X-ray fluorescence, using the fundamental parameters method for matrix correction, is used routinely for quantitative analysis
in our group. The accuracy of this method has been demonstrated with a Nd:YAG (Nd:Y3Al5O12) laser crystal. The neodymium concentration was determined relatively to the yttrium concentration by X-ray fluorescence
using241Am for excitation. The yttrium-aluminum ratio was confirmed by scanning electron microscopy (SEM-EDX). Although there was
a 10% discrepancy for yttrium due to the uncertainty in the neutron capture cross-section, the fluorescence results for aluminum
and neodymium were in very good agreement with neutron activation analysis (NAA) results. Based on this agreement, the Nd:YAG
sample will be used in future NAA calibration measurements.
The quantitative interpretation of X-ray fluorescence (XRF) data is often difficult because of matrix effects. The intensity
of fluorescence measured for a given element is not only dependent on the element's concentration, but also on the mass absorption
coefficients of the sample for the excitation and fluorescence radiation. Also, there are interelement effects in which high-energy
fluorescence from heavier elements is absorbed by ligher elements, with a resulting enhancement of their fluorescence. Recent
theoretical treatments of this problem have shown that X-ray fluorescence data can be corrected for these matrix effects by
calculations based on first principles. Fundamental constants, available in atomic physics data tables, are the only parameters
needed. It is not necessary to make empirical calibrations. In this paper we report the application of this correctional procedure
to alloys and alumina-supported catalysts. We also discuss how it may be applied to other matrices. A description is given
of a low-background spectrometer which uses monochromatic AgKα radiation for excitation. Matrix corrections by first principles can be easily applied to data from instruments of this type
because fluorescence excitation cross-sections and mass absorption coefficients can be accurately defined for monochromatic
Authors:J. Dale, L. Hulett, S. Pendyala and W. Lyon
In using positrons as analytical tools the experimenter has two quite different options. The first and more obvious is to duplicate electron methods with positrons and see what differences (if any) result. The second is to exploit a unique characteristic of positrons, such as the formation and decay of the positronium atom, to study chemical composition and surface characteristics. Because positrons do not exist freely in our world, they must be obtained from radioactive sources or nuclear interactions. Source intensity has consequently been a limiting factor in experiments that attempt to duplicate electron applications. Some methods of producing and moderating positrons that have been developed here (and elsewhere) are described as well as results from studies using the sources. Surface measurements require less intense sources and yield useful data on materials such as xeolites, silica gels, graphite and alumina. Experimental apparatus, data and interpretation will be discussed.
Authors:L. Hulett, Jun Xu, S. McLuckey and D. Schrader
A review is given of the ionization of organic moecules by monoenergetic positrons having energies in the range of 0.5–15 eV. Two mechanisms, unique to positrons, are described. If the kinetic energy of the positron is above the positronium formation threshold, such that electrons can be removed from the molecules to form free positronium atoms, the ionization/fragmentation behavior can be explained qualitatively by a modification of the Ore gap theory. To explain how positrons can ionize and fragment molecules when their kinetic energies are below the positronium formation threshold, it is necessary to assume that energy is transferred to the molecule by the annihilation process. Ionization cross sections for positrons having kinetic energies below the positronium formation threshold are sensitive to molecular size, structure and bond types. Continuing work involves a search for positronium compound formation and measurements of the kinetic energy distributions of ions.
Authors:L. Hulett, John Dale, H. Dunn and P. Murty
The purpose of this paper is to demonstrate that complicated mixtures of solids can be characterized to a rather high degree
if a coordinated examination by non-destructive methods is used. The techniques discussed are X-ray fluorescence, scanning
electron microscopy, photoelectron spectroscopy, transmission electron microscopy, electron diffraction and X-ray diffraction.
The application of these methods to the characterization of corrosion scale on an inconel coupon is illustrated. The types
of information accumulated were elemental composition, chemical forms of elements, special distributions of elements and compounds
in the scale, sizes of particles that made up the scale, variations in composition of particle surfaces from that of their
interiors, and composition of scale-alloy interface.