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The importance of small-angle neutron scattering (SANS) in biological, chemical, physical, and engineering research mandates that all intense neutron sources be equipped with SANS instruments. Four existing instruments are described, and the general differences between pulsed-source and reactor-based instrument designs are discussed. The basic geometries are identical, but dynamic range is achieved by using a broad band of wavelengths (with time-of-flight analysis) rather than by moving the detector. This allows a more optimized collimation system. Data acquisition requirements at a pulsed source are more severe, requiring large, fast histogramming memories. Data reduction is also more complex, as all wave length-dependent and angle-dependent backgrounds and non-linearities must be accounted for before data can be transformed to intensity vs Q.A comparison is shown between the Los Alamos pulsed instrument and D-11 (Institute Laue-Langevin), and examples from the four major topics of the conference are shown. The general conclusion is that reactor-based instruments remain superior at very low Q or if only a narrow range of Q is required, but that the current generation of pulsed-source instruments is competitive at moderate Q and may be faster when a wide range of Q is required. In principle, a user should choose which facility to use on the basis of optimizing the experiment; in practice the tradeoffs are not severe and the choice is usually made on the basis of availability.
Detailed comparisons of measurements made on small-angle neutron scattering instruments at pulsed spallation and reactor sources show that the results from the two types of instruments are comparable. It is further demonstrated that spallation instruments are preferable for measurements in the mid-momentum transfer domain or when a large domain is needed. 8 refs., 2 figs.
We studied the design and performance of a small-angle neutron scattering (SANS) instrument for a proposed 1 MW, 60 Hz long pulsed spallation source at the Los Alamos Neutron Science Center (LANSCE). An analysis of the effects of source characteristics and chopper performance combined with instrument simulations using the LANSCE Monte Carlo instrument simulations package shows that the T0 chopper should be no more than 5 m from the source with the frame overlap and frame definition choppers at 5.6 and greater than 7 m, respectively. The study showed that an optimal pulse structure has an exponential decaying tail with [tau] ≈ 750 [mu]s. The Monte Carlo simulations were used to optimize the LPSS SANS, showing that an optimal length is 18 m. The simulations show that an instrument with variable length is best to match the needs of a given measurement. The performance of the optimized LPSS instrument was found to be comparable with present world standard instruments.
The applications of the use of cold neutrons for condensed matter research at accelerator-based spallation sources are less well-known than that of the use of cold neutrons at reactor sources. This book represents the outcome of the first international workshop on “Materials Research Using Cold Neutrons at Pulsed Sources”. It consists of overviews of the present status and future trends in research opportunities using cold neutrons, as well as topical papers focusing on the areas of small-angle neutron scattering, reflectivity and spectroscopic studies of a rich variety of materials, from adsorbed molecules to thin films to coal to metallic glasses. These papers will benefit researchers who are interested in the characterization of microscopic properties of advanced materials.
In this chapter, neutron experimental techniques are described. The chapter covers the basics of neutron scattering, neutron source characteristics, diffraction techniques, inelastic neutron scattering, instruments for semi-macroscopic structure analysis, detectors, optics, choppers, and some concepts for instrument design. Techniques for both steady and pulsed sources are described, with more emphasis on the latter in view of the recent worldwide trend toward pulsed sources, generally pulsed spallation sources. The source character, even within the class of pulsed sources, has a significant influence on instrument design. Inelasticity effects in the total scattering technique are clearly specified in the chapter. Some details of chopper instruments, including resolution effects, are described, since this class of instruments has recently received considerable innovative development and use over a wide range of science. Recent developments in scintillation detectors are discussed as an alternative technology to more conventional 3He detectors. Optical components are becoming more and more important not only for neutron transport but also for background reduction. Neutron spin-echo techniques are presented as an example of the exploitation of polarized neutrons.
Neutron and synchrotron facilities, which are beyond the scale of the laboratory, and supported on a national level in countries throughout the world. These tools for probing micro- and nano-structure research and on fast dynamics research of atomic location in materials have been key in the development of new polymer-based materials. Different from several existing professional books on neutron science, this book focuses on theory, instrumentation, an applications. The book is divided into five parts: Part 