Letter of Interest

A Damping Wiggler-based Project Beamline for X-ray Absorption Spectroscopy (DWXAS)

Contents

  1. Letter of Interest
    1. A Damping Wiggler-based Project Beamline for X-ray Absorption Spectroscopy (DWXAS)
      1. Executive summary
      2. The scientific case for the beamline
        1. Chemical and Energy Sciences
        2. Earth and Environmental Science
        3. Life Science
        4. Materials Science
      3. The technical requirements and specifications of the beamline
        1. Beam characteristics
        2. Heat load management
        3. End stations
        4. Time structure of the stored current
      4. Meeting the needs of the User Community
      5. Choice of source
      6. Proposal Team membership

Executive summary

We submit this Letter of Interest supporting the development of the DWXAS Project Beamline at NSLS-II dedicated to X-ray Absorption Spectroscopy (XAS) and to offer our services as the Beamline Advisory Team for that beamline. This beamline shall be built on canted NSLS-II DW90 damping wiggler and shall consist of two experimental end stations initially sharing the same source – one dedicated to microbeam XAS and the other dedicated to bulk XAS and supporting a wide variety of experimental configurations. The primary design criterion of this beamline is the delivery of world-leading flux over a very broad energy range combined with high stability of beam position and energy calibration over its entire range of operation. This beamline will deliver a substantial improvement in the state of the art of XAS measurement, enabling novel research that is currently unfeasible or impossible, such as speciation of environmental contaminants at natural concentrations, operando studies of dilute commercial catalysts, or in vivo studies of metal-related human disease. In this Letter, we outline the scientific case for this beamline, with emphasis on its cross-disciplinary relevance. We then summarize the technical requirements of the beamline, demonstrate the impact the beamline will have on its sizable user community, and justify the choice of the DW90 source. Research and development efforts required of the NSLS-II team are called out throughout this document.

The scientific case for the beamline

X-ray Absorption Spectroscopy (XAS) is a fundamental tool in the repertoire of synchrotron methods. XAS is an element specific measure of local atomic and electronic structure; it can be applied to nearly every element in the periodic table; and it is sensitive to small minority components of samples. The measurement and interpretation of an XAS spectrum depends neither on symmetry nor periodicity, thus XAS is equally applicable to matter in all forms, including structured and amorphous solids, liquids, colloids, films, surface sorbed complexes, and others. Consequently, XAS addresses the needs of a surprisingly broad array of scientific disciplines at all synchrotrons around the world, playing a key role in many diverse and essential research areas, such as remediation of contaminated soils and groundwaters, development of materials for battery and other energy technologies, management of nuclear waste stockpiles, design of industrial catalysts, characterization of materials for electronic devices, development of organometallic pharmaceuticals, and many others.

The primary limitation to the state of the art of XAS measurement is simply stated. We are limited by photon flux. Limited flux imposes experimental limits on sample concentration or sample abundance. High energy resolution detection of the XAS fluorescence signal is characterized by low detector efficiency and thus is also flux limited. The speed of measurement of system dynamics by XAS is flux limited on all time scales from the sub-minute to the ultrafast. The NSLS-II DW90 source presents a significant advance in this metric compared to extant sources worldwide. It will have a profound impact on the many scientific disciplines which use XAS, pushing back these flux-imposed limitations and setting a new standard for the state of the art of XAS measurement.

Along with exceptional flux, this beamline will provide a beam of very high quality as measured by several other criteria. High energy stability and excellent energy resolution are required for accurate chemical speciation. High positional stability on the sample is required to probe sample heterogeneity down to the micron length scale. Because the scientific community served by this beamline is so diverse, this beamline will provide highly versatile and robust end station instrumentation. These various characteristics of a high quality beam combined with the exceptional flux of the DW90 source will address the many scientific drivers relevant to scientific communities which are already heavily invested in XAS beamlines at synchrotrons worldwide.

