Surface analysis by x-ray photoelectron spectroscopy (XPS)
The surface sensitivity of XPS applied to radioactive materials gives insight into chemical composition of surfaces and interfaces relevant for the safety case study of a nuclear waste repository. Examples are the interaction of dissolved radionuclides with surfaces of corrosion products, colloids of the groundwater, and mineral phases by sorption or incorporation.
Chemical interaction of a solid material with its environment takes place at its surface, which mostly results in the formation of an interface phase or a layer with a chemical composition differing from that of the bulk. The thickness of such layers may range from monolayers to some nanometers. Knowledge of surface layer properties is essential for understanding of chemical processes occurring at the surface. To identify elements, their valence, and chemical bonding states in the outermost atomic layers of a sample, surface-sensitive x-ray photoelectron spectroscopy (XPS) is applied.
During XPS, the surface of a sample is irradiated by soft x-rays and the photoelectrons leaving the sample are analyzed. The energy of the emitted photoelectrons is essentially given by the difference between x-ray energy and the binding energy of the electrons at their initial atomic energy levels, thus characteristic for each element. By use of the measured photoelectron energies and intensities, the elements and their abundances are identified.
Chemical bonds cause a small shift of the energy of atomic electron levels, measured by high energy resolution XPS as "chemical shift" of the photoelectron energy. Comparison with data from well-known compounds allows drawing conclusion about chemical bonding states at the sample surface.
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Scanning electron microscopy (SEM)
Electron microscopy applied to radioactive materials gives insight into microscopic/ nanoscopic structures relevant for safety assessment of a nuclear repository.
SEM analyses are performed at INE using a FEI QUANTA 650 FEG environmental scanning electron microscope located in the radioactive control area (Fig. 1). In SEM, a fine electron probe is focussed on the surface of a sample and scanned in a two-dimensional raster. At each point, various emitted signals can be detected and contribute to a corresponding point in an image. So-called secondary electrons (SE) enable the observation of surface topography with a lateral resolution down to few nanometers, whereas images produced by means of elastically backscattered electrons (BSE) present an elemental contrast. Furthermore, the detection of characteristic x-rays allows quantitative determination of atomic concentrations at a micrometer scale (energy-dispersive x-ray spectroscopy, EDX).
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Surface analysis by atomic force microscopy (AFM)
A prerequisite of a detailed understanding of chemical reactions at surfaces are high-resolution microscopic techniques. The atomic force microscope (AFM) gives quantitative insight into surface properties and topography. Advantages of this technique are that no special sample preparation is necessary and measurements with high lateral resolution can be performed at ambient pressure and in solutions.
Surface processes like e.g. crystal growth or dissolution of minerals, can be observed directly in solution in realtime. The incorporation of radionuclides into minerals can be observed and analysed by changes of the crystal growth.
During atomic force microscopy, the surface of the sample is scanned by a nanoscopic sized tip with radius of 2 nm. Spatial resolution in z-direction is about 0.1 nm, and in x,y directions given by the tip radius.
The tip is bonded to a relatively long and elastic bar (ca. 0.1 mm ) with well-known spring constant. The position of the tip is measured by a reflecting laser beam and a segmented photodiode. The bar with the tip is attached to a piezoelectric quartz and moved by electric potentials.
Sensing of the sample surface is performed by essentially two techniques. Either the tip is scanned in permanent contact with the sample surface or the tip oscillates perpendicular to the surface while scanning. If the tip approaches to the surface, attenuation and phase shift of the tip in relation to the exciting piezoelectric quartz occurs, which is detected by the laser and the segmented photodiode. Advantage of the latter technique is that it has less impact onto the sample surface, which is favourable especially in the case of sensitive samples.
|Dr. Dieter Schild
|Dr. Frank Heberling (AFM)
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X-ray diffraction (XRD) is a non-destructive analytical technique used to identify and characterize solid samples. Information on the crystallographic structure can be obtained without complex sample preparation and with low amount of sample. For any given crystalline material, both the position and the intensity of the lines on the diffractogram are indicative of a particular phase (“fingerprint”). Quantitative information on a sample can be obtained by applying a full pattern analysis technique, such as the Rietveld method for example.
Powder X-ray diffractograms are collected in reflection mode using a D8 ADVANCE (Bruker) diffractometer located in the controlled area of INE. X-rays are produced from a Cu anode and the intensity of the scattered radiation is detected with an energy dispersive detector (SOL-X). The sample stage concept allows analyses of powders, oriented or textured samples (such as sheet silicates), air-sensitive and/or active samples (closed and airtight sample holder).
Information provided by XRD is of key importance in various areas of the research activities at INE. Besides determining the mineralogical composition of natural samples, XRD is used to characterize samples after corrosion experiments (nuclear waste glass, cement waste form…), the purity of synthetic samples from (co)precipitation experiments (retention of radionuclides in solid phases, identification of precipitates).
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