ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010
R. L. Withers
Structured diffuse-scattering intensities, whether of compositional or of pure displacive origin, static or dynamic, contain important information about the symmetry of the individual compositional and/or displacive modes responsible for the observed intensities. However, the interpretation of the experimental data is very often impeded by the lack of a symmetry-based approach to the analysis of the structured diffuse-scattering distributions. Recently, we have demonstrated the existence of systematic phonon selection rules for diffuse scattering that depend on the symmetries of the mode and the scattering vector, and not on the specific structure. Here, we show that such symmetry analysis can be successfully extended and also applied to structure-dependent diffuse scattering associated with `disordered' materials: the combination of theoretically determined, diffuse-scattering extinction conditions with the concept of non-characteristic orbits proves to be very useful in the interpretation of the observed diffuse-scattering extinctions. The utility of this approach is illustrated by the analysis of diffuse-scattering data from ThAsSe, FeOF and FeF2. The essential part of the associated calculations are performed by the computer programs NEUTRON (systematic phonon extinction rules in inelastic scattering) and NONCHAR (non-characteristic orbits of space groups) that are available on the Bilbao crystallographic server (http://www.cryst.ehu.es). [source]

Neutron Diffraction of Zirconia-Dispersed Alumina with Increasing Stress and Temperature

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2008
Geoffrey A. Carter
Neutron and X-ray diffraction were used to investigate changes in the phase composition and lattice parameters with temperature and pressure of a 20-wt% zirconia-dispersed alumina composite. Neutron diffraction was used to determine the variation of tetragonal/monoclinic zirconia phase abundance for the bulk with stress. The bulk tetragonal/monoclinic ratio decreased by 40% for a stress of 550 MPa. A difference in the tetragonal/monoclinic ratio between the near-surface and the bulk of the processed samples is believed to contribute to observed variations in nonuniform strain in the matrix, which has the effect of prestressing the sample. Placing a stress on the sample initially releases this prestressing. [source]

Structural analysis of lithium lanthanum titanate with perovskite structure

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2009
Koji Ohara
Abstract Neutron and high-energy X-ray diffraction analysis of polycrystalline La4/3-xLi3xTi2O6 have been performed to clarify the extent of disorder of the distribution of La and Li ions and to understand the relation of these distributions to ionic conduction. The distributions of the La and Li ions in a 10 × 10 × 20 cubic box (i.e., 10 × 10 × 10 unit cell) super-structure, in which Ti and O atoms are fixed onto their regular sites, were obtained by the reverse Monte Carlo (RMC) structural modelling of both diffraction data sets. When the occupancy of La ions in the planes perpendicular to the c-axis is analysed, one can find a La-rich and La-poor layers alternating, which is consistent with the results of earlier Rietveld analysis (Stramare et al., Chem. Mater. 15, 3974 (2003) [1]). Of particular interest, the Li ions are found mainly on the interstitial sites between the O-3 triangle plane of the TiO6 octahedron and a La ion, which is different from the earlier work (Yashima et al., J. Am. Chem. Soc. 127, 3491 (2005) [2]). (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Neutron and electron diffraction studies of La(Zn1/2Ti1/2)O3 perovskite

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2006
Yi Hu
The crystallography and microwave dielectric properties of La(Zn1/2Ti1/2)O3 (LZT) ceramics prepared via the mixed-oxide route were investigated in this study. While samples were largely single phase, small amounts of ZnO impurity were detected in sintered pellets. Observed reflections in electron and neutron diffraction patterns indicate that the symmetry of LZT is P21/n. The B site is ordered on {110} or pseudocubic {111}, but the presence of the pseudocubic (111) reflection is in itself insufficient to indicate the existence of such order. Rietveld refinements of the neutron diffraction data yield an excellent fit for such a model. The structure is highly twinned, with variants related through common {211} composition planes and 90° rotations about ,011,. The microwave dielectric properties measured were ,r = 34, Qf = 36,090 and ,f = ,70,MK,1. [source]

Unambiguous determination of H-atom positions: comparing results from neutron and high-resolution X-ray crystallography

