We present developments on a hard X-ray wavefront sensing instrument for characterizing and monitoring the beam of the European X-ray Free Electron Lasers (EuXFEL). The pulsed nature of the intense X-ray beam delivered by this new class of facility gives rise to strong challenges for the optics and their diagnostic. In the frame of the EUCALL project Work Package 7, we are developing a sensor able to observe the beam in the X-ray energy range [8-40] keV without altering it. The sensor is based on the speckle tracking principle and employs two semi-transparent optics optimized such that their X-ray absorption is reduced. Furthermore, this instrument requires a scattering object with small random features placed in the beam and two cameras to record images of the beam at two different propagation distances. The analysis of the speckle pattern and its distortion from one image to the other allows absolute or differential wavefront recovery from pulse to pulse. Herein, we introduce the stakes and challenges of wavefront sensing at an XFEL source and explain the strategies adopted to fulfil the high requirements set by such a source.
We present a systematic study in which multilayers of different composition (W/Si, Mo/Si, Pd/B4C), periodicity (from
2.5 to 5.5 nm), and numbers of layers have been characterised. Particularly, we investigated the intrinsic quality
(roughness and reflectivity) as well as the performance (flatness and coherence of the outgoing beam) as a
monochromator for synchrotron radiation hard X-ray micro-imaging. The results indicate that the material composition
is the dominating factor for the performance. This is of high importance for synchrotron-based hard X-ray imaging
which has become a widely applied tool for probing the microstructure of bulk samples. The high spatial resolution and
different contrast modalities available here strongly depend on using coherent beams from highly brilliant sources. In
order to satisfy the demand for a high flux of quasi-monochromatic photons, multilayer-coated mirrors are commonly
used as monochromators. Their properties present a good tradeoff between spectral bandwidth and photon flux density.
Since the photon flux density at the sample position is higher than with standard crystal monochromators, better spatial
resolution can be reached. This comes at the cost of reduced energy resolution and stronger non-uniformities in the
incoming beam profile. By helping scientists and engineers specify the design parameters of multilayer monochromators,
our results can contribute to a better exploitation of the advantages of multilayer monochromators over crystal-based
devices; i.e., larger spectral bandwidth and high photon flux density for X-ray imaging.
Graded depth multi-layer coatings have the potential to optimise the performance of X-ray reflective surfaces for improved energy response. A study of deposition techniques on silicon substrates representative of the XEUS High Performance Pore Optics (HPO) technology has been carried out. Measurements at synchrotron radiation facilities have been used to confirm the excellent performance improvements achievable with Mo/Si and W/Si multilayers. Future activities that will be necessary to implement such coatings in the HPO assembly sequence are highlighted. Further coating developments that may allow an optimisation of the XEUS effective area in light of potential changes to science requirements and telescope configurations are also identified. Finally an initial measurement of effects of radiation damage within the multilayers is reported.
Periodic multilayers deposited by Distributed Electron Cyclotron Resonance (DECR) sputtering were studied with synchrotron radiation at the ESRF bending magnet beam line BM5. In situ reflectivity measurements at a photon energy of 20keV have been carried out on these samples during a specific heat treatment. A dedicated furnace has been developed to heat the multilayers under vacuum from room temperature up to 550°C. [Ru/B4C]70 and [W/B4C]40 samples with repetition periods of about 4nm were chosen. Simulations of reflectivity measurements were performed to understand the evolution of layer thicknesses and interface widths. Additional ex-situ reflectivity measurements were done at 8keV before and after the annealing experiments to investigate irreversible effects. We will discuss the heat impact on the layered structure and in which way multilayer optics could be thermally pre-treated before their installation on synchrotron beam lines.
We present the theoretical design, the fabrication, and the performance of double gradient multilayers to be installed on a Kirkpatrick-Baez focusing system for the ESRF bending magnet beam line BM5. The lateral and the depth gradient of the two coatings were chosen in such a way as to obtain a flat reflectivity response of about 25% after two reflections over an energy range from 12keV to 14keV and at an angle of incidence of 0.5deg at the mirror center. Both mirrors were coated with a non-periodic Ru/B4C structure containing 71 individual layers. The overall depth gradient was identical for both multilayers and optimized at the mirror center while the lateral gradient was adapted to the different focal lengths of each of the two KB elements.
