Multishell Analysis Research Articles

Ultra-high resolution and multi-shell diffusion MRI of intact ex vivo human brains using kT-dSTEAM at 9.4T

Diffusion MRI (dMRI) in ex vivo human brain specimens is an important research tool for neuroanatomical investigations and the validation of dMRI techniques. Many ex vivo dMRI applications have benefited from very high dMRI resolutions achievable on small-bore preclinical or animal MRI scanners for small tissue samples. However, the investigation of entire human brains post mortem provides the important context of entire white matter (WM) network systems and entire gray matter (GM) areas connected through these systems. The investigation of intact ex vivo human brains in large bore systems creates challenges due to the limited gradient performance and transmit radio-frequency (B1+) inhomogeneities, specially at ultra-high field (UHF, 7T and higher). To overcome these issues, it is necessary to tailor ex vivo diffusion-weighted sequences specifically for high resolution and high diffusion-weighting. Here, we present kT-dSTEAM, which achieves B1+ homogenization across whole human brain specimens using parallel transmit (pTx) on a 9.4T MR system. We use kT-dSTEAM to obtain multi-shell high b-value and high resolution diffusion-weighted data in ex vivo whole human brains. Isotropic whole brain data can be acquired at high b-value (6000–8000 s/mm2) at high resolution (1000 μm) and at moderate b-value (3000 s/mm2) at ultra-high isotropic resolution (400 μm). As an illustration of the advantages of the ultra-high resolution, tractography across the WM/GM border shows less of the unwanted gyral crown bias, and more high-curvature paths connecting the sulcal wall than at lower resolution. The kT-dSTEAM also allows for acquisition of T1 and T2 weighted images suitable for estimating quantitative T1 and T2 maps. Finally, multi-shell analysis of kT-dSTEAM data at variable mixing time (TM) is shown as an approach for ex vivo data analysis which is adapted to the strengths of STEAM diffusion-weighting. Here, we use this gain for multi-orientation modelling and crossing-fiber tractography. We show that multi-shell data allows superior multiple orientation tractography of known crossing fiber structures in the brain stem.

Atom geometry of nanostructured Fe films grown on c(2 × 2)-N/Cu(1 0 0) surface: An investigation by X-ray absorption spectroscopy with multishell analysis

We investigated the growth of Fe nanostructured films on c(2 × 2)-N/Cu(1 0 0) surface with Fe K-edge X-ray absorption fine structure (XAFS) in the near edge and in the extended energy region. The high photon flux of the incident X-rays allowed us to perform multishell analysis of the XAFS oscillations for Fe coverage Θ Fe < 1 ML. This data analysis yields a detailed investigation of the atom geometry and some insights in the film morphology. At Θ N < 0.5 ML (N saturation coverage) there is absence of contribution to XAFS from N atoms. First shell analysis of linearly polarized XAFS gives Fe–Fe (or Fe–Cu) bond length values varying between R 1 = 2.526 ± 0.006 Å at the highest Fe coverage (3 ML ) and R 1 = 2.58 ± 0.01 Å at Θ Fe = 0.5 ML, Θ N = 0.3 ML, with incidence angle Θ = 35°. These values are different from the case of bcc Fe ( R = 2.48 Å), and compatible with fcc Fe ( R 1 = 2.52 Å) and fcc Cu ( R 1 = 2.55 Å). At the Fe lowest coverage ( Θ Fe = 0.5 ML) the dependence of R 1 on the incidence angle indicates expansion of the outmost layer. Near edge spectra and multishell analysis can be well reproduced by fcc geometry with high degree of static disorder. At N saturation pre-coverage ( Θ N = 0.5 ML) the XAFS analysis has to keep into account the Fe–N bonding. The results suggest two different adsorption sites: one with Fe in a fcc hollow site, surrounded by other metal atoms as nearest neighbours, and one resulting from an exchange with a Cu atom underneath the N layer.

