56,024 research outputs found
Ga self-interstitials in GaN investigated by ab-initio calculations of the electronic g-tensor
No full textIn GaN, an exceptionally important role is played by Ga-interstitials being mobile at room temperature: Despite rather large formation energy, they have been observed in irradiated uurtzite-GaN: at least two similar, but clearly distict ODEPR-signals L5 and L6/L6* have been identified (via exceptionally large, nearly isotropic hyperfine splittings of about 4 GHz) as interstitial Ga-i(2+) in two diffent lattice configurations. However, judging from experimental data and total energy calculations alone, the exact microscopic configuration remains unclear. In this theoretical work, the situation is elucidated by ab-initio calculating the complete set of EPR parameters, hyperfine splittings as well as g-tensors, for the stable structural configurations of the Ga-interstitial using a gauge-including projector augmented plane wave (GI-PAW) approach. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Uranium and thorium in soils, mineral sands, water and food samples in a tin mining area in Nigeria with elevated activity
No full textThe activity concentrations of uranium and thorium have been determined in soils and mineral sands from the Nigerian tin mining area of Bisichi, located in the Jos Plateau, and from two control areas in Nigeria (Jos City and Akure) using high-purity germanium detectors (HPGe). High resolution sector field inductively coupled plasma mass spectroscopy (HR-SF-ICP-MS) was used to determine uranium and thorium in liquids and foodstuffs consumed locally in the mining area. The activities of uranium and thorium measured in the soils and mineral sands from Bisichi ranged from 8.7 kBq kg−1 to 51 kBq kg−1 for 238U and from 16.8 kBq kg−1 to 98 kBq kg−1 for 232Th, respectively. These values were significantly higher than those in the control areas of Jos City and Akure and than the reference values reported in the literature. They even exceeded the concentrations reported for areas of high natural radioactive background. Radionuclide concentrations in samples of the local foodstuffs and in water samples collected in Bisichi were found to be higher than UNSCEAR reference values. The results reveal the pollution potential of the mining activities on the surrounding areas
Inventory and geochemical host phases of natural radionuclides in tin mining materials from Nigeria
No full textOn the Nigerian Jos Plateau tin mining is extensively carried out in open pit style. Several types of materials occurring there (raw materials, waste, and soil) were analysed radiometrically. The geochemical host phases of the natural radionuclides were determined by a sequential extraction procedure according to the European BCR standard. It was found that especially easily mobilisable 228Ra must be taken into consideration as a radioactive contaminant for the mining are
Gerstmann-Sträussler-Scheinker Disease
No full textGerstmann-Sträussler-Scheinker disease (GSS) is a hereditary form of prion disease. GSS, in particular the form caused by the PRNP gene P102L mutation, is transmissible to primates and rodents. Thus, GSS is a unique disease that is both genetic and transmissible; however, the exact nature of the transmissible agent is not clear. The clinical picture of GSS comprises cerebellar ataxia, dementia and pyramidal and extrapyramidal signs and symptoms. However, the disease is heterogeneous and in different families and different mutations the clinical picture may vary. The neuropathological picture is characterized by the presence of amyloid plaques – mainly multicentric plaques. There are several models of GSS in transgenic mice and in Drosophila sp. In mice produced with an overexpressed transgene that carries the P101L mutation (corresponding to the P102L mutation in humans), “spontaneous” neurodegeneration is observed and this, in turn, is transmissible but to transgenic mice with a low copy number. In contrast, P101L transgenic mice produced by means of reciprocal recombination show no spontaneous neurodegeneration, but instead become more susceptible to transmission of human GSS following inoculation.Choroba Gerstmanna-Sträusslera-Scheinkera (GSS) jest genetycznie uwarunkowaną chorobą wywoływaną przez priony. Jest ona unikalna, ponieważ udało się przepasażować GSS na naczelne i gryzonie przynajmniej z mózgu obarczonego mutacją kodonu 102. Tym samym jest to jedyne schorzenie jednocześnie genetycznie uwarunkowane i zakaźne, aczkolwiek natura czynnika infekcyjnego (prionu) nadal stanowi przedmiot dyskusji. W obrazie klinicznym GSS dominuje postępująca ataksja móżdżkowa z towarzyszącym otępieniem i objawami piramidowo-pozapiramidowymi. Jest to jednak choroba heterogenna, o różnym obrazie klinicznym u nosicieli różnych mutacji, a nawet u nosicieli tej samej mutacji. Obraz neuropatologiczny obejmuje obecność PrPd – immunododatnich złogów amyloidu pod postacią blaszek, zwłaszcza tzw. blaszek wielordzeniowych. Istnieje kilka modeli GSS. U myszy transgenicznych z nadekspresją zmutowanego genu kodującego PrP obserwuje się spontaniczną chorobę zwyrodnieniową, pasażowalną na myszy transgeniczne o niskiej liczbie transgenu. U myszy transgenicznych uzyskanych drogą wzajemnej rekombinacji, a więc bez nadeskpresji, nie występuje choroba spontaniczna, niemniej stają się one wrażliwe na zakażenie GSS
