Jessica A. Gaskin

First results from a next-generation off-plane X-ray diffraction grating

Future NASA X-ray spectroscopy missions will require high throughput, high resolving power grating spectrometers. Off-plane reflection gratings are capable of meeting the performance requirements needed to realize the scientific goals of these missions. We have identified a novel grating fabrication method that utilizes common lithographic and microfabrication techniques to produce the high fidelity groove profile necessary to achieve this performance. Application of this process has produced an initial pre-master that exhibits a radial (variable line spacing along the groove dimension), high density (> 6000 grooves/mm), laminar profile. This pre-master has been tested for diffraction efficiency at the BESSY II synchrotron light facility and diffracts up to 55 % of incident light into usable spectral orders. Furthermore, tests of spectral resolving power show that these gratings are capable of obtaining resolving powers well above 1300 (λ/Δλ) with limitations due to the test apparatus, not the gratings. Obtaining these results has provided confidence that this fabrication process is capable of producing off-plane reflection gratings for the next generation of X-ray observatories.

The Mass, Baryonic Fraction, and X-Ray Temperature of the Luminous, High-Redshift Cluster of Galaxies MS 0451.6–0305

We present new Chandra X-ray observations of the luminous and cosmologically significant X-ray cluster of galaxies, MS 0451.6-0305, at z = 0.5386. Spectral imaging data for the cluster are consistent with an isothermal cluster of (10.0-10.6) ± 1.6 keV, with an intracluster Fe abundance of (0.32-0.40) ± 0.13 solar. The systematic uncertainties, arising from calibration and model uncertainties, of the temperature determination are nearly the same size as the statistical uncertainties, since the time-dependent correction for absorption on the detector is uncertain for these data. We discuss the effects of this correction on the spectral fitting. The effects of statistics and fitting assumptions of two-dimensional models for the X-ray surface brightness are thoroughly explored. This cluster appears to be elongated, and so we quantify the effects of assuming an ellipsoidal gas distribution on the gas mass and the total gravitating mass estimates. These data are also jointly fitted with previous Sunyaev-Zeldovich observations to obtain an estimate of the clusters distance (DA = 1219 ± 387 Mpc, statistical followed by systematic uncertainties) assuming spherical symmetry. If, instead, we assume a Hubble constant, the X-ray and Sunyaev-Zeldovich data are used together to test the consistency of an ellipsoidal gas distribution and to weakly constrain the intrinsic axis ratio. The mass derived from the X-ray data is consistent with the weak-lensing mass and is only marginally less than the mass determined from the optical velocities. We confirm that this cluster is very hot and massive, further supporting the conclusion of previous analyses that the universe has a low matter density and that cluster properties have not evolved much since z ~ 0.5. Furthermore, the presence of iron in this high-redshift cluster at an abundance that is the same as that of low-redshift clusters implies that there has been very little evolution of the cluster iron abundance since z ~ 0.5. We discuss the possible detection of a faint, soft, extended component that may be the by-product of hierarchical structure formation.

The X-Ray Surveyor Mission: A Concept Study

NASAs Chandra X-ray Observatory continues to provide an unparalleled means for exploring the high-energy universe. With its half-arcsecond angular resolution, Chandra studies have deepened our understanding of galaxy clusters, active galactic nuclei, galaxies, supernova remnants, neutron stars, black holes, and solar system objects. As we look beyond Chandra, it is clear that comparable or even better angular resolution with greatly increased photon throughput is essential to address ever more demanding science questions—such as the formation and growth of black hole seeds at very high redshifts; the emergence of the first galaxy groups; and details of feedback over a large range of scales from galaxies to galaxy clusters. Recently, we initiated a concept study for such a mission, dubbed X-ray Surveyor. The X-ray Surveyor strawman payload is comprised of a high-resolution mirror assembly and an instrument set, which may include an X-ray microcalorimeter, a high-definition imager, and a dispersive grating spectrometer and its readout. The mirror assembly will consist of highly nested, thin, grazing-incidence mirrors, for which a number of technical approaches are currently under development—including adjustable X-ray optics, differential deposition, and new polishing techniques applied to a variety of substrates. This study benefits from previous studies of large missions carried out over the past two decades and, in most areas, points to mission requirements no more stringent than those of Chandra.

