Charles J. Glinka

The 30 m small-angle Neutron scattering instruments at the National Institute of Standards and Technology

Two high-resolution, general-purpose, small-angle neutron scattering instruments have been constructed at the National Institute of Standards and Technologys Center for Neutron Research. The instruments are 30 m long and utilize mechanical velocity selectors, pinhole collimation and high-data-rate two-dimensional position-sensitive neutron detectors. The incident wavelength, wavelength resolution and effective length of the instruments are independently variable, under computer control, and provide considerable flexibility in optimizing beam intensity and resolution. The measurement range of the instruments extends from 0.0015 to 0.6 A−1 in scattering wavevector, corresponding to structure in materials from 10 A to nearly 4000 A. The design and characteristics of the instruments, and their modes of operation, are described, and data are presented which demonstrate their performance.

Design and performance of a thermal-neutron double-crystal diffractometer for USANS at NIST

An ultra-high-resolution small-angle neutron scattering (USANS) double-crystal diffractometer (DCD) is now in operation at the NIST Center for Neutron Research (NCNR). The instrument uses multiple reflections from large silicon (220) perfect single crystals, before and after the sample, to produce both high beam intensity and a low instrument background suitable for small-angle scattering measurements. The minimum detector background to beam intensity ratio (noise-to-signal, N/S) for q ≥ 5 × 10 -4 A -1 is 4 × 10 -7 . The instrument uses 2.38 A wavelength neutrons on a dedicated thermal neutron beam port, producing a peak flux on the sample of 17300 cm -2 s -1 . The typical measurement range of the instrument extends from 3 × 10 -5 A -1 to 5 × 10 -3 A -1 in scattering wavevector (q), providing information on material structure over the size range from 0.1 μm to 20 μm. This paper describes the design and characteristics of the instrument, the mode of operation, and presents data that demonstrate the instruments performance.

Focusing cold neutrons with multiple biconcave lenses for small-angle neutron scattering

The focusing of a cold neutron beam by multiple biconcave lenses has recently been proposed as a practical means of extending the lower limit of Q in conventional, long flight-path small-angle neutron scattering (SANS) instruments. To test the feasibility of this approach, we have carried out extensive measurements on one of the 30 m SANS instruments at NIST of the focusing characteristics of a set of 28 biconcave MgF2 lenses. The focused beam profile has been measured over several orders of magnitude using high resolution neutron auto-radiography. The focusing lens configuration outperforms the pinhole collimation at Qmin lower than 0.004 A-1. At Qmin = 0.001 A-1, the intensity gain of the lens configuration over the pinhole collimation is greater than one order of magnitude.

Properties of nanoporous silica thin films determined by high-resolution x-ray reflectivity and small-angle neutron scattering

A new methodology based on a novel combination of a high-resolution specular x-ray reflectivity and small-angle neutron scattering has been developed to evaluate the structural properties of low-dielectric-constant porous silica thin films about one micrometer thick supported on silicon wafer substrates. To complement these results, film composition was determined by high-energy ion scattering techniques. For the example thin film presented here, the overall film density was found to be (0.55±0.01) g/cm3 with a pore wall density of (1.16±0.05) g/cm3 and a porosity of (53±1)%. The characteristic average dimension for the pores was found to be (65±1) A. It was determined that (22.1±0.5)% of the pores had connective paths to the free surface. The mass fraction of water absorption was (3.0±0.5)% and the coefficient of thermal expansion was (60±20)×10−6/°C from room temperature to 175 °C. Lastly, model fitting of the specular x-ray reflectivity data indicated the presence of a thin surface layer with an increa...

Morphology of fast-tumbling bicelles: a small angle neutron scattering and NMR study

Bilayered micelles, or bicelles, which consist of a mixture of long- and short-chain phospholipids, are a popular model membrane system. Depending on composition, concentration, and temperature, bicelle mixtures may adopt an isotropic phase or form an aligned phase in magnetic fields. Well-resolved (1)H NMR spectra are observed in the isotropic or so-called fast-tumbling bicelle phase, over the range of temperatures investigated (10-40 degrees C), for molar ratios of long-chain lipid to short-chain lipid between 0.20 and 1.0. Small angle neutron scattering data of this phase are consistent with the model in which bicelles were proposed to be disk-shaped. The experimentally determined dimensions are roughly consistent with the predictions of R.R. Vold and R.S. Prosser (J. Magn. Reson. B 113 (1996)). Differential paramagnetic shifts of head group resonances of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC), induced by the addition of Eu(3+), are also consistent with the bicelle model in which DHPC is believed to be primarily sequestered to bicelle rims. Selective irradiation of the DHPC aliphatic methyl resonances results in no detectable magnetization transfer to the corresponding DMPC methyl resonances (and vice versa) in bicelles, which also suggests that DHPC and DMPC are largely sequestered in the bicelle. Finally, (1)H spectra of the antibacterial peptide indolicidin (ILPWKWPWWPWRR-NH(2)) are compared, in a DPC micellar phase and the above fast-tumbling bicellar phases for a variety of compositions. The spectra exhibit adequate resolution and improved dispersion of amide and aromatic resonances in certain bicelle mixtures.

