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[en] Small angle neutron scattering (SANS) was used to investigate interaction forces between hemoglobin (Hb) molecules contained within human red cells. The scattering separately attributable to cell membranes and intracellular Hb was identified. A series of D₂O-H₂O contrast variation measurements were made in order to establish conditions for which scattering from the cell membrane is minimized (approximately 15% D₂O). Measurements then were performed to examine changes in intermolecular Hb interactions occurring when the cells are contracted or swollen by varying the ionic strength of the suspension buffer. The scattering cross-sections were fitted to structure factors computed by a mean spherical approximation, and molecular parameters thereby extracted. Oxygenation studies on normal cells were performed, and results contrasted with those of similar studies of erythrocytes obtained from sickle cell disease patients

[en] We applied SANS to study nanoscale structures formed from tubulin and either of two similar peptides, cryptophycin1 and dolastatin10. A simple bead ring model adequately describes the cryptophycin-tubulin data, indicating high monodispersity and non-association of the rings. In contrast, dolastatin-tubulin samples show secondary assembly of larger structures. Analysis indicates that these macrostructures may contain locally stacked rings

[en] The product k p c is the relevant thermophysical parameter for the heating up of material walls in contact with plasma and, apart from the emissivity, it is also the decisive thermal parameter for the quantitative IR thermography; here k, p, and c are the thermal conductivity, density and specific heat capacity, respectively, of the wall material. The Photoacoustic Calorimetry (PAC) provides the diagnostic means to measure this quantity k p c directly. First results of PAC measurements with graphite limiter plates exposed in ASDEX to about 600 discharges are presented and compared to the unused graphite material. Further prospects of the nondestructive photoacoustic techniques in plasma-surface interaction problems are discussed, in relation to coatings/claddings and redeposition layers. (orig.)

[en] Graphite limiters which had been incorporated in the Tokamaks ASDEX and JET were analysed by frequency- dependent photoacoustics. A depth dependence was found for the effusivity (kρc)^1/2, the thermal parameter governing transient surface heating. The thermal depth profile observed before plasma exposure is related to the surface roughness and porosity. After exposure to the plasmas and depending on the position of the sample in the Tokamaks, changes of the effusivity to both lower and higher values are measured and may be related to lattice damage from energetic particle bombardment and to morphological modifications in regions which experienced higher heat loads. (orig.)

[en] A brief review is given of a frequency-dependent thermal wave technique with audio-acoustic detection which is used to measure the thermal properties of solid state samples in comparison to reference materials of known properties. Two artificial graphites (Schunk and Ebe FP1591 and Carbone Lorraine 5890PT) which have been used as limiter materials in the tokamak devices ASDEX and JET are analysed. A depth-dependence is found for the effusivity √kρc, the thermal parameter governing the temperature rise of the limiter surface after short intense heat pulses. The absolute value of the effective effusivity at the surface and that of the subsurface layers are determined. This thermal depth profile can be represented by a three-layer model and is related qualitatively to the surface roughness and volume porosity of the graphite plates. (orig.)

[en] The graphite limiters have to absorb heat loads of the order of 10 MW/m² with pulse durations up to a few seconds. With the help of a non-destructive and non-contact thermal wave technique with audio-acoustic detection, the effective thermal structures of two graphite limiter samples (Schunk and Ebe FP159I, and Carbone Lorraine 5890PT) have been analysed. (orig./MM)

[en] Counterions are critical to the self-assembly of RNA tertiary structure because they neutralize the large electrostatic forces which oppose the folding process. Changes in the size and shape of the Azoarcus group I ribozyme as a function of Mg²⁺ and Na⁺ concentration were followed by small angle neutron scattering. In low salt buffer, the RNA was expanded, with an average radius of gyration (R_g) of 53 ± 1 Angstroms. A highly cooperative transition to a compact form (R_g = 31.5 ± 0.5 Angstroms) was observed between 1.6 and 1.7 mM MgCl₂. The collapse transition, which is unusually sharp in Mg²⁺, has the characteristics of a first-order phase transition. Partial digestion with ribonuclease T1 under identical conditions showed that this transition correlated with the assembly of double helices in the ribozyme core. Fivefold higher Mg²⁺ concentrations were required for self-splicing, indicating that compaction occurs before native tertiary interactions are fully stabilized. No further decrease in Rg was observed between 1.7 and 20 mM MgCl₂, indicating that the intermediates have the same dimensions as the native ribozyme, within the uncertainty of the data (± 1 Angstroms). A more gradual transition to a final R_g of approximately 33.5 Angstroms was observed between 0.45 and 2 M NaCl. This confirms the expectation that monovalent ions not only are less efficient in charge neutralization but also contract the RNA less efficiently than multivalent ions

[en] Multiple small-angle neutron scattering was used to characterize the microstructure evolution of porous silica as a function of thermal processing. This new technique offers a statistically significant determination of microstructure morphology in the 0.08-10 μm range, which was previously inaccessible without increasing the resolution of the currently available scattering spectroscopy beamlines. All of the scatterers, which in the present work are pores within ceramic bodies, are measured whether they are open or closed. Earlier mercury porosimetry and nitrogen desorption measurements of the microporous silica system indicated that there are two major populations of pores in this material, differing in pore size by approximately an order of magnitude. In this work, it was found that densification during the intermediate stages of sintering is accompanied by coarsening to radii >0.22 μm within the population of large pores in microporous silica. In the late stages of sintering, there are no detectable pores with radii >0.08 μm, and radii of gyration equal to 33 and 28 nm were measured. Finally, Monte Carlo simulations were carried out to complement the neutron-scattering measurements, to predict the influence on the results of the bimodal distribution, and to explore the sensitivity of the multiple-scattering method. (orig.)

[en] The small angle neutron scattering technique is a valuable method for the characterization of morphology of various materials. It can probe inhomogeneities in the sample (whether occurring naturally or introduced through isotopic substitution) at a length scale from the atomic size (nanometers) to the macroscopic (micrometers) size. This work provides an overview of the small angle neutron scattering facilities at the National Institute of Standards and Technology and a review of the technique as it has been applied to polymer systems, biological macromolecules, ceramic, and metallic materials. Specific examples have been included