Performance of a Thin-Window Silicon Drift Detector X-Ray Fluorescence Spectrometer

Several sets of hexagonal Silicon Drift Detector (SDD) arrays were produced by Brookhaven National Laboratory (BNL) and by the commercial vendor, KETEK. These detector arrays were tested at BNL. Each array consists of 14 independent SDD detectors (pixels) and two additional test pixels located at two corners of the array. The side of the detector upon which the X-ray radiation is incident (window side) has a thin junction covering the entire active area. The opposite side (device side) contains a drift-field electrode structure in the form of a hexagonal spiral and an electron collecting anode. There are four guard rings surrounding the 14-pixel array area on each side of the detector. Within each array, seven pixels have aluminum field plates - interrupted spirals that stabilize the electric potential under the Si- SiO2 interface, while the other seven do not. Three bias voltages are applied to control the drift field in the silicon volume; one is applied to a rectifying contact surrounding the central anode (one for each pixel), one is applied to the detector entrance window (common to the full array), and a third bias is applied to a contact on the outer portion of the spiral, common to all pixels in the array. Some arrays were recently tested in NSLS beam line U3C at BNL. For this work, we installed the complete assemblies in the vacuum and cooled them to -27degC. During this beam run, we collected spectra for energies ranging between 350 and 900 eV in several pixels, some with field plates and others without. The detailed testing results of several arrays are reported here.

Beyond Chandra: the x-ray Surveyor

Over the past 16 years, NASAs Chandra X-ray Observatory has provided an unparalleled means for exploring the high energy universe with its half-arcsecond angular resolution. Chandra studies have deepened our understanding of galaxy clusters, active galactic nuclei, galaxies, supernova remnants, planets, and solar system objects addressing most, if not all, areas of current interest in astronomy and astrophysics. As we look beyond Chandra, it is clear that comparable or even better angular resolution with greatly increased photon throughput is essential to address even more demanding science questions, such as the formation and subsequent growth of black hole seeds at very high redshift; the emergence of the first galaxy groups; and details of feedback over a large range of scales from galaxies to galaxy clusters. Recently, NASA Marshall Space Flight Center, together with the Smithsonian Astrophysical Observatory, has initiated a concept study for such a mission now named the X-ray Surveyor. This concept study starts with a baseline payload consisting of a high resolution X-ray telescope and an instrument set which may include an X-ray calorimeter, a wide-field imager and a dispersive grating spectrometer and readout. The telescope would consist of highly nested thin shells, for which a number of technical approaches are currently under development, including adjustable X-ray optics, differential deposition, and modern polishing techniques applied to a variety of substrates. In many areas, the mission requirements would be no more stringent than those of Chandra, and the study takes advantage of similar studies for other large area missions carried out over the past two decades. Initial assessments indicate that such an X-ray mission is scientifically compelling, technically feasible, and worthy of a high prioritization by the next American National Academy of Sciences Decadal Survey for Astronomy and Astrophysics.

ASIC for SDD-Based X-Ray Spectrometers

We present an application-specific integrated circuit (ASIC) for high-resolution x-ray spectrometers (XRS). The ASIC reads out signals from pixelated silicon drift detectors (SDDs). The pixel does not have an integrated field effect transistor (FET); rather, readout is accomplished by wire-bonding the anodes to the inputs of the ASIC. The ASIC dissipates 32 mW, and offers 16 channels of low-noise charge amplification, high-order shaping with baseline stabilization, discrimination, a novel pile-up rejector, and peak detection with an analog memory. The readout is sparse and based on custom low-power tristatable low-voltage differential signaling (LPT-LVDS). A unit of 64 SDD pixels, read out by four ASICs, covers an area of 12.8 cm2 and dissipates with the sensor biased about 15 mW/cm2. As a tile-based system, the 64-pixel units cover a large detection area. Our preliminary measurements at -44°C show a FWHM of 145 eV at the 5.9 keV peak of a 55Fe source, and less than 80 eV on a test-pulse line at 200 eV.