Chemical and Energy Sciences

The synchrotron Chemical and Energy Sciences (CES) community consists of scientists who use synchrotron radiation in the areas of chemical transformations, catalysis, electrochemistry, energy conversion, and hydrogen storage. The December 20, 2007 BESAC report Directing Matter and Energy: Five Challenges for Science and the Imagination the following “Grand Challenges” were outlined for the new era of energy science: (1) control of materials properties at the electron level, (2) emergence of bulk properties at the nanoscale, (3) design of materials with tailored properties, (4) characterization and control of non-equilibrium materials, and (5) creation of new nanotechnologies mastering energy and information and rivaling those of living things.

The key scientific drivers for chemical sciences in the synchrotron catalysis and electrochemistry fields are real time monitoring of chemical reactions, design of new or more efficient chemical processes, and rational design of high-activity, high-selectivity, low-cost catalysts. To meet this last critical goal, a fundamental, atomic-scale understanding of the physical and chemical properties of catalysis and reaction intermediates is required. Much is known from synchrotron studies about materials structure and properties of many model systems at specific external conditions. Real systems and processes, however, are extremely complex and thus require development of techniques for the characterization of such systems in-situ, as they evolve in time with a changing environment.

A central mission of energy research is development of methods for exploiting, converting and optimizing existing and recently developed energy sources. Methods of analysis based on synchrotron radiation are required across this field and impact the development of renewable energy conversion technologies, the design of portable devices for energy delivery, and and the understanding of nanocomposite materials for various energy applications.

Catalysis serves as a cross-cutting feature in each of these disciplines, thus many technical capabilities required in designing their synchrotron-based lines of research (as well as many challenges toward their realization) are expected to be similar. CES researchers will use the DWXAS Project Beamline to study engineered materials which are structured on sub-nm to hundreds of nm length scales; interactions between nanoparticles and supports; real-time changes of the structure in the process of catalysis; time-resolved studies of kinetics of chemical transformations at the time scale from seconds to hours. CES researchers will use this best in its class beamline and its well-planned, dedicated infrastructure to develop and test novel experimental methods in CES fields including combined x-ray diffraction and absorption techniques and the emerging techniques for in situ characterizations of real catalysts. A useful analogy is the close interaction between the catalysis industry and European synchrotrons, which has resulted in the development of new experimental techniques such as QEXAFS (Quick Scanning EXAFS) and combined XAFS/XRD methods at DESY, the ESRF, and the SLS.

Due to its exceptional flux, the DWXAS Project Beamline will enable studies of real catalysts with low metal loadings and of lithium-ion batteries with faster (from minutes to seconds) discharge/charge cycles. Many such systems cannot be efficiently studied today because of insufficient flux. Increasing the metal loading of catalysts or reducing the speed of battery discharge/charge cycles to make them accessible for current experimental facilities also changes their reactions and intermediate stages. The flux of the DW90 source will allow, for example, measurement of the 0.1% metal loadings of Pt on $\gamma$-Al2O3 used in commercial catalysts. Additionally, the increased energy range allows to measure such catalytically important materials as Pd, Ag, and In at low (0.1 wt. %) loadings. A high flux source is also essential for combined XAFS/XRD measurements during chemical reactions. Beam position and energy stability combined with high flux allow the use of Diffraction Anomalous Fine Structure (DAFS), a powerful technique to study heterogeneous or multi-site catalytic or battery systems such as metal nanoparticles, metal oxides, and others.

Earth and Environmental Science

Applications of synchrotron-based XAS and microscopy techniques brought revolutionary improvement to our understanding of many geologically and biologically important processes in the environment in the last decade. Research areas range from low pressure, water-sensitive stratospheric conditions to high pressure-temperature (PT) conditions that mimic the deep Earth and other planetary bodies and from oxygen-rich soils to anoxic sediments. These studies include in-situ chemical speciation and mineral characterization under a wide range of physico-chemical conditions and were impossible to study without XAS and other synchrotron techniques.

