John G. Barker

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.

SANS Polarization Analysis with Nuclear-Spin-Polarized 3He

A neutron spin filter based on transmission through nuclear-spin-polarized 3He gas has been applied to polarization analysis of small angle neutron scattering (SANS). Such spin filters, which are based on the large spin dependence of the absorption of neutrons by 3He, make SANS polarization analysis possible because of their large angular acceptance. In the present experiment, a 3He-based analyzer was employed to separate nuclear scattering into its coherent and spin-incoherent components. Polarized 3He analyzers were prepared by two different optical pumping methods and installed on the NG3 SANS instrument at the NIST Center for Neutron Research (NCNR). Measurements were taken on cellophane tape and silica gel, for which the scattering is almost completely incoherent and coherent, respectively, and on a combined sample. For the combined sample, separation of the coherent part from the incoherent part was successfully demonstrated using polarization analysis.

Neutron transmission of single-crystal magnesium fluoride

The neutron attenuation through single-crystal magnesium fluoride has been measured as a function of wavelength at both room temperature and 77 K. These data confirm a total cross section that is lower than that of MgO for wavelengths greater than 0.2 nm. MgF2 cooled to 77 K is a slightly better filter of epithermal neutrons than MgO for obtaining a thermal neutron beam.

Effect of re-wetting on silica aerogel structure: a SANS study

Small angle neutron scattering (SANS) has been used to study (1) the ability to refill the pores of a silica aerogel (ρ=0.3 g/cm3) with water and (2) any effect of the water and the re-wetting process on the aerogel structure on the 1 nm to 100 nm scale. A sample filled with a D2O:H2O solution which was chosen to match the scattering properties of silica shows no scattering at length scales <40 nm down to the detection limit (~2 nm) of the experiment. This lack of scattering demonstrates that the mesopores of the silica aerogel were successfully filled with the contrast-matching fluid. SANS spectra for samples filled with air or D2O are similar, indicating that the aerogel structure as detected by this technique is not affected by the reintroduction of water into the pores. Results suggest that some aerogels may be suited for applications in aqueous environments such as catalysis or chemical sensing.

Survey of background scattering from materials found in small-angle neutron scattering

The paper describes quantitative methods for determining the background scattering from materials, including gases, liquids and solids, found in small-angle neutron scattering measurements. By understanding the dependence of the background on material type, thickness, wavelength and temperature, the investigator can design experiments that minimize the background scattering and thus improve the statistical accuracy of the background-corrected data.

DCD USANS and SESANS: a comparison of two neutron scattering techniques applicable for the study of large‐scale structures

This paper provides a comparison of the capabilities of two techniques for extending the range of conventional small-angle neutron scattering (SANS) towards the micrometre length scale, namely the double-crystal diffraction ultra-small-angle neutron scattering (DCD USANS) technique, which uses perfect silicon crystals in Bragg reflection, and spin-echo SANS (SESANS), a method that uses the spin precessions of a polarized neutron beam. Both methods encode the scattering angle to very high precision. Based on round-robin test measurements, the strengths and weaknesses of the two techniques are discussed with respect to the measurement of the particle size of monodisperse scatterers, and potential performance gains for state-of-the-art DCD USANS and SESANS instruments are investigated.

The morphology of nanoscale deposits of rutheniumoxide in silica aerogels

Abstract Ruthenium oxide was deposited on the internal surfaces of silica aerogel by solution impregnation of a Ru(III)tris-acetylacetonate solution followed by reduction and oxidation. The structure of the Ru phase was determined by contrast-matching SANS, which allows independent collection of scattered intensity from SiO2 and Ru oxide components of composite samples. The structure of the Ru oxide deposit depends strongly on the oxidation temperature and very little on the reduction temperature. The Ru oxide forms particles, rather than a conformal layer, which coarsen from a few nanometers after oxidation at room temperature to several tens of nm after oxidation at 250 °C.

Characterization of multi-phase aerogels by contrast-matching SANS

Contrast-matching small-angle neutron scattering (SANS) has been used to independently characterize the structures of two-phase aerogels consisting of a metal oxide supported on silica aerogel. Ru oxide was formed on silica aerogel by oxidation at 400°C of ruthenocene, impregnated from the vapor phase, or Ru(III)acetylacetonate, impregnated via an acetone solution. Fe oxide was formed by vapor-phase impregnation of ferrocene, followed by reduction at 400°C and exposure to ambient air. The Ru oxide forms particles several tens of nanometers in diameter in the aerogel mesopores. The Fe oxide forms a coating which mimics the structure of the support. The silica aerogel structure is not affected by the deposition and heat treatment processes.

Structure of Ru–Ti oxide aerogels: a SANS study

Small-angle neutron scattering (SANS) has been used to characterize Ru–Ti oxide aerogels annealed under various atmospheres and temperatures. The pores were filled with H2O/D2O solutions that match the scattering properties of either RuO2 or TiO2, therefore scattering from the remaining unmatched phase is measured independently. The structure of the RuO2 and TiO2 components of the aerogel detected by SANS depends more on annealing temperature than annealing atmosphere. The results are consistent with a branched, polymeric network made up of 8–16 nm TiO2 particles and <5-nm RuO2 particles. The SANS results are supported by TEM. This study demonstrates that contrast-matching SANS is complementary to other techniques for characterizing the structure of two-phase aerogels.

Swelling effects in semidilute block copolymer solutions

Small‐angle neutron scattering is used to investigate swelling effects in disordered block copolymer solutions in the semidilute regime. Results are compared with Leibler’s mean‐field analysis for incompressible copolymer melts rescaled by Olvera to account for chain swelling. Semidilute solutions of two copolymers in toluene are considered: poly(styrene‐b‐methyl methacrylate) and poly(methyl methacrylate‐b‐methyl methacrylate‐d8). The copolymers are found to exhibit the predicted scaling behavior for the characteristic crossover distance (or blob size) as a function of copolymer concentration, φ. However, the shift in the position of the scattering maximum, q*, is seen to have a weaker dependence on concentration than that predicted theoretically. For both systems investigated the peak position scales approximately with concentration as q*∼φ0.05.