A Dynamic Subfilter-scale Stress Model for Large Eddy Simulations Based on Physical Flow Scales
No full textWe propose a new definition of the length scale in an eddy-viscosity model for large-eddy simulations (LES). This formulation extends and generalizes a previous proposal [Piomelli, Rouhi and Geurts, Proc. ETMM10, 2014], in which the LES length scale was expressed in terms of the integral length-scale of turbulence determined by the flow characteristics and explicitly decoupled from the simulation grid; this approach was named Integral Length-Scale Approximation (ILSA). As in the original ILSA, the model coefficient was determined by the user, and required to maintain a desired contribution of the unresolved, subfilter scales (SFS) to the global transport. We propose a local formulation (local ILSA) in which the model coefficient is local in space, allowing a precise control over SFS activity as a function of location. This new formulation preserves the properties of the global model; application to channel flow and backward-facing step verifies its features and accuracy
Large-eddy simulation of a separated flow with a sub-filter scale model based on the integral length-scale
No full textA new sub-filter scale model for large-eddy simulations, which uses a length-scale proportional to the integral scale of the turbulence instead of the grid resolution to parametrize the modelled stresses, will be assessed in the prediction of the flow of a boundary-layer over a rough surface, which includes separation and reattachment
Near Wall PIV-Measurements on the Windward Slope of a Hill
No full textThe turbulent flow over periodic hills was measured near to the wall, using planar Particle-Image-Velocimetry (PIV) at high spatial resolution. Our focus is on the near wall turbulence structure on the windward slope of the hill. For large-eddy simulation (LES) we suspect that, if this was not predicted accurately, it affects the prediction of the velocity profiles over the hill crest which in turn will affect the recirculation length downstream of the hill. Regarding the time averaged velocities, we were able to resolve the linear viscous region of the boundary layer. The velocity distribution and also the Reynolds stress does not comply with the law of the wall as it is valid for a turbulent boundary layer at equilibrium
Energy dissipation and flux laws for unsteady turbulence
No full textDirect Numerical Simulations of spatially periodic unsteady turbulence show that the high Reynolds number scalings of the instantaneous energy dissipation rate and interscale energy flux at intermediate wavenumbers are qualitatively different from the well-known cornerstone scalings of equilibrium turbulence where and are time-dependent rms velocity and integral length-scales. Instead, they both scale as where and are length and velocity scales characterizing initial/overall unsteady turbulence conditions
Direct numerical simulation of turbulent Couette-Poiseuille flow with zero skin friction
No full textThe near-wall scaling of mean velocity U(y) is addressed for the case of zero skin friction on one wall of a fully turbulent channel flow. The present DNS results can be added to the evidence in support of the conjecture that U is proportional to √yw in the region just above the wall at which the mean shear dU/dy = 0
Measurement techniques for tracer kinetic studies with stable isotopes of zirconium
No full textBiokinetic models are used in radiation protection to assess internal radiation doses. Experiments with stable isotopes as tracers can be performed to obtain characteristic parameters of these models. Two methods for the measurement of zirconium isotopes in human biological samples are presented - thermal ionisation mass spectrometry (TIMS) and proton nuclear activation analysis (PNA). Descriptions include sample preparation, operating conditions, relative uncertainties and method detection limits as well as important properties of both methods
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