P.L. Cowan

A suite of programs for calculating x‐ray absorption, reflection, and diffraction performance for a variety of materials at arbitrary wavelengths

One of the most useful characteristics of synchrotron radiation is the wide spectral distribution of the source. For applications involving tuned monochromatic beams it is often helpful to predict the x‐ray optical characteristics of a sample or the beam line optics at a particular wavelength. In contrast to this desire stands the fact that tabulated values for the optical parameters of interest are generally available only at wavelengths corresponding to typical x‐ray tube sources. We have developed a suite of fortran programs which calculate photoabsorption cross sections and atomic scattering factors for materials of arbitrary, uniform composition or for arbitrary layered materials. Further, the suite includes programs for calculation of x‐ray diffraction or reflection from such materials. These programs are of use for experimental planning, data analysis, and predictions of performance of beam line optical elements.

A UHV compatible two-crystal monochromator for synchrotron radiation

Abstract For obvious reasons, monochromatic synchrotron radiation studies at soft X-ray energies require that the monochromator be ultra-high vacuum compatible. We will describe the design, testing and performance of a linkage based, two-crystal, non-dispersive monochromator for use at synchrotrns. The use of mechanical linkage reducesthe degrees of freedom and minimized the number of motion feedthroughs into the vacuum. The linkage chosen maintains a constant output beam position, and assures that the second crystal is always properly positioned. Pertinent design features include direct angular encoding and a feedback-controlled correction system to eliminate the effects of mechanical imperfections.

A high energy resolution X-ray spectroscopy synchrotron radiation beamline for the energy range 800–5000 eV

A beamline for X-ray spectroscopy of atomic and molecular gases and condensed matter has been designed and installed at the National Synchrotron Light Source. The beamline is UHV compatible to allow windowless operation for improved flux at low photon energies. A double axis crystal monochromator is employed with a collimating premirror and a focusing postmirror. Paris of beryl, quartz or silicon crystals define an energy band width of <0.4 eV at an arbitrary energy above 0.8 keV. The premirror acts as a tuneable low-pass filter to minimize heat loading on the first monochromator crystal. At the present operating parameters of NSLS, a flux of 109–1013 photons/s of highly monochromatic X-rays can be focused onto a 1 mm diameter spot. Initial experimental results are presented.

PIN diodes as detectors in the energy region 500 eV–10 keV

Abstract PIN diodes offer several advantages over ion chambers in signal-to-noise ratio, size, and ultra-high vacuum compatibility. We have evaluated several commercially available PIN diodes, suitable for use in the X-ray region 500 eV ⩽ E ⩽ 10 keV, using both brehmsstrahlung and characteristic lines from X-ray tubes between 1.29 and 8.9 keV. We present data on response functions, noise levels, and linearity of response.

Multilayer-coated mirrors as power filters in synchrotron radiation beamlines

Multilayer-coated mirrors, rather than conventional total-reflection mirrors, have been proposed as a means to reduce power incident on the first optical element of high resolution monochromators. We have designed, fabricated, installed and characterized a multilayer pre-mirror specifically for the 800 to 4000 eV range for the X24-A bending magnet beamline at the National Synchrotron Light Source. Various aspects of this application are discussed, including power and thermal considerations, beamline layout considerations and constraints, choice of multilayer materials and substrate, and techniques to ensure lateral uniformity of the multilayer. Results of a preliminary characterization of a mirror coated with a SiC/V multilayer and installed in the beamline are also discussed. 9 refs., 5 figs.

Performance of a high‐energy‐resolution, tender x‐ray synchrotron radiation beamline (invited)

Beamline X‐24A at the National Synchrotron Light Source was designed for optimal performance in the x‐ray spectral region 500–5000 eV. This choice of energy range placed a number of constraints on the beamline design, requiring a crystal monochromator in a windowless UHV environment. Although this increased the complexity of the design, there were compelling scientific reasons for our desire to work in this range. In addition to tunability over the selected energy range, a primary goal was to obtain the highest possible energy resolution in the primary beam. We have achieved incident energy resolution significantly better than the typical core‐level lifetime broadening for this energy range. This has permitted studies of processes that are not broadened by lifetimes, such as resonant scattering and back‐reflection x‐ray standing‐wave effects. In addition to high resolution, it was designed to collect and focus as much flux as possible from the bending magnet source.

Selective Excitation of X-ray Emission Spectra

While x-ray emission spectroscopy is a relatively old technique, it takes on important new characteristics when samples are selectively excited by a carefully prepared beam of x-rays. This method is made practical by the availability of synchrotron radiation sources and it extends the capabilities of the traditional technique. Both the energy and the polarization of the excitation x-rays can have significant effects on the emission spectra. Selective control of these parameters can provide specific information on both the electronic structure and the atomic structure of molecules or solids. Examples of the new types of studies include: Satellite free x-ray emission, multi-vacancy excitation, polarized x-ray excitation and emission, and state selective excitation of neutral atoms and molecules.

Performance of a tuneable secondary x-ray spectrometer

An efficient, high‐resolution secondary x‐ray spectrometer is critical to studies of x‐ray‐excited fluorescent x‐ray emission spectra. Even with the highest available incident flux of x rays, the signal count rate can become unacceptably low when dispersed by an analyzing crystal. This problem is most serious in studies of gas targets, or at low energies where fluorescence yields are low. We have characterized the performance of a spectrometer based on the Rowland circle geometry with a variable‐radius curved (Johann) analyzing crystal. A position‐sensitive detector was used so that counts at a range of points on the Rowland circle corresponding to different wavelengths can be recorded in parallel. The efficiency of the spectrometer permits the observation of weak processes, such as subthreshold elastic and elastic x‐ray scattering from gases. Energy resolution at low energies is sufficient to allow observations of spectral peak widths which are narrower than lifetime broadening widths. Polarization depen...

The naval research laboratory materials analysis beam line at the national synchrotron light source

Abstract The optical and mechanical designs of a monochromatic beam line being built by the Naval Research Laboratory on the X-ray ring at the National Synchrotron Light Source (NSLS) are described. The line contains a dynamically bent collimating first mirror, a fixed-exit two-crystal monochromator designed and built at the National Bureau of Standards (NBS), and a bent cylindrical mirror to focus radiation to a spot about 1.1 × 1.7 mm2 in the 5–10 keV range. A throughput of approximately 3×109 photons/s. mA is expected with a E ΔE of approximately 5×103 with silicon crystals. The hutch will contain a six-circle diffractometer since crystallography and other X-ray scattering measurements will constitute the primary uses of the line. EXAFS and detector calibration experiments are also planned.

New two dimensional position sensitive proportional detectors using charge division

Abstract Several two dimensional position sensitive proportional counters have been built. The cathodes can encode the position of the event in one or two dimensions using capacitative charge division techniques; a backgammon configuration encodes in one dimension with anode encoding of the second dimension, or a new cathode pattern can be used to encode in two dimensions. Details of the construction and performance are given.