The highest priority in the environmental sciences is to identify contaminant speciation, the biogeochemical processes that alter that speciation, the transport of chemicals at different length scales (submicron to several millimeters), and the bioaccumulation of contaminants. However, many of the current studies have been limited to concentrated or model systems or to the most heavily contaminated environments. The DWXAS Project Beamline will enable these studies at environmentally relevant concentrations. This high-flux microbeam station will enable characterization of the full spatial and compositional heterogeneity of natural systems.

The key issues in high-pressure Earth science studies are to identify the chemical states of different elements under high PT conditions, such as those relevant to the core-mantle boundary, and to examine their variations in different mineral phases in diamond anvil cell (DAC) environments. The DAC is currently of limited applicability for XAS studies as the cell itself is highly absorbing at lower energies. The high-flux and broad energy range of the DW90 source will allow novel uses of DAC-based or large-volume press XAS studies. For instance, high PT XAS studies only recently attempted on hydrothermal systems will become routine on this beamline.

Geochemical reactions in the environment occur at a range of time scales, from the sub-nanosecond to many years. While equilibrium states of many reactions are well known, the kinetics of several reactions, and the influence of different environmental variables on reaction rates are not well understood. The majority of kinetic studies have again been limited to homogeneous model systems. Reaction rates in natural samples will remain poorly explored until the exceptional flux of this beamline becomes available.

A topic of growing importance in environmental research is the fate and transport of mercury in terrestrial and aquatic systems. Mercury is a waste product of industrial activity, coal mining, and coal consumption as well as a ground contaminant at Department of Energy nuclear production facilities. It is typically present in natural systems at low concentrations yet poses substantial human health risks at levels well below measurement limits on most existing XAS beamlines. The chemistry of mercury is complex, highly sensitive to thermodynamic parameters including concentration, and highly variable in a heterogeneous natural setting. The DWXAS Project Beamline will provide adequate flux to understand the fate and transport of this critical environmental contaminant.

Life Science

Metalloenzymes are nature's catalysts and are able to carry out remarkably difficult chemical transformations which often pose significant challenges in a laboratory setting (including the need for high temperatures, high pressures, and complex synthetic procedures). These enzymatic catalysts include nitrogenase, which converts dinitrogen to ammonia, the oxygen-evolving complex in photosystem II, which is responsible for oxygen production through water oxidation, and methane monooxygenase, which converts methane to methanol. In all these examples, a metal-based active site is the center of catalytic activity. Many XAS studies have already provided key insights into the resting structures of these catalytic metalloprotein centers, and in some cases the intermediates. However, the detailed mechanisms remain elusive and understanding how nature carries out these complex reactions is a major challenge in the biological sciences. These are questions for which synchrotron-based spectroscopy and the capabilities of the DW90 source are well-poised to provide key insights.

In particular the high flux and beam stability afforded by DW90 will enable time-resolved studies on these enzymes in the microsecond time-scale by using stop-flow mixing devices and varying the distance of the x-ray source from the mixing device. This will create a unique ability to characterize reactive intermediates.

With high-flux and high resolution coupled to a crystal analyzer, site-selective XAS of these systems will be possible. Many of these metalloenzymes contain several metal sites with in the active site cluster (i.e. four manganese in photosystem II and two irons in methane monooxygenase) which are known to cycle through different oxidation states during turnover. By using site-selective XAS, one can separate different oxidation states within a multi-centered metalloprotein active site. This application also has great utility for defining high-valent intermediates in freeze-quenched mononuclear metalloprotein centers (including both heme and non-heme iron enzymes), where complete conversion to an intermediate species is often impossible. Site-selective XAS has a large dependence on the 3p-3d exchange correlation and thus a dependence on spin and oxidation state, but is severely flux limited at current sources. It relies on XAS measured via $K\beta$ emission lines, which are almost an order of magnitude weaker than the main $K\alpha$ line, while the analyzer required for their detection is highly inefficient. For a dilute metalloprotein, site-selective measurements are currently prohibitive, but the high flux of DW90 source will make them feasible and will be an important compliment to the time-resolved studies described above.

