X-ray Mirrors Research Articles

Side-bounce beamlines using single-reflection diamond monochromators at Cornell High Energy Synchrotron Source

The design and implementation of new beamlines featuring side-bounce (single-reflection) diamond monochromators at Cornell High Energy Synchrotron Source (CHESS) are described. Undulator radiation is monochromated using aninterchangeable set of diamond crystal plates reflecting radiation in the horizontal (synchrotron) plane, where each crystal plate is set to one of the low-index Bragg reflections (111, 220, 311 and 400) in either Bragg or Laue reflection geometries. At the nominal Bragg angle of 18° these reflections deliver monochromated X-rays with photon energies of 9.7, 15.9, 18.65 and 22.5 keV, respectively. An X-ray mirror downstream of the diamond monochromator is used for rejection of higher radiation harmonics and for initial focusing of themonochromated beam. The characteristics of the X-ray beam entering the experimental station were measured experimentally and compared with the results of simulations. A reasonable agreement is demonstrated. It is shown thatthe use of selected high-dislocation-density `mosaic' diamond single-crystal plates produced using the chemical vapor deposition method yields a few-fold enhancement in the flux density of the monochromated beam in comparison with that delivered by perfect crystals under the same conditions. At present, the Functional Materials Beamline at CHESS, which is used for time-resolved in situ characterization of soft materials during processing, has been outfitted with the described setup.

Development of a one-dimensional differential deposition system for X-ray mirror figure correction

A thin-film deposition system was developed for the surface coating of X-ray mirrors of up to 1 m in length. With two coating process areas and four sputtering cathodes, various combinations employing a single layer, multilayered, and co-sputtered thin films are possible. Furthermore, it is possible to correct and modify the mirror surface shape by controlling the speed of the substrate stage. In this study, to evaluate the performance of the proposed coating system, the static coating distribution was measured to check the vertical direction. In a 10 mm area, a 0.9% peak-to-valley error and 0.2% root mean square error occurred. A differential deposition test was also performed for the horizontal direction (stage scan direction). In this study, arbitrary shapes were deposited on 100-mm and 400-mm-long mirrors. After removing the measurement error, the deposition error was less than 1 nm (peak-to-valley). The results demonstrate that this system can correct the surface of an X-ray mirror with ultra-high precision.

Cadmium Zinc Telluride detectors for a next-generation hard X-ray telescope

We are currently developing Cadmium Zinc Telluride (CZT) detectors for a next-generation space-borne hard X-ray telescope which can follow up on the highly successful NuSTAR (Nuclear Spectroscopic Telescope Array) mission. Since the launch of NuSTAR in 2012, there have been major advances in the area of X-ray mirrors, and state-of-the-art X-ray mirrors can improve on NuSTAR’s angular resolution of ∼ 1arcmin Half Power Diameter (HPD) to 15” or even 5” HPD. Consequently, the size of the detector pixels must be reduced to match this resolution. This paper presents detailed simulations of relatively thin (1mm thick) CZT detectors with hexagonal pixels at a next-neighbor distance of 150 μm. The simulations account for the non-negligible spatial extent of the deposition of the energy of the incident photon, and include detailed modeling of the spreading of the free charge carriers as they move toward the detector electrodes. We discuss methods to reconstruct the energies of the incident photons, and the locations where the photons hit the detector. We show that the charge recorded in the brightest pixel and six adjacent pixels suffices to obtain excellent energy and spatial resolutions. The simulation results are being used to guide the design of a hybrid application-specific integrated circuit (ASIC)-CZT detector package.

Measuring the atomic scattering factors near the iridium L-edges for the Athena silicon pore optics reflector

Athena, a future high-energy mission, is expected to consist of a large aperture x-ray mirror with a focal length of 12 m. The mirror surface is to be coated with iridium and a low Z overcoat. To define the effective area of the x-ray telescope, the atomic scattering factors of iridium with an energy resolution less than that (2.5 eV) of the x-ray integral field unit are needed. We measured the reflectance of the silicon pore optics mirror plate coated with iridium in the energy range of 9 to 15 keV and that near the iridium L-edges in steps of 10 and 1.5 eV, respectively, at the synchrotron beamline SPring-8. The L3, L2, and L1 edges were clearly detected around 11,215, 12,824, and 13,428 eV, respectively. The measured scattering factors were ∼3 % smaller than the corresponding values reported by Henke et al., likely due to the presence of an overlayer on the iridium coating, and were consistent with those measured by Graessle et al. The angular dependence of the reflectivity measured indicates that the iridium surface was extremely smooth, with a surface roughness of 0.3 nm.