SANS Study on the Effect of Lanthanide Ions and Charged Lipids on the Morphology of Phospholipid Mixtures

The structural phase behavior of phospholipid mixtures consisting of short-chain (dihexanoyl phosphatidylcholine) and long-chain lipids (dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol), with and without lanthanide ions was investigated by small-angle neutron scattering (SANS). SANS profiles were obtained from 10 degrees C to 55 degrees C using lipid concentrations ranging from 0.0025 g/ml to 0.25 g/ml. The results reveal a wealth of distinct morphologies, including lamellae, multi-lamellar vesicles, unilamellar vesicles, and bicellar disks.

Spontaneously Formed Monodisperse Biomimetic Unilamellar Vesicles: The Effect of Charge, Dilution, and Time

Using small-angle neutron scattering and dynamic light scattering, we have constructed partial structural phase diagrams of lipid mixtures composed of the phosphatidylcholines dimyristoyl and dihexanoyl doped with calcium ions (Ca2+) and/or the negatively charged lipid, dimyristoyl phosphatidylglycerol (DMPG). For dilute solutions (lipid concentration < or =1 wt %), spontaneously forming unilamellar vesicles (ULVs) were found, and their polydispersity was determined to be approximately 20%. The stability of the Ca2+- or DMPG-doped ULVs was monitored over a period of 4 days and their structural parameters (e.g., average outer radius, ) were found to be insensitive to the lipid concentration (Clp). However, doping the dimyristoyl/dihexanoyl system with both Ca2+ and DMPG resulted in ULVs whose was found to be Clp dependent. The of DMPG-doped ULVs remained unchanged over an extended period of time (at least 4 days), a good indication of their stability.

Shearing apparatus for neutron scattering studies on fluids: Preliminary results for colloidal suspensions

A Couette-type concentric cylinder apparatus to investigate liquids at equilibrium and under shear has been constructed and tested. The apparatus is designed for a neutron facility and is optimized as a general purpose adjunct to the small-angle neutron scattering (SANS) equipment. It is versatile and rugged; a wide range of shear rates and operating temperatures can be covered; and controls are fully automated. Test results with sheared colloidal suspensions of 91-nm polystyrene spheres are presented. Evidence of shear-induced structure changes is clear.

A SANS Study of Organoclay Dispersions

Small-angle neutron scattering (SANS) is used to investigate the dispersion in toluene of various forms of the complex, Cloisite C15A. Cloisite is a commercially important exchanged clay prepared from montmorillonite and the cation di-tallow ammonium. Points discussed include estimates of the extent to which the complex is dispersed, the amount of organic clay platelet surface layer coverage, and possible network formation of the complex in the solvent. From power-law plots of the scattered neutron intensity versus the wave vector, it is estimated that C15A is well dispersed into clusters which consist of a stack of between three and seven di-tallow-coated montmorillonite platelets. The removal of excess di-tallow from the surface layer reduces the number of platelets in the cluster. Substitution of dimethydioctodecyl ammonium for the di-tallow molecule promotes network formation. It is demonstrated that SANS is a powerful tool for examining these complicated organic/inorganic systems.

Critical behavior and magnetic ordering in amorphous TbFe2

The zero field small angle neutron scattering from amorphous TbFe2 above the magnetization‐determined TC =409 K shows a conventional Lorentzian line shape with a spin correlation length which increases to only about 135 A at and just below TC. There is only a weak indication of a cusp in the scattering in the vicinity of TC, and below TC the scattering continues to increase with decreasing temperature, finally saturating below about 40 K. The Lorentzian behavior observed above TC gradually evolves into a power law dependence I∝Q−n with n≊3 for temperatures below about 300 K. This subcritical scattering can also be approximately represented by the sum of Lorentzian (L) and Lorentzian‐squared (L2) terms as suggested for random anisotropy field systems and yields a correlation length of 122 A at 295 K. On application of a field in the range 2–12 kG at 295 K, the overall scattering at low Q is sharply suppressed indicating an increase in the ferromagnetic component with field. The residual magnetic scattering...