The beam line will further impact the life sciences by allowing examination of metal-related diseases at conditions that are relevant in vivo. Metals are invoked in numerous neurodegenerative diseases (including Parkinson's, Alzheimer's and Amyotrophic Lateral Sclerosis). It is thought that metals interact with neuronal proteins and catalyze aberrant redox chemistry resulting in protein aggregation. However, most studies of the metal-protein interactions require concentrations that are much higher than what is biologically relevant (resulting in characterization of additional and/or exogenous metal binding sites). The ability to examine concentrations on the micromolar to submicromolar levels will greatly enhance our understanding of these diseases.

Materials Science

One of the semiconductor industry's “Grand Challenges” is to develop an alternative to the SiO2 gate dielectric that has enabled scaling (increasing integrated circuit device density, according to Moore’s Law) of metal-oxide-semiconductor-field-effect transistors (MOSFETs) for the past 40 years. The challenge originates from the quest for integrated circuits exhibiting higher speed and lower power consumption, no longer attainable with ultra-thin (sub 2nm) SiO2 gate dielectrics due to their high direct tunneling leakage currents. This paradigm of Moore’s Law for next generation nano-electronics is relevant to other industries as well that are constantly in search for lighter, more efficient, less toxic, smaller, and hence less costly solutions to their applications. Future solutions to this general problem will come from state-of-the-art advances utilizing new materials and nano-engineering. They will also need to address the relevance of size, strain, and boundary effects on the local electronic and atomic structure from a host of industrial driven applications.

To bring these new materials to market requires quantitative local structural analysis on the atomic scale with the ability to measure lattice parameters, near neighbor bonds, local strain fields, local chemistry, and atomic symmetry in the materials’ natural device state and to relate these structural measurements to physical properties. As these device layers are typically buried, they are inaccessible to many common microscopies, and techniques such as x-ray diffraction loose their sensitivity when layer thickness becomes less than a few lattice constants. Such information is provided, however, in a non-destructive way by high quality XAS data assuming that such high quality data can be collected. With the flux of a typical bending magnet, high quality XAFS data on 5 nm films can be collected over the course of several days, with difficulties arising from the background due to the substrate and capping layers that support the film and from the low count rates due to the limited amount of material (thinness of the films and diluteness of the dopants) – often in the ppm range or below. In order to obtain good signal to background, the films must be studied at glancing incidence near the critical angle, which requires beam sizes as small as 100 μm in either the vertical or horizontal dimension, adding to the existing experimental challenges. The NSLS-II DW90 source offers an optimal solution to the requirement of such high brightness coupled with the unique ability of the DWXAS Project Beamline for stable energy scanning and minimal beam motion all with a highly focused beam. It is estimated that samples typically now requiring 4 days for high quality data acquisition should be routinely measured in several hours. This achievement will increase the relevance to industry, as industrial partners typically require the characterization of a large number of samples per beam run.

The technical requirements and specifications of the beamline

In this section, we outline the most salient specifications of this beamline. The DWXAS Project Beamline is described in detail in Part 2, Chapter 5 of the NSLS-II Preliminary Design Report (PDR). Design of this beamline has been led by Paul Northrup, Experimental Facilities Division, NSLS-II Project. Reviews have been conducted by the Technical Review Committee, the NSLS-II Experimental Facilities Advisory Committee, and the DOE (CD-2), each leading to an improved design. A community workshop on January 16, 2008 further refined the technical design and scientific mission and formed this proposal team. This team has already begun serving an advisory capacity by overseeing developments since the January 2008 revision of the PDR, including studies of heat loads on optical equipment and design revisions in response to the recent decision to reposition the shield wall and extend the experimental floor. As shown in the following rendering, the first optical enclosure and experimental hutches fit well within the proscribed space on the NSLS-II floor, with adequate room for control stations both upstream and downstream of the experimental hutches.