Structure properties of nanostructured Fe films grown on c(2 × 2) N/Cu(1 0 0) self-organised surface

The growth of Fe on the c(2×2) N/Cu(100) surface was studied with X-ray absorption fine structure (XAFS) assisted by LEED. Previous STM work had shown that atomic N on Cu(100) forms periodic arrays of c(2×2) N islands with a size of about 5nm×5nm channelling the growth of Fe onto the clean Cu areas. From XAFS multishell analysis we describe the growth in terms of face-centred cubic (fcc) phase with strong lattice distortion, deviation from layer by layer growth, and presence of complex surface phases, some with different geometry. The differences between the Fe growth mode on the clean and on the N-saturated Cu(100) surface are explained in terms of Fe–N atom interactions, and we obtained the adsorption geometry of Fe on the N-saturated surface.

The Role Of Ge In Cluster Formation In B And Bf2 Implanted Si Wafers After Ge Pre-Amorphization

ABSTRACTCluster formation in high dose B, BF2 implanted Si wafers is an important problem in silicon doping, since it is one of the leading causes of the electrical deactivation of the dopant. In this study, we used Ge pre-amorphized, ultra low energy B and BF2 implanted Si wafers in order to probe these clusters from a local structural point of view. Ge K-edge x-ray absorption spectroscopy (XAFS) is a powerful tool in obtaining local structural information around the Ge atom. The effects of different implant species with various implant doses and annealing conditions on the cluster formation are presented using Ge K-edge multi-shell XAFS analysis. The non-linear least-squares fits to the Ge K-edge Fourier Transformed (FT) XAFS data using calculated standards from multiple scattering simulations around the Ge atom reveal formed Ge-B clusters for the Rapid Thermal Processing (RTP) annealed wafers. The results also indicate the laser annealing process on the other hand is blocking the formation of these Ge induced Ge-B clusters.

Thermal–hydraulic analysis for a sodium-heated steam generator using a multi-shell method

A multi-shell analysis method has been applied to predict the Thermal–hydraulics in a steam generator of a liquid metal reactor. The method is intended to improve the calculation accuracy for temperature profiles at a wide range of heat exchange rates in a large sized steam generator for the future plant. The calculation accuracy has been examined using experimental 50 MWth steam generator test data that were measured during 1970's–1980's. The calculation results of temperature profiles were compared with experimental data at 20, 30, 50, 75, 90, and 100% of nominal operating condition. Responses for the stepwise flow rate variations were also evaluated. In conclusion, the calculation accuracy for the temperature profile in the steam generator was improved by using the multi-shell analysis method for a wide range of heat exchange rates.

The Local Structure of Antimony in High Dose Antimony Implants in Silicon by XAFS and SIMS.

AbstractOne of the important challenges in semiconductor industry is to sustain high concentration of dopant atoms electrically active in very small areas. In investigating the optimum post implantation treatment methods that will help to attain these conditions, the local structural information around the dopant atom is crucial. In this study, we have used secondary ion mass spectroscopy (SIMS) and x-ray absorption fine structure spectroscopy (XAFS) to obtain the concentration depth profiles and the local structural information around the Sb atom in laser thermal annealed (LTA) Sb implants in Si wafers. The Sb implant doses used in this work are 6.4x1015/cm2 and 2.0x1016/cm2. The XAFS results for the 6.4x1015/cm2 Sb dose sample do not exhibit any rhombohedral-Sb precipitation as the Fourier Transformed (FT) data can be fit successfully using only substitutional-Sb in the Si lattice sites. However, a multi-shell analysis of the Fourier Transformed (FT) data for the 2.0x1016/cm2 Sb dose sample clearly indicates there is a substantial contribution from the Sb-Sb scattering, which is a signature of precipitated form of Sb.