Christian Schroer, Marion Kuhlmann, Til Florian Gunzler, Bruno Lengeler, Matthias Richwin, Bernd Griesebock, Dirk Lutzenkirchen-Hecht, Ronald Frahm, Eric Ziegler, Ali Mashayekhi, Dean Haeffner, Jan-Dierk Grunwaldt, Alfons Baiker
Hard x-ray absorption spectroscopy is combined with scanning microtomography to reconstruct full near edge spectra of an elemental species at each location on an arbitrary virtual section through a sample. These spectra reveal the local concentrations of different chemical compounds of the absorbing element inside the sample and give insight into the oxidation state, the local atomic structure, and the local projected free density of states. The method is implemented by combining a quick scanning monochromator and data acquisition system with a scanning microprobe setup based on refractive x-ray lenses. The full XANES spectra reconstructed at each point of the tomographic slice allow to detect slight variations in concentrations of chemical compounds, such as metallic and monovalent copper. The method is applied to the analysis of a heterogeneous catalyst.
We derive mathematical relations for hard X-ray moire wavefront analysis with a grating interferometer. In particular, the first derivative of the wavefront phase profile and the local radius of curvature of the wavefront are related to the position and inclination of the observed moiré fringes.
Focusing optics are now poised to dramatically improve the sensitivity and angular resolution at energies above 10 keV to levels that were previously unachievable by the past generation of background limited collimated and coded-aperture instruments. Active balloon programs (HEFT), possible Explorer-class satellites (NuSTAR - currently under Phase A study), and major X-ray observatories (Con-X HXT) using focusing optics will play a major role in future observations of a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters. These instruments call for low cost, grazing incidence optics coated with depth-graded multilayer films that can be nested to achieve large collecting areas. Our approach to building such instruments is to mount segmented mirror shells with our novel error-compensating, monolithic assembly and alignment (EMAAL) procedure. This process involves constraining the mirror segments to successive layers of graphite rods that are precisely machined to the required conic-approximation Wolter-I geometry. We present results of our continued development of thermally formed glass substrates that have been used to build three HEFT telescopes and are proposed for NuSTAR. We demonstrate how our experience in manufacturing complete HEFT telescopes, as well as our experience developing higher performance prototype optics, will lead to the successful production of telescopes that meet the NuSTAR design goals.
We have determined experimentally optical constants for eight thin film materials that can be used in hard X-ray multilayer coatings. Thin film samples of Ni.97V.03, Mo, W, Pt, C, B4C, Si and SiC were deposited by magnetron sputtering onto superpolished optical flats. Optical constants were determined from fits to reflectance-vs-incidence angle measurements made using synchrotron radiation over the energy range E=35-180 keV. We have also measured the X-ray reflectance of a prototype W/SiC multilayer coating over the energy range E=35-100 keV, and we compare the measured reflectance with a calculation using the newly derived optical constants.
Complete hard X-ray optics modules are currently being produced for the High Energy Focusing Telescope (HEFT), a balloon born mission that will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented layers are made of thermally-formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. Our novel mounting technique involves constraining these mirror segments to successive layers of precisely machined graphite spacers. We report the production and calibration of the first HEFT optics module.
This paper outlines an in-depth study of the W/Si coated mirrors for the High Energy Focusing Telescope (HEFT). We present data taken at 8, 40 and 60 keV obtained at the Danish Space Research Institute and the European Synchrotron Radiation Facility in Grenoble. The set of samples were chosen to cover the parameter space of sample type, sample size and coating type. The investigation includes a study of the interfacial roughness across the sample surface, as substrates and later as coated, and an analysis of the roughness correlation in the W/Si coatings for N = 10 deposited bilayers. The powerlaw graded flight coating for the HEFT mirrors is studied for uniformity and scatter, as well as its performance at high energies.
A double-reflection multilayer monochromator is being developed at BM5 in order to fulfill two different functions. As a primary monochromator, it provides higher bandpass and higher photon flux than the Si(111) Bragg crystal monochromator. In combination with the crystal monochromator, it rejects the harmonics and the beam exit can be kept fixed. An additional aim is to preserve the beam coherence. Design issues and performances evaluated on the beamline are presented.