[Figure 1]

Figure 1 Beamline layout

Beam characteristics

The DWXAS Project Beamline requires high flux with a continuous spectrum over a very broad energy range. This energy range should cover edge energies of a large fraction of the periodic table so as to provide the broadest possible scientific relevance. A range from 5.5 (below the chromium K edge) to 35 keV is required for normal operations. Given the requirements of managing the very high on-axis power, energies below 5.5 keV will likely be inaccessible. Higher energies, up to 90 keV, will be available using higher order reflections of the monochromator.

A collimating mirror placed before the monochromator will help define the energy resolution. The heat-bearing monochromator must deliver energy resolution of ΔE/E≈10-4, typical for an XAS experiment. A secondary monochromator in-line with the heat-bearing monochromator can be used when needed to refine the energy resolution to ΔE/E≈10-5 or better with an appropriate crystal choice.

The ability to characterize sample heterogeneity over many lengths scales is a central design goal of this beamline. A toroidal mirror placed after the monochromator is designed to focus the full beam of 5 x 55 mm to a 200 micron spot. Additionally, focusing optics in the Kirkpatrick-Baez geometry can be used to provide a microfocused spot as small as 1 micron. These optics along with beam-defining slits will access this full range of spatial resolutions.

Excellent stability is required. The position of the beam delivered to the experimental hutch should vary by under 5 microns in normal operating conditions. This level of positional stability is required for optimal performance of the microfocusing optics, for measurement of heterogeneous bulk samples, and for the use of specialized sample environments. The heat-bearing monochromator should offer scan-to-scan variation in energy calibration under 0.05 eV and maintain absolute energy calibration over its entire angular range. The stored current at NSLS-II will be highly stable in service of the needs of the Nanoprobe Project Beamline. The DWXAS Project Beamline requires a much lower threshold of stability, but will certainly benefit by this aspect of the NSLS-II design. This beamline will also benefit by top-off operations, which will allow maintenance of a constant heat load on all optical elements and help to meet the targets of positional and energy stability.

Heat load management

The principle engineering challenge of this beamline is to manage the very high heat load delivered by the DW90 source. This is critical both for equipment protection and for meeting the positional and energy stability requirements of the beamline. The full solution to heat load management, outlined briefly here, is detailed in a BNL Technical Report that is included as an appendix. To accommodate a future upgrade to canted wigglers allowing simultaneous operation of the microbeam and bulk EXAFS hutches, the DWXAS Project Beamline will be built on one 3.5 m DW90 damping wiggler segment, thus halving the heat load problem compared to any attempt to build on a full 7 m DW90. Much of the heat will be absorbed by the front end and the white beam slits – all performance statistics for the beamline assume a 1 mrad horizontal aperture and a 0.17 mrad vertical aperture. The water-cooled collimating mirror along with a series of cooled graphite filters will reduce the heat load on the first monochromator crystal below 700 W, a manageable level. As explained in the technical report, mirror angle and graphite filter thickness will be configured appropriate to different energy ranges, such that flux of the order of 1013 photons/second can be delivered throughout the operating range of the beamline while attaining all technical requirements of positional and energy stability.

The target of 700 W is, admittedly, high, but not unreasonably so compared to beamlines already operating or under development throughout the world. The undulator at APS beamline 20ID delivers only 300 W to the first crystal, but in a smaller footprint such that the areal power density is comparable to this beamline. The SPEAR3 wiggler beamline 11-2 has operated as high as 1300W (or 14.4W/mm2) on its first crystal with no discernible distortion to the Si(111) rocking curve. Wiggler beamlines at the Canadian Light Source, the Australian Synchrotron, and Diamond are designed to deliver between 550 and 1000 W to the monochromator with commensurately large heat loads on their collimating mirrors. The design specifications proposed for this beamline – a 700 W monochromator footprint along with around 2 kW at low energy on the collimating mirror – are quite manageable and well within the range of extant beamline designs.

