G. S. Knapp

An energy dispersive x-ray absorption spectroscopy beamline, X6A, at NSLS

An energy dispersive x‐ray absorption spectroscopy instrument has been built at the X6A beam port of the x‐ray ring at the National Synchrotron Light Source (NSLS). This instrument allows the collection of extended x‐ray‐absorption fine structure and/or x‐ray absorption near‐edge structure spectra for many elements on the millisecond time scale. The beamline employs a four‐point crystal bender and a rectangular Si 220 crystal to access incident energies between 6.5 and 21 keV. Because the polychromator focuses the synchrotron beam to a narrow 100‐μm line, this experimental apparatus is ideal for x‐ray absorption spectroscopy experiments in special environments such as at high pressures, for in situ experiments, and/or for very small samples. In this manuscript we will describe the instrument design and present data with which to evaluate the instrument. This beamline is available through the NSLS user proposal system.

Development of a laboratory EXAFS facility

The EXAFS technique is a powerful new structural tool, particularly useful for studies of disordered or otherwise complex materials for which x-ray diffraction techniques are difficult or unfeasible. At the present time, most EXAFS experiments are carried out at a synchrotron facility because of the larger fluxes available. We have developed an in-laboratory apparatus utilizing a focusing crystal technique which increases available fluxes two to three orders of magnitude over previous laboratory facilities, so that EXAFS measurements can be carried out quickly and accurately in the laboratory. We will discuss the principles of the focusing monochromator and we will also illustrate the experimental method with examples, including studies of chemical solutions, defect crystalline solids, and high-temperature superconductors.

The historical development of cryogenically cooled monochromators for third-generation synchrotron radiation sources

In the period of the late-1980s, before the construction of multi-GeV third-generation storage rings with their intense insertion-device sources, the perceived number one problem for X-ray instrumentation was proper cooling of the first optical element in the beamline. This article, first given as an acceptance speech for the Compton Award ceremony at the Advanced Photon Source, presents a somewhat historical and anecdotal overview of how cryogenically cooled monochromator optics have been developed to provide a monochromator cooling solution adequate for todays power levels. A series of workshops and international collaborations were the key components for the progress and final success of this development.

XANES study of 3d oxides: Dependence on crystal structure

Abstract XANES measurements are reported for a number of transition metal oxides. Oxide phases, in which the transition element could be widely varied (within the 3d series) while preserving the crystal structure, were systematically examined. The materials examined include monoxides, perovskites, zircons and spinels. For those samples of a given oxide phase, the near edge spectra are nearly identical but spectra for different phases are dissimilar. These observations are consistent with the simplest view of the x-ray absorption process, namely that dipole selection rules are obeyed and spectral features predominately result from transitions between the K shell and empty states with p-character.

Performance of a liquid‐nitrogen‐cooled, thin silicon crystal monochromator on a high‐power, focused wiggler synchrotron beam

An experiment was performed on beamline BL 3 at the European Synchrotron Radiation Facility to test the diffraction performance of a novel internally liquid‐nitrogen‐cooled, thin silicon crystal monochromator exposed to high heat loads. The beam parameters were chosen to closely match the conditions expected, in terms of absorbed power and beam profile, at the Advanced Photon Source (APS) for the closed gap undulator at 7 GeV and 100 mA. The cooled crystal was oriented at 11.4° in the symmetric Bragg geometry to diffract 30 keV x‐rays from the Si(333) planes. The source was a 44‐pole wiggler with the insertion device gap set at 25.0 mm corresponding to a deflection parameter, K, of 4.2. A tunable toroidal mirror was used to focus the wiggler beam onto the crystal. Double‐crystal rocking curves were measured at several power values using different attenuators. The maximum total power absorbed by the 0.6‐mm‐thick crystal was 154 W at a storage ring current of 136 mA. The peak power density at normal inciden...

EXAFS studies of silicate glasses containing uranium

Abstract Sodium silicate glasses containing hexavalent uranium ions have been studied using the EXAFS technique. The U 6+ ions appear in the uranyl configuration with two oxygen atoms at 1.85 A and four to five at 2.2–2.3 A. In the glasses (0.25Na 2 O·0.75SiO 2 ) 1− x (UO 3 ) x with x = 0.02 to x = 0.1, planar (or nearly planar) uranium containing clusters, wi U-U distances of 3.3 A, are observed. A layered model is proposed to describe these glasses.

On the use of electronic tilt sensors as angle encoders for synchrotron applications

We have tested several electrolytic tilt sensors produced by Applied Geomechanics, Inc. A pair of sensors were tested by mounting them on the Chi circle of a Huber 4 circle diffractometer. The angles were scanned in 1° intervals over a range of ±35°. In these tests the resolution was about ±5 μrad but the repeatability depended on angle and varied from 70 μrad (one standard deviation) at large angles to 7 μrad at small angles. This type of tilt sensor may be too slow (2–10 s settling time) as a primary angle encoder for a monochromator or diffractometer, but for a system run by stepping motors, they would prove quite useful as secondary angle encoders. Other models which have a narrower angular range would be useful in setting and tuning focusing mirrors.

A new powder diffraction method for linear detectors

In this article we describe a new type of powder diffractometer that gives a significant improvement in count rate over existing methods. The key to the new technique is the use of a flat, diffracted beam monochromator in combination with a position sensitive detector. The incident x rays fall on a flat plate or capillary sample and multiple diffraction lines are reflected by the monochromator out of the equatorial plane onto a position sensitive detector. Since the monochromator is tuned for the incident beam energy, background from fluorescence or other scattering is eliminated and data can be recorded over a range of several degrees thereby providing a large improvement in counting efficiency over conventional diffractometers. This new method is also compared to a previous article which employed a focusing analyzer crystal and position sensitive detector.

Low‐temperature heat‐pulse calorimetry for self‐heating radioactive samples

A heat‐pulse technique is described for measuring low‐temperature heat capacities of radioactive self‐heating samples in the range 2–60 K. The method also provides a simple alternative to traditional heat‐pulse methods for measuring nonradioactive samples and is attractive because a heat switch is not required. Data for copper indicate an overall accuracy of about 1%. Heat capacity data for plutonium‐242 between 4 and 13 K are also presented.

A simple sagittal focusing crystal which utilizes a bimetallic strip

A sagittal focusing monochromator which utilizes a bimetallic strip as its active focusing member has been designed and has undergone preliminary testing. The crystal bender is very easy to use as only setting the temperature of the bimetallic strip is necessary to adjust the focus. The mechanism utilizes bending rods mounted on live centers which prevent twisting of the crystal. A finite element analysis has been done on a new ribbed crystal which, based on the analysis, would have very small aberrations.