We report on the coating of depth graded W/Si multilayers on the thermally slumped glass substrates for the HEFT flight telescopes. The coatings consists of several hundred bilayers in an optimized graded power law design with stringent requirements on uniformity and interfacial roughness. We present the details of the planar magnetron sputtering facility including the optimization of power, Ar pressure and collimating geometry which allows us to coat the several thousand mirror segments required for each telescope module on a time schedule consistent with the current HEFT balloon project as well as future hard X-ray satellite projects. Results are presented on the uniformity, interfacial roughness, and reflectivity and scatter at hard X-ray energies.
We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard X-ray reflective coatings operating in the energy range 100 - 200 keV. Grazing incidence X-ray reflectance at E=8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, while synchrotron radiation was used to measure the hard X-ray reflectance of a depth-graded multilayer designed specifically for use in the range E~150 - 170 keV. We have modeled the hard X-ray reflectance using newly-derived optical constants, which we determined from reflectance-vs-incidence angle measurements also made using synchrotron radiation, in the range E=120 - 180 keV. We describe our experimental investigation in detail, compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard X-ray nuclear line telescope.
Recent progress in the design and the manufacturing of wide bandpass x-ray multilayers has opened up new possibilities in hard x-ray optics, particularly in astrophysics and synchrotron x-ray applications.
In contrast to previous design based on semi-empirical laws or extensive computer calculation, the recent development of an analytical expression has greatly improved the design of aperiodic multilayer mirrors, allowing to generate any given spectral dependence of the reflectivity. In practice, an approximate differential equation is used to derive an in-depth multilayer composition profile whose reflectivity response approaches the desired one. Based on this asymptotic solution, usually sufficiently close to the final solution, the optimum multilayer composition profile is calculated numerically using a classical downhill algorithm.
We have studied the intrinsic characteristics of depth-graded multilayers using the procedure described above, with an emphasis put on the important case of a flat reflectivity response over a large but limited spectral range. The performance of depth-graded multilayer mirrors manufactured at ESRF and characterized at the BM5 beamline are presented. The necessity to account at the design level for deposition process parameters, such as chemical composition and thickness errors, and for the effective optical constants is highlighted.
A planar magnetron sputtering facility has been established at the Danish Space Research Institute (DSRI) for the production coating of depth graded multilayers on the thermally slumped glass segments which form the basis for the hard X-ray telescope on the HEFT balloon project. The facility is capable of coating 20-45 mirrors segments in each run. The coatings are optimized W/Si coatings. The paper describes the facility, the results of the calibration and presents data for the X-ray testing of flight mirrors.
We describe the fabrication and testing of a novel type of tunable transmission hard x-ray optics. The diffractive elements are generated by electron beam lithography and chemical wet etching of <110> oriented silicon substrates. Structures with widths down to 100 nm and extreme aspect ratios were obtained using this method. By tilting the lenses with respect to the x-ray beam, the effective path through the phase shifting structures can be varied. This makes it possible to optimize the diffraction efficiency for a wide range of photon energies, and to obtain effective aspect ratios not accessible with untilted optics. The diffraction efficiency of a Fresnel lens was measured for various energies between 8 keV and 29 keV. Values close to the theoretical limit (approx. 35%) were obtained. The described technique provides focusing in one direction only. For two-dimensional focusing, two linear lenses with different focal lengths and orthogonal orientations can be placed along the optical axis. Depending on the coherence properties of the source, such an arrangement can improve the resolution and flux compared to a single circular zone plate. The wet etching technique is also applied to the fabrication of linear gratings with pseudo-random pitch, which will be used as one-dimensional decoherers to adapt the coherence of a synchrotron beam in a defined way. Linear gratings with uniform line density can be used as beam splitters for applications such as holography or interferometry.
The analysis of the roughness of B4C films of different thickness as well as W/B4C multilayer mirrors of different periods is performed basing on AFM and x-ray scattering (XRS) measurements. It is demonstrated that the linear model of a film growth is able to describe the whole set of experimental data including films at initial island stage of growth, if suppose the relaxation processes of a film surface to depend on the film thickness. New approach to the inverse problem of x-ray reflectometry consisting in inferring the dielectric constant profile from the reflectivity data is shortly discussed.