End stations

Because the user community for this beamline comes from such a broad range of scientific disciplines, each bringing specific experimental needs to the beamline, the experimental hutches must be designed with en eye towards flexibility and multiplicity of purpose. One experimental hutch will house the Kirkpatrick-Baez optics. This microbeam sample station will be housed in a large-volume, controlled-atmosphere enclosure, which will protect both the optics and the sample from exposure to oxygen.

[Figure 2]

Figure 2 Interior of bulk XAS hutch

The second hutch will be large, accommodating three in-line experimental stations. The first station will be a conventional bench-top XAS setup with ample room for instrumentation for high- or low-temperature, in situ, or operando experimentation. The second station will be a similar enclosure to the one used with the microbeam and will provide a variety of operating modes, including: (1) N2/H for oxygen sensitive samples, (2) special atmosphere for in situ chemical control, (3) ventilation for hazardous gas mitigation, or (4) dead air for nanoparticle dispersion control. The enclosure will include glove arms for manual sample access and a load-lock for safe sample transfer. The final station will be a large empty space at the end of the hutch which can be used to accommodate large instrumentation. Example uses of this space might include (1) a goniometer for DAFS or reflectivity-XAS measurements, (2) a high-field magnet, (3) an inelastic scattering spectrometer for resonant inelastic or X-ray Raman studies, (4) a laser for ultrafast chemistry investigated via pump-probe techniques, or (5) a large-volume, high-pressure apparatus.

The possibility of sample damage under the intense beam from the DW90 source is a serious concern. There are several strategies for minimizing this problem. One is to minimize time of exposure. By slewing the monochromator continuously throughout an XAS scan, entire spectra can be measured in 10s of seconds without compromising beamline performance criteria. These fast scans, combined with the small spot provided by the toroidal focusing mirror, allow the user to raster over a sample, measuring high quality spectra at each spot and moving to the next spot before significant beam-induced damage accumulates. At the microbeam station, the use of continuously slewing translation stages will serve the same purpose for fluorescence imaging experiments. When allowed by the experiment, the use of cryogenics slows the rate of damage caused by free radicals from hydrolysis or other beam-induced phenomena. High-volume, high-throughput cryogenic equipment is required for this beamline.

This experimental hutch must be highly efficient and very user-friendly. To that end, extensive in-hutch automation is required. This automation should include, but certainly not be limited to, automated gas handling for ion chambers and other instrumentation, accurately encoded motors on all tables and sample stages, well-characterized look-up tables for positions of optics and other instrumentation, and simple robotics for sample exchange.

To fully serve the needs of the user community, a significant investment in detector technology is required. Energy discriminating detection will be a workhorse at the beamline, although current detector technology suffers from limited count rate and so will not take full advantage of the superior flux of the damping wiggler source. Recent developments in silicon drift detectors show promise for handling higher flux, as do the high-rate EXAFS detector arrays currently under development by BNL Instrumentation.

Wavelength dispersive detection is an area in which this beamline will excel. In many cases, the use of fluorescence mode XAS to measure small chemical edge shifts, to differentiate subtle features of an XAS spectrum, or to measure small minority components of a system is limited either by the natural line width of the absorbing element or by the energy resolution of the detector. A spectrometer based on an array of bent-Laue analyzers can measure the final state fluorescence with high energy resolution, as low as 3eV at 10keV, thereby increasing the resolution of the standard fluorescence XAS measurement while amplifying throughput by subtending a large fraction of the solid angle about the sample. This analyzer array will access all fluorescence energies attainable by the beamline as a simple, self contained spectrometer requiring minimal setup for each edge of interest. Leveraging the exceptional flux of the DWXAS Project Beamline, this spectrometer will provide a unique user facility for superior sensitivity to small differences in chemical composition and local structure around absorbing atoms and to small concentrations of absorbing species.