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2010
Anna S. Gardberg
The locations of H atoms in biological structures can be difficult to determine using X-ray diffraction methods. Neutron diffraction offers a relatively greater scattering magnitude from H and D atoms. Here, 1.65,Å resolution neutron diffraction studies of fully perdeuterated and selectively CH3 -protonated perdeuterated crystals of Pyrococcus furiosus rubredoxin (D-rubredoxin and HD-rubredoxin, respectively) at room temperature (RT) are described, as well as 1.1,Å resolution X-ray diffraction studies of the same protein at both RT and 100,K. The two techniques are quantitatively compared in terms of their power to directly provide atomic positions for D atoms and analyze the role played by atomic thermal motion by computing the , level at the D-atom coordinate in simulated-annealing composite D-OMIT maps. It is shown that 1.65,Å resolution RT neutron data for perdeuterated rubredoxin are ,8 times more likely overall to provide high-confidence positions for D atoms than 1.1,Å resolution X-ray data at 100,K or RT. At or above the 1.0, level, the joint X-ray/neutron (XN) structures define 342/378 (90%) and 291/365 (80%) of the D-atom positions for D-rubredoxin and HD-rubredoxin, respectively. The X-ray-only 1.1,Å resolution 100,K structures determine only 19/388 (5%) and 8/388 (2%) of the D-atom positions above the 1.0, level for D-rubredoxin and HD-rubredoxin, respectively. Furthermore, the improved model obtained from joint XN refinement yielded improved electron-density maps, permitting the location of more D atoms than electron-density maps from models refined against X-ray data only. [source]

Optical studies of defects generated in neutron-irradiated Cz-Si during HP-HT treatment

CRYSTAL RESEARCH AND TECHNOLOGY, Issue 4-5 2005
B. Surma
Abstract Neutron-irradiated Czochralski grown silicon subjected to heat treatment (HT) at 350 °C and 1000 °C under enhanced hydrostatic pressure (HP) was studied in this work. It has been shown that external hydrostatic pressure enhances the creation of VO2 defects in neutron irradiated silicon subjected to the HP - HT treatment at 350 °C. Enhanced formation of platelet-like oxygen precipitates was found in the samples treated at 1000 °C under 1.1 GPa. This effect was more pronounced in the samples with VO2 defects. Presented results seem to suggest that probably HP helps to transform VO2 to some kind of defects or change alone VO2 defects in the form that can act as an additional nucleus for an additional oxygen precipitation at 1000°C. No correlation between the plate-like oxygen precipitates related absorption at 1225 cm -1 and dislocation-related emission has been confirmed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Inversion of time-dependent nuclear well-logging data using neural networks

GEOPHYSICAL PROSPECTING, Issue 1 2008
Laura Carmine
ABSTRACT The purpose of this work was to investigate a new and fast inversion methodology for the prediction of subsurface formation properties such as porosity, salinity and oil saturation, using time-dependent nuclear well logging data. Although the ultimate aim is to apply the technique to real-field data, an initial investigation as described in this paper, was first required; this has been carried out using simulation results from the time-dependent radiation transport problem within a borehole. Simulated neutron and ,-ray fluxes at two sodium iodide (NaI) detectors, one near and one far from a pulsed neutron source emitting at ,14 MeV, were used for the investigation. A total of 67 energy groups from the BUGLE96 cross section library together with 567 property combinations were employed for the original flux response generation, achieved by solving numerically the time-dependent Boltzmann radiation transport equation in its even parity form. Material property combinations (scenarios) and their correspondent teaching outputs (flux response at detectors) are used to train the Artificial Neural Networks (ANNs) and test data is used to assess the accuracy of the ANNs. The trained networks are then used to produce a surrogate model of the expensive, in terms of computational time and resources, forward model with which a simple inversion method is applied to calculate material properties from the time evolution of flux responses at the two detectors. The inversion technique uses a fast surrogate model comprising 8026 artificial neural networks, which consist of an input layer with three input units (neurons) for porosity, salinity and oil saturation; and two hidden layers and one output neuron representing the scalar photon or neutron flux prediction at the detector. This is the first time this technique has been applied to invert pulsed neutron logging tool information and the results produced are very promising. The next step in the procedure is to apply the methodology to real data. [source]

Energy Group optimization for forward and inverse problems in nuclear engineering: application to downwell-logging problems

