The Interpretation of X-Ray and Electron Signals Generated in Thin or Layered Targets

This work outlines the development of a comprehensive theory for the electron probe microanalyser and scanning electron microscope or SEM, that is intended to serve as a framework of understanding for those employing electron beam methods and as a basis for improved correction procedures. There is particular emphasis on applications to layered and non-uniform specimens. Starting from a simple Gaussian depth distribution of electrons and making assumptions about the X-ray production, a series of predictions of X-ray and electron signals are made for various target configurations. When compared with experimental measurements a series of interesting discoveries follow, which, taken altogether, lead to a more refined model with the promise of more accurate analyses and a better understanding of the physics involved

A Survey of Electron Probe Microanalysis Using Soft Radiations: Difficulties and Presentation of a New Computer Program for Wavelength Dispersive Spectrometry

This paper aims to demonstrate that on-line peak integral technique with wavelength dispersive spectroscopy (WDS) provides accurate results with intensity measurement counting times as short as one or two minutes, owing to the high counting rates obtained with multilayer analyzers. A great advantage of a new computer program using this technique (available on SUN/UNIX work-stations operating Cameca SX-50 microprobes) consists in the original way that peak overlaps are treated. For each analytical point, overlapping counts emerging from an element B (B counts) are removed on-line from the measured raw counts in order to obtain the net counts corresponding to the element to be analyzed (element A). B counts are first measured on a proper standard containing B but not A. The effects of chemical bonding on the shape and the shift of peaks is clearly seen in the analysis of fluorine in topaz and lithium fluoride. Self-absorption effects, which usually distort the high energy side of L-series soft radiations, are generally inconsistent with the direct measurements of peak area fork-ratio determination. A method based on the conventional area/peak factor concept is proposed for this purpose

Analytical Description of X-Ray Peaks: Application to L X-Ray Spectra Processing of Lanthanide Elements by Means of the Electron Probe Micro-Analyzer

The shape of Lα X-ray peaks analyzed by means of a LiF (200 plane) monochromator was described by a pseudo-Voigt function: P(λ) = 0.35 P1(λ)+ 0.65 P2(λ) where P1(λ) and P2(λ) are a Gaussian and a Lorentzian distribution centered at the same wavelength, with the same amplitude and half-width and in relative proportion 0.35 and 0.65 respectively. For peaks occurring at wavelength greater than ≃ 0.17 nm, a Gaussian offset was added in order to correct the asymmetry of peaks resulting from the monochromator mounting within the spectrometer. The effective wavelength resolution was obtained by quadrature addition of the instrumental resolution and the natural width of the X-ray peaks. It has been shown that the difference in peak width of the L emission peaks of the lanthanide elements resulted from their difference in their natural widths. For these elements, the Lβ2, Lγ1 and Lγ2 were found to be accompanied by non-diagram lines, Lβ14, Lγ9 and Lγ10 respectively. The wavelength separation distances Lβ14-Lβ2, Lγ9-Lγ1 and Lγ10-Lγ2 were found consistent with the distances derived from the plasmon theory

Complementary Surface Characterization of Chalcopyrite by Electron Microscopy, Electron Spectroscopy, and Optical Reflectance

Surface oxidation of polished natural specimens of chalcopyrite (CuFeS2) at temperature between 23°C and 300°C in air has been characterized by Auger electron and X-ray photoelectron spectroscopies. The reaction products consisted of an outer iron oxide layer and an intermediate copper rich sulfide layer. Several different oxides and sulfides were consistent with the electron spectroscopy data, so specimens were analyzed as a function of time and temperature at selected 20 μm diameter areas with an optical microreflectometer (OMR). Since the optical properties of a compound are unique, a reflectance model with three homogeneous layers was used to calculate reflectance curves by varying the compound in and thickness of each layer. The reaction products were modelled as Cu5FeS4 in contact with the CuFeS2 and Cu2S as an intermediate layer between Cu5FeS4 and the outer oxide. The outer oxide was most consistent with Fe3O4. Relative layer thicknesses were calculated from a series of balanced chemical equations, and Cu5FeS4 was much thicker than Cu2S with total thickness increasing with increasing temperature. The total film layer thicknesses calculated at 23°C were between 10nm and 35nm. At 200°C the film thickness varied from 8nm to 51nm with greater thicknesses associated with longer reaction times. Thicknesses at 300°C ranged from 12nm to 85nm

Optical Microreflectometry and Microscopy of Chalcopyrite Specimens: Reflectance Calculation and Comparison to Backscattered Electron Microscopy

