Due primarily to the absence of Bremsstrahlung radiation, fluorescent spectra obtained with x-ray excitation of a sample have a lower background than those obtained with electron excitation. This can be used to detect elements in liquids at very low concentrations under certain conditions. The technique consists of placing a small measured volume of liquid to be analyzed for trace elements on a thin, low atomic number film; evaporating the liquid to dryness; placing it under an x-ray beam; and obtaining a fluorescent spectra. The residue from the evaporated liquid contains any trace elements present, which then produce characteristic peaks in the fluorescent spectrum. These peaks can be used to measure concentrations quantitatively.

This technique has been around for some time, but with detection limits at the parts per million (ppm) level. Although there are many definitions of detection limit, the most commonly used one is from Burton:

X-ray microscopy is a rapidly developing field stimulated by the development of brilliant X-ray sources and high resolution X-ray lenses. It provides higher resolution than optical microscopy and higher penetration power than electron microscopy. Therefore, X-ray microscopy allows high resolution imaging of thick hydrated samples. The two dominating processes determining the contrast in X-ray microscopy are photoelectric absorption and phase shift. For this reason X-ray microscopy can be performed in amplitude and phase contrast. The Göttingen X-ray microscope at the BESSY electron storage ring in Berlin is operating in both contrast modes and is used for different application fields, for example in biology, biophysics, medicine, colloid chemistry, and soil sciences.

Especially biological objects are sensitive to ionizing radiation. Theoretical investigations show that X-ray images of frozen-hydrated specimen can be obtained without radiation induced artifacts. Therefore, an object stage for cryogenic specimen was developed and implemented on the Gottingen transmission X-ray microscope (TXM) at the electron storage ring BESSY.

The Center for X-ray Optics (CXRO) built and operates a high-resolution soft x-ray microscope (XM-1) at the Advanced Light Source in Berkeley. We report on the use of this instrument in a variety of scientific fields, including biology, civil engineering and environmental sciences.

The microscope is a conventional (full field) x-ray microscope, which uses zone plate lenses to provide high resolution transmission images. The optical setup is similar to the Göttingen x-ray microscope, operated at the BESSY synchrotron radiation facility in Berlin, Germany. A condenser zone plate, fabricated by the Göttingen group, is illuminating the sample and an objective zone plate, fabricated by Erik Anderson (CXRO), is forming an enlarged image on an x-ray CCD camera. While the optical path of the microscope is in vacuum, the sample is at atmospheric pressure, flushed by helium. The spatial resolution of our microscope is 43 nm, measured as the distance from 10%-90% intensity in the image of a knife-edge.

With the steady improvement of x-ray optics with high resolution and efficiency, and continued development or adaptation of different imaging and measuring techniques, soft x-ray microscopy has emerged as a powerful method to image and analyze fully hydrated specimens of several micrometer thickness at sub-optical resolution (for a recent overview, see ref. 1). We report on experiments performed with the cryo scanning transmission x-ray microscope (cryo-STXM), which has recently come into operation at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory.

Cryo-STXM uses x-rays with energies between the absorption edge of Carbon (E = 284 eV) and Oxygen (E = 543 eV) from the soft x-ray undulator at the NSLS. Fully hydrated specimens such as eucaryotic cells in water or ice layers of up to 10 micrometer thickness can be imaged without any additional need for contrast enhancing techniques.

Ultra-thin unstained sections of femoral neck bone from normal and ovariectomised mice were imaged in a scanning transmission x-ray microscope. The images were formed using soft x-ray energies at the calcium L and carbon K edges. The aim of the study was to quantitatively map the calcium, protein and embedding medium in normal and ovariectomised (osteoporotic) mice to compare the quantity and distribution of mineral (via calcium), collagen (via protein) and hydrated content (via embedding medium substitution).

A set of seven images were taken of both the normal and ovariectomised mouse sections. These images were processed using x-ray optical constants to produce mass thickness maps. The thickness of the sections from the normal mouse and ovariectomised mice were 205nm and 375nm respectively. The images show embedding medium, protein and calcium maps of near complete sections of the majority of the femoral neck at equivalent neck positions in the normal (la, 2a & 3a) and ovariectomised (lb, 2b & 3b) mouse bone sections.

