Bart Vekemans

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Use of microscopic XRF for non‐destructive analysis in art and archaeometry

The various application possibilities of microscopic x-ray fluorescence and associated methods for the characterization and provenance analysis of objects and materials of cultural heritage value are discussed by means of a number of case studies. They include the trace analysis of historic glass, the analysis of corroded coins and statues in bronze and silver and the study of inks on historical documents. These studies are conducted by means of micro-XRF instruments installed at synchrotron beamlines and by means of laboratory equipment, some of which is easily transportable to the museum or archaeological site where the objects of interest are located. Copyright

Comparison of several background compensation methods useful for evaluation of energy-dispersive X-ray fluorescence spectra

Abstract A number of background estimation and modelling strategies suitable for evaluating energy-dispersive X-ray spectra by means of non-linear least squares fitting are evaluated and intercompared. As background modelling functions, exponential and linear combinations of mutually orthogonal polynomials are considered. These functions allow the shape of the background to be determined together with the photopeak intensities. As background estimation algorithms, an iterative stripping algorithm and a background channel selection procedure which is also based on the use of orthogonal polynomials are studied. The last two methods calculate the spectral background prior to the actual fitting process. For the intercomparison, the various methods were incorporated in the software package AXIL (Analysis of X-ray spectra by means of Iterative Least Squares). By using simulated spectra in which the intensity of all lines is a priori known, the accuracy and noise-sensitivity of the different background compensation strategies are evaluated. The method in which the background is modelled as a linear combination of orthogonal polynomials is identified as being the most robust and yielding the most accurate results.

Processing of three-dimensional microscopic X-ray fluorescence data

A novel polycapillary based confocal X-ray fluorescence (XRF) technique was applied for the first time at the ID18F beamline of the ESRF to obtain directly three-dimensional (3D) compositional information of an inclusion inside a natural diamond sample (KK200). Preliminary analysis of the results on the basis of the trace elements Sr, Y/Zr, and Th suggest three phases, two of which were identified by earlier micro-Raman spectroscopy as larnite (β-Ca2SiO4) and CaSiO3-walstromite, two minerals with significantly different Ca content. In order to support this multiphase model for the investigated inclusion the data set was analysed combining the conventional multivariate method of PCA and K-means clustering procedure after application of instrument specific routines such as spectral evaluation and normalization. Through the knowledge of the full spatial 3D structure of the different phases, it was possible to correct for absorption of the fluorescent radiation in the different phases of the inclusion and the surrounding diamond.

Automated segmentation of μ-XRF image sets

The combined use of principal component analysis (PCA) and K-means clustering (KMC) for the segmentation and (semi-)quantitative calibration of multivariate μ-XRF data sets is proposed and evaluated. The usefulness of the method is compared with that of using PCA or KMC separately by discussing image sets derived from geological and archeometric samples.

A compact small-beam XRF instrument for in-situ analysis of objects of historical and/or artistic value

Abstract The analytical characteristics, possibilities and limitations of a compact and easily transportable small-beam XRF instrument are described. The instrument consists of a compact, mini-focus Mo X-ray tube that is collimated to produce a sub-mm beam and a peltier-cooled PIN diode detector. Relative MDLs in highly scattering matrices are situated in the 10–100-ppm range; for metallic matrices featuring strong matrix lines, the MDLs of the instrument are approximately a factor 2 higher. Since only a small irradiation area is required, a simple micro-polishing technique that may be performed in situ in combination with the measurements is shown to be effective for the determination of the bulk composition of corroded bronze objects. As an example, a series of Egyptian bronze objects date from XXIInd Egyptian Dynasty (ca. 1090 bc ) to the Roman era (30 bc to 640 ad ) was analyzed in order to contribute to the very limited database on Cu–alloy compositions from this period.

Trace analysis for distinguishing between Venetian and facon-de- Venise glass vessels of the 16th and 17th century{

In this paper, the differences and similarities in chemical composition between facon-de-Venise glass excavated in Antwerp and two types of Venetian glass, both from the sixteenth and seventeenth centuries, are documented. On the basis of typological criteria alone it is not possible to distinguish imported Venetian glass from locally produced glass. For this purpose, the major element composition of Antwerp and Venetian glass was studied with EPXMA (electron probe X-ray microanalysis) and, in a second phase, these objects were also analysed for their trace element content by means of SR-XRF (synchrotron radiation induced X-ray fluorescence analysis) and LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry). The largest group of Antwerp objects has a composition that significantly differs from truly Venetian compositions, pointing to an important local production. However, a few objects were found that have a major ncomposition very similar to Venetian Cristallo and a larger group corresponds to the Venetian Vitrum Blanchum composition. In order to be able to make a more clear distinction between the Antwerp Cristallo and Venetian Cristallo glass types, to verify that indeed the Antwerp and Venetian Vitrum Blanchum compositions are identical and to understand better the causes of the compositional differences among the groups, the concentrations of a number of trace elements, possibly providing clues on the raw materials, were determined.

A micro-XRF spectrometer based on a rotating anode generator and capillary optics

Abstract The analytical characteristics of a laboratory-scale micro-X-ray fluorescence spectrometer, based on a rotating anode X-ray generator and capillary optics, are described. Usually, a microbeam 15 μm in diameter, derived from a copper or molybdenum anode operated at 45 kV, is used for sample irradiation. Elemental yields around the 1 count s−1(μg cm−2)−1 level are obtained, corresponding to absolute detection limits for thin samples in the 0.05–1 pg range and to relative MDL levels of 3–10 ppm for thick organic samples. The use of the instrument for studying (trace) element migration in Roman glass and for the nondestructive analysis of decorations on Japanese 18th century porcelain vases is described.

Monte Carlo simulation of X-ray fluorescence spectra: Part 4. Photon scattering at high X-ray energies

Abstract The photon scattering model of a Monte Carlo simulation code for synchrotron radiation X-ray fluorescence (SRXRF) spectrometers is evaluated at high X-ray energies (60–100 keV) by means of a series of validation experiments performed at Beamline BW5 of HASYLAB. Using monochromatic X-rays, Compton/Rayleigh multiple scattering experiments were performed on polypropylene, Al and Cu samples. Especially in the case of the first two matrices multiple Compton scattering occurs with high probability. This work demonstrates that the simulation model provides a reliable estimate of the spectral distribution of the multiply scattered linearly polarized photon beam as observed by an HPGe detector. Next to variations in sample composition and thickness, the ability of the code to simulate various detection geometries has also been verified. As an application of the code, the achievable detection limits of SRXRF for rare earth elements as obtained with white beam and monochromatic (80 keV) excitation are compared.

Micro-heterogeneity study of trace elements in USGS, MPI-DING and NIST glass reference materials by means of synchrotron micro-XRF

Synchrotron μ-XRF (X-ray fluorescence analysis), a trace level micro analytical method, allows the quantitative study of the nature and degree of heterogeneity of inorganic trace constituents in reference materials with a homogeneous matrix. In the present study, glass materials of NIST, MPI-DING and USGS containing trace levels of heavy metals are considered. The measurements involve an extensive series of local analyses, performed in identical conditions at different locations on the material. This procedure is employed to measure the degree of micro-heterogeneity of several existing reference materials and to evaluate their suitability for calibration of trace-level micro-analytical methods. For a number of the trace elements present in the various reference materials, a minimum representative mass for homogeneous measurements is calculated.