GEOPHYSICAL PROSPECTING, Issue 2 2006
Elsa Aristodemou
ABSTRACT Simulating radiation transport of neutral particles (neutrons and ,-ray photons) within subsurface formations has been an area of research in the nuclear well-logging community since the 1960s, with many researchers exploiting existing computational tools already available within the nuclear reactor community. Deterministic codes became a popular tool, with the radiation transport equation being solved using a discretization of phase-space of the problem (energy, angle, space and time). The energy discretization in such codes is based on the multigroup approximation, or equivalently the discrete finite-difference energy approximation. One of the uncertainties, therefore, of simulating radiation transport problems, has become the multigroup energy structure. The nuclear reactor community has tackled the problem by optimizing existing nuclear cross-sectional libraries using a variety of group-collapsing codes, whilst the nuclear well-logging community has relied, until now, on libraries used in the nuclear reactor community. However, although the utilization of such libraries has been extremely useful in the past, it has also become clear that a larger number of energy groups were available than was necessary for the well-logging problems. It was obvious, therefore, that a multigroup energy structure specific to the needs of the nuclear well-logging community needed to be established. This would have the benefit of reducing computational time (the ultimate aim of this work) for both the stochastic and deterministic calculations since computational time increases with the number of energy groups. We, therefore, present in this study two methodologies that enable the optimization of any multigroup neutron,, energy structure. Although we test our theoretical approaches on nuclear well-logging synthetic data, the methodologies can be applied to other radiation transport problems that use the multigroup energy approximation. The first approach considers the effect of collapsing the neutron groups by solving the forward transport problem directly using the deterministic code EVENT, and obtaining neutron and ,-ray fluxes deterministically for the different group-collapsing options. The best collapsing option is chosen as the one which minimizes the effect on the ,-ray spectrum. During this methodology, parallel processing is implemented to reduce computational times. The second approach uses the uncollapsed output from neural network simulations in order to estimate the new, collapsed fluxes for the different collapsing cases. Subsequently, an inversion technique is used which calculates the properties of the subsurface, based on the collapsed fluxes. The best collapsing option is chosen as the one that predicts the subsurface properties with a minimal error. The fundamental difference between the two methodologies relates to their effect on the generated ,-rays. The first methodology takes the generation of ,-rays fully into account by solving the transport equation directly. The second methodology assumes that the reduction of the neutron groups has no effect on the ,-ray fluxes. It does, however, utilize an inversion scheme to predict the subsurface properties reliably, and it looks at the effect of collapsing the neutron groups on these predictions. Although the second procedure is favoured because of (a) the speed with which a solution can be obtained and (b) the application of an inversion scheme, its results need to be validated against a physically more stringent methodology. A comparison of the two methodologies is therefore given. [source]

Pair distribution functions calculated from interatomic potential models using the General Utility Lattice Program

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2007
Elizabeth R. Cope
A new module has been developed for the widely used General Utility Lattice Program (GULP). The phonon-based theory developed by Chung & Thorpe [Phys. Rev. B (1999), 59, 4807,4812] to calculate pair distribution function (PDF) peak widths has been utilized to give a selection of commonly used correlation functions. A numerical library of neutron scattering information is now available within GULP, and is used to produce results that can be compared with neutron scattering experimental data. The influence of different phonon modes on the PDF can be assessed by excluding modes above or below a cut-off frequency. Results are presented for sample crystallographic systems, MgO, SrTiO3 and ,-cristobalite, as well as CaxSr1,xTiO3 at x = 0.5, which makes use of the capability to handle partial occupancies to compare different Ca/Sr ordering arrangements with a disordered model in which every Ca/Sr site has 50% occupancy of both species. [source]

A deconvolution method for the reconstruction of underlying profiles measured using large sampling volumes

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2006
Y.-S. Xiong
A deconvolution method for diffraction measurements based on a statistical learning technique is presented. The radial-basis function network is used to model the underlying function. A full probabilistic description of the measurement is introduced, incorporating a Bayesian algorithm based on an evidence framework. This method allows predictions of both the convolution and the underlying function from noisy measurements. In addition, the method can provide an estimation of the prediction uncertainty, i.e. error-bars. In order to assess the capability of the method, the model was tested first on synthetic data of controllable quality and sparsity; it is shown that the method works very well, even for inaccurately measured (noisy) data. Subsequently, the deconvolution method was applied to real data sets typical of neutron and synchrotron residual stress (strain) data, recovering features not immediately evident in the large-gauge-volume measurements themselves. Finally, the extent to which short-period components are lost as a function of the measurement gauge dimensions is discussed. The results seem to indicate that for a triangular sensor-sensitivity function, measurements are best made with a gauge of a width approximately equal to the wavelength of the expected strain variation, but with a significant level of overlap (,80%) between successive points; this is contrary to current practice for neutron strain measurements. [source]