Time structure of the stored current

The high flux of the damping wiggler source is extremely attractive for probing system dynamics on micro- to pico-second time scales using pump-probe methods. By virtue of the high flux, this beamline could be competitive with current state of the art, dedicated ultrafast science facilities at the APS and the Swiss Light Source. To this end, the camshaft mode for the stored current described in Section 2.1.1 of the Preliminary Design Report is highly desirable. With this mode, ultrafast science would be a natural area for expansion of the capabilities of this beamline.

Meeting the needs of the User Community

Large, active, and highly productive XAS communities exist at every synchrotron in the United States and around the world. Every synchrotron has included one or more XAS beamlines in the initial phase of beamline construction to meet the enormous demand for XAS measurements. Illustrative of this demand, the XAS effort at NSLS involves 13 beamlines on the two rings, 10 of which offer XAS or μXAS at hard x-ray energies. Of 3295 on-site visitors at NSLS in FY2006, 653 or 19.8% worked at the 10 hard x-ray XAS/μXAS beamlines. Of about 2250 currently active users of NSLS, 505 or 22.4% are users of those beamlines. Of about 2700 publications reported in the last three years from measurements at NSLS, 15% reported data measured at those 10 beamlines. These numbers well justify one of six NSLS-II Project Beamlines serving the XAS community. XAS users come from a wide variety of scientific disciplines – catalysis and energy science, life science, condensed matter physics, environmental science, geology, and others – and from government, academia, and industry. In the US, they receive funding from nearly every scientific funding resource available from the US government.

The DWXAS Project Beamline is identified as a critical instrumentation need in the whitepapers from the recent Scientific Strategic Planning Workshops for both Earth and Environmental Sciences and Chemical and Energy Sciences. Additionally, access to the DWXAS Project Beamline is a component of the synchrotron research needs identified in the whitepapers from the Workshops on Life Sciences and Hard Condensed Matter and Materials Physics. Attendance at the January 16, 2008 Technique-Based Workshop for this beamline exceeded 50 people.

Nationwide, the community of XAS users is growing. Evidence of this can be found in the popularity of XAS training courses in recent years at NSLS, SSRL, and APS. Members of this proposal team have been on organizing committees of 12 XAS schools at NSLS, SSRL, and APS since 2003. Attendance in those courses has ranged from 20 to 40 students and every instance has been oversubscribed. Courses at NSLS, SSRL, and APS are scheduled for 2008.

Extrapolating from the demand for XAS at NSLS and at other synchrotron, the demand for XAS at NSLS-II will hugely exceed the capacity that the DWXAS Project Beamline can provide. We envision that the DWXAS Project Beamline will be the centerpiece of a suite of absorption spectroscopy beamlines at NSLS-II serving all energies from 100 eV to 90 keV, all length scales from the bulk to the submicron, and all time scales from the static to the picosecond. Future upgrades to this DWXAS Project Beamline will go a long way towards meeting those many needs by using the second canted damping wiggler to provide dedicated, independent beamlines for the microbeam and bulk XAS stations. We also plan to insert a side-deflecting mirror off-axis between the canted wigglers, creating a side branch delivering more than 1014 photons/second in the 2 to 6 keV range. This suite of beamlines serving the XAS community will be fleshed out by developing capabilities for: (1) ultrafast measurement on this beamline, (2) subsecond quick XAS on another DW90, (3) micron and sub-micron probes on three-pole wiggler and undulator sources, (4) soft and mid-range absorption spectroscopy on soft bend, (5) conventional and subsecond quick-XAS on three-pole wigglers.

