray astronomy missions. Sidewalls of the micropores through a thin wafer with a typical thickness of hundreds of micrometers and a pore width of ∼20 μm are used for x-ray mirrors. However, burr structures observed after etching with a typical height of a few micrometers at the micropore edges are known to significantly reduce x-ray reflectivity. A new grinding and chemical mechanical polishing process is introduced to remove the burr structures. Both sides of the silicon wafer were ground and precisely polished after etching. X-ray reflectivity measurements confirmed an increase of reflectivity by 2-15 times at incident angles of 0.8-0.2deg. The surface microroughness worsened from 2.0±0.2 nm rms to 7.8-0.8+0.6 nm rms; however, an additional annealing recovered the smooth surface and the estimated surface microroughness was <1.4 nm rms. This new process enables not only removing the burr structures but also choosing a flat part of the sidewalls for better angular resolution.

Subpixel response of SOI pixel sensor for X-ray astronomy with pinned depleted diode: first result from mesh experiment

We have been developing a monolithic active pixel sensor, “XRPIX”, for the Japan led future X-ray astronomy mission “FORCE” observing the X-ray sky in the energy band of 1–80 keV with angular resolution of better than 15′′. XRPIX is an upper part of a stack of two sensors of an imager system onboard FORCE, and covers the X-ray energy band lower than 20 keV . The XRPIX device consists of a fully depleted high-resistivity silicon sensor layer for X-ray detection, a low resistivity silicon layer for CMOS readout circuit, and a buried oxide layer in between, which is fabricated with 0.2 μm CMOS silicon-on-insulator (SOI) technology. Each pixel has a trigger circuit with which we can achieve a 10 μs time resolution, a few orders of magnitude higher than that with X-ray astronomy CCDs. We recently introduced a new type of a device structure, a pinned depleted diode (PDD), in the XRPIX device, and succeeded in improving the spectral performance, especially in a readout mode using the trigger function. In this paper, we apply a mesh experiment to the XRPIX devices for the first time in order to study the spectral response of the PDD device at the subpixel resolution. We confirmed that the PDD structure solves the significant degradation of the charge collection efficiency at the pixel boundaries and in the region under the pixel circuits, which is found in the single SOI structure, the conventional type of the device structure. On the other hand, the spectral line profiles are skewed with low energy tails and the line peaks slightly shift near the pixel boundaries, which contribute to a degradation of the energy resolution.

Full-shell x-ray optics development at NASA Marshall Space Flight Center.

NASA's Marshall Space Flight Center (MSFC) maintains an active research program toward the development of high-resolution, lightweight, grazing-incidence x-ray optics to serve the needs of future x-ray astronomy missions such as Lynx. MSFC development efforts include both direct fabrication (diamond turning and deterministic computer-controlled polishing) of mirror shells and replication of mirror shells (from figured, polished mandrels). Both techniques produce full-circumference monolithic (primary + secondary) shells that share the advantages of inherent stability, ease of assembly, and low production cost. However, to achieve high-angular resolution, MSFC is exploring significant technology advances needed to control sources of figure error including fabrication- and coating-induced stresses and mounting-induced distortions.

Hyper-velocity impact risk assessment and mitigation strategies in the context of future X-ray astronomy missions

Future X-ray astronomy missions will be based on instruments with apertures much larger than those used up to now. Therefore, the risk posed by hyper-velocity dust grains in the space environment to the onboard instrumentation will increase, especially when a larger aperture is combined with a longer focal length. Starting from the lessons learned from the XMM and Swift satellites, we review the question of hyper-velocity impacts and discuss the expected impact-rate, risk of damage and possible mitigation strategies in the context of LOFT, eROSITA and ATHENA.

Stress manipulated coating for fabricating lightweight X-ray telescope mirrors.