To fill the gap in energy resolution dE/E between a few percent for multilayer x-ray optics and a few 10-4 for perfect crystal optics we have developed narrow bandpass multilayers consisting of Al2O3 and B4C layers. Their resolving power was precisely determined on the ESRF bending magnet beamline BM5 using a white beam and a Si(111) analyzer crystal. Scans in the (n,+m) and in the (n,-m) scattering geometry return consistent results. With a sample of 680 double layers we have obtained a spectral resolution of 0.27% at energies around 12 keV which is in good agreement with earlier studies using monochromatic x-rays.
Using the technology that has been developed over many years for the fabrication of glass micro-channel plates, a prototype micro-pore optic has been produced that is a very light and compact implementation of a Wolter-I optic for X-ray imaging. With this prototype true Wolter-I imaging has been observed for the first time in a micro-pore optic. Individual fibers in the plates are found to be quite good, with a surface roughness permitting application at medium X-ray energies. The image quality and effective area is however seriously reduced by random tilt errors of multifibers in the plates. If this limitation can be overcome, this technology would allow very light and compact X-ray telescopes to be built. A design is presented that already provides a considerable effective area for soft X-rays using the properties of the surfaces obtained in this program.
Presently there is a gap in energy resolution (Delta) E/E between a few percent for multilayer x-ray optics and a few 10-4 for perfect crystal optics. One approach to bridge this gap is the development of high-resolution multilayers. We will report on recent advances in this field and discuss both the capabilities and the limitations of this solution. The deposition of hundreds of layers of hard materials is a considerable challenge for the coating system, and stability issues have to be considered with particular care. We have shown theoretically and experimentally that this challenge can be met with a combination of Al2O3 and B4C. With 680 bilayers we reached a spectral resolution < 0.3% and a peak reflectivity of almost 50% for 12 keV x- rays. The disagreement with the diffraction properties of a perfect multilayer system could be accurately described by instabilities during the deposition process. With improved stability, such systems can provide still better performances.
The potentialities of the x-ray scattering methods (XRS) for quantitative testing of supersmooth surfaces, thin films, and multilayer structures are discussed. The results of the surface roughness study with the use of XRS technique in hard and sort x-ray spectral regions are compared with independent measurements of the roughness by atomic force microscopy (AFM). It is demonstrated that the results obtained by XRS and AFM are in a very good agreement in spite of different physical principles and underlying the methods. XRS technique is applied for the roughness study of thin films which are used in applications for x-ray and UV optics. The XRS method is demonstrated to enable quantitative evaluation of PSD functions of both the film interfaces and the correlation between the substrate and film roughnesses. X-ray investigations of the correlation of the roughnesses of short-period multilayer structures are discussed as well. The use of the whispering gallery effect is demonstrated to extend the XRS method to control of the concave surface roughness.
A novel type of micro-pore optics for the X-ray regime has been developed. These optics have a radial design instead of the square packing in the more traditional Lobster-eye optics. With such a design true imaging, without a crucifix in the focus, can be achieved. We demonstrate that the walls inside the square pores are good enough to produce sub- arcminute focussing up to photon energies above 10 keV. The current performance of the optics is limited by large-scale distortions of the plates, probably caused by the method to fuse the fibers together.
We have measured the hard X-ray reflectivity and imaging performance from depth graded W/Si multilayer coated mirror segments mounted in a single reflection cylindrical prototype for the hard X-ray telescopes to be flown on the High Energy Focusing Telescope (HEFT) balloon mission. Data have been obtained in the energy range from 18 - 170 keV at the European Synchrotron Radiation Facility and at the Danish Space Research Institute at 8 keV. The modeling of the reflectivity data demonstrate that the multilayer structure can be well described by the intended power law distribution of the bilayer thicknesses optimized for the telescope performance and we find that all the data is consistent with an interfacial width of 4.5 angstroms. We have also demonstrated that the required 5% uniformity of the coatings is obtained over the mirror surface and we have shown that it is feasible to use similar W/Si coatings for much higher energies than the nominal energy range of HEFT leading the way for designing Gamma-ray telescopes for future astronomical applications. Finally we have demonstrate 35 arcsecond Half Power Diameter imaging performance of the one bounce prototype throughout the energy range of the HEFT telescopes.
