Alexandre S. Simionovici

Internal elemental microanalysis combining X-ray fluorescence, Compton and transmission tomography

Conventional x-ray transmission tomography provides the spatial distribution of the absorption coefficient inside a sample. Other tomographic techniques, based on the detection of photons coming from fluorescent emission, Compton and Rayleigh scattering, are used for obtaining information on the internal elemental composition of the sample. However, the reconstruction problem for these techniques is generally much more difficult than that of transmission tomography, mainly due to self-absorption effects in the sample. In this article an approach to the reconstruction problem is presented, which integrates the information from the three types of signals. This method provides the quantitative spatial distribution of all elements that emit detectable fluorescent lines (Z15 in usual experimental conditions), even when the absorption effects are strong, and the spatial distribution of the global density of the lighter elements. The use of this technique is demonstrated on the reconstruction of a grain of the martian meteorite NWA817, mainly composed of low Z elements not measured in fluorescence and for which this method provides a unique insight. The measurement was done at the ID22 beamline of the European Synchrotron Radiation Facility.

Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft

Can you spot a speck of space dust? NASAs Stardust spacecraft has been collecting cosmic dust: Aerogel tiles and aluminum foil sat for nearly 200 days in the interstellar dust stream before returning to Earth. Citizen scientists identified most of the 71 tracks where particles were caught in the aerogel, and scanning electron microscopy revealed 25 craterlike features where particles punched through the foil. By performing trajectory and composition analysis, Westphal et al. report that seven of the particles may have an interstellar origin. These dust particles have surprisingly diverse mineral content and structure as compared with models of interstellar dust based on previous astronomical observations. Science, this issue p. 786 Analysis of seven particles captured by aerogel and foil reveals diverse characteristics not conforming to a single model. Seven particles captured by the Stardust Interstellar Dust Collector and returned to Earth for laboratory analysis have features consistent with an origin in the contemporary interstellar dust stream. More than 50 spacecraft debris particles were also identified. The interstellar dust candidates are readily distinguished from debris impacts on the basis of elemental composition and/or impact trajectory. The seven candidate interstellar particles are diverse in elemental composition, crystal structure, and size. The presence of crystalline grains and multiple iron-bearing phases, including sulfide, in some particles indicates that individual interstellar particles diverge from any one representative model of interstellar dust inferred from astronomical observations and theory.

Synchrotron hard x-ray microprobe: Fluorescence imaging of single cells

Imaging of trace elements in single cells was achieved by synchrotron-induced x-ray fluorescence (SXRF) in the hard x-ray range. Monochromatic and “pink” excitations at 14 keV were used with compound refractive lenses resulting in a 1×10 μm2 beam size. The experiment shows that SXRF is well suited for microanalysis of freeze-dried cells, and demonstrated high accuracy in quantitative imaging of trace element in cells treated with pharmacological doses of an iodine-labeled anticancer drug.

High-resolution X-ray fluorescence microtomography of homogeneous samples

First experimental results of fluorescence microtomography with 6 /spl mu/m resolution obtained at ESRF, Grenoble, France, are described. The setup comprises high-quality optics (monochromator, mirror, focusing lenses) coupled to the high-energy/brilliance/coherence of the ID 22 undulator beamline. The tomographic setup allows precise measurements in the pencil-beam geometry. The image reconstruction is based on a modification of the algebraic reconstruction method but includes simplifications of the model. The quality and precision of the two-dimensional reconstructed elemental images of a phantom sample are encouraging. The method will be further refined and applied for the analysis of more complex inhomogeneous samples.

Nondestructive three-dimensional elemental microanalysis by combined helical x-ray microtomographies

A nondestructive x-ray technique combining simultaneous transmission, fluorescence, and Compton microtomography has been developed. Simultaneous three-dimensional structural information and three-dimensional internal elemental composition maps down to trace concentration levels have been obtained by a helical scan of the sample through an x-ray microbeam. With this method quantitative three-dimensional chemical distributions can be obtained at (sub)micrometric resolution in a nondestructive and noninvasive way, opening unique possibilities for the microanalysis of rare and fragile samples from several research fields.

Compression behavior of nanocrystalline anatase TiO2

Abstract We present a synchrotron X-ray diffraction study of pressure-induced changes in nanocrystalline anatase (with a crystallite size of 30–40xa0nm) to 35xa0GPa. The nanoanatase was observed to a pressure above 20xa0GPa. Direct transformation to the baddeleyite-TiO 2 polymorph was seen at 18xa0GPa. A fit of the pressure versus volume data to a Birch–Murnaghan equation yielded the following parameters: zero-pressure volume, V 0 =136.15xa0A 3 , bulk modulus, K T =243(3)xa0GPa, and the pressure derivative of bulk modulus, K ′=4 (fixed). The bulk modulus value obtained for the nanocrystalline anatase is about 35% larger than that of the macrocrystalline counterpart.

Evidence for fractional crystallization of wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins.

Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa24–26) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa6–Fa10) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa28–Fa38) belt. The wadsleyite–ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa24–26), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa24–26). Our findings could erase the possibility of the resulting unrealistic time scales of the high-pressure regime reported recently from other shocked L-6 chondrites.

Dissolution of strontianite at high P-T conditions: An in-situ synchrotron X-ray fluorescence study

Abstract In-situ measurements of the amount of dissolution of carbonate minerals at high pressures (up to 3.6 GPa) and temperatures (up to 523 K) are reported. Using an externally heated diamond anvil cell (DAC) and synchrotron X-ray fluorescence (SXRF), the extent of dissolution of strontianite (SrCO3) has been followed as a function of time by monitoring the fluorescence of Sr cations in the fluid surrounding the crystal. This work demonstrates that Sr2+ concentrations as low as 1000 ppm can be detected and measured in-situ in a DAC, using a forward transmission geometry. The preliminary data presented here indicate that this technique has high potential for determining solution composition in high-pressure and high-temperature geochemical studies.

Iron oxidation states in silicate glass fragments and glass inclusions with a XANES micro-probe

Abstract The iron oxidation states and its environment in volcanic silicate glasses have been studied by combining micro-X-ray absorption near edge structure (XANES) experiments at the iron K-edge in silicate glass fragments and glass inclusions trapped during the crystal growth. Experiments were performed at the ESRF (ID22 beam-line) – a third generation synchrotron – using a focused beam (2×10 μm 2 ) together with a high brilliance and a good energy resolution. The selected silicate glasses, basaltic in composition, contain between 5.5 and 8 wt% total Fe. In all XANES spectra, pre-peaks exhibit a multi-component structure, which is the convolution of information related to the valence state of iron (2+,3+), the site geometry and transition types. The Fe3+/ΣFe ratio of silicate glass fragments deduced from the XANES spectra is correlated to values determined by chemistry on bulk samples within the range 0.05–0.85. XANES spectra from glass inclusions hosted in natural olivine crystals may be interpreted considering the glass fragments as reference samples.

Beryllium parabolic refractive x-ray lenses

Recently, we have been able to fabricate high quality parabolic refractive x-ray lenses made of beryllium. We report first experimental results in both full field microscopy and microbeam production using these new lenses. In full field microscopy, undistorted images of test patterns were recorded in a field of view of 450 μm full width half maximum at 12keV with 10 fold magnification. A significant improvement of the lateral resolution as compared to imaging with aluminium refractive lenses was achieved. Microbeam characteristics were determined at 12keV demagnifying a high β undulator source 82 times. The lateral beam size was measured by fluorescence knife-edge. Microbeam characteristics, such as flux, lateral beam size, and low intensity background are discussed.