R. Berliner

Curved crystal optics and the resolution formalism: programs and optimization procedures

Abstract The problem of computing the neutron optics of curved-crystal spectrometers is reviewed. The conditions of focusing in scattering and the limitations to achievable resolutions are discussed. Experience with programs for resolution-intensity optimization is presented. Computational examples are given of high resolution configurations with bent crystals and open beams, showing significant intensity gain over conventional techniques. Quasi-elastic scattering measurements at energy transfer resolutions below 10 μeV are shown to be feasible at high luminosity. Curved monochromators for high resolution neutron powder diffraction, combining the beam focusing onto sample with focusing in scattering, are also discussed.

Development and testing of a flash analog-to-digital converter based system for pulse shape discrimination of nuclear radiation pulses

A digital signal processor (DSP) based system is under development for analyzing and processing pulses produced by radiation detectors. The system is designed to replace conventional pulse-type, analog-to-digital converters. It is capable of capturing the complete radiation induced pulse in digital form. Subsequently, all the pulse features can be analyzed digitally (e.g., pulse validation, energy information, dynamic threshold determination, pulse duration, pulse shape analysis, and noise reduction). A prototype system has been built and performance parameters have been evaluated. >

Two-bent-crystal technique in neutron small angle scattering

The design of two-bent-crystal small angle neutron scattering configurations is discussed. Neutron optics computations for experiment optimization are presented. An arrangement implemented at the Missouri University Research Reactor is described. It consists of a curved pyrolytic graphite premonochromator followed by two curved thin silicon wafers diffracting from (111) and (220) planes, respectively.

Application of neutron scattering to Portland cement

Abstract Portland cement concrete is actually a composite material consisting of fine and coarse stone aggregate in a matrix of hydrated calcium silicates and aluminates. The aggregate to binder mass ratio is typically 3: 1. At a price of roughly 0.05 US dollars per kg, concrete is often the lowest cost material for construction, and hence it is widely used. In the United States, the annual consumption of Portland cement is approximately 80 million metric tons, or roughly 500 million metric tons of concrete. The largest single market is streets and highways which accounts for 30 percent of total consumption 1.11. Consequently, transportation agencies like the Federal Highway Administration have a major interest in research to better understand and improve this material.

Position-sensitive analysis in curved-crystal three-axis neutron spectrometry (quasielastic scattering case)

The use of bent perfect crystals in focusing three-axis neutron spectrometry with position-sensitive detection (PSD) is analyzed on the basis of a phase-space theory. With PSD, the usual sequential scans are replaced by simultaneous scans. The case of high resolution in energy transfer is considered in detail. With commercial thin silicon wafers, the achievable resolutions are in the 10–150 µeV range, depending on neutron energy. Resolutions around 10 µeV are obtained on the peak of cold-source spectra. To take advantage of the possibilities offered by silicon wafers, PSD with spatial resolution well below 1 mm will be needed. With PSD analysis, a new kind of focusing exists that allows the thickness of bent perfect analyzer crystals to be increased, providing an intensity gain at no resolution loss. With multilamella assemblies a gain in count rate by the number of lamellae in the packet is achievable. Results of a demonstration experiment are presented, confirming that under the right conditions, a multilamella analyzer may resemble a single bent wafer, the individual curves of many wafers having been combined to provide the intensity gain.

A perfect match for high resolution neutron powder diffraction: Position sensitive detection and focusing monochromators

Abstract Position sensitive detectors (PSDs) have been in use at MURR for nearly 20 years, while focusing monochromators have been developed only during the past six years. Incremental improvements in both systems have led to a high resolution powder diffraction system with a fast data acquisition rate for small samples (typically one gram of material). This is achieved by matching sample size to detector spatial resolution, by focusing in real space (two dimensional) and focusing in scattering with bent silicon monochromators. Fully open beams and large area detectors make the performance of this method superior to the conventional one with soller collimators and flat mosaic monochromators (vertically focusing). The use of inexpensive perfect Si crystals and linear position sensitive elements as well as the elimination of multiple Soller slit assemblies also make this an extremely economical alternative. Improvements in electronics, data acquisition software and new, doubly focusing devices have opened the door to widespread use of high resolution powder diffraction at small and medium flux facilities.

Investigation of Portland Cement and Pure C 3 S Using 1-D Sans

The effect of contrast matching in ordinary Portland cement (OPC) and pure C{sub 2}S has been investigated as a function of time after hydration using the 1-D small-angle scattering technique. The observed scattering response of OPC hydrated with 100% D{sub 2}O increased in time, while the response of OPC hydrated with a null water mixture did not. The increase in scattering is therefore attributed to the formation of bubbles in the water-filled pores of the hydrating cement paste. The addition of 1% CaCl{sub 2} to OPC increased the kinetics of bubble formation considerably. Based on similar contrast measurements, the authors have concluded that bubbles do not form in pure C{sub 3}S. However, the addition of 1% CaCl{sub 2} was found to induce bubble formation in the hydrated C{sub 3}S compound.

SANS wavelength selection using a multilayer neutron monochromator

A small angle neutron scattering instrument is currently being designed at the University of Missouri Research Reactor. An anti-parallel arrangement of two Ni-Ti multilayers will serve as a neutron monochromator for this instrument. The first multilayer will be flat and in soller, or transmission, geometry. The second will be curved and in the conventional reflection geometry. The curvature of the second multilayer is designed to eliminate resolution asymmetry and increase the incident neutron intensity. The intensities are still too low with this arrangement, but can be increased two or threefold by depositing a second or third bilayer group with a spacing slightly offset from the first group.

High-resolution neutron scattering with commercial thin silicon wafers as focusing monochromators

Abstract Quasielastic scattering measurements with commercial silicon wafers as focusing monochromator and analyzer in a three-axis spectrometer showed energy-transfer resolutions in the 10–100 μeV range (fwhm). Resolutions were better and intensities higher than in a conventional arrangement with Soller collimators and pyrolytic graphite (PG) monochromator and analyzer. Resolution remained high when extended-plate samples were used in focusing orientation. At cold sources, this technique would give 10–20 μeV resolutions at neutron energies near the peak of the spectrum. Projections of resolution ellipsoids were determined by diffraction from powder samples. The orientations of the ellipsoids are controlled by horizontal curvatures and can be rotated 90° for Q -resolution. A monochromator unit with remote control of horizontal curvature and fixed vertical curvature set to spatial focusing at the sample position was also tested. The strong vertical focusing gave a significant gain in integrated intensities, but worsened the resolution because of second-order aberrations.