Hard x-ray focusing devices based on laterally graded multilayers are key elements to fully exploit the advantages of third generation synchrotron sources. We have developed a design method to produce laterally graded multilayers using sputter deposition techniques. The multilayers are adapted to the given application by the proper choice of layer materials, d spacing, and the partition of the two constituent materials. The optimization of all relevant parameters yields an ab initio estimation of the desired layer thickness gradient. The performance and the accuracy of this method are demonstrated. The experimental lateral thickness errors could be reduced below 0.5% RMS over a total length of 300 mm. Reflectivity measurements at different energies are in good agreement with theoretical simulations. During focusing experiments at 13 keV a spot size of 1 micrometer and a gain in flux of 1000 were achieved.
A new approach is proposed for the design of wide band-pass multilayer optical elements for the hard x-ray spectral region. The method, based on the combination of analytical and numerical methods, solves the inverse problem consisting of inferring the composition profile of a depth-graded multilayer coating. First, assuming the multilayer d-spacing profile to be a monotone function of the depth and the d- spacing gradient to be large enough, we derived the differential equation that describes the change of period necessary to guarantee a given spectral reflectivity profile. Then, a computer code using an algorithm of steepest descent was used to refine numerically the multilayer period profile, each layer thickness being treated as an independent variable. When using the solution to the differential equation as a starting point of the direct problem, a many-fold decrease of computer time could be obtained. At each step, the spectral dependence of reflectivity was accurately computed using a standard matrix method. Simulations of the particular case of constant reflectivity and maximum integrated reflectivity over a wide spectral range are presented. The best choice of material pairs for comprising a depth-graded multilayer structure is discussed from the viewpoint of maximum achievable reflectivity and least number of bi-layers. Features of depth-graded multilayer mirrors, which are distinctive from conventional periodic mirrors, are examined.
Recent progress in the manufacturing of X-ray multilayers has opened up new possibilities in the field of hard x-ray optics allowing to produce wide bandpass optical elements through the design of depth-graded multilayer coatings. However, the inverse problem consisting of inferring the composition profile of the multilayer has only been addressed in a semi- empirical way, which encouraged us to develop a new (theoretical) approach. Our method is composed of three steps. First, we use an approximate analytical expression to describe the x-ray reflectivity spectral dependence of any arbitrary graded-multilayer structure. Such an approximation is obtained by assuming the multilayer d-spacing profile to be a monotone function of the depth. The d-spacing gradient is also assumed to be large enough, so that the reflection condition for each given energy is fulfilled in a depth zone that is small compared to the thickness of the multilayer stack. On this basis we could derive the differential equation that describes the change of period necessary to guarantee a given spectral reflectivity profile. Then, a computer code, written for solving the inverse problem, lead us to the desired multilayer period profile. Finally, the effective spectral dependence of reflectivity was accurately computed using a standard matrix method. Simulations of the particular case of constant reflectivity over a wide spectral range are presented. Possible applications of wide bandpass multilayers in synchrotron and astrophysics hard x-ray optics are discussed.
We present the x-ray performance on an ESRF synchrotron beamline of a focusing device based on the dynamical bending of a flat silicon plate coated with a 2.5 m d-spacing W/Si multilayer. The mirror was shaped by trial and error to a cylindrical ellipse using an optical profilometer. In a first experiment the device was bent to a 71-m radius to account for the demagnification factor and the energy of operation. With a monochromatic incident beam set at 9 keV a vertical spot size of 4.5 micrometers was obtained at 41 m from the source and 1 m from the multilayer, which agreed closely with theoretical expectations. Such good agreement was due to small residual slope error with respect to the ellipse: around 2 (mu) rad over a 150-mm length for radii greater than 50 m. Moreover, as the divergence of the incident beam was larger than the rocking curve width, less than 25% of the mirror could contribute, reducing the distortion to an even lower figure. With the same geometrical parameters the device exposed to the white beam (monochromator removed) lead to a vertical spot size of about 7 micrometers . Here the whole mirror surface cold reflect, which caused more distortion to the incoming beam but also gave rise to a much higher intensity. The gain in flux obtained with a gradient of d-spacing along the mirror surface is discussed. Finally, results with a Kirpatrick-Baez arrangement let expect in the near future a flux gain greater than 104 with a 10 micrometers by 10 micrometers focal spot.