DarpanX: A python package for modeling X-ray reflectivity of multilayer mirrors

Multilayer X-ray mirrors consist of a coating of a large number of alternate layers of high Z and low Z materials with a typical thickness of 10-100 Angstrom, on a suitable substrate. Such coatings play an important role in enhancing the reflectivity of X-ray mirrors by allowing reflections at angles much larger than the critical angle of X-ray reflection for the given materials. Coating with an equal thickness of each bilayer enhances the reflectivity at discrete energies, satisfying Bragg condition. However, by systematically varying the bilayer thickness in the multilayer stack, it is possible to design X-ray mirrors having enhanced reflectivity over a broad energy range. One of the most important applications of such a depth graded multilayer mirror is to realize hard X-ray telescopes for astronomical purposes. Design of such multilayer X-ray mirrors and their characterization with X-ray reflectivity measurements require appropriate software tools. We have initiated the development of hard X-ray optics for future Indian X-ray astronomical missions, and in this context, we have developed a program, DarpanX, to calculate X-ray reflectivity for single and multilayer mirrors. It can be used as a stand-alone tool for designing multilayer mirrors with required characteristics. But more importantly, it has been implemented as a local model for the popular X-ray spectral fitting program, XSPEC, and thus can be used for accurate fitting of the experimentally measured X-ray reflectivity data. DarpanX is implemented as a Python 3 module, and an API is provided to access the underlying algorithms. Here we present details of DarpanX implementation and its validation for different type multilayer structures. We also demonstrate the model fitting capability of DarpanX for experimental X-ray reflectivity measurements of single and multilayer samples.

Simulation of X-Ray Television System Output Signal Based on CMOS-Matrixes

Various X-ray television systems are used for radiation control of materials and products, both indirect conversion of radiation images and direct conversion systems. X-ray television system type "X-ray screen - optics - CMOS-matrix" have a number of advantages over others: the ability to change the X-ray scintillation screen, which allows you to change the size of the working field and other system parameters; short image acquisition time; simplicity of construction; small dimensions and weight; low cost. The development of digital X-ray television systems requires the solution of important scientific and technical problems, which include: increasing the signal-to-noise ratio and, accordingly, the sensitivity of control, increasing the quantum efficiency of image converters, optimizing operating modes to ensure high quality radiation images.The aim of this research was to develop a model of X-ray television system. A physical and topological model of X-ray image transformation in X-ray television systems of the “X-ray screen — optics — CMOS-matrix” type has been developed during research. The math model allows to calculate the characteristics of X-rays at the output of the X-ray tube and the object under study, the intensity of the screen, the light intensity on the surface of the CMOS matrix after passing the optics, calculate the charge packet in a pixel and the output signal systems. The block diagram of the X-ray television system includes the following components: X-ray machine, object, scintillator (screen), mirror and lens with CMOS-matrix. The first stage of creating a through mathematical model is to simulate X-rays at a certain distance from the anode of the X-ray tube. The second stage of creating an end-to-end mathematical model of X-ray television system based on CMOS matrices of the optical range is the modeling of the process of converting X-rays, which corresponds to the shadow X-ray image of the object, into visible range radiation.The developed physical and topological model of X-ray image transformation in X-ray television systems of the type "X-ray screen - optics - CMOS-matrix" allows us to calculate the characteristics of X-ray television system at the design stage, which will save time and money on system development and also select the optimal modes of operation for the study of certain materials. The found results are a perspective direction in development of technology of research of materials that will allow to reduce cost of installations for researches.