Small-angle neutron and X-ray scattering of dispersions of oleic-acid-coated magnetic iron particles

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2004
Karen Butter
This paper describes the characterization of dispersions of oleic-acid-coated magnetic iron particles by small-angle neutron and X-ray scattering (SANS and SAXS). Both oxidized and non-oxidized dilute samples were studied by SANS at different contrasts. The non-oxidized samples are found to consist of non-interacting superparamagnetic single dipolar particles, with a lognormal distribution of iron cores, surrounded by a surfactant shell, which is partially penetrated by solvent. This model is supported by SAXS measurements on the same dispersion. Small iron particles are expected to oxidize upon exposure to air. SANS was used to study the effect of this oxidation, both on single particles, as well as on interparticle interactions. It is found that on exposure to air, a non-magnetic oxide layer is formed around the iron cores, which causes an increase of particle size. In addition, particles are found to aggregate upon oxidation, presumably because the surfactant density on the particle surfaces is decreased. [source]

PRINSAS, a Windows-based computer program for the processing and interpretation of small-angle scattering data tailored to the analysis of sedimentary rocks

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2004
Alan L. Hinde
PRINSAS is a Windows program that takes as input raw (post-reduction) small-angle neutron and small-angle X-ray scattering (SANS and SAXS) data obtained from various worldwide facilities, displays the raw curves in interactive log,log plots, and allows processing of the raw curves. Separate raw SANS and ultra-small-angle neutron scattering (USANS) curves can be combined into complete scattering curves for an individual sample. The combined curves can be interpreted and information inferred about sample structure, using built-in functions. These have been tailored for geological samples and other porous media, and include the ability to obtain an arbitrary distribution of scatterer sizes, the corresponding specific surface area of scatterers, and porosity (when the scatterers are pores), assuming spherical scatterers. A fractal model may also be assumed and the fractal dimension obtained. A utility for calculating scattering length density from the component oxides is included in the program. [source]

Spin-echo small-angle neutron scattering in neutron reflectometry

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2003
M. Theo Rekveldt
A new method to perform neutron reflectometry measurements is discussed. The method is based on Larmor precession of polarized neutrons in the spin-echo mode. Two different modes of application are discussed, giving successively information about the structure perpendicular to and parallel to the sample plane. After describing the method, simulated results of the perpendicular mode are compared with those obtained by conventional neutron reflectometry methods. In this comparison, first the results obtained by the SESANS method are translated to those obtained by conventional methods. After that, more specific applications of the new method are considered. In the comparison, attention is focused on measuring statistics and resolution as readily comparable quantities. [source]

Cationic disorder, microstructure and dielectric response of ferroelectric SBT ceramics

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-2 2003
Ch. Muller
Polycrystalline samples of SrBi2Ta2O9 (acronym SBT) have been prepared by means of solid-state reaction either using a classical route or by mechanochemical activation. For each compound, a structural analysis of the ferroelectric orthorhombic phase (space group A21am) has been performed from the fitting of neutron and/or X-ray powder diffraction data using the Rietveld method. A cationic disorder on Bi3+ and Sr2+ crystallographic sites has been revealed, the Sr atoms occupying the Bi sites and vice versa. From diffraction peak broadening analyses of high-resolution synchrotron X-ray diffraction data, it has been shown that the two grinding methods (manual or mechanical) induce local strains, the average apparent strain being three times larger for the mechanically ground sample. In order to link microstructure and ferroelectric properties, the dielectric constant has been measured as a function of the temperature. It appears that the position and the shape of the dielectric anomaly strongly depend upon the composition and the route used to elaborate the powders. More exactly, for the mechanically ground powder, the large apparent strain, probably correlated to the strong cation exchange revealed by the structural refinement, leads to a significantly enhanced dielectric response. [source]

Complementarity of small-angle neutron and X-ray scattering methods for the quantitative structural and dynamical specification of dendritic macromolecules

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2003
Alexander I. Kuklin
The structural characteristics of polycarbosilane dendrimers with different molecular architecture were determined in solutions by small angle neutron and X-ray scattering. The same linear dimensions were sized up for the dendrimers both in benzene and chloroform. A solvent molecules penetration inside dendrimer structure in amount up to 30 vol.% was found from the comparison of the partial and effective scattering volume for the dendrimers in solution. [source]

