Recently developed Critical-Angle Transmission (CAT) grating technology - in combination with x-ray CCD
cameras and large collecting-area focusing optics - will enable a new generation of soft x-ray spectrometers
with unprecedented resolving power and effective area and with at least an order of magnitude improvement in
figures-of-merit for emission and absorption line detection. This technology will be essential to address a number
of high-priority questions identified in the Astro2010 Decadal Survey “New Worlds New Horizons” and open the
door to a new discovery space. CAT gratings combine the advantages of soft x-ray transmission gratings (low
mass, relaxed figure and alignment tolerances, transparent at high energies) and blazed reflection gratings (high
broad band diffraction efficiency, utilization of higher diffraction orders to increase resolving power). We report
on progress in the fabrication of large-area (31× 31 mm2) free-standing gratings with two levels of low-blockage
support structures using highly anisotropic deep reactive-ion etching.
Diffraction grating-based, wavelength dispersive high-resolution soft x-ray spectroscopy of celestial sources promises
to reveal crucial data for the study of the Warm-Hot Intergalactic Medium, the Interstellar Medium, warm absorption
and outflows in Active Galactic Nuclei, coronal emission from stars, and other areas of interest to the
astrophysics community. Our recently developed critical-angle transmission (CAT) gratings combine the advantages
of the Chandra high and medium energy transmission gratings (low mass, high tolerance of misalignments
and figure errors, polarization insensitivity) with those of blazed reflection gratings (high broad band diffraction
efficiency, high resolution through use of higher diffraction orders) such as the ones on XMM-Newton. Extensive
instrument and system configuration studies have shown that a CAT grating-based spectrometer is an outstanding
instrument capable of delivering resolving power on the order of 5,000 and high effective area, even with a
telescope point-spread function on the order of many arc-seconds. We have fabricated freestanding, ultra-high
aspect-ratio CAT grating bars from silicon-on-insulator wafers using both wet and dry etch processes. The 200
nm-period grating bars are supported by an integrated Level 1 support mesh, and a coarser external Level 2
support mesh. The resulting grating membrane is mounted to a frame, resulting in a grating facet. Many such
facets comprise a grating array that provides light-weight coverage of large-area telescope apertures. Here we
present fabrication results on the integration of CAT gratings and the different high-throughput support mesh
levels and on membrane-frame bonding. We also summarize recent x-ray data analysis of 3 and 6 micron deep
wet-etched CAT grating prototypes.
High-resolution spectroscopy at energies below 1 keV covers the lines of C, N, O, Ne and Fe ions, and is central
to studies of the Interstellar Medium, the Warm Hot Intergalactic Medium, warm absorption and outflows
in Active Galactic Nuclei, coronal emission from stars, etc. The large collecting area, long focal length, and 5
arcsecond half power diameter telescope point-spread function of the International X-ray Observatory will present
unprecedented opportunity for a grating spectrometer to address these areas at the forefront of astronomy and
astrophysics. We present the current status of a transmission grating spectrometer based on recently developed
high-efficiency critical-angle transmission (CAT) gratings that combine the traditional advantages of blazed
reflection and transmission gratings. The optical design places light-weight grating arrays close to the telescope
mirrors, which maximizes dispersion distance and thus spectral resolution and minimizes demands on mirror
performance. It merges features from the Chandra High Energy Transmission Grating Spectrometer and the
XMM-Newton Reflection Grating Spectrometer, and provides resolving power R = E/ΔE = 3000 - 5000 (full
width half max) and effective area >1000 cm2 in the soft x-ray band. We discuss recent results on ray-tracing
and optimization of the optical design, instrument configuration studies, and grating fabrication.
We present a high-resolution soft x-ray grating spectrometer concept for the International X-Ray Observatory
(IXO) that meets or exceeds the minimum requirements for effective area (> 1, 000 cm2 for E < 1 keV) and
spectral resolution (E/▵E > 3, 000). At the heart of the spectrometer is an array of recently developed highefficiency
blazed transmission gratings, the so-called critical-angle transmission (CAT) gratings. They combine
the advantages of traditional transmission gratings (very low mass, extremely relaxed alignment and flatness tolerances)
with those of x-ray reflection gratings (high efficiency due to blazing in the direction of grazing-incidence
reflection). In addition, a CAT grating spectrometer is well-suited for co-existence with energy-dispersive highenergy
focal plane detectors, since most high-energy x rays are neither absorbed, nor diffracted, and contribute
to the effective area at the telescope focus. Since our initial successful x-ray demonstrations of the CAT grating
concept with large-period and lower aspect-ratio prototypes, we have now microfabricated 200 nm-period silicon
CAT gratings comprised of grating bars with the required dimensions (6 micron tall, 40 nm wide, aspect ratio
150), optimized for the 0.3 to 1.0 keV energy band. Preliminary analysis of recent x-ray tests show blazing
behavior up to 1.28 keV in accordance with predictions.
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