Synthesis and structural analysis of Mo/B periodical multilayer X-ray mirrors for beyond extreme ultraviolet optics

The synthesis and structural analysis of Mo/B periodical multilayer X-ray mirrors (PMMs) for beyond extreme ultraviolet (BEUV) optics was performed. The PMMs were deposited by a combination of pulsed DC and radio frequency (RF) magnetron sputtering. The structure was analyzed by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and grazing incidence X-ray reflectometry. The formation of 0.35 nm-thick interlayers comprised of a mixture of molybdenum borides was observed at the Mo/B interfaces. Furthermore, a low interface roughness of 0.3–0.4 nm was reported. The temperature of the substrate increased due to the increase in the sputtering power. This resulted in an increase in the thickness of the interlayers and the interface roughness; subsequently, the optical properties of the PMM deteriorated. Theoretical calculations were performed based on the real structure of the PMM to predict the reflectivity at a working wavelength of 6.7 nm. The reflectivity of approximately 53% was two times higher than that of the conventional B4C-based BEUV mirrors. Based on our study results, it can be concluded that the as-synthesized PMMs will perform better than the traditional mirrors and can be effectively used for the development of the next generation of BEUV lithography.

Use of a ray-tracing simulation to characterize ghost rays in the FOXSI rocket experiment

Imaging X-rays by direct focusing offers greater sensitivity and a higher dynamic range compared to techniques based on indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload that uses seven sets of nested Wolter-I figured mirrors to observe the Sun in hard X-rays through direct focusing. Characterizing the performance of these optics is critical to optimize their performance and to understand their resulting data. In this paper, we present a ray-tracing simulation we created and developed to study Wolter-I X-ray mirrors. We validated the accuracy of the ray-tracing simulation by modeling the FOXSI rocket optics. We found satisfactory agreements between the simulation predictions and laboratory data measured on the optics. We used the ray-tracing simulation to characterize a background pattern of singly reflected rays (i.e., ghost rays) generated by photons at certain incident angles reflecting on only one of a two-segment Wolter-I figure and still reaching the focal plane. We used the results of the ray-tracing simulation to understand, and to formulate a set of strategies that can be used to mitigate, the impact of ghost rays on the FOXSI optical modules. These strategies include the optimization of aperture plates placed at the entrance and exit of the smallest Wolter-I mirror used in FOXSI, a honeycomb type collimator, and a wedge absorber placed at the telescope aperture. The ray-tracing simulation proved to be a reliable set of tools to study Wolter-I X-ray optics. It can be used in many applications, including astrophysics, material sciences, and medical imaging.

XL-Calibur – a second-generation balloon-borne hard X-ray polarimetry mission

XL-Calibur is a hard X-ray (15-80 keV) polarimetry mission operating from a stabilised balloon-borne platform in the stratosphere. It builds on heritage from the X-Calibur mission, which observed the accreting neutron star GX 301 - 2 from Antarctica, between December 29th 2018 and January 1st 2019. The XL-Calibur design incorporates an X-ray mirror, which focusses X-rays onto a polarimeter comprising a beryllium rod surrounded by Cadmium Zinc Telluride (CZT) detectors. The polarimeter is housed in an anticoincidence shield to mitigate background from particles present in the stratosphere. The mirror and polarimeter-shield assembly are mounted at opposite ends of a 12 m long lightweight truss, which is pointed with arcsecond precision by WASP – the Wallops Arc Second Pointer. The XL-Calibur mission will achieve a substantially improved sensitivity over X-Calibur by using a larger effective area X-ray mirror, reducing background through thinner CZT detectors, and improved anticoincidence shielding. When observing a 1 Crab source for tdaydays, the Minimum Detectable Polarisation (at 99% confidence level) is ∼2%·tday−1/2. The energy resolution at 40 keV is ∼5.9 keV. The aim of this paper is to describe the design and performance of the XL-Caliburmission, as well as the foreseen science programme.

Comparative Study of Pd/B4C X-ray Multilayer Mirrors Fabricated by Magnetron Sputtering with Kr and Ar Gas.

