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Results for “Scattering, Small Angle”

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PubMed2006

Detrended fluctuation analysis in x-ray photon correlation spectroscopy for determining coarsening dynamics in alloys.

We study the dynamics of precipitate coarsening in phase-separating alloys at late stages of phase separation by x-ray photon correlation spectroscopy (XPCS). For analyzing time series of fluctuating speckle intensities from small-angle scattering of coherent x rays, the method of detrended fluctuation analysis (DFA), which is ideal for determining power-law correlations, is applied. We discuss the application of DFA with respect to XPCS data by means of simulated time series. In particular, the effects of different signal-to-noise ratios are examined. Results from measurements of the two model systems Al-6 at. % Ag at 140 degrees C and Al-9 at. % Zn at 0 degrees C are presented. Since the DFA effectively removes adulterating trends in the data, quantitative agreement with Monte Carlo simulations is obtained. It is verified that two different coarsening mechanisms are predominant in the two systems--coarsening either by diffusion of single atoms or by movement of whole precipitates.

Journal Article↗

PubMed2000

Resonant Rayleigh scattering of exciton-polaritons in multiple quantum wells.

A theoretical concept of resonant Rayleigh scattering (RRS) of exciton-polaritons in multiple quantum wells (QWs) is presented. The optical coupling between excitons in different QWs can strongly affect the RRS dynamics, giving rise to characteristic temporal oscillations on a picosecond scale. Bragg and anti-Bragg arranged QW structures with the same excitonic parameters are predicted to have drastically different RRS spectra. Experimental data on the RRS from multiple QWs show the predicted strong temporal oscillations at small scattering angles, which are well explained by the presented theory.

Journal Article↗

PubMed2002

Mobility transition of solid rare gases in confined environments.

We report the results of x-ray diffraction and small angle scattering studies of Ar and Kr confined in sol-gel and Vycor glasses. The confined liquid crystallizes in a disordered hcp structure on freezing. Upon further cooling a sharp transition occurs at a reduced temperature of T/T(m) approximately 0.65, where the crystalline structure disappears and the total scattering decreases. This behavior marks the onset of a well-defined mobility transition, where the confined sample migrates out of the pore space.

Journal Article↗

PubMed2004

Dynamical scaling of the structure factor of some non-Euclidean systems.

Predictions of nonlinear theories on dynamics of new phase formation have been examined for the hydration of calcium silicates with light water and heavy water. In the case of hydration with light water, reasonable agreement has been observed with dynamical scaling hypothesis with a new measure of the characteristic length. The characteristic length does not follow a power law relation with time. Hydrating mass is found to be mass fractal throughout hydration, with mass fractal dimension increasing with time. But, in the case of hydration with heavy water, no agreement has been observed with the scaling hypothesis. Hydrating mass undergoes transition from mass fractal to surface fractal and finally again to mass fractal. The qualitative features of the kinetics of hydration, as measured in small-angle scattering experiments, are strikingly different for hydration with light water and heavy water.

Journal Article↗

PubMed2006

Analytical study of diffusive relativistic shock acceleration.

Particle acceleration in relativistic shocks is studied analytically in the test-particle, small-angle scattering limit, for an arbitrary velocity-angle diffusion function D. The particle spectral index s is found to be sensitive to D, particularly downstream and at certain angles. The analysis, confirmed numerically, justifies and generalizes previous results for isotropic diffusion. It can be used to test collisionless shock models and to observationally constrain D. For example, strongly forward- or backward-enhanced diffusion downstream is ruled out by gamma-ray burst afterglow observations.

Journal Article↗

PubMed2001

Analysis of chord-length distributions.

A closed-form analytical solution for the inversion of the integral equation relating small-angle scattering intensity distributions of two-phase systems to chord-length distributions is presented. The result is generalized to arbitrary derivatives of higher order of the autocorrelation function and to arbitrary projections of the scattering intensity (including slit collimation). This inverse transformation offers an elegant way to investigate the impact of certain features, e.g. singularities, in the chord-length distribution or its higher-order derivatives on the scattering curve, e.g. oscillatory components in the asymptotic behavior at a large scattering vector. Several examples are discussed.

Journal Article↗

PubMed2004

A large-area CMOS imager as an X-ray detector for synchrotron radiation experiments.

