X-ray observation of micro-failures in granular piles approaching an avalanche

An X-ray imaging technique is used to probe the stability of 3-dimensional granular packs in a slowly rotating drum. Well before the surface reaches the avalanche angle, we observe intermittent plastic events associated with collective rearrangements of the grains located in the vicinity of the free surface. The energy released by these discrete events grows as the system approaches the avalanche threshold. By testing various preparation methods, we show that the pre-avalanche dynamics is not solely controlled by the difference between the free surface inclination and the avalanche angle. As a consequence, the measure of the pre-avalanche dynamics is unlikely to serve as a tool for predicting macroscopic avalanches.

The geometry and the number of contacts of monodisperse sphere packs using X-ray tomography

The three-dimensional structure of very large samples of monodisperse bead packs is studied by means of X-Ray Computed Tomography. We retrieve the coordinatesofeach bead inthe pack and wecalculate the average coordination number by using the tomographic images to single out the neighbors in contact. The results are compared with the average coordination number obtained in Aste et al. (2005) by using a deconvolution technique. We show that the coordination number increases with the packing fraction, varying between 6.9 and 8.2 for packing fractions between 0.59 and 0.64. © 2005 Taylor & Francis Group.

Evidence for X(5) critical point symmetry in 128Ce

Lifetimes of excited states in 128Ce were measured using the recoil distance Doppler-shift (RDDS) and the Doppler-shift attenuation (DSAM) methods. The experiments were performed at the Wright Nuclear Structure Laboratory of Yale University. Excited states of 128Ce were populated in the 100Mo(32Si,4n) reaction at 120 MeV and the nuclear γ decay was measured with an array of eight Clover detectors positioned at forward and backward angles. The deduced yrast transition strengths together with the energies of the levels within the ground-state (gs) band of 128Ce are in agreement with the predicted values for the X(5) critical point symmetry. Thus, we suggest 128Ce as a benchmark X(5) nucleus in the mass A ≈ 130 region. © World Scientific Publishing Company.

Concentration dependence of the Er3+ visible and infrared luminescence in Y2-x Erx O3 thin films on Si

Y2-x Erx O3 thin films, with x varying between 0 and 0.72, have been successfully grown on crystalline silicon (c-Si) substrates by radio-frequency magnetron cosputtering of Y2 O 3 and Er2 O3 targets. As-deposited films are polycrystalline, showing the body-centered cubic structure of Y2 O3, and show only a slight lattice parameter contraction when x is increased, owing to the insertion of Er ions. All the films exhibit intense Er-related optical emission at room temperature both in the visible and infrared regions. By studying the optical properties for different excitation conditions and for different Er contents, all the mechanisms (i.e., cross relaxations, up-conversions, and energy transfers to impurities) responsible for the photoluminescence (PL) emission have been identified, and the existence of two different well-defined Er concentration regimes has been demonstrated. In the low concentration regime (x up to 0.05, Er-doped regime), the visible PL emission reaches its highest intensity, owing to the influence of up-conversions, thus giving the possibility of using Y2-x Er x O3 films as an up-converting layer in the rear of silicon solar cells. However, most of the excited Er ions populate the first two excited levels 4I11/2 and 4I13/2, and above a certain excitation flux a population inversion condition between the former and the latter is achieved, opening the route for the realization of amplifiers at 2.75 μm. Instead, in the high concentration regime (Er-compound regime), an increase in the nonradiative decay rates is observed, owing to the occurrence of cross relaxations or energy transfers to impurities. As a consequence, the PL emission at 1.54 μm becomes the most intense, thus determining possible applications for Y2-x Erx O 3 as an infrared emitting material. © 2009 American Institute of Physics.

Local structure of Mn in (La1-xHox) 2/3Ca1/3MnO3 studied by X-ray absorption fine structure

Results of X-ray absorption fine structure measurements in manganites (La1-xHox)2/3Ca1/3MnO3 with 0.15 < x < 0.50 are presented. When LaMnO3 is doped with a, divalent element such as Ca2+, substituting for La3+, holes are induced in the filled Mn d orbitais. This leads to a, strong ferromagnetic coupling between Mn sites. Ca ions in La1-xCa xMnO3 introduce a distortion of the crystal lattice and mixed valence Mn ions (Mn3+ and Mn4+). On the other hand, in manganites (La1-xHox)2/3Ca 1/3MnO3 the substitution of La for Ho causes a lattice distortion and induces a disorder, which reduces a magnetic interaction. The ferromagnetic transition temperature and conductivity decrease very quickly with increasing x. The magnetic and transport properties of compounds depend on the local atomic structure around Mn ions. The information on the bond lengths and Debye-Waller factor are obtained from the extended X-ray absorption fine structure (EXAFS) data analysis. The charge state of Mn is determined from the position of the absorption edge in X-ray absorption near edge structure (XANES) data. XAFS results are in good agreement with magnetic characteristics of the studied materials.