We discuss the scheme and test performances of the recently commissioned MEPHISTO system in its final configuration. The tests show that the improvements in the electron optics system with respect to other instruments in the same class made it possible to reach lateral resolutions in the 50 nm range. They also demonstrate rather good spectromicroscopy and spectroscopy performances, reliability and flexibility of operation.

This instrument belongs to the class of the electron-imaging systems, like the PEEM (PhotoElectron Emission Microscope) [1] and the X-PEEM (X-ray PhotoElectron Emission Microscope) Microscope [2-5], Our new instrument, called MEPHISTO from the French acronym “Microscope a Emission de Photo électrons par Illumination Synchrotronique de type Onduleur” (Photoelectron Emission Microscope by Synchrotron Undulator Illumination) shares the good characteristics of the instruments of its class: real-time chemical imaging, fast taking of local x-ray absorption spectra, high spatial resolution, large dynamic range, flexibility and user-friendliness.

The phase of a coherent wave is an intuitively well understood concept and efforts to measure the phase of a wave are a staple of visible, x-ray, electron and atom optics. However, in a subtle way, phase is often measured where it is not even defined. An example of this is adaptive optics where the atmospheric phase distortion of light from astronomical objects is measured using polychromatic light. The assumption buried under this work is that it is possible to sensibly talk about phase even though the context of the discussion does not permit phase to be defined according to elementary concepts.

We have been undertaking an ongoing program investigating the nature of wavefields and exploring their characterisation using more robust approaches than standard techniques such as interferometry In a very recent paper we have developed a new viewpoint on phase defined via the Poynting vector of the radiation and where the phase acts as a set of potentials leading to the Poynting vector field.

We have developed a state-of-the-art x-ray microprobe with many unprecedented capabilities for a broad range of scientific applications. The combination of the high brilliance third-generation synchrotron radiation sources and the development of high performance x-ray focusing optics have led to the construction of a x-ray microprobe at Argonne National Laboratory with a FWHM focal size less than 0.25 μm and photon flux exceeding 5×l010 photons/cm2/sec/0.01% BW. The microprobe is very flexible and versatile and can be used simultaneously to perform x-ray diffraction microscopy, fluorescence microscopy, spectromicroscopy, and 3D tomography with submicron spatial resolution. It has been used for a broad range of applications since its commissioning in December of 1996. For example, it was used in the study of the symbiotic relation between plant root and fungi, platinum-based anticancer agents, integrated laser/modulator microdevices, electromigration in Al wires in semiconductor devices, and submicron spatial resolution threedimensional tomography of a single cell. The capabilities of the microprobe are described below, and applications using these capabilities will be presented.

X-ray absorption spectroscopy (XANES and EXAFS) applied with x-ray microprobe instrumentation (figures 1) can be used for studying the electronic structure of specific elements in complex materials in a spatially-resolved manner (figure 2). Such techniques are valuable in a wide range of studies including hydrothermal fluid processes, migration and encapsulation of toxic and radioactive wastes, and redox evolution of solar system bodies.

One of the major technical challenges in this work is the production of high flux microbeams from high power, hard x-ray synchrotron sources. Some of the microbeam technologies under development include tapered glass capillaries, zone plates, and elliptical mirrors. Each approach has advantages and disadvantages and the optimum microfocusing device depends on the particular experiment.

One of the most versatile of these devices is the dynamically bent, elliptical mirror, especially when a pair of mirrors are arranged in a Kirkpatrick-Baez (KB) geometry to provide two-dimensional focusing. This versatility derives mainly from four attributes.

Coral aragonite encodes a rich, complex record of ambient environmental conditions (including temperature, salinity, pollutants) during skeletogenesis. Annual growth banding in appropriate species provides an internal calendar, and the rapidity of skeletal growth permits temporal sampling to at least a sub-monthly basis. The longevity of reef coral colonies can provide continuous centuries-long environmental records. The presence of well-dated Pleistocene coral reefs at sites across the world ocean extends this record into the more distant past. Thus corals are a remarkable resource for the investigation of environmental conditions over perhaps 7 orders of magnitude of time.

