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Calibration and Performance of the REgolith X-Ray Imaging Spectrometer (REXIS) Aboard NASA’s OSIRIS-REx Mission to Bennu
The REgolith X-ray Imaging Spectrometer (REXIS) instrument on board NASA’s
OSIRIS-REx
mission to the asteroid Bennu is a Class-D student collaboration experiment designed to detect fluoresced X-rays from the asteroid’s surface to measure elemental abundances. In July and November 2019 REXIS collected ∼615 hours of integrated exposure time of Bennu’s sun-illuminated surface from terminator orbits. As reported in Hoak et al. (Results from the REgolith X-ray Imaging Spectrometer (REXIS) at Bennu,
2021
) the REXIS data do not contain a clear signal of X-ray fluorescence from the asteroid, in part due to the low incident solar X-ray flux during periods of observation. To support the evaluation of the upper limits on the detectable X-ray signal that may provide insights for the properties of Bennu’s regolith, we present an overview of the REXIS instrument, its operation, and details of its in-flight calibration on astrophysical X-ray sources. This calibration includes the serendipitous detection of the transient X-ray binary MAXI J0637-430 during Bennu observations, demonstrating the operational success of REXIS at the asteroid. We convey some lessons learned for future X-ray spectroscopy imaging investigations of asteroid surfaces.
Journal Article
The BepiColombo Mercury Imaging X-Ray Spectrometer: Science Goals, Instrument Performance and Operations
The Mercury Imaging X-ray Spectrometer is a highly novel instrument that is designed to map Mercury’s elemental composition from orbit at two angular resolutions. By observing the fluorescence X-rays generated when solar-coronal X-rays and charged particles interact with the surface regolith, MIXS will be able to measure the atomic composition of the upper ∼10-20 μm of Mercury’s surface on the day-side. Through precipitating particles on the night-side, MIXS will also determine the dynamic interaction of the planet’s surface with the surrounding space environment.
MIXS is composed of two complementary elements: MIXS-C is a collimated instrument which will achieve global coverage at a similar spatial resolution to that achieved (in the northern hemisphere only – i.e. ∼ 50 – 100 km) by MESSENGER; MIXS-T is the first ever X-ray telescope to be sent to another planet and will, during periods of high solar activity (or intense precipitation of charged particles), reveal the X-ray flux from Mercury at better than 10 km resolution. The design, performance, scientific goals and operations plans of the instrument are discussed, including the initial results from commissioning in space.
Journal Article
Femtosecond x-ray spectroscopy of an electrocyclic ring-opening reaction
The ultrafast light-activated electrocyclic ring-opening reaction of 1,3-cyclohexadiene is a fundamental prototype of photochemical pericyclic reactions. Generally, these reactions are thought to proceed through an intermediate excited-state minimum (the so-called pericyclic minimum), which leads to isomerization via nonadiabatic relaxation to the ground state of the photoproduct. Here, we used femtosecond (fs) soft x-ray spectroscopy near the carbon K-edge (~284 electron volts) on a table-top apparatus to directly reveal the valence electronic structure of this transient intermediate state. The core-to-valence spectroscopic signature of the pericyclic minimum observed in the experiment was characterized, in combination with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbital energy levels. We show that this transient valence electronic structure arises within 60 ± 20 fs after ultraviolet photoexcitation and decays with a time constant of 110 ± 60 fs.
Journal Article
Physicochemical characterization of free troposphere and marine boundary layer ice-nucleating particles collected by aircraft in the eastern North Atlantic
Atmospheric ice nucleation impacts the hydrological cycle and climate by
modifying the radiative properties of clouds. To improve our predictive
understanding of ice formation, ambient ice-nucleating particles (INPs) need
to be collected and characterized. Measurements of INPs at lower latitudes
in a remote marine region are scarce. The Aerosol and Cloud Experiments in
the Eastern North Atlantic (ACE-ENA) campaign, in the region of the Azores
islands, provided the opportunity to collect particles in the marine
boundary layer (MBL) and free troposphere (FT) by aircraft during the
campaign's summer and winter intensive operation period. The particle
population in samples collected was examined by scanning transmission X-ray
microscopy with near-edge X-ray absorption fine structure spectroscopy. The
identified INPs were analyzed by scanning electron microscopy with
energy-dispersive X-ray analysis. We observed differences in the particle
population characteristics in terms of particle diversity, mixing state, and
organic volume fraction between seasons, mostly due to dry intrusion events
during winter, as well as between the sampling locations of the MBL and FT. These
differences are also reflected in the temperature and humidity conditions
under which water uptake, immersion freezing (IMF), and deposition ice
nucleation (DIN) proceed. Identified INPs reflect typical particle types
within the particle population on the samples and include sea salt, sea salt
with sulfates, and mineral dust, all associated with organic matter, as well as
carbonaceous particles. IMF and DIN kinetics are analyzed with respect to
heterogeneous ice nucleation rate coefficients, Jhet, and ice
nucleation active site density, ns, as a function of the water
criterion Δaw. DIN is also analyzed in terms of contact angles
following classical nucleation theory. Derived MBL IMF kinetics agree with
previous ACE-ENA ground-site INP measurements. FT particle samples show
greater ice nucleation propensity compared to MBL particle samples. This
study emphasizes that the types of INPs can vary seasonally and with
altitude depending on sampling location, thereby showing different ice
nucleation propensities, which is crucial information when representing mixed-phase
cloud and cirrus cloud microphysics in models.