The performance of a CMOS flatpanel imager from Hamamatsu Photonics (C7942) has been tested in various synchrotron radiation experiments. This detector has a detection area of about 120 mm x 120 mm with 0.05 mm pixels, and a frame rate of 2 s(-1). The commercially available product was insensitive to X-rays with an energy lower than 15 keV, but slight modifications solved this problem. Images obtained in small-angle scattering, protein crystallography and medical imaging experiments were all of high quality. The fast readout and the large area are advantageous in real-time imaging. Although its noise level is higher than the area detectors that are currently used in synchrotron radiation experiments, it is particularly useful in experiments where other bulky detectors cannot be used. Its relatively low price (about 30,000 US dollars) makes it a unique option in the choice of detectors.

Animals↗

PubMed2005

Troika II: a versatile beamline for the study of liquid and solid interfaces.

The Troika II beamline at the European Synchrotron Radiation Facility was conceived as a versatile beamline for the study of liquid and solid interfaces, combining grazing-incidence diffraction, X-ray reflectivity and grazing-incidence small-angle scattering in a single instrument. Scattering experiments can be performed both in horizontal and in vertical scattering geometry. Additional options are the use of analyzer crystals for high-resolution studies in both the horizontal and the vertical scattering geometry as well as the use of a horizontal microfocusing mirror for experiments requiring very high flux onto the sample. Here, the way in which the features of the beamline have been exploited in selected recent experiments is described.

Equipment Design↗

PubMed2005

Structure and mechanical properties of carbon fibres: a review of recent microbeam diffraction studies with synchrotron radiation.

Hard X-ray beams with beam sizes in the sub-micrometre range are frequently available at third-generation synchrotron radiation sources, enabling new insights into the structure of materials at different levels of hierarchy by applying novel techniques such as scanning microbeam diffraction or scanning small-angle scattering. The high brilliance of the radiation from wiggler and undulator sources allows in situ experiments such as mechanical testing of single fibres to be performed, and even the combination of microbeam scanning with in situ testing is feasible. Three different experiments on single carbon fibres are presented: an in situ tensile test using a 10 microm collimated beam, a scanning experiment applying a 3 microm beam from a tapered glass capillary, and a scanning experiment with simultaneous in situ bending with a 0.1 microm beam provided by a waveguide structure.

Journal Article↗

PubMed1998

D2AM, a beamline with a high-intensity point-focusing fixed-exit monochromator for multiwavelength anomalous diffraction experiments.

D2AM is a french CRG beamline installed at the ESRF (European Synchrotron Radiation Facility) in Grenoble, with half of the time dedicated to biological macromolecule crystallography and half to materials science studies (diffraction, wide-angle and small-angle scattering). It is constructed at the front-end BM02 of the ESRF storage ring, using the X-ray beam from a 0.8 T bending magnet. D2AM entered into routine operation at the end of 1994, and is used either for single-wavelength or for multiwavelength anomalous diffraction studies. The beam is monochromated by an Si[111] two-crystal monochromator with a resolution of about 2 x 10(-4). The first crystal is water cooled. The X-ray photon energy covers the range between 6.5 keV (lambda approximately 1.9 A) and 17 keV (lambda approximately 0.7 A), a domain of energy with many K or L absorption edges of heavy atoms of interest for biological macromolecules studies and in materials science. The X-ray beam is focused in the vertical plane by two long curved mirrors and in the horizontal plane by the second crystal of the monochromator which is given an adjustable sagittal curvature. A spot size of 0.3 x 0.1 mm (FWHM) is measured at the sample position. Both mirrors are cut out of a 6"-diameter 1.1 m-long Si single crystal, polished and coated with a 400 A Pt thin film. The rugosity is better than 4 A r.m.s. and the longitudinal slope error is better than 5 x 10(-6) rad r.m.s. The first mirror is water cooled, the second is not. The beam intensity on the sample is about 10(11) photon s(-1) on a 0.3 x 0.3 mm focus area at 100 mA in the storage ring of the ESRF. The harmonic rejection ratio obtained with the two mirrors is better than 10(-5) for lambda/3. The combined optical system, mirror/monochromating-crystals/mirror, used on D2AM constitutes altogether a high-intensity point-focusing fixed-exit monochromator, which has the additional property that the energy resolution is not dependent on the beam divergence in use. Its stability and resolution are perfectly adapted to multiwavelength anomalous diffraction studies. The alignment of the mirrors and the monochromator is fully automated, taking 5 min, with the exception of the adjustment of the sagittal focusing. During multiwavelength diffraction experiments the wavelength is changed by a fast single monochromator rotation. Neither realignment of the mirrors nor readjustment of the beam focusing are necessary. The stability and reproducibility of the selected X-ray photon energy is better than 0.5 eV.