The past few years have seen considerable use of Sr/Ca ratios in scleractinian corals to reconstruct sea surface temperatures (SSTs) to document global change. The detailed correlation between Sr/Ca ratios in living corals and measured ambient water temperature is often remarkable, as is the match in older corals between Sr/Ca ratios and such other SST temperature proxies as 18O /16O ratios.

X-ray microfluorescence (XRMF) is spatially resolved x-ray fluorescence (XRF) analysis of elemental composition. Formerly available only at synchrotron sources, recent development of commercial instruments has brought a tamer version of this technique into the size and price range of the academic laboratory. Our Kevex Omicron system features a primary beam diameter as small as 30 μm, with an energy dispersive spectrometer collecting the x-ray spectrum from Na to U. The micro-positioning stage and optical microscope permit selective spatially resolved analysis on the surfaces of objects as large as a desktop dictionary. The large sample stage, functional spatial resolution, and non-destructive analysis present significant opportunities for elemental analysis of a variety of archaeological materials. In particular, objects which for aesthetic, religious, regulatory, or economic reasons cannot be subjected to any alteration in sample preparation may still be analyzed by XRMF. Our laboratory has been conducting a number of investigations which illustrate the potential impact of XRMF technology on archaeometry.

We use the Scanning Transmission X-ray Microscope (STXM) at the National Synchrotron Light Source (NSLS) (1) to acquire images and spectra of polymers. To interpret data correctly, the effects X-ray radiation has on polymers must be understood. We have therefore started to characterize radiation damage in a variety of carbonyl containing polymers in two ways. First, we want to ascertain the critical dose for mass loss and the critical dose for the carbonyl in a variety of polymers and relate the critical dose to the polymer structure. (The critical dose is the radiation dose at which the optical density of the material is decreased by 1/e of its original value.) We also want to understand the damage mechanism. STXM acquires images and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra by using X-ray photons to excite inner shell electrons to unoccupied valence orbitals or to the continuum.

Characterization of deformed crystals containing extensive inhomogeneities and localized lattice reorientations presents a formidable challenge, particularly when the available sample volume is limited and/or irregular in configuration. The crystals analyzed in this work were originally high-symmetry single crystal tungsten rods, 6.9 mm in diameter and 102 mm long, which had been ballistically launched against steel blocks. At ordnance velocities (1.5 km/sec), the rods evert as they penetrate the block, resulting in a cylindrical cavity partially lined with a hollow tube of tungsten, with a slug resting at the bottom. These tungsten residuals contained many localizations, fractures, and assemblages of discrete sub-entities, typically 100-200 microns in size, within a macroscopic flow pattern extending over many millimeters. The measured cavity depths displayed a strong crystallographic symmetry dependence. Because the post-event samples, which were either blocks containing polished axial sections through the cavity or loose ejecta, were both massive and frangible, area-resolved X-ray diffraction, utilizing a fine-beam Laue technique with a precision specimen translator to permit correlation with micrographs, was used to map and characterize the lattice sub-structure.

The ESRF source properties offer the required coherence and brilliance to extend X-ray microscopy techniques, which have been extensively used in the soft X-ray region, to higher energies. This will enable new investigations such as the study of thicker specimens compared to the soft X-ray region, access to absorption edges of elements of major interest in the biological and materials sciences, in particular from Potassium to Chromium, the use of X-ray fluorescence for trace element mapping with high spatial resolution and the application of phase contrast.

Two microscopes are under development at the ESRF to cover a wide range of biological and material sciences applications:

A scanning transmission X-ray microscope (STXM) working in the photon energy range of 0.5-8keV. Whilst in its simplest form this might consist of taking multiple images of a single sample region at different incident energies, an interesting extension is to perform highly spatially resolved XAS scans on small regions of the sample.