Benefits of polarized small-angle neutron scattering on magnetic nanometer scale structure modeling

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2003
André Heinemann
Recent use of polarized neutron technique in small-angle scattering (SANS) have led to impressive results in the case of magnetic nanometer-scale structure analysis. In some particular cases this method offers the possibility to survey structure models with the necessary accuracy for the first time. The different cross sections for spin-up and spin-down neutron scattering on magnetic precipitates can be combined with the method of chemical contrast variation. All data fitting using structure models will benefit of that kind of constraints. The analysis of the interference term of nuclear and magnetic scattering respectively enables the extraction of additional information about the composition and magnetization profiles of the samples. Here we place emphasis on the difference of spin-up and spin-down neutron scattering intensities to obtain this information. This technique profits by the clear distinction between magnetic and nonmagnetic scattering contributions and the strong auxiliary conditions for model fitting procedures. Depending on the relative orientations of the external magnetic field, the local magnetization of the precipitates and the scattering vector, significant scattering patterns can be scrutinized. Beside general formulas for some special cases of present experimental interest we exercise the approach to a nontrivial case of data obtained from polarised SANS experiments at the Berlin Neutron Scattering Center (BENSC). [source]

Simultaneous light and small-angle neutron scattering on aggregating concentrated colloidal suspensions

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2003
Sara Romer
A new sample environment has been developed in order to perform light and small-angle neutron scattering (SANS) simultaneously on colloidal systems. The combination of SANS and diffusing wave spectroscopy (DWS) is of particular use in the high-concentration regime. DWS provides information on the local dynamic properties of the individual particles, whereas SANS gives access to the structural properties on similar length scales. The combination of both methods thus allows one to obtain structural and dynamic information over a very large range of length and time scales. Using this new setup, the onset of aggregation and the sol,gel transition in concentrated destabilized polystyrene sphere suspensions have been investigated. At the gel point, a dramatic change of the particle dynamics from diffusion to a subdiffusive arrested motion is observed. However, while the DWS measurements indicate that dramatic changes in the local dynamics occur over a long period, the SANS pattern quickly reaches its final appearance. The SANS experiments thus indicate that a fluid-like structure is arrested in the course of the gel formation. The data are found to be in good qualitative agreement with computer simulations. [source]

FOX, `free objects for crystallography': a modular approach to ab initio structure determination from powder diffraction

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2002
Vincent Favre-Nicolin
A new program has been developed for ab initio crystal structure determination from powder diffraction data (X-ray and neutron). It uses global-optimization algorithms to solve the structure by performing trials in direct space. It is a modular program, capable of using several criteria for evaluating each trial configuration (e.g. multi-pattern). It is also modular in the description of the crystal content, with the possibility of describing building blocks in the sample, such as polyhedra or molecules, and with automatic adaptive handling of special positions and sharing of identical atoms between neighbouring building blocks. It can therefore find the correct structure without any assumption about the connectivity of the building blocks and is suitable for any kind of material. Several optimization algorithms (simulated annealing, parallel tempering) are available, with the possibility of choosing the convergence criterion as a combination of available cost functions. This program is freely available for Linux and Windows platforms; it is also fully `open source', which, combined with an object-oriented design and a complete developer documentation, ensures its future evolution. [source]

Outcomes of the International Union of Crystallography Commission on Powder Diffraction Round Robin on Quantitative Phase Analysis: samples 1a to 1h