Journal Article↗

PubMed2000

A model of troponin-I in complex with troponin-C using hybrid experimental data: the inhibitory region is a beta-hairpin.

We present a model for the skeletal muscle troponin-C (TnC)/troponin-I (TnI) interaction, a critical molecular switch that is responsible for calcium-dependent regulation of the contractile mechanism. Despite concerted efforts by multiple groups for more than a decade, attempts to crystallize troponin-C in complex with troponin-I, or in the ternary troponin-complex, have not yet delivered a high-resolution structure. Many groups have pursued different experimental strategies, such as X-ray crystallography, NMR, small-angle scattering, chemical cross-linking, and fluorescent resonance energy transfer (FRET) to gain insights into the nature of the TnC/TnI interaction. We have integrated the results of these experiments to develop a model of the TnC/TnI interaction, using an atomic model of TnC as a scaffold. The TnI sequence was fit to each of two alternate neutron scattering envelopes: one that winds about TnC in a left-handed sense (Model L), and another that winds about TnC in a right-handed sense (Model R). Information from crystallography and NMR experiments was used to define segments of the models. Tests show that both models are consistent with available cross-linking and FRET data. The inhibitory region TnI(95-114) is modeled as a flexible beta-hairpin, and in both models it is localized to the same region on the central helix of TnC. The sequence of the inhibitory region is similar to that of a beta-hairpin region of the actin-binding protein profilin. This similarity supports our model and suggests the possibility of using an available profilin/actin crystal structure to model the TnI/actin interaction. We propose that the beta-hairpin is an important structural motif that communicates the Ca2+-activated troponin regulatory signal to actin.

Amino Acid Sequence↗

PubMed1978

Studies on the conformational properties of the high-mobility-group chromosomal protein HMG 17 and its interaction with DNA.

The conformation of the non-histone chromatin protein, HMG 17, has been studied using circular dichroism, infrared and nuclear magnetic resonance spectroscopies, and by small-angle scattering. The results show that in free solution this protein has little or no secondary or tertiary structure in contrast to the other high-mobility-group proteins, HMG 1 and 2, which exhibit highly ordered structures. Protein HMG 17 binds to calf thymus DNA in an ionic-dependent manner, precipitating the DNA at high protein/DNA ratio. The nuclear magnetic resonance data suggest that the principle DNA-binding segment of HMG 17 is that between about residues 15 and 40.

Animals↗

PubMed2002

Diffraction enhanced imaging contrast mechanisms in breast cancer specimens.

We have investigated the contrast mechanisms of the refraction angle, and the apparent absorption images obtained from the diffraction enhanced imaging technique (DEI) and have correlated them with the absorption contrast of conventional radiography. The contrast of both the DEI refraction angle image and the radiograph have the same dependence on density differences of the tissues in the visualization of cancer; in radiography these differences directly relate to the contrast while in the DEI refraction angle image it is the density difference and thickness gradient that gives the refraction angle. We show that the density difference of fibrils in breast cancer as measured by absorption images correlate well with the density difference derived from refraction angle images of DEI. In addition we find that the DEI apparent absorption image and the image obtained with the DEI system at the top of the reflectivity curve have much greater contrast than that of the normal radiograph (x8 to 33-fold higher). This is due to the rejection of small angle scattering (extinction) from the fibrils enhancing the contrast.

Absorption↗

PubMedDate not supplied

Characterization of tissue via coherent-to-Compton scattering ratio: sensitivity considerations.

It is known that the ratio (R) of the detected coherent and Compton scattered photons from bone can be used in order to determine its mineral density. This technique utilizes the dependence of the coherent scattering on the effective atomic number (Z) of the scattering medium. It is generally accepted that a small scatter angle is preferred in order to ensure adequate counting statistics by favoring the detection of more coherent photons. Moreover, it has been assumed that a change in the scatter angle does not affect the sensitivity of the measurement. Our theoretical calculations for 60-keV photons and for the range of Z that corresponds to trabecular bone, indicate that increasing the scatter angle results in a stronger power dependence of the measured ratio on Z. This implies that by increasing the scatter angle, smaller changes in the mineral density can be detected, thus improving the sensitivity of the measurement. This effect was investigated experimentally by using a collimated beam of 59.54-keV photons from Am-241 (44.4 GBq) and a collimated intrinsic germanium detector. Solutions of K2HPO4 with different concentrations were used in order to simulate trabecular bone. The scatter spectra were recorded for all solutions at six scatter angles between 37 degrees and 98 degrees and the value of R was computed for each spectrum. The sensitivity of the measurement, evaluated from these experiments increased, with the increase of the scatter angle.