The manufacturing of x-ray multilayers calls for a precise control of layers having individual thicknesses in the nanometer range and with minimum diffusion at the interfaces. Visible light kinetic ellipsometry and grazing incidence hard x-ray reflectometry (8 keV) have been used under a plasma sputtering environment. The growth of W/Si multilayers illustrates the capabilities of both methods. Thickness, density and roughness values have been derived for each layer by simulation of the experimental data. Those values fit very well the grazing x-ray reflectivity data obtained on the final multilayer exposed to the atmosphere, except for the last 2 layers as a result of oxidation.
A W/Si double multilayer monochromator has been installed on the ESRF BM29 (multipurpose XAFS) beamline with the specific task of suppressing higher order harmonics from the synchrotron x-rays of a bending magnet. This novel technique uses an identical pari of multilayers which were deposited by a method based on distributed electron cyclotron resonance plasma sputtering. In situ growth monitoring enabled the clear identification of a WSi chemical interface with an approximate width of 1nm: the relative size of the interface was found to severely limit the ability of the multilayer to completely reject specific harmonics. In total, the monochromator enabled suppression of all higher order harmonics to less than six orders of magnitude with an approximately 30 percent throughput in the first Bragg peak (at 8 keV). The use of such a multilayer monochromator for harmonic rejection in tandem with a crystal monochromator replaces the traditional method of deploying large grazing incidence mirrors; particular advantages for the multilayers are the significant reduction in size and cost.
Multilayers are the optical elements of choice in any situation where flux rather than resolution is desired. They can be tailored to optimize either reflectivity at fixed or variable energy, or heat resistance, or bandpass, or harmonic rejection. The present state of the art of x-ray multilayers is presented, and the possible applications are reviewed, including the use of multilayers as soft x-ray polarizers, large-passband elements for hard x-rays, and power filters. In the situation of very intense beams the increase of temperature can produce significant changes of reflectivity, which have been extensively studied at the microstructure level in some cases, such as W/C, W/Si, and Mo/Si. Existing and prospective solutions are detailed, including heat treatment prior to x-ray exposure, use of compound materials, efficient cooling, and modification of the electric field distribution by nonperiodic arrangements.
Supermirrors are multilayer structures where the thickness of the layers down through the structure changes so that wide-band reflection occurs. The principles were developed in the mid-70s and have been used extensively for neutron optics. Absorption in the upper layers limits the attainable reflectivity for x rays. For hard x rays (>= 15 keV), the absorption, however, is low enough that it is possible to design supermirrors with 10 - 70% reflectivity in a band approximately equals 3 times the width of the total reflection regime. Supermirrors of W/Si and Ni/C have been successfully fabricated and characterized. The measured x-ray reflectivities are well accounted for by the standard dynamical theories of multilayer reflection. Hard x ray applications that could benefit from x-ray supermirror coatings include focusing and imaging instrumentation for astrophysics, collimating and focusing devices for synchrotron radiation, and particle filtering in plasma diagnostics.
This paper presents a new type of X-ray reflectometer that measures the grazing incidence reflectivity at an energy of 8 keV for angles from 0 - 2.5 degree(s) simultaneously. With an acquisition time of 5 s this allows to analyze the layer growth during a deposition process with a thickness accuracy in the sub-nm range. As a second independent method we are using kinetic and spectroscopic ellipsometry to follow the same deposition process.
Supermirrors are multilayer structures where the thickness of the layers down through the structure changes so that wide-band reflection occurs. The principles were developed in the mid-70's and have been used extensively for neutron optics. Absorption in the upper layers limits the attainable reflectivity for X-rays. For hard X-rays (>= 15 keV), the absorption, however, is low enough that it is possible to design supermirrors with 10 - 70% reflectivity in a band approximately equals 3 times the width of the total reflection regime. Supermirrors of W/Si and Ni/C have been successfully fabricated and characterized. The measured X-ray reflectivities are well accounted for by the standard dynamical theories of multilayer reflection. Hard X-ray applications that could benefit from X-ray supermirror coatings include focusing and imaging instrumentation for astrophysics, and collimating and focusing device for synchrotron radiation.