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2001
Ian C. Madsen
The International Union of Crystallography (IUCr) Commission on Powder Diffraction (CPD) has sponsored a round robin on the determination of quantitative phase abundance from diffraction data. Specifically, the aims of the round robin were (i) to document the methods and strategies commonly employed in quantitative phase analysis (QPA), especially those involving powder diffraction, (ii) to assess levels of accuracy, precision and lower limits of detection, (iii) to identify specific problem areas and develop practical solutions, (iv) to formulate recommended procedures for QPA using diffraction data, and (v) to create a standard set of samples for future reference. Some of the analytical issues which have been addressed include (a) the type of analysis (integrated intensities or full-profile, Rietveld or full-profile, database of observed patterns) and (b) the type of instrument used, including geometry and radiation (X-ray, neutron or synchrotron). While the samples used in the round robin covered a wide range of analytical complexity, this paper reports the results for only the sample 1 mixtures. Sample 1 is a simple three-phase system prepared with eight different compositions covering a wide range of abundance for each phase. The component phases were chosen to minimize sample-related problems, such as the degree of crystallinity, preferred orientation and microabsorption. However, these were still issues that needed to be addressed by the analysts. The results returned indicate a great deal of variation in the ability of the participating laboratories to perform QPA of this simple three-component system. These differences result from such problems as (i) use of unsuitable reference intensity ratios, (ii) errors in whole-pattern refinement software operation and in interpretation of results, (iii) operator errors in the use of the Rietveld method, often arising from a lack of crystallographic understanding, and (iv) application of excessive microabsorption correction. Another major area for concern is the calculation of errors in phase abundance determination, with wide variations in reported values between participants. Few details of methodology used to derive these errors were supplied and many participants provided no measure of error at all. [source]

String Fit: a new structurally oriented X-ray and neutron reflectivity evaluation technique

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2001
Erich Politsch
A novel method for the analysis of neutron and X-ray reflectivity measurements is presented. In contrast to existing methods, the new data fitting approach is structurally oriented and therefore only requires information about the chemical structure of studied molecules and no other ad hoc assumptions. Apart from the inversion of reflectivity into scattering length density profile, the inversion of scattering length density profile into molecular arrangement is addressed systematically for non-trivial molecular conformations for the first time. This includes the calculation of structural characteristics, such as radius of gyration or chain order parameters, based on measured reflectograms. Another important option is the possibility to evaluate simultaneously neutron and X-ray reflectograms of a given sample. For better convergence, especially for complex simultaneous evaluations, an effective extension of the normally used least-squares deviation function is introduced. Different simulated molecular ensembles are used to illustrate the features of the new approach; typically, excellent agreement between the simulated starting and final deduced data sets is achieved. [source]

pH dependent self assembly of ,-amyloid(10-35) and ,-amyloid(10-35)-PEG3000

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2000
P. Thiyagarajan
Small angle neutron and x-ray scattering (SANS/SAXS) studies were conducted on the structure of the aggregates formed from both the truncated model peptide ,-Amyloid(10-35) (A,10-35) and a block copolymer ,-Amyloid(10-35)-PEG3000 (A,10-35 -PEG) in D2O at pHs from 3.0 to 7.0. These studies indicate that A,10-35 aggregates into rod-like particles (fibril) and their radii are strongly dependent on the pH of the solution. The fibril-fibril association in A,10-35 solutions is less at pH < 5.6, but becomes larger at higher pH. A,10-35 -PEG also assembles into rod-like particles whose radius is larger by about 30 Å than that for A,10-35 fibril at pH 4.2, while it is about 23 Å larger at higher pH. Contrast matching SAXS/SANS experiments that eliminate the coherent scattering from PEG reveal that PEG moiety is located at the periphery of the fibril. Also the mass per unit length of the peptide portion is similar for both A,10-35 and A,10-35 fibrils at pH 5.6. The mass per unit length of the rods from SANS provides key information on the packing of A,10-35 peptides in the fibril. [source]

Multilevel structure of reinforcing silica and carbon

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2000
D.W. Schaefer
Using small-angle x-ray (SAXS), neutron (SANS), x-ray diffraction and light scattering, we study the structure of colloidal silica and carbon on length scales from 4 Å < q,1 < 107 Å where q is the magnitude of the scattering vector. These materials consist of primary particles of the order of 100 Å, aggregated into micron-sized aggregates that in turn are agglomerated into 100 µ agglomerates. The diffraction data show that the primary particles in precipitated silica are composed of highly defective amorphous silica with little intermediate-range order (order on the scale of several bond distances). On the next level of morphology, primary particles arise by a complex nucleation process in which primordial nuclei briefly aggregate into rough particles that subsequently smooth out to become the seeds for the primaries. The primaries aggregate to strongly bonded clusters by a complex process involving kinetic growth, mechanical disintegration and restructuring. Finally, the small-angle scattering (SAS) data lead us to postulate that the aggregates cluster into porous, rough-surfaced, non-mass-fractal agglomerates that can be broken down to the more strongly bonded aggregates by application of shear. We find similar structure in pelletized carbon blacks. In this case we show a linear scaling relation between the primary and aggregate sizes. We attribute the scaling to mechanical processing that deforms the fractal aggregates down to the maximum size able to withstand the compaction stress. Finally, we rationalize the observed structure based on empirical optimization by filler suppliers and some recent theoretical ideas due to Witten, Rubenstein and Colby. [source]