Elongation of Ganglioside (GD1a) Micellar Structure Depending on Cholesterol Content

The influence of cholesterol with various molar fractions on the structure of disialoganglioside (GD1a)/cholesterol mixed micelle was investigated by using synchrotron radiation small-angle X-ray scattering (SR-SAXS). The radius of gyration (Rg) of the micelles, estimated from the obtained data, was increased with increasing cholesterol content. A multi-shell model analysis for micelles, that was successfully applied to the structure estimation of GD1 micelles by Hirai et al. (1996), revealed that the cholesterol-dependent increase of Rg of the micelles is due to an elongation of its prolate spherocylindrical structure. In the present analysis, we assumed the micelle of the ganglioside/cholesterol mixture as a triple-shell structure taking into account the high electron density of the steroid ring of cholesterol that is inserted in the hydrophobic region of the micelle. The examined molar ratios of [ganglioside]/[cholesterol] were 1:0, 1:0.05, 1:0.1, 1:0.2, 1:0.4, 1:0.7 and 1:1. The ganglioside concentration for all samples was constant at 0.5 % w/v.

EXAFS study of local structure at the alkali ions in superconducting K2RbC60, K2CsC60, Rb2CsC60, and Rb3C60.

We have studied the local site assignment as well as the structural and dynamic properties of the alkali ions in superconducting K{sub 2}RbC{sub 60}, K{sub 2}CsC{sub 60}, Rb{sub 2}CsC{sub 60}, and Rb{sub 3}C{sub 60} compounds by x-ray absorption fine structure (EXAFS) measurements at the alkali ions K, Rb, and Cs in the temperature range 14{endash}300 K. In the ternary {ital A}{sub 2}{ital B}C{sub 60} systems a strong preferential site occupation is observed, where the smaller alkali ions ({ital A}) are placed in tetrahedral sites and the larger ones ({ital B}) in octahedral sites. This site ordering allows a separate structural analysis of tetrahedral and octahedral sites. A quantitative analysis of the tetrahedral site ({ital A}) dopants yields the neighbor distances {ital R}({ital A}-{ital C}), coordination numbers {ital N}{sub {ital C}} and second cumulants {sigma}{sup 2}. A similar analysis of the tetrahedral sites in Rb{sub 3}C{sub 60} was performed by creating difference spectra with the K{sub 2}RbC{sub 60} system. A multishell analysis of the octahedral sites ({ital B}) yields an off-center position of the {ital B} dopants. The off-center displacement scales with the size of the octahedral site and inversely with the ionic radius of the {ital B} dopant. A preferentialmore » displacement in the (1,1,1) direction is discussed in conjunction with possible carbon neighbor configurations at the octahedral sites. {copyright} {ital 1996 The American Physical Society.}« less

Structure of epitaxial ferromagnetic layers by multishell analysis of polarization dependent XAFS data

X-ray absorption fine structure (XAFS) has been often used to determine the structure of metastable magnetic films and multilayers deposited on single crystal substrates. These metastable phases provide important extensions of the family of simple elemental magnetic crystals and give new elements for the comparison with the predictions of electronic structure calculations. Further the well-known strengths of XAFS, a quantitative simulation of the spectra including distant shells contributions allows to identify unambiguously stacking sequences and the symmetry of the adsorption sites at the interface. This is now possible by using multiple scattering calculations which take the polarization of the light into account. It is illustrated by the CoCu system. The stable structure of cobalt is hexagonal closed packed (hcp), but cobalt can be stabilized in a distorted face centered cubic structure (fcc) by epitaxy on Cu(001). The deviation from the isotropic fcc structure is here determined precisely by polarization dependent XAFS. On the other hand, for a cobalt film grown on a (111) oriented copper single crystal, a complete simulation of the X-ray absorption spectra, using a multiple scattering formalism, clearly shows that the thick films have a local hcp stacking. The analysis of the thinner films reveals details of the growth of cobalt on Cu(111): the first two deposited layers follow the fcc (111) copper lattice, whereas the third one already starts an hcp (0001) stacking.

EXAFS studies of superconducting A 2BC 60 compounds

In the present work we have studied the X-ray absorption fine structure of the potassium and rubidium K-edges as well as the L III-edge of caesium in K 2RbC 60, K 2CsC 60 and Rb 2CsC 60. A quantitative analysis for the first neighbour shell of the tetrahedral site (A) dopants yields the nearest neighbour distances R(A t−C), coordination numbers N c and second cumulants σ 2. A multishell analysis of the octahedral sites (B) including the relevant scattering paths shows an off-center position of the (B) dopants.