Front-End ASIC for High Resolution X-Ray Spectrometers

We present an application specific integrated circuit (ASIC) for high-resolution X-ray spectrometers. The ASIC is designed to read out signals from a pixelated silicon drift detector (SDD). Each hexagonal pixel has an area of 15 mm2 and an anode capacitance of less than 100 fF. There is no integrated Field Effect transistor (FET) in the pixel, rather, the readout is done by wire-bonding the anodes to the inputs of the ASIC. The ASIC provides 14 channels of low-noise charge amplification, high-order shaping with baseline stabilization, and peak detection with analog memory. The readout is sparse and based on low voltage differential signaling. An interposer provides all the interconnections required to bias and operate the system. The channel dissipates 1.6 mW. The complete 14-pixel unit covers an area of 210 mm2, dissipates 12 mW cm-2, and can be tiled to cover an arbitrarily large detection area. We measured a preliminary resolution of 172 eV at -35 degC on the 6 keV peak of a 55Fe source.

Development of Thin-Window Silicon Drift Detector for X-ray spectroscopy

A new set of thin-window silicon drift detectors composed of an array of hexagonal shaped detectors has been designed, constructed and tested for X-ray spectroscopy. Each individual ThinWinSDD has a thin entrance window on one side and a spiral shaped hexagonal cathode around a center anode on the other side. To produce the thin entrance window a 10 keV implantation of boron through a 500 A silicon dioxide was used. The implantation was followed by an annealing at 700degC for 30 min and a reactive ion etching step to ensure the removal of silicon dioxide from the smallest feature (5 mum). An aluminum layer is coated in the same vacuum system after back-sputtering. This step involves removing the native oxide that has formed on the top of the silicon substrate and then sputtering a 1100 A thick layer of aluminum onto the X-ray entrance window. The aluminum layer must be thick enough to block visible light, but thin enough to be transparent to soft X-rays down to 280 eV. We discuss first test results that include detector leakage current measurements and the response for multiple detectors exposed to the National Synchrotron Light Sources UV beam line U3C located at Brookhaven National Laboratory for X-ray energies as low as 280 eV.

Differential deposition to correct surface figure deviations in astronomical grazing-incidence x-ray optics

A coating technique is being developed to correct the surface figure deviations in grazing-incidence X-ray optics. These optics are typically designed to have precise conic profiles, and any deviation in this profile, as a result of fabrication, results in a degradation of the imaging performance. To correct the mirror profiles, physical vapor deposition has been utilized to selectively deposit a filler material inside the mirror shell. The technique, termed differential deposition, has been implemented as a proof of concept on miniature X-ray optics developed at MSFC for medical-imaging applications. The technique is now being transferred to larger grazing-incidence optics suitable for astronomy.

X-ray optic developments at NASA's MSFC

NASAs Marshall Space Flight Center (MSFC) has a successful history of fabricating optics for astronomical x-ray telescopes. In recent years optics have been created using electroforming replication for missions such as the balloon payload HERO (High energy replicated optics) and the rocket payload FOXSI (Focusing Optics x-ray Solar Imager). The same replication process is currently being used in the creation seven x-ray mirror modules (one module comprising of 28 nested shells) for the Russian ART-XC (Astronomical Rontgen Telescope) instrument aboard the Spectrum-Roentgen-Gamma mission and for large-diameter mirror shells for the Micro-X rocket payload. In addition to MSFCs optics fabrication, there are also several areas of research and development to create the high resolution light weight optics which are required by future x-ray telescopes. Differential deposition is one technique which aims to improve the angular resolution of lightweight optics through depositing a filler material to smooth out fabrication imperfections. Following on from proof of concept studies, two new purpose built coating chambers are being assembled to apply this deposition technique to astronomical x-ray optics. Furthermore, MSFC aims to broaden its optics fabrication through the recent acquisition of a Zeeko IRP 600 robotic polishing machine. This paper will provide a summary of the current missions and research and development being undertaken at NASAs MSFC.