Investigation of human pharmacoscintigraphic behavior of two tablets and a capsule formulation of a high dose, poorly water soluble/highly permeable drug (efavirenz)

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 11 2007
J. ZH.
Abstract Human pharmacoscintigraphic behavior of two tablets and a capsule formulation of a high dose, poorly water soluble, highly permeable, micronized drug (efavirenz) was investigated. The tablets and capsule, prepared with samarium oxide and neutron activated to produce radioactive samarium-153, were evaluated for their in vivo disintegration and gastrointestinal (GI) transit in healthy subjects under fasted condition. Scintigraphic images were acquired to coincide with blood sampling times to assess the plasma concentration-time profile in relation to in vivo disintegration and GI transit. The mean gastric emptying times were approximately the same for all three formulations. Although in vivo dosage form disintegration was faster for Tablet A as compared to Tablet B and was similar between Tablet A and the capsule, Tablet A showed a slower rate and extent of drug absorption than Tablet B and the capsule. The results of this study eliminated the initial hypothesis that the difference in in vivo performance between the two tablet formulations is due to a different rate of in vivo disintegration and suggest that for this drug the in vivo dissolution rate of the drug from its disintegrated dosage form was a more important factor affecting the rate and extent of drug absorption. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 2970,2977, 2007 [source]

New model calculations for the production rates of cosmogenic nuclides in iron meteorites

METEORITICS & PLANETARY SCIENCE, Issue 4 2009
Katja AMMON
The model usually describes the production rates for cosmogenic radionuclides within their uncertainties; exceptions are 53Mn and 60Fe, possibly due to normalization problems. When an average S content of about 1 ± 0.5% is assumed for Grant and Carbo samples, which is consistent with our earlier study, the model predictions for 3He, 21Ne, and 38Ar are in agreement. For 4He the model has to be adjusted by 24%, possibly a result of our rather crude approximation for the primary galactic , particles. For reasons not yet understood the modeled 36Ar/38Ar ratio is about 30,40% higher than the ratio typically measured in iron meteorites. Currently, the only reasonable explanation for this discrepancy is the lack of experimentally determined neutron induced cross sections and therefore the uncertainties of the model itself. However, the new model predictions, though not yet perfect, enable determining the radius of the meteoroid, the exposure age, the sulphur content of the studied sample as well as the terrestrial residence time. The determination of exposure ages is of special interest because of the still open question whether the GCR was constant over long time scales. Therefore we will discuss in detail the differences between exposure ages determined with different cosmogenic nuclides. With the new model we can calculate exposure ages that are based on the production rates (cm3STP/(gMa)) of noble gases only. These exposure ages, referred to as noble gas exposure ages or simply 3,4He, 21Ne, or 36,38Ar ages, are calculated assuming the current GCR flux. Besides calculating noble gas ages we were also able to improve the 41K- 40K-and the 36Cl- 36Ar dating methods with the new model. Note that we distinguish between 36Ar ages (calculated via 36Ar production rates only) and 36Cl- 36Ar ages. Exposure ages for Grant and Carbo, calculated with the revised 41K- 40K method, are 628 ± 30 Ma and 841 ± 19 Ma, respectively. For Grant this is equal to the ages obtained using 3He, 21Ne, and 38Ar but higher than the 36Ar- and 36Cl- 36Ar ages by ,30%. For Carbo the 41K- 40K age is ,40% lower than the ages obtained using 3He, 21Ne, and 38Ar but equal to the 36Ar age. These differences can either be explained by our still insufficient knowledge of the neutron-induced cross sections or by a long-term variation of the GCR. [source]

Neutron capture effects on samarium, europium, and gadolinium in Apollo 15 deep drill-core samples