Journal Article
Research on the Polishing Process of Wolter-I Type Grazing Incidence Mirrors
As the demand for solar X-ray observation devices continues to rise, the Wolter-I type grazing incidence mirror has emerged as a critical component in these instruments, particularly for high-precision imaging. This mirror efficiently focuses X-rays, enabling astronomers to detect fainter celestial signals. It plays a key role in providing essential data for understanding the origin and evolution of the universe. The operating principle of the Wolter-I type grazing incidence mirror is based on grazing incidence reflection. This reflection guides X-rays to a focal point through a specific surface structure, enabling high-resolution imaging. This paper presents the design of a super-precision optical processing system for the Wolter-I type mirror. The system features a coaxial Confocal structure, consisting of a rotating parabolic surface and a rotating hyperbolic surface. It also includes a radial adjustment fixture and an automatic polishing fluid supply device. The paper outlines the imaging principles of the Wolter-I mirror, analyzes the impact of surface shape accuracy on imaging performance, and selects microcrystalline glass as the mirror substrate. Using the custom-designed processing system, the rough-turned workpiece undergoes several steps. After diamond wheel grinding, the workpiece is subjected to 160 hours of rough polishing. It then undergoes 720 hours of fine and super-fine polishing, using cerium oxide polishing fluids with particle sizes of W2, W1, and W0.8, respectively. The final surface shape accuracy of the mirror is characterized by a peak-to-valley (PV) value of 253 nm, a root mean square (RMS) value of 3.5 nm, and a root mean square roughness (Rq) of 4.6 nm. These values meet the requirements for composite extreme ultraviolet-soft X-ray telescopes. Experimental results show that the designed super-precision optical processing system effectively improves surface shape accuracy. It is well-suited for processing the unique internal surfaces of Wolter-I type grazing incidence mirrors. This system enhances the mirror's imaging performance and lays a solid foundation for future high-resolution X-ray astronomical observations. Future research will focus on optimizing the processing techniques further, exploring the impact of different materials on imaging quality, and developing more advanced optical systems to meet emerging observational needs.
Journal Article
Micro-spectroscopic and freezing characterization of ice-nucleating particles collected in the marine boundary layer in the eastern North Atlantic
Formation of atmospheric ice plays a crucial role in the microphysical evolution of mixed-phase and cirrus clouds and thus climate. How aerosol
particles impact ice crystal formation by acting as ice-nucleating particles (INPs) is a subject of intense research activities. To improve
understanding of atmospheric INPs, we examined daytime and nighttime particles collected during the Aerosol and Cloud Experiments in the Eastern
North Atlantic (ACE-ENA) field campaign conducted in summer 2017. Collected particles, representative of a remote marine environment, were
investigated for their propensity to serve as INPs in the immersion freezing (IMF) and deposition ice nucleation (DIN) modes. The particle
population was characterized by chemical imaging techniques such as computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM/EDX) and scanning transmission X-ray microscopy with near-edge X-ray absorption fine-structure spectroscopy (STXM/NEXAFS). Four major particle-type classes were identified where internally mixed inorganic–organic particles make up the majority of the analyzed particles. Following ice nucleation experiments, individual INPs were identified and characterized by SEM/EDX. The identified INP types belong to
the major particle-type classes consisting of fresh sea salt with organics or processed sea salt containing dust and sulfur with organics. Ice
nucleation experiments show IMF events at temperatures as low as 231 K, including the subsaturated regime. DIN events were observed at lower temperatures of 210 to 231 K. IMF and DIN observations were analyzed with regard to activated INP fraction, ice-nucleation active site (INAS) densities, and a water activity-based immersion freezing model (ABIFM) yielding heterogeneous ice nucleation rate coefficients. Observed IMF
and DIN events of ice formation and corresponding derived freezing rates demonstrate that the marine boundary layer aerosol particles can serve as INPs under typical mixed-phase and cirrus cloud conditions. The derived IMF and DIN parameterizations allow for implementation in cloud and climate
models to evaluate predictive effects of atmospheric ice crystal formation.
Journal Article