Harmonics as an alternative method for measuring I-0 during x-ray absorption spectroscopy experiments at laboratory scale

In the recent years, the advent of an efficient and compact laboratory-scale spectrometer for x-ray absorption spectroscopy experiments has been extensively reported in the literature. Such modern instruments offer the advantage to routinely use x-ray absorption spectroscopy on systematic studies, which is usually unconceivable at synchrotron radiation source facilities due to often limited time access. However, one limiting factor is the fact that due to laboratory x-ray source brightness compared to a synchrotron, two separate measures of the incoming and transmitted x-ray intensities, i.e., the so-called I-0 and I-1, respectively, are usually required. Herein, we introduce and discuss an alternative approach for measuring I-0 and I-1 simultaneously. Based on the usage of harmonics arising naturally from the use of monochromator crystals, the reliability and robustness of our proposed approach is demonstrated through experiments at the Co K-edge measured using Co metal foil and at the Nd L-3-edge measured in Nd2O3.Peer reviewe

Charge compensation mechanisms in Nd-doped UO2 samples for stoichiometric and hypo-stoichiometric conditions : Lack of miscibility gap

The evolution of the crystal lattice of samples made of UO2 doped with different concentrations of Nd in stoichiometric and hypo-stoichiometric conditions has been systematically studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The substitution of a trivalent cation for the U4+ initial position is responsible for creating local structural disorder and changes in the oxidation states. In this scenario, the lattice parameter is affected and the concentration of U5+ and formation of oxygen vacancies as well, since these are the mechanisms necessary to maintain the charge neutrality. The systematic oxidation of U4+ as predominant charge compensation mechanism over the formation of vacancies can be reduced by performing a thermal treatment under reducing conditions. This paper presents an experimental characterization of the uranium oxidation state mixture and local structure using XAS techniques in samples with chemical formula (U1-yNdy)O2-x, with y = 0.04, 0.17 and 0.25 (Peer reviewe

Ambient-noise tomography of the wider Vienna Basin region

We present a new 3-D shear-velocity model for the top 30 km of the crust in the wider Vienna Basin region based on surface waves extracted from ambient-noise cross-correlations. We use continuous seismic records of 63 broad-band stations of the AlpArray project to retrieve interstation Green’s functions from ambient-noise cross-correlations in the period range from 5 to 25 s. From these Green’s functions, we measure Rayleigh group traveltimes, utilizing all four components of the cross-correlation tensor, which are associated with Rayleigh waves (ZZ, RR, RZ and ZR), to exploit multiple measurements per station pair. A set of selection criteria is applied to ensure that we use high-quality recordings of fundamental Rayleigh modes. We regionalize the interstation group velocities in a 5 km × 5 km grid with an average path density of ∼20 paths per cell. From the resulting group-velocity maps, we extract local 1-D dispersion curves for each cell and invert all cells independently to retrieve the crustal shear-velocity structure of the study area. The resulting model provides a previously unachieved lateral resolution of seismic velocities in the region of ∼15 km. As major features, we image the Vienna Basin and Little Hungarian Plain as low-velocity anomalies, and the Bohemian Massif with high velocities. The edges of these features are marked with prominent velocity contrasts correlated with faults, such as the Alpine Front and Vienna Basin transfer fault system. The observed structures correlate well with surface geology, gravitational anomalies and the few known crystalline basement depths from boreholes. For depths larger than those reached by boreholes, the new model allows new insight into the complex structure of the Vienna Basin and surrounding areas, including deep low-velocity zones, which we image with previously unachieved detail. This model may be used in the future to interpret the deeper structures and tectonic evolution of the wider Vienna Basin region, evaluate natural resources, model wave propagation and improve earthquake locations, among others

Arrival angles of teleseismic fundamental mode Rayleigh waves across the AlpArray

The dense AlpArray network allows studying seismic wave propagation with high spatial resolution. Here we introduce an array approach to measure arrival angles of teleseismic Rayleigh waves. The approach combines the advantages of phase correlation as in the two-station method with array beamforming to obtain the phase-velocity vector. 20 earthquakes from the first two years of the AlpArray project are selected, and spatial patterns of arrival-angle deviations across the AlpArray are shown in maps, depending on period and earthquake location. The cause of these intriguing spatial patterns is discussed. A simple wave-propagation modelling example using an isolated anomaly and a Gaussian beam solution suggests that much of the complexity can be explained as a result of wave interference after passing a structural anomaly along the wave paths. This indicates that arrival-angle information constitutes useful additional information on the Earth structure, beyond what is currently used in inversions