In this paper wepresent a method to correct the surface profile of an X-ray mirror by using a stress manipulated coating on the back side of mirror shells. The ability to fabricate a thin walled mirror by some replication process is required if future affordable X-ray space missions are to have ~30 times the effective area of the current best X-ray observatory, i.e., the Chandra X-ray Observatory (CXO). Thus, some process is necessary for using replicated X-ray optics to make the next generation X-ray observatory. However, although the surface roughness of sub-100 μm length scales can be replicated, no known replication technique can make 1 arc-second or better CXO-like optics. Yet, because the images produced by the CXO are so exquisite, many X-ray astronomers are not willing to settle for less in the future. Therefore, a post replication technique must be developed to make future major X-ray astronomy missions possible. In this paper, we describe a technique based on DC magnetron sputtering. For figure correction, we apply a controlled bias voltage on the surface during the sputtering. We show that we can produce, in 1-D, shape changes large enough (1 μm over 10 mm) to correct the typical figure errors in replicated optics. We demonstrate reproducibility on an order of 0.6%, and stability over weeks on a scale of less than 1 μm over 10 mm. For these tests, we used 200 μm thick pieces of D263 Schott glass, about 5 mm x 20 mm. In addition to the basic concept of controlling the stress with the coating, we describe a new optimization software design to calculate the stress distribution for a desired surface profile. We show that the combination of the stress optimization software coupled with the coating process, can reduce the slope error of a 5 mm x 20 mm glass sample by a factor of ten.

Development and characterization of the latest X-ray SOI pixel sensor for a future astronomical mission

We have been developing active pixel sensors based on silicon-on-insulator technology for future X-ray astronomy missions. Recently we fabricated the new prototype named “XRPIX2”, and investigated its spectroscopic performance. For comparison and evaluation of different chip designs, XRPIX2 consists of 3 pixel types: Small Pixel, Large Pixel 1, and Large Pixel 2. In Small Pixel, we found that the gains of the 68% pixels are within 1.4% of the mean value, and the energy resolution is 656eV (FWHM) for 8keV X-rays, which is the best spectroscopic performance in our development. The pixel pitch of Large Pixel 1 and Large Pixel 2 is twice as large as that of Small Pixel. Charge sharing events are successfully reduced for Large Pixel 1. Moreover Large Pixel 2 has multiple nodes for charge collection in a pixel. We confirmed that the multi-nodes structure is effective to increase charge collection efficiency.

The Czech Contribution to Future Large X-Ray Astronomy Telescopes: Recent Progress

We briefly review the recent status of the Czech contribution to future space X-ray astronomy missions with emphasis on the development of new technologies and test samples of X-ray mirrors with precise surfaces based on new materials and alternative designs. We report on further investigations and tests of X-ray optical arrangements, such as Kirkpatrick-Baez systems and Multi-Foil Optics.

Optics requirements for x-ray astronomy and developments at the Marshall Space Flight Center

X-ray optics have revolutionized x-ray astronomy; the degree of background suppression that these afford has led to a tremendous increase in sensitivity. The current Chandra observatory has the same collecting area (∼103cm2) as that of the non-imaging UHURU observatory, the first x-ray observatory which was launched in 1970, but has five orders of magnitude more sensitivity due to its focusing optics. In addition, its 0.5″ angular resolution has revealed a wealth of structure in many cosmic x-ray sources.The Chandra observatory achieved its resolution by using relatively thick pieces of Zerodur glass, which were meticulously figured and polished to form the four-shell nested array. The resulting optical assembly weighed >1000kg, and cost approximately $0.5B. The challenge for future x-ray astronomy missions is to greatly increase the collecting area (by one or more orders of magnitude) while ultimately maintaining sub-arcsecond angular resolution, and all within realistic mass and budget constraints.