LiMn2-xTixO4 spinel-type compounds (x ≤ 1): Structural, electrical and magnetic properties

LiMn2-xTixO4 compounds with 0 ≤ x ≤ 1 were prepared by solid state reaction and Pechini technique. Powder X-ray diffraction showed that all samples crystallize with the spinel crystal structure (S.G. Fd3-m). The cubic unit-cell parameter increases with the Ti content. The influence of the Ti content and cationic distribution on the magnetic properties of the compounds was studied by measuring the temperature and magnetic field dependences of the magnetization: substitution by non-magnetic d0 Ti4+ ions appeared to weaken the magnetic interactions between the manganese ions. The electrical properties of LiMnTiO4 were studied by AC impedance spectroscopy and DC polarisation measurements, which revealed the electronic character of the conduction process. © 2006 Elsevier B.V. All rights reserved.

Cortical thickness estimation of the proximal femur from multi-view Dual-Energy X-ray Absorptiometry (DXA)

Hip fracture is the leading cause of acute orthopaedic hospital admission amongst the elderly, with around a third of patients not surviving one year post-fracture. Although various preventative therapies are available, patient selection is difficult. The current state-of-the-art risk assessment tool (FRAX) ignores focal structural defects, such as cortical bone thinning, a critical component in characterizing hip fragility. Cortical thickness can be measured using CT, but this is expensive and involves a significant radiation dose. Instead, Dual-Energy X-ray Absorptiometry (DXA) is currently the preferred imaging modality for assessing hip fracture risk and is used routinely in clinical practice. Our ambition is to develop a tool to measure cortical thickness using multi-view DXA instead of CT. In this initial study, we work with digitally reconstructed radiographs (DRRs) derived from CT data as a surrogate for DXA scans: this enables us to compare directly the thickness estimates with the gold standard CT results. Our approach involves a model-based femoral shape reconstruction followed by a data-driven algorithm to extract numerous cortical thickness point estimates. In a series of experiments on the shaft and trochanteric regions of 48 proximal femurs, we validated our algorithm and established its performance limits using 20 views in the range 0°-171°: estimation errors were 0:19 ± 0:53mm (mean +/- one standard deviation). In a more clinically viable protocol using four views in the range 0°-51°, where no other bony structures obstruct the projection of the femur, measurement errors were -0:07 ± 0:79 mm. © 2013 SPIE.

Improved switching uniformity and low-voltage operation in TaO x-based RRAM using Ge reactive layer

Significant improvements in the spatial and temporal uniformities of device switching parameters are successfully demonstrated in Ge/TaOx bilayer-based resistive switching devices, as compared with non-Ge devices. In addition, the reported Ge/TaOx devices also show significant reductions in the operation voltages. Influence of the Ge layer on the resistive switching of TaOx-based resistive random access memory is investigated by X-ray spectroscopy and the theory of Gibbs free energy. Higher uniformity is attributed to the confinement of the filamentary switching process. The presence of a larger number of interface traps, which will create a beneficial electric field to facilitate the drift of oxygen vacancies, is believed to be responsible for the lower operation voltages in the Ge/TaO x devices. © 1980-2012 IEEE.

Isolation of Y- and X-linked SCAR markers in yellow catfish and application in the production of all-male populations

P>Sex controls have been performed in some farmed fish species because of significant growth differences between females and males. In yellow catfish (Pelteobagrus fulvidraco), adult males are three times larger than female adults. In this study, six Y- and X-linked amplified fragment length polymorphism fragments were screened by sex-genotype pool bulked segregant analysis and individual screening. Interestingly, sequence analysis identified two pairs of allelic genes, Pf33 and Pf62. Furthermore, the cloned flanking sequences revealed several Y- and X-specific polymorphisms, and four Y-linked or X-linked sequence characterized amplified region (SCAR) primer pairs were designed and converted into Y- and X-linked SCAR markers. Consequently, these markers were successfully used to identify genetic sex and YY super-males, and applied to all-male population production. Thus, we developed a novel and simple technique to help commercial production of YY super-males and all-male populations in the yellow catfish.