The Water Recovery X-ray Rocket (WRXR) is a sounding rocket payload that launched from the Kwajalein Atoll in April 2018 and was the first NASA astrophysics sounding rocket payload to be recovered from water. WRXR's primary instrument is a grating spectrometer that consists of a mechanical collimator, X-ray reflection gratings, grazing-incidence mirrors, and a hybrid CMOS detector. We present here the design of the WRXR spectrometer’s gratings and mirrors.
We will introduce SCIL as a full-wafer soft-stamp base nanoimprint technique with the advantages of being able to replicate sub-10nm features and perform overlay alignment with sub-micron precision over 200mm wafers. The combination of PDMS based soft stamps and an inorganic crosslinking imprint resist leads to a very long stamp lifetime of over 700 imprints, as demonstrated in the AutoSCIL 200 high volume production tool. Initial applications for wafer based NIL mainly required only a single, first mask, patterning step. For optical applications high refractive index material which can be directly patterned with high fidelity and low shrinkage are desired and initial results of inorganic resists that reach a refractive index of n=1.8 are demonstrated. As NIL and the applications develop, overlay alignment is the next step. Here we will discuss the developments ongoing to integrate wafer scale overlay alignment in the AutoSCIL production platform and which would achieve ~1 micrometer overlay alignment over 200mm wafers. Two applications that make use of the ability of NIL to replicate large area nano-patterns (X-ray mirrors) and the combination of micro- and sub-20nm patterns in one layer / pattern (cell proliferation templates) are discussed.
Future soft X-ray spectroscopy missions have science requirements that demand higher instrument throughput and higher resolution than currently available technology. A key element in such spectrometers are dispersive elements such as diffraction gratings. Our group at Penn State University develops and fabricates off-plane reflection gratings in an effort to achieve the level of performance required by future missions. We present here efficiency measurements made in the 0.2 – 1.3 keV energy band at the Advanced Light Source (ALS) at Lawrence Berkley National Laboratory for one such grating, which was fabricated to achieve the high-throughput required for future observatories. This grating was replicated from a grating master using UV-nanoimprint techniques which are suitable for mass-production and is coated in a layer of gold. Total absolute diffraction efficiency was measured to be ~55-65% across the energy range, with relative diffraction efficiency approaching 90%. These results represent the first successful demonstration of off-plane grating replicas produced via these fabrication techniques and exceed the grating efficiency requirements for future X-ray missions.
The Water Recovery X-ray Rocket (WRXR) is a sounding rocket payload that will launch from the Kwajalein Atoll in April 2018 and seeks to be the first astrophysics sounding rocket payload to be water recovered by NASA. WRXR's primary instrument is a grating spectrometer that consists of a mechanical collimator, X-ray reflection gratings, grazing-incidence mirrors, and a hybrid CMOS detector. The instrument will obtain a spectrum of the diffuse soft X-ray emission from the northern part of the Vela supernova remnant and is optimized for 3rd and 4th order OVII emission. Utilizing a field of view of 3.25° × 3.25° and resolving power of λ/δλ ≈40-50 in the lines of interest, the WRXR spectrometer aims to achieve the most highly-resolved spectrum of Vela's diffuse soft X-ray emission. This paper presents introductions to the payload and the science target.
Off-plane reflection gratings require high-fidelity, custom groove profiles to perform with high spectral resolution in a Wolter-I optical system. This places a premium on exploring lithographic techniques in nanofabrication to produce state-of-the-art gratings. The fabrication recipe currently being pursued involves electron-beam lithography (EBL) and reactive ion etching (RIE) to define the groove profile, wet anisotropic etching in silicon to achieve blazed grooves and UV-nanoimprint lithography (UV-NIL) to replicate the final product. A process involving grayscale EBL and thermal reflow known as thermally activated selective topography equilibration (TASTE) is also being investigated as an alternative method to fabricate these gratings. However, a master grating fabricated entirely in soft polymeric resist through the TASTE process requires imprinting procedures other than UV-NIL to explored. A commerically available process called substrate conformal imprint lithography (SCIL) has been identified as a possible solution to this problem. SCIL also has the ability to replicate etched silicon gratings with reduced trapped air defects as compared to UV-NIL, where it is difficult to achieve conformal contact over large areas. As a result, SCIL has the potential to replace UV-NIL in the current grating fabrication recipe.
We present the first results from the Off-plane Grating Rocket for Extended Source Spectroscopy (OGRESS) sounding rocket payload based at the University of Iowa. OGRESS is designed to perform moderate resolution (R~10- 40) spectroscopy of diffuse celestial x-ray sources between 0.3 – 1.2 keV. A wire grid focuser constrains light from diffuse sources into a converging beam that feeds an array of off-plane diffraction gratings. The spectrum is focused onto Gaseous Electron Multiplier (GEM) detectors. OGRESS launched on the morning of May 2, 2015 and collected data for ~5 minutes before returning via parachute. OGRESS observed the Cygnus Loop supernova remnant with the goal of obtaining the most accurate physical diagnostics thus far recorded. During the flight, OGRESS had an unexpectedly high count rate which manifested as a highly uniform signal across the active area of the detector, swamping the expected spectrum from Cygnus. Efforts are still in progress to identify the source of this uniform signal and to discover if a usable spectrum can be extracted from the raw flight data.
Photon counting detector systems on sounding rocket payloads often require interfacing asynchronous outputs with a synchronously clocked telemetry stream. Though this can be handled with an on-board computer, there are several low cost alternatives including custom hardware, microcontrollers, and Field-Programmable Gate Arrays (FPGAs). This paper outlines how a telemetry interface for detectors on a sounding rocket with asynchronous parallel digital output can be implemented using low cost FPGAs and minimal custom hardware. It also discusses how this system can be tested with a simulated telemetry chain in the small laboratory setting.