Americium↗

PubMed1986

X-ray diffraction from magnetically oriented solutions of macromolecular assemblies.

A simple system was developed for obtaining x-ray diffraction patterns from magnetically oriented solutions of macromolecular assemblies. A small permanent magnet was designed that produces a magnetic field of 16 kilogauss in a volume of 1 cubic millimeter and is mountable on most x-ray cameras. Many subcellular structures have sufficient diamagnetic anisotropy that they exhibit orientation in dilute solution when placed between the poles of the magnet. Diffraction from solutions oriented in this magnet can provide substantially more structural information than small-angle scattering from isotropic solutions. In favorable cases, such as dilute solutions of filamentous bacteriophages, it is possible to produce oriented fiber diffraction patterns from which intensities along layer lines can be measured to 7-angstrom resolution. The magnetically induced birefringence observed in solutions of other macromolecular assemblies suggests that this technique may have broad applicability to subcellular structures.

Bacteriophages↗

PubMed2004

Shear-induced concentration fluctuations and form factor changes in polymer solution in the good-solvent regime.

Small-Angle Scattering from sheared semidilute polymer solution is reported in the good-solvent regime, at variance with former light and neutron measurements in the theta regime. First, concentration fluctuations are observed: the scattering increases noticeably along the flow at low q, but at variance with former results for the theta-solvent regime, no demixing is observed at higher shear. Here, instead, the effects follow a time-temperature superposition and saturate above a Weissenberg number around 5, like the stress which is known to present a plateau for these systems. Using the Zero Average Contrast technique, we have also measured the form factor, which displays the same saturation effect reaching a deformation ratio of the order of 2. These results agree with the Convective Constraint Release models (CCR) elaborated in order to predict the stress effects in the non-Newtonian regime (Marrucci-Ianniruberto) and their extension predicting also the scattering (Likhtman-Milner-McLeish).

Biophysics↗

PubMed2000

Large-scale shape changes in proteins and macromolecular complexes.

Proteins and RNA undergo intricate motions as they carry out functions in biological systems. These motions frequently entail large-scale conformational changes that induce changes in the surface structure, or shape, of a molecule. This review describes the experimental characterization of large-scale shape changes in proteins and macromolecular complexes and the effects of such changes on macromolecular behavior. We describe several important results that have been obtained by using small-angle scattering, which is emerging as a powerful technique for determining macromolecular shapes and elucidating the quaternary structure of macromolecular assemblies.

Animals↗

PubMed2002

Investigation into the interaction of recombinant human serum albumin with Re-lipopolysaccharide and lipid A.

The interaction of bacterial endotoxins, deep rough mutant lipopolysaccharide LPS Re and the 'endotoxic principle' lipid A, with recombinant human serum albumin (rHSA) was investigated with a variety of physical techniques and biological assays. With Fourier-transform infrared spectroscopy and differential scanning calorimetry, the influence of albumin on the acyl chain melting behavior of the endotoxins was measured. Also, the effect on the functional groups of the endotoxins, in particular with respect to their orientation, was studied, including competition experiments with polymyxin B. Furthermore, the influence of endotoxin binding to rHSA on the protein's secondary structure was investigated. The results indicate a non-electrostatic binding with no change of the backbone orientation of LPS and only a slight change of the secondary structure of rHSA. Correspondingly, the amount of charge neutralization of the endotoxins due to rHSA measured by the electrophoretic mobility exhibited only a slight reduction of the surface potential. From these measurements and isothermal titration calorimetry, the lipid:protein binding stoichiometry was estimated to [LPS]:[rHSA], 10:1 molar. The determination of the aggregate structure of the endotoxins by X-ray small-angle scattering exhibited a complex change of a cubic into a non-lamellar structure. No influence of rHSA on endotoxin intercalation into phospholipid liposomes induced by lipopolysaccharide-binding protein could be detected by fluorescence resonance energy transfer. Finally, the LPS-induced cytokine production of human mononuclear cells was only slightly increased at high molar rHSA excess, while the coagulation of amebocyte lysate in the Limulus test yielded a complex change due to rHSA binding of LPS.

Algorithms↗