X-ray imaging started over a century ago. For several decades its only form was absorption radiography, in which contrast is due to local variations in beam attenuation. About forty years ago, a new form of X-ray imagery, Bragg-diffraction imaging or X-ray topography, developed into practical use. It directly reveals crystal defects in the bulk of large single crystals, and paved the way to microelectronics by leading to the growth of large, practically perfect, crystals. The advent of third-generation synchrotron radiation sources of X-rays such as ESRF and APS is now making possible, through the coherence of the X-ray beams, a novel form of radiography, in which contrast arises from phase variations across the transmitted beam, associated with optical path length differences, through Fresnel diffraction. Phase radiography and its three-dimensional companion, X-ray phase tomography, are providing new information on the mechanics of composites as well as on biological materials.

Recent MXRF reports have demonstrated marked x-ray flux enhancements over traditional aperturecollimating systems when using a total reflection capillary optic. In the present work, several standard reference materials (SRMs) were examined with a Kevex Omicron MXRF system to compare the elemental sensitivities achieved with an aperture x-ray guide to those obtained with a monolithic polycapillary optic (X-ray Optical Systems). This study is relevant to the MXRF community using commercial aperture-based instruments where trace elemental concentrations in a sample sometimes mandate the need for a higher primary x-ray dose, but the flexibility of incorporating various sized apertures for other samples is still maintained.

By scanning the x-ray beam over a nickel/gold knife edge, the capillary focal spot diameter at the time of this work was determined to be ∼400 μm, while a 300 μm aperture provided a beam size of ∼550 μm due to beam divergence past the aperture.

Now that correctors for objective lens spherical aberration are becoming feasible, questions have been raised about the usefulness of high-resolution transmission electron microscopy at zero Cs and the possible difficulties of interpretation of such images. In general, high resolution TEM images are interpreted either by comparison with simulations from model structures or by contrast transfer functions (CTFs) to determine the weight (and sense) of spatial contributions to images from corresponding diffracted beams. At zero Cs, HREM image simulations will work, but a projected charge density theory should be used (instead of CTF theory) to interpret images. Both theories use approximations; CTF theory relies on kinematic scattering and PCD theory on zero Cs and limited defocus.

One of the most challenging tasks for high-resolution electron microscopy (HREM) is the atomistic investigation of defects and interfaces in thin films of semiconductors and ceramics. In particular, the application of these materials in electronic devices requires the understanding of the atomistic structure responsible for electronic and optical properties. The imaging of these structures requires a point resolution down to 0.1 nm. For example the smallest atomic spacing of GaAs in the (110) projection is 0.14 nm. According to Scherzer the point resolution of the TEM is proportional to spherical-aberration coefficient Cs1/4 and to the wave length λ3/4. Commercial medium-voltage microscopes up to 400 kV offer only a point resolution of 0.16 nm due to the high spherical-aberration constant Cs of the electromagnetic objective lens. Decreasing the wave length λ by increasing the electron energy improves the point resolution, but has the drawback of severe radiation damage and high costs.

Effective use of the ANL 300kV CFEG/A AEM requires a knowledge of the probe current and size distribution during operation. In most cases this can be calculated once basic parameters of the system are experimentally determined. Central to this is a determination of the source brightness of the Cold Field Emission Gun (CFEG) under standard operating conditions of the instrument. For the last ten years most probe calculations of CFEG's have been done assuming a brightness parameter of β∼×109 A/cm2/sterad2 at 100 kV, while few if any experimental values have been reported at 300 kV. Using β and the relationship that the operating probe diameter is obtained by adding in quadrature the Gaussian probe size (dg, the CFEG source contribution), and the diameters of least confusion due to spherical abberation (ds) and diffraction limits (dd) one obtains the total probe size dt as :

Although, critical performance criteria of TEM objective lenses, such as spherical and chromatic aberration and, more recently, 3-fold astigmatism, are of common concern, the important properties of TEM energy filters are less well-known and not often measured or quoted. In an effort to bring these to light, we have developed methods for automatically measuring and tuning three optical properties important to practical filter performance: non-isochromaticity, geometric distortion, and non-achromaticity.

A filter is perfectly isochromatic if the range of electron energies that contribute to an energy-filtered image is the same across all image pixels. In practice, energy filters display some degree of nonisochromaticity, i.e. the energy-loss range forming the periphery of the image differs by anywhere from a fraction of an eV to as much as few eV from that forming the center of the field of view.