Ultrathin Pd/B4C multilayers are suitable X-ray mirrors working at the photon energy region of 7–20 keV. To further improve the layer structure, Pd/B4C multilayers with a d-spacing of 2.5 nm were fabricated by magnetron sputtering using the heavy noble gas Kr and compared with the conventional ones fabricated by Ar. Although the Kr-sputtering process can work at a lower pressure, the interface width—especially the interface roughness—is a little larger than that made by Ar. A stronger polycrystallization and a lower content of sputter gas atoms were found in the Kr-made sample, which can be explained by the joint effect from less recoiled particles and lower sputtering pressure. A good reflectance of 68% of the Kr made multilayer was measured at 10 keV, which is only slightly lower than that of the Ar made sample (71%).

Comparative study of single-layer, bilayer, and trilayer mirrors with enhanced x-ray reflectance in 0.5- to 8-keV energy region

Effective area is one of the most important parameters of x-ray telescopes. It can be increased by enlarging the entrance aperture or maximizing the reflectivity through the proper designing and optimization of the reflecting coating. A method to increase the reflectivity of grazing incidence x-ray mirrors in the 0.5- to 8-keV energy region is analyzed. The idea consists in the use of a trilayer reflecting coating instead of single-layer one (e.g., C/Ni/Pt mirror instead of Pt one). Deposition of low-absorbing medium-Z and low-Z layers onto the top of strongly absorbing high-Z material results in essential increase in the reflectivity while keeping the same width of the reflectivity plateau. In particular, C/Ni/Pt trilayer mirror demonstrates enhancement of the double reflection coefficient by a factor achieving 1.5 to 3.5 compared to that of Pt-coated mirror. The effective area of a telescope is also considerably increased. The experimental results are in a very good agreement with the theoretical predictions. In addition, the C/Ni/Pt trilayer mirror exhibits a reasonable thermal stability and a relatively low compressive stress of about −550 MPa.

Development of x-ray mirror foils using a hot plastic deformation process.

In this paper, we present a silicon reflector developed through a hot plastic deformation process and used as a lightweight, high-angular-resolution x-ray mirror. We deformed the silicon substrate using conical dies with a curvature radius of 100 mm. The measured radii of the reflector were approximately 100 µm greater than the design values. Due to a gap between the die and the reflector toward the edge, it is probable that the substrate did not reach the yield point, and an elastic spring back occurred. In addition, we have evaluated the x-ray imaging capability of the plastically deformed silicon reflector for the first time, to the best of our knowledge. The estimated angular resolution is 1.76 arc min from the entire reflector, and 0.52 arc min in the best region. For the enhancement of the imaging capability, we may improve the shape of die and determine the best parameter set for the deformation.

Silicon Direct Bonding for the European Space Agency's Next X-ray Observatory

Silicon direct bonding is a manufacturing process that is used in the fabrication of electronic, optical and mechanical microsystems. The chemical bonds are providing an intimate surface contact and strong structural stiffness that is for this reason also suitable for large assemblies. Together with the knowledge of the surface properties and of X-ray optics, we are making mirror imaging systems.The manufacturing process resorts to the highest-grade 12 inch silicon wafers with double Super Polished and near perfect plan parallel surfaces. With readily available semiconductor manufacturing and lithographic tools the wafers are processed into mirror plates with micrometer accuracy channel structures, yet maintaining the nanometer accurate figure of the silicon wafer. Multiple channeled X-ray mirrors shall subsequently be curved and stacked to form an optical component.We want to use such optical components for a ~3m diameter segmented optical system that would be part of the Athena observatory, European Space Agency's (ESA) next L-class mission, scheduled to launch in 2032. Athena will observe the hot and energetic Universe and answer two big scientific questions: How does ordinary matter assemble into the large-scale structures we see today? and how do black holes grow and shape the Universe?The scientific objectives of Athena require the largest X-ray optic ever flown. Silicon Pore Optics (SPO) is the selected technology that produces the building blocks for Athena's modular mirror. The segmented Athena optic will be made of about 700 X-ray mirror modules all co-aligned to a common focus, with radii of curvature varying from 0.25 m to 1.25 m. Furthermore, SPO has also shown to be a versatile technology that can be further developed for gamma-ray optics, medical applications and for material research.While considerable progress has been made on the performance of the optic, we are also upscaling towards an automated production line including mirror manufacturing, coating, cleaning, stacking and stack integration into mirror modules. The complete production line features many metrology steps to ensure quality and high yield. From the start, the development process has looked not just at the production of a single sample of high quality but at all the processes eventually required for the production of a complete system, and this in an effort to ensure that issues (mechanical, thermal, environment, supply chain etc) are identified early and addressed in a manner compatible with the core goal of producing high quality optics.In this paper we will discuss the challenges and present solutions that we have addressed. Cleanliness assessment, and bond strength enhancement are for instance two current topics of active development.