Technology developments needed for future X-ray astronomy missions

X-ray astronomy is in a privileged situation with the successful missions Chandra and XMM-Newton for more than 10 years in orbit, and Astro-H in the building phase. Over the past 10 years ESA, NASA, and YAXA studies have been made of follow-up missions, like Constellation-X, XEUS, IXO, and ATHENA. This presentation will highlight the technological challenges encountered to build X-ray optics and instrumentation for these types of missions. The optics requires an order of magnitude more collecting area (>5m2) for a few seconds of arc spatial resolution. This drives the focal length of the telescope (∼25m), and thereby the complexity of the spacecraft. Furthermore new technologies are required to realize such an optic within a reasonable mass. The detectors require significant improvement in field of view (number of pixels), energy resolution, and count rate ability. This tends to be possible by the use of Si-based imaging arrays with a large number of pixels, high detection efficiency, and high count rate ability at one side, and the development of modest imaging arrays of cryogenic sensors with very high energy resolution and good detection efficiency at the other side. The cryogenic detectors require further development of cooling systems based on mechanical coolers, like employed for the 1st time on Planck, and planned for Astro-H. The biggest challenge for the realization of such a mission is however not technical. That challenge is that the realization of this future X-ray astronomy mission will require coordination between scientists and Space Agencies on a Global scale.

Development of Superconducting Multilayer Wiring for Large Arrays of TES X-Ray Microcalorimeters

We are developing TES (Transition Edge Sensor) X-ray Microcalorimeters for future X-ray astronomy missions such as DIOS (Diffuse Intergalactic Oxygen Surveyor). Standard wiring for a large-format array requires narrow and closely-packed configuration. This can cause deleterious crosstalk especially in the Frequency Domain Multiplexing readout. Hence we have employed multilayer wiring technique. In this paper, we tested a deposition of a TES film on the multilayer wiring by sputtering. Our first trial showed that the TES pixels have large residual resistances >50 mΩ and small critical currents of 30 µA).

SQUID multiplexing using baseband feedback for space application of transition-edgesensor microcalorimeters

A microcalorimeter array based on a transition-edge sensor (TES) thermometer is apromising imaging spectrometer for use in future x-ray astronomy missions. A TESmicrocalorimeter achieves eV energy resolution and an array of>100 pixels also provides a moderate imaging capability. For a large format array, signalmultiplexing at the low temperature stage is mandatory in order to reduce heat loads fromcold stage preamplifiers and through wirings. We are developing frequency divisionmultiplexing (FDM). In FDM, each TES is ac-biased with a different carrier frequency.Signals from several pixels are summed and then read out by one dc SQUID(superconducting quantum interference device). The maximum number of multiplexedpixels is limited by the bandwidth of a SQUID in a flux-locked loop. Assuming 1 m cablelength between the room temperature and the cold stage, the bandwidth is only<1 MHz with a standard flux-locked loop, due to the delay and phase shift of wirings.We report our development of baseband feedback, a new feedback scheme thatovercomes the bandwidth limitation. In baseband feedback, the signal ( kHz) from the TES is sent back to the SQUID after the phase of carrier frequency (∼1 MHz) has been adjusted. We demonstrated open-loop gain of 8 for 10 kHzsignal at 5 MHz carrier frequency, which indicates the possibility of∼40-pixel multiplexing of the TES signal.

Development of Position-Sensitive Transition-Edge Sensor X-Ray Detectors

We report on the development of position-sensitive transition-edge sensors (PoST's) for future X-ray astronomy missions such as the International X-ray Observatory (IXO), under study by NASA and ESA. PoST's consist of multiple absorbers each with a different thermal coupling to one or more transition-edge sensors (TESs). This results in a characteristic pulse shape for each absorber element and allows position discrimination. PoST development is motivated by a desire to achieve maximum focal-plane area with the fewest number of readout channels. We report detailed characterization of our single TES PoST's or Hydras, which consist of four electroplated Au/Bi absorbers coupled to a low noise Mo/Au TES. Using a numerical model of the Hydra we fit to measured complex impedance curves and determine device parameters that allow us to accurately reproduce the measured pulse shapes and noise spectra. Results from Hydras with different internal thermal conductances reveal the trade-offs in optimizing for energy resolution or position-sensitivity. We report a best achievable energy resolution of < 6.0 eV across all pixels for a device with transition temperature of 86 mK, coupled with straightforward position discrimination by rise-time.