Hydration effect on titanium linkage to silicalite framework

An EXAFS multishell analysis of a titanium silicalite with an MFI structure reveals that the double Si–O–Ti bridges formed between [TiOx] species and the [SiO4] framework tetrahedra are partially hydrolysed in the presence of water which results in the conversion from an edge- to a corner-sharing type of bonding between the two structural units.

Surface structure determination by SEXAFS: The reliability of bond lengths and coordination numbers from multi-shell analyses

Any structural determination from measurements of surface extended X-ray absorption fine structure (SEXAFS) is dependent on deriving sufficient, reliable structural parameters from experimental spectra. The reliability with which a multi-shell analysis (based on computation of the EXAFS spectrum and comparison with experimental data) can produce bond lengths and coordination numbers has been tested. Furthermore, the limitations placed on such an analysis by superimposing typical experimental levels of random noise onto the input data set has been investigated, both in terms of the number of parameters which can be reproduced and their associated errors. Significance tests have also been applied to a multi-shell analysis of experimental SEXAFS measurements of chlorine adsorbed on silver {110}. These tests on both real and simulated data show that a number of (reproducible) bond lengths may be expected from a multi-shell analysis and with a sufficient accuracy to distinguish between shells separated by as little as ≈ 0.3 A ̊ , for a maximum k of 8.7 Å −1. The effective coordination number of the nearest-neighbour shell can be reproduced within an error limit of ±0.2. Large correlations between Debye-Waller factors, coordination numbers and inelastic terms in the EXAFS amplitude place a high degree of inaccuracy, in some cases as high as ±3, on the values obtained for the effective coordination number of higher shells, which cannot by themselves be used to infer the adsorption site geometry of an adatom.

Discrimination between domain wall and uniformly incommensurate structures by surface extended x-ray absorption fine structure: Adsorption of chlorine on Ag{110}

A well-established problem in surface structure determination is the experimental difficulty in distinguishing a uniformly incommensurate structure from a domain wall structure. Surface extended x-ray absorption fine structure (SEXAFS), as a local structural probe, should provide an unambiguous means of making the distinction. We have studied chlorine adsorbed on Ag{110} by SEXAFS at several coverages. The Cl–Ag nearest-neighbor distance, 2.56±0.04 Å, is found to be substantially shorter than on the {111} and {100} faces, and is independent of coverage. At θ=0.75 an incommensurate phase is formed. A multishell analysis (up to 5.6 Å) allowed a complete structural determination and has, for the first time, confirmed our expectation. The Cl–Cl spacing unambiguously favors a domain wall structure. Structural models have been tested by laser simulation experiments, and by kinematic analysis of the low-energy electron diffraction data.

Oxygen and carbon isotopes of individual shells of planktonic foraminifera from Ontong-Java plateau, equatorial pacific

Analyses of single shells of Orbulina universa and two other species show ranges of 2‰ in δ 18O and δ 13C at various levels in a box core from the western equatorial Pacific. Seasonal and inter-annual variations can account for only about half of this range; variations in depths of calcification and in metabolic effects must account for the rest. The large range must introduce considerable noise in multi-shell analysis unless the number of shells in the sample is of the order of 50 or more. We did not find a change in the range downcore; nor did we find any evidence for the deglacial “meltwater spike”.

Investigation of pointed domes—I. Theory

A theoretical method of shell analysis has been utilized and, with the help of certain innovations, the study of pointed domes subjected to pointed loading is presented in this paper. To derive an analytical expression for the pointed dome, the general theory of shell of revolution is used to take into account the effect of membrane and bending stress field. Additionally the general classical approach of analytical multi-shell analysis is utilized to obtain the solution. This solution is applied to a pointed dome with a given dimension and the membrane stress field was calculated. The results thus obtained give a good approximation of the internal force field in the central portion of the dome. However this is not true near the lower boundary and near the kinked vertex of the dome. The values of internal meridional bending moment and meridional shear have also been calculated. These indicate that the bending field diminishes in magnitude as we move away from the kinked region.