Long-term performance and durability of Ir/B4C multilayer x-ray mirrors: focusing on composition, structure, and reflectivity properties

Multilayer (ML) thin film coatings have shown promise in achieving hard x-ray nanofocusing with high reflectivity and high resolution. The chemical, structural, and long-term stability of Ir/B4C MLs, which are of great interest to the synchrotron and astrophysics communities, are not yet fully understood. The evolution of the x-ray performance of Ir/B4C ML mirrors was monitored over 5 years, and the chemical and structural properties were investigated in depth. Reflectivity scans reveal significant alteration in the energy range of 3.4 to 10 keV over this period. Furthermore, thickness and density degradation of B4C layers were observed in scanning electron transmission microscopy results. The oxidation of B4C occurs only for the top layers, whereas the buried B4C layers go through various complex chemical modifications. The x-ray reflectivity model of Ir/B4C structure was modified, based on the experimental findings, and resulted in good understanding of the long-term reflectivity performance of the x-ray mirror coatings.

Silicon Direct Bonding for the European Space Agency's Next X-ray Observatory

Silicon direct bonding is a manufacturing process used in the fabrication of electronic, optical and mechanical microsystems. Chemical bonds are providing an intimate surface contact and strong structural stiffness that is, for this reason, also suitable for large assemblies. Based on the knowledge of the surface properties and of X-ray optics, we are making mirror imaging systems. The manufacturing process resorts to the highest-grade 12 inch silicon wafers with double-sided super polished and nearly perfect plan parallel surfaces. With readily available semiconductor manufacturing and lithographic tools, the wafers are processed into mirror plates with micrometer accuracy channel structures, yet maintaining the nanometer accurate figure of the silicon wafer. Multiple channeled X-ray mirrors shall subsequently be curved and stacked to form an optical component. We want to use such optical components for a ~3 m diameter segmented optical system that would be part of the Athena observatory.

Research on ultra-smooth machining technique for monocrystalline silicon substrate

X-ray mirrors are an important part of the beam line in advanced light sources and have a stringent surface roughness, and monocrystalline silicon is the preferred substrate material for X-ray mirror. A method of improving the surface roughness of X-ray mirror, based on optimized density and dynamic viscosity of the polishing fluid, is analyzed and tested experimentally. The experimental results show that the surface roughness of monocrystalline silicon substrates was reduced significantly, with the root mean square of surface roughness decreasing from 0.420 nm to 0.272 nm, corresponding to an improvement of approximately 32%.

Multi-pitch self-calibration measurement using a nano-accuracy surface profiler for X-ray mirror metrology.

For high accuracy X-ray mirror measurement, the analysis and corrections of minute systematic errors of the measuring instrument are required. As an X-ray mirror metrology tool, the nano-accuracy surface profiler (NSP) consists of two autocollimators (AC) serving its reference and sample beams, in which the sample-beam AC maintains a fixed distance from the mirror. In this work, the multi-pitch self-calibration method is applied to an NSP instrument to reconstruct both the mirror slope and the instrument error of the sample-beam AC through a series of x scans and pitch angle scans. It is more technically sound to apply this multi-pitch self-calibration method to a working-distance-fixed slope scanner, such as the NSP. First of all, we introduce the principle of the multi-pitch self-calibration method, discuss its ambiguities, and provide our regularization illustrated with simulations. Second, some real measurements of a spherical mirror with 10-mrad total slope are demonstrated to verify the effectiveness of the multi-pitch self-calibration technique with an NSP. Furthermore, the experimental reconstruction of the low- and high-frequency signals of the instrument error with different settings in x and pitch steps are addressed and studied in terms of repeatability, reproducibility, self-consistency, and effectiveness in compensation for single-pitch scans.