The X-ray quantum efficiency measurement of high resistivity CCDs

The CCD247 is the second generation of high-resistivity device to be manufactured in e2v technologies plc development programme. Intended for infrared astronomy, the latest devices are fabricated on high resistivity (∼8 kΩ cm) bulk silicon, allowing for a greater device thickness whilst maintaining full depletion when ‘thinned’ to a thickness of 150 μm. In the case of the front illuminated variant, depletion of up to 300 μm is achievable by applying a gate to substrate potential of up to 120 V, whilst retaining adequate spectral performance. The increased depletion depth of high-resistivity CCDs greatly improves the quantum efficiency (QE) for incident X-ray photons of energies above 5 keV, making such a device beneficial in future X-ray astronomy missions and other applications. Here we describe the experimental setup and present results of X-ray QE measurements taken in the energy range 2–20 keV for a front illuminated CCD247, showing QE in excess of 80% at 10 keV. Results for the first generation CCD217 and swept-charge device (1500 Ω cm epitaxial silicon) are also presented.

EURECA: European-Japanese Microcalorimeter Array

The EURECA project aims to demonstrate technological readiness of a micro-calorimeter array for application in future X-ray astronomy missions, like Constellation-X, EDGE, and XEUS. The prototype instrument consists of a 5×5 pixel array of TES-based micro-calorimeters read out by two SQUID-amplifier channels using frequency-domain-multiplexing (FDM) with digital base-band feedback. The detector array is cooled by a cryogen-free cryostat consisting of a pulse tube cooler and a two stage ADR. Initial tests of the system at the PTB beam line of the BESSY synchrotron showed stable performance and an X-ray energy resolution of 1.5 eV at 250 eV for read-out of one TES-pixel only. Next step is deployment of FDM to read-out the full array. Full performance demonstration is expected end 2008.

Optimizing Arrays of Position-Sensitive TESs

We are developing position-sensitive transition-edge sensors (PoSTs) for future X-ray astronomy missions such as NASAs Constellation-X. The PoST consists of 1 or more transitions-edge sensors (TESs) thermally connected to a large X-ray absorber, which through heat diffusion, gives rise to position dependence. The development of PoSTs is motivated by the desire to achieve the largest focal-plane coverage with the fewest number of readout channels. In this paper, we investigate optimized signal processing algorithms for single channel PoSTs or ‘Hydras’ (consisting of 4-absorbers connected to 1 TES). Using simulated data, we investigate the impact different parameters such as the thermal conductances and the inductance of the bias circuit have on device performance.

Characterizing the Superconducting-to-Normal Transition in Mo/Au Transition-Edge Sensor Bilayers

We are developing arrays of Mo/Au bilayer transition-edge sensors (TES’s) for future X-ray astronomy missions such as NASA’s Constellation-X. The physical properties of the superconducting-to-normal transition in our TES bilayers, while often reproducible and characterized, are not well understood. The addition of normal metal features on top of the bilayer are found to change the shape and temperature of the transition, and they typically reduce the unexplained ‘excess’ noise. In order to understand and potentially optimize the properties of the transition, we have been studying the temperature, widths and current dependence of these transitions. We report on the characterization of devices both deposited on silicon substrates and suspended on thin silicon nitride membranes. This includes key device parameters such as the logarithmic resistance sensitivity with temperature α, and the logarithmic resistance sensitivity with current β, and their correlation with excess noise.

