John Gilfrich

Integral Reflection Coefficient of X-ray Spectrometer Crystals

Analyzing crystals used in x-ray spectrometers have widely varying diffraction efficiencies. When employed in x-ray fluorescence analysis, the parameter which defines the efficiency is the integral reflection coefficient. This parameter has been measured using a single crystal spectrometer, as a function of wavelength, for a number of crystals commonly used. A recent adaptation of an existing diffraction theory is shown to make possible the calculation of integral reflection coefficients which agree with measured values.

Bragg diffractors with graded-thickness multilayers

Abstract Possible designs, characteristics and uses of multilayer diffraction elements in which the layer thickness is non-uniform are discussed.

Layered Synthetic Microstructures as Dispersing Devices in X-ray Spectrometers

Layered synthetic microstructures (LSMs) are useful dispersing devices for x-ray spectroscopy. They can be produced with virtually any layer spacing (d) greater than approximately 10 Å and they have high diffraction efficiency. Integral reflection coefficients for such structures made up of alternating layers of a transition metal and carbon are 3 to 10 times greater than values for other dispersing elements used in the moderate to long wavelength region of the x-ray spectrum. Resolving power of the LSMs is somewhat poorer than crystals at this time but is sufficient to permit significant applications.

Graded-Layer-Thickness Bragg X-Ray Reflectors

Multilayer x-ray reflectors which have layers that vary in thickness, either in depth or laterally along the surface, were prepared by sputtering. Alternating layers of W and C were deposited on flexible mica and smooth silicon-wafer substrates. X-ray diffraction properties of the multilayers were measured in the 1.54-8.34 A region. The mutlilayer on mica represents a dual-spacing depth-graded device. Diffraction from both the multilayer and mica was observed. A smoothly-depth-graded multilayer on silicon has a relatively-wide rocking curve which agrees well with diffraction theory. A laterally-graded multilayer on silicon has a bilayer thickness (d value) which varies linearly from 24.8 to 29.2 A in 6 cm.

Evaluation of Commercial Energy Dispersion X-Ray Analyzers for Water Pollution

Seven typical commercial energy dispersion x-ray analyzers were evaluated for application to the measurement of elemental concentration in polluted water. Sensitivity and detection limits were measured on single- (or dual-) element standards. Results for 400-sec counting intervals indicate that the better energy dispersive analyzers have limits of detection between 10 and a few hundred µg/cm2. Many pollutants in natural water are present at >10 ppb, which means that adequate material can be obtained from 100 to 1000 ml samples; some elements of interest such as Cd and Sn are generally present at <1 ppb, which makes practical detection marginal for the energy dispersion method. When x-ray calibration curves were prepared from individual-element gravimetric standards and used to analyze multiple-element samples (also prepared gravimetrically), the agreement between x-ray and gravimetric results showed an average relative deviation of 16%.

X-Ray Fluorescence Analysis

X-ray fluorescence analysis has come a long way in the sixty years since Moseley began his classic experiments on the relation of x-ray wavelength to atomic number.(1) During this period several significant milestones were reached: in 1923 Coster and Von Hevesy confirmed the existence of element 72, Hafnium, from the x-ray spectra of Norwegian zircon;(2) in the 1930’s concentrations of a fraction of a percent could be measured; in the mid-1940’s the availability of high-powered sealed x-ray tubes, large single crystals, and geiger counters enabled Friedman and Birks to demonstrate a practical system for chemical analysis.(3) in the last twenty years, the technique has developed to a high degree of sophistication and is presently accepted as one of the most useful rapid and economical analytical methods available. The method is simple and straightforward with commercial equipment and a high vacuum is not required.

Comparison of measured and calculated values for the diffraction line profiles and integral reflection coefficients for multiple diffraction orders of multilayer structures

Double-crystal and single-crystal spectrometer measurements of line profile and integral reflection coefficient versus diffraction order are presented. These results are compared with theoretical predictions. The ability of the use of an intermediate layer in the theoretical model to explain the results is emphasized.

Characterization of Multilayer Structures for Soft X-ray Dispersion

The use of multilayer structures for soft x-ray spectroscopy requires an accurate knowledge of their reflection properties. In this paper, measurements and calculations will be presented for the single crystal integral reflection coefficients of several multilayer structures. Experimental results will be presented for wavelengths from about 0.8 to 1.4 nm. The measured integral reflection coefficients were smaller than those calculated for a perfect multilayer structure, in agreement with previous results. This reduction of diffraction efficiency is due to imperfections in the multilayer structure. Introduction of appropriate defect structure into the computational methods will be discussed. Evidence will be presented that the effect of substrate roughness is dominant.

From the (former) editor ‘One more time’

If you read this page regularly, you should remember that I said my goodbyes in the last issue, but in view of the fact that this is being written while I still hold the position of editor-in-chief (early November 1997), it seemed appropriate that I present a few more thoughts on my tenure.

Multilayered structures as dispersing devices in X-ray spectrometry

Abstract Artificial Bragg diffractors (multilayers) are useful dispersing devices for the quantification of boron to silicon (atomic numbers 5–14). This paper describes the preparation, characterization and recent efforts to use such devices in that application, particularly where dispersing crystals of appropriate spacing for wavelength-dispersive spectrometers are not available.