METEORITICS & PLANETARY SCIENCE, Issue 3 2000
Hiroshi HIDAKA
Large isotopic deviations of 150Sm/149Sm, 156Gd/155Gd, and 158Gd/157Gd derived from neutron capture effects were observed in all samples. Although neutron capture products in lunar samples were investigated extensively in the 1970s, our precise isotopic measurements resulted in several new findings. The neutron fluence in the Apollo 15 drill core is a function of depth with a symmetric peak at 190 g/cm2 depth from the surface, confirming the results of earlier investigations. Neutron fluence values calculated from the isotopic shifts by comparison to artificially irradiated standard reagents were (5.16,7.49) × 1016 n/cm2. These values are 1.3 to 1.4x larger than those previously reported. Variations of ,Sm/,Gd with depth are interpreted as being due to variations in the neutron energy spectrum. Here ,Sm and ,Gd are defined as in previous studies of lunar neutron stratigraphy. Our data suggest that the neutron is more thermalized at the lower layers than it is at the upper layers. In addition to large isotopic shifts for 149Sm, 150Sm, 155Gd, 156Gd, 157Gd, and 158Gd, isotopic enrichments of 152Gd and 154Gd derived from neutron capture for 151Eu and 153Eu, respectively, were also observed in all samples. [source]

Minimal models of cooling neutron stars with accreted envelopes

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
A. D. Kaminker
ABSTRACT We study the ,minimal' cooling scenario of superfluid neutron stars with nucleon cores, where the direct Urca process is forbidden and enhanced cooling is produced by neutrino emission due to the Cooper pairing of neutrons. Extending our recent previous work, we include the effects of surface accreted envelopes of light elements. We employ the phenomenological density-dependent critical temperatures Tcp(,) and Tcnt(,) of singlet-state proton and triplet-state neutron pairing in a stellar core, as well as the critical temperature Tcns(,) of singlet-state neutron pairing in a stellar crust. We show that the presence of accreted envelopes simplifies the interpretation of observations of thermal radiation from isolated neutron stars in the scenario of our recent previous work and widens the class of models for nucleon superfluidity in neutron star interiors consistent with the observations. [source]

Reduced pore diameter fluctuations of macroporous silicon fabricated from neutron-transmutation-doped material

PHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 7 2010
Stefan L. Schweizer
Abstract The precision of photo-electrochemical etching of perfectly-ordered macropores in single-crystalline silicon is limited by pore diameter fluctuations due to doping variations of the starting wafer (striations). The doping variation originates from the rotation during crystal growth in the float-zone or Czochralski process, respectively. Experimentally, variations of the pore diameter up to 7% can occur. These so-called striations limit performance of possible applications of macroporous silicon. As doping inhomogeneities are the reason for the striations, uniformly doped silicon wafers by neutron transmutation doping were used for the first time. Photoelectrochemical etching of neutron transmutated silicon has been carried out and the pore diameter fluctuation has been reduced by about 40% compared to standard doped float-zone wafers. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Structure of gallium-doped mayenite and its reduction behaviour

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2008
Luis Palacios
Abstract The isovalent Al/Ga substitution in mayenite has been studied by preparing the (Ca12Al14,xGax O32)O1,5 series. Single phase compounds can be prepared up to x = 1 and the structure of this compound has been studied by joint Rietveld analysis of neutron and X-ray powder diffraction data. The results showed that gallium substitutes aluminium, which provokes the expansion of the unit cell, as expected. (Ca12Al13GaO32)O1,5 has been treated at high temperature under strong reducing conditions to prepare an electride derivative. Unfortunately, instead the reduction leaving electrons into the mayenite cages, the Ga-doped mayenite undergoes decomposition. The initial compound transforms to Ca3Al2O6, (Ca12Al14O32)O1,5 and amorphous Ga metal, which is likely deposited on the mayenite crystal surfaces. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Positron lifetime calculations for stacking fault tetrahedra in copper

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007
S. Van Petegem
Abstract We investigate the positron response of simulated stacking fault tetrahedra in Cu. We find that in both the regular and truncated stacking fault tetrahedra positron lifetimes are about 10-15 ps larger than the bulk lifetime. This is an indication that the vacancy-like defects observed in neutron irradiated Cu by positron annihilation lifetime measurements do not arise during the formation of the stacking fault tetrahedra but rather during the interaction of the stacking fault tetrahedra with other defects. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Radiation effects in sapphire optical fibers

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2007
Dan Sporea
Abstract Radiation effects on sapphire optical fibers were investigated using off-line gamma-ray, neutron and electron beam irradiation and on-line proton irradiation. The optical attenuation of these optical fibers was studied from UV to IR (200 nm , 1700 nm). For the in-situ measurements radioluminescence was also investigated as function of the total irradiation dose. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]