E33 超薄肉X線望遠鏡基板の旋削加工について(OS-11 超精密加工(2))

An improvement of angular resolution is one of the most important problems in the development of high throughput X-ray telescope for future X-ray astronomy missions. We have developed a new method to make very thin mirror by diamond turning using special support structure made of water solvable UV-curing resin. One of a critical process to minimize the figure error is to control of internal strain. Using this method, we have made test conical mirrors with the diameter of 150 mm, the height of 100 mm, grazing angle of 0.6 degree and thickness of 0.3 mm. The mechanical design of this mirror and support structure, process of diamond machining including chucking and separation, and X-ray test for the mirror performance and their results are described.

Theoretical noise analysis on a position-sensitiveMetallic Magnetic Calorimeter

We report on the theoretical noise analysis for a position-sensitive MetallicMagnetic Calorimter (MMC), consisting of MMC read-out at both ends of a large X-rayabsorber. Such devices are under consideration as alternatives to other cryogenic technologiesfor future X-ray astronomy missions. We use a finite-element model (FEM) to numericallycalculate the signal and noise response at the detector outputs and investigate the correlationsbetween the noise measured at each MMC coupled by the absorber. We then calculate, using theoptimal filter concept, the theoretical energy and position resolution across the detector anddiscuss the trade-offs involved in optimising the detector design for energy resolution, positionresolution and count rate. The results show, theoretically, the position-sensitive MMC conceptoffers impressive spectral and spatial resolving capabilities compared to pixel arrays and similarposition-sensitive cryogenic technologies using Transition Edge Sensor (TES) read-out.

The MPE X-ray test facility PANTER: Calibration of hard X-ray (15–50 kev) optics

The Max-Planck-Institut fur extraterrestrische Physik (MPE) in Garching, Germany, uses its large X-ray beam line facility PANTER for testing X-ray astronomical instrumentation. A number of telescopes, gratings, filters, and detectors, e.g. for astronomical satellite missions like Exosat, ROSAT, Chandra (LETG), BeppoSAX, SOHO (CDS), XMM-Newton, ABRIXAS, Swift (XRT), have been successfully calibrated in the soft X-ray energy range (<15keV). Moreover, measurements with mirror test samples for new missions like ROSITA and XEUS have been carried out at PANTER. Here we report on an extension of the energy range, enabling calibrations of hard X-ray optics over the energy range 15–50 keV. Several future X-ray astronomy missions (e.g., Simbol-X, Constellation-X, XEUS) have been proposed, which make use of hard X-ray optics based on multilayer coatings. Such optics are currently being developed by the Osservatorio Astronomico di Brera (OAB), Milano, Italy, and the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA, USA. These optics have been tested at the PANTER facility with a broad energy band beam (up to 50 keV) using the XMM-Newton EPIC-pn flight spare CCD camera with its good intrinsic energy resolution, and also with monochromatic X-rays between C-K (0.277 keV) and Cu-Kα (8.04 keV).

Pre-flight performance and radiation hardness of the Tokyo Tech pico-satellite Cute-1.7

The Cute-1.7 was launched successfully in February 2006 as a piggyback satellite of the Astro-F mission. The Cute-1.7 dimensions are 10 × 10 × 20 cm 3 box with a total mass of 3.6 kg. It is the second pico-satellite to have been developed completely by students of the Tokyo Institute of Technology (Tokyo Tech.) after the successful launch of the CUTE-I in June 2003. The goals of the Cute-1.7 mission are two-fold: (1) to validate high-performance, commercially available products for the first time in space. We particularly use personal digital assistants (PDAs) as a main computer in orbit (2) to demonstrate new potential uses for small satellites in various space studies, as proposed by the “satellite-core” concept. For the Cute-1.7 mission, we will carry avalanche photo diodes (APDs) as a high-count particle monitor in low-Earth orbit. Here we present details of various ground tests and pre-flight performance of the Cute-1.7 immediately before the launch. Results of the Cute-1.7 mission will provide quick feedback for space applications of APDs in Japan's future X-ray astronomy mission NeXT.