A model was developed to calculate the optical reflectance of an absorbing substrate covered by multiple thin layers of absorbing materials. Both multiple homogeneous thin layers and thin surface layers of mixed phases were modeled. Reflectance versus wavelength was measured for polished chalcopyrite (CuFeS2) and compared to calculated data. The identity and thickness of surface compounds used to calculate reflectance curves were partially determined using X-ray photoelectron and Auger electron spectroscopies. Very good agreement between theoretical and experimental reflectance curves were observed as a function of surface composition. The hue (color) and luminosity (brightness) of the polished surface were also calculated from both experimental and theoretical curves and were found to also be valuable for evaluating surface composition. Contrast in optical photomicrographs resulting from both luminosity and hue was illustrated. Secondary and backscattered electron microscopy were also used to image chalcopyrite polished surfaces which were naturally oxidized by an exposure before and after ion etching. For a substrate covered with thin layers, the resulting backscattered coefficient was calculated as a function of the backscattered coefficient for the surface and the substrate, respectively. The variations of the relative difference between the effective backscattered coefficients vs the primary beam energy exhibited a maximum for a critical thickness difference of the surface layer. The dependence of the variations in thickness of the oxidized layer with the crystallographic orientation changes of the substrate as well as the resulting contrasts of the optical and electron images were discussed

Spectral Decomposition of Wavelength Dispersive X-Ray Spectra: Implications for Quantitative Analysis in the Electron Probe Microanalyzer

The line shapes of Kα, Lα,β and Mα X-ray peaks of pure elements were analyzed by means of commercial wavelength dispersive spectrometers (WDS) attached to an electron probe micro-analyzer (EPMA). A pseudo-Voigt function, i.e., a linear combination of Gaussian and Lorentzian distributions, was used as a fitting profile for the X-ray peaks, with Gaussian offsets incorporated in the short wavelength (high energy) side to describe the observed asymmetry. The asymmetry of X-ray peaks resulting from both instrumental distortions and satellite bands may lead to discrepancies in quantitative analysis with the EPMA as a function of the procedure used for deriving X-ray intensities from WDS spectra, e.g., peak height, peak area, or peak decomposition. These effects have been illustrated by analyzing gold-copper metallic alloys and minerals containing gold at trace levels

Scanning Mechanical Microscopy of Laser Ablated Volumes Related to Inductively Coupled Plasma-Mass Spectrometry

Scanning mechanical microscopy based on the point by point sampling of the target surface was used to characterize volumes of minerals ablated by laser pulses (Nd: YAG, = 1064 nm, 140 μs pulse-width). Differentiated volumes resulting from vaporization and exfoliation mechanisms were selectively measured. Ablated volumes of natural pyrite (cubic FeS2), marcasite (orthorhombic FeS2) and arsenopyrite AsFeS, were transported into an inductively coupled plasma torch for subsequent mass analysis. The log of the S34 Fe57, and As75 mass intensities was linearly correlated with the log of the dimensions of the vaporized crater induced by the laser shots while large particles had no effect on the measured intensities. A memory effect for As was observed when a nylon tube was used to carry the ablated materials into the plasma torch. The memory effect was decreased by using a copper tube resulting probably from a difference in the electrical properties of the tubing systems leading to a lower adsorption of As within the copper tube than for the case of the nylon tube

Electron Microprobe Analysis and Proton Induced X-Ray Spectrometry Applied to Trace Element Analysis in Sulfides: Problems and Prospects

The complementary techniques of EPMA and micro-PIXE are reviewed in the context of spatially resolved trace element analysis of sulfide minerals. Attention is focussed on methods of standardization and of fitting EDX spectra. Sphalerites and chalcopyrites from various sources are used as specimens. For Ag in chalcopyrites, the two techniques agree well. Sphalerites pose problems such as Zn-Fe replacement and the presence of minor elements, both of which influence matrix corrections ; these are addressed in detail. The necessity for absorbers in the micro-PIXE work prevents detection of minor elements lighter than Zn ; these are determined by EPMA and the results used in the micro-PIXE fitting and matrix corrections. For Cd, Ag, Ga, Ge there is acceptable agreement between the two techniques given uncertainties and constraints on samples, but EPMA results for Hg are notably lower than micro-PIXE results. The improvement in detection limits afforded by micro-PIXE over EPMA in these sulfide minerals ranges from ~ 3 for Ga, Ge, Hg to 10-30 for Se, Ag, Cd, In ; possible further gains are discussed for both techniques

Cathodoluminescence Applied to the Microcharacterization of Mineral Materials: A Present Status in Experimentation and Interpretation

Experimentation and interpretation of cathodoluminescence (CL) microscopy and spectroscopy applied to the microcharacterization of material minerals are reviewed. The origins of the intrinsic (host lattice) and extrinsic (impurities) luminescence emissions in crystals are briefly discussed. Merits and limitations of the available techniques are illustrated. CL emission changes as a function of the incident electron dose are illustrated for the case of natural quartz and sphalerite (ZnS) crystals. These effects are discussed in terms of the development of bulk charging, production of heat, diffusion of impurities, and creation of lattice defects induced by the incident ionizing particles. Although CL emission is mostly extrinsic in origin there is no general rule for identifying the nature of impurities from the CL emission spectra of minerals. However there is potential for using CL spectroscopy for trace element analysis as presented for the case of minerals containing rare-earth luminescent ions. The CL emission is a signature of the crystal-chemistry properties of minerals and hence contains potential genetic information. Some of the applications of CL emissions in the geosciences are summarized