Choice of source

Wigglers are ideal XAS sources as they combine high flux with continuous, incoherent radiation over a very broad energy range. In contrast, XAS using an undulator source is a more challenging proposition. The beam coherence of an undulator is undesirable in an XAS measurement. Furthermore, using an undulator requires either tracking the undulator gap with the monochromator energy or tapering the gap to broaden the energy band delivered to the monochromator. Each of these solutions is problematic. Tracking the gap is relatively slow, adding to the overhead of each scan and limiting any quick scanning strategy involving continuous slewing of the monochromator. Furthermore, errors in the synchronization between the undulator and the monochromator as the scan progresses can introduce measurement nonlinearities due to changing harmonic content or other issues. Tapering the gap is used successfully at the APS for XAS measurements. The high ring energy allows a 1000–1500 eV bandpass even with modest tapering. At NSLS-II, with its much lower ring energy, the maximum taper of the NSLS-II U14 and U20 undulators would allow a bandpass of only 200–300 eV, too small for extended XAS, and with significantly reduced flux compared to the DW90 source. Furthermore, the energy coverages of each harmonic of the those undulators are rather narrow and have regions of significantly diminished flux at energies of critical importance to an XAS program.

With its continuous spectrum, a wiggler avoids the many problems an NSLS-II undulator would present for XAS measurements. Heat load management, the central technical challenge for the DW90 source, is demonstrated to be tractable. With its exceptional flux, the NSLS-II DW90 damping wiggler will outperform any existing undulator or wiggler for x-ray absorption spectroscopy. As an example, recent work at APS beamline 10ID (Environ. Sci. Technol. (2006), 40(7), 2262) used an APS undulator, one of the world's highest flux sources, and a microbeam from Kirkpatrick-Baez optics to study uranium incorporation in an ancient calcite at natural concentration. Extended XAS data were measured, allowing a detailed analysis identifying the mechanism of uranium incorporation into the calcite structure. This work demonstrates a plausible strategy for long-term uranium sequestration – a major interest of the Department of Energy. Under the experimental conditions reported, about 1011 photons per second were delivered into a 10 micron spot, requiring 2½ days of continuous data collection. The NSLS-II three-pole wiggler will deliver only about 5x109 photons per second in the Kirkpatrick-Baez geometry, requiring months of continuous data collection to obtain similar data quality. With a microbeam flux of 1012 photons per second, this same experiment could be performed using the DW90 microbeam station in only 6 hours. This is one example of an experiment that is challenging by today's state of the art, but which will become feasible or even routine on the NSLS-II damping wiggler.

Proposal Team membership

The team submitting this proposal offers itself as the initial DWXAS Project Beamline Advisory Team. This team reflects the diversity of the XAS user community. The spokesperson is an expert in XAS analytical technique and practice. Other members are strong voices for the diverse user communities to be served by this beamline. Our final member is an expert in synchrotron and XAS instrumentation. Together, we are advocates for the XAS community in its breadth and diversity and will ensure that this beamline will serve its scientific focus.

Spokesperson

Bruce Ravel
Scientist, Synchrotron Methods Group, Ceramics Division
National Institute of Standards and Technology

Chemical and Energy Science

Anatoly Frenkel
Professor, Department of Physics
Yeshiva University

Earth and Environmental Science

Satish Myneni
Assistant Professor of Geosciences
Department of Geosciences, Princeton University Princeton

Materials Science

Joseph Woicik
Scientist, Synchrotron Methods Group, Ceramics Division
National Institute of Standards and Technology.

Life Science

Serena DeBeer-George
Scientist, Stanford Synchrotron Radiation Laboratory
Stanford Linear Accelerator Center

Instrumentation

A. Jeremy Kropf
Physicist, Chemical Sciences and Engineering
Argonne National Laboratory

NSLSII/XasBeamLine/ProjectBeamline/BAT/LetterOfInterest (last edited 2009-10-09 19:51:15 by localhost)