Multilayers can play a role in any situation where flux, rather than resolution, is desired. They are used in various classical optical arrangements ranging from double monochromators to grazing incidence mirrors. Beam focusing ca also be obtained by multilayer coating on a substrate that is either polished to a given shape, or dynamically bent, or grooved as a Bragg- Fresnel lens. With hard x rays, absorption plays a lesser role, allowing production of a very large bandpass through an aperiodic multilayer design, However, the sputtered, nanometer- thick individual layers are often amorphous, a cause of structural instability in case of a temperature rise. When the beam is intense, which is often the case with a synchrotron x-ray source, a number of solutions are envisaged, including efficient cooling, use of compound materials, and modification of the electric field distribution by nonperiodic arrangement. Results on the performance of multilayers and examples of applications are presented.
Distributed electron cyclotron resonance (DECR) plasma sputtering was used for depositing W/Si multilayers for x-rya optics. The argon plasma used in the sputtering process was excited by the DECR method. The DECR argon plasma diffusing the middle of the deposition chamber was characterized with a Langmuir probe. The ionization rate was found to be 7 X 10-4. This allowed us to sputter W and Si with a large range of target bias values at low pressures. Deposited single layers and W/Si multilayers were characterized with grazing incidence x-ray reflectometry. As an illustration of the capabilities, data is shown for a W/Si multilayer with period d equals 3.0 nm and an interface roughness (sigma) < 0.47 nm.
By varying the thickness of the layers in a multilayer down through the structure, it is possible to produce wide-band reflectors. We report measurements and modeling of the reflectivity of Ni/C, Mo/Si and W/Si supermirrors, at energies ranging from 8 to 130 keV, and discuss the performance of two possible applications: a Kirkpatrick-Baez telescope, and a multiwavelength hard X-ray focusing reflector. The supermirrors perform as expected, and model-fits over the full range have been attempted with some success. We conclude that the supermirror coatings do indeed look very promising as hard x-ray optics for synchrotron applications, while some work on highly nested structures and supermirror coatings on very thin large substrates is necessary, before the feasibility of employing large-area supermirrors for hard X-ray astronomy is determined.
This paper presents measurements of specular and non-specular reflectivities of a W/Si multilayer with period d equals 135.1 angstroms. Angular dispersive measurements were performed at 8.05 keV and 59.3 keV, while energy dispersive measurements were made in the range of 17 keV to 130 keV. At an incidence angle of 1.57 mrad the fourth order Bragg- reflection is found at an energy of 125 keV with a reflectivity in excess of 50% and a bandwidth (FWHM) of 3%.
Fe/C multilayers were rf-sputtered on two silicon substrates with substantial difference of surface quality. Microroughness and figure were measured for each substrate before and after layer deposition by using a scanning tunnelling microscope, an optical microscope interferometer, and a Fizeau interferometer. Although the optical quality of both substrates is better than that of current wafers, differences of x-ray reflectivity and peak shape are still observed. The better sample was exposed to an x-ray wiggler beam of 4.6 keV critical energy, so that the multilayer surface received a power density of 5W/mm2 during a 4 hour period, the sample being cooled from the backside. The reflection properties of the sample before and after x ray exposure are compared. At first glance we did not see any significant changes in reflectivity or bandpass, which is very promising for the use of multilayers on high-power synchrotron x-ray beamlines.
The present paper outlines the requirements for the performance of x-ray mirrors at the European Synchrotron Radiation Facility (ESRF), to be built in Grenoble, France. It is shown that present-day surface preparation techniques are about adequate to achieve conservation of the source emittance, although some improvements are needed in special cases. It is much harder to conserve brilliance, where thermal deformation is the major obstacle. Here substantial research and development efforts are
absolutely indispensable. Two possible ways are indicated to solve the heat problem: cryogenic cooling of silicon-based mirrors and adaptive
optical systems. In the first case thermal deformations are drastically reduced, and in the second they can be compensated by mechanical forces.
Our results are based on theoretical considerations of scattering by nonideal surfaces and on a thermomechanical analysis, which are also given.
For layered synthetic microstructures the technological problems appear to be still more difficult. Because the critical photon energy of the ESRF 6 GeV storage ring and of most of its insertion devices is between 10 and 20 keV or even higher, the discussion is limited to hard x-ray optics.
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