Super-resolution surface slope metrology of x-ray mirrors.

We present experimental, analytical, and numerical methods developed for reconstruction (deconvolution) of one-dimensional (1D) surface slope profiles over the spatial frequency range where the raw data are significantly perturbed due to the limited resolution of the measurement instrument. We characterize the spatial resolution properties of a profiler with the instrument's transfer function (ITF). To precisely measure the ITF, we apply a recently developed method utilizing test surfaces with 1D linear chirped height profiles of constant slope amplitude. Based on the results of the ITF calibration, we determine parameters of an analytical model for the ITF that is used in the original reconstruction software. Here, we treat surface slope metrology data obtained with the Optical Surface Measuring System (OSMS), using as a sensor an electronic autocollimator (AC) ELCOMAT-3000. The spatial resolution of the OSMS is limited by the size of the AC light-beam-collimating aperture. For the purposes of this investigation, the OSMS is equipped with a circular aperture with a diameter of 2.5 mm. This is a typical arrangement of most AC-based slope profilers developed for surface slope metrology of state-of-the-art x-ray mirrors. Using the example of surface slope metrology of two state-of-the-art elliptically shaped x-ray focusing mirrors, we demonstrate that the developed data reconstruction procedure allows us to significantly improve the accuracy of surface slope metrology with the OSMS over the spatial wavelength range from ∼1.6 mm to 7 mm. Thus, the amplitude of the quasi-periodic error characteristic of the deterministic polishing process used appears to be higher by a factor of ∼2 than is apparent from the rough metrology data. Underestimation of the surface slope errors in this spatial wavelength range can lead to serious errors in the expected performance of x-ray mirrors in synchrotron beamlines, especially at modern light sources utilizing coherent x rays, where the perturbations can lead to increased speckle-like intensity variation.

Operation of a bending magnet beamline in large energy bandwidth mode for non-resonant X-ray emission spectroscopy

Two new operation modes, “pink beam” excitation, which has the energy bandwidth defined by X-ray mirrors and filters, and “broadband beam”, where the energy bandwidth is defined by the reflection from a multilayer monochromator, were implemented at the SuperXAS beamline of the Swiss Light Source (SLS). These setups allow measuring non-resonance X-ray emission spectra (XES) with 2–3 orders of magnitude higher incident flux than non-resonant XES measurements with a monochromatic incident beam. For the broadband beam mode, a Mo/Si multilayer structure was designed, with which the energy can be tuned in the 5–17 keV range. The multilayer demonstrates a relatively large energy bandwidth of 4 ± 0.2% through the whole energy range and a reflectivity of 23–60%, which increases with energy. We show that by using pink beam mode one can investigate the electronic structure of photocatalytic intermediates through time-resolved core-to-core XES experiments of diluted samples with a concentration of the element of interest of ~1 mM. Broadband beam mode is optimal for valence-to-core XES experiments and allow to avoid the excitation of additional KLβ satellites that can complicate the interpretation of spectra.

Performance simulation of the soft gamma-ray concentrator

The soft gamma-ray concentrator is a telescope mission concept utilizing a suitable arrangement of bent multilayer structures of alternating low- and high-density materials. This lens is able to channel gamma-ray photons via total external reflection and concentrate the incident radiation to a point. The channeling technique offers the potential for concentrating gamma rays with focal lengths 100 keV, beyond the reach of current grazing-incidence hard x-ray mirrors. For the performance estimation of such an instrument, we have developed a flexible set of computer modeling tools to compute the optical properties of multilayer structures, predict the channeling efficiency for a given multilayer configuration, and aid in the optimization of potential gamma-ray concentrator-based telescope designs. This modeling includes the multilayer optical properties calculated by the IMD software, the ray tracing using an IDL code, and the focal plane detector simulation by MEGAlib. We illustrate the potential of this approach by presenting simulated astronomical observations from a balloon-borne platform. The final result, including simulated effective area, instrument sensitivity, and polarization performance, shows that the gamma-ray concentrator will provide greatly increased sensitivity for next-generation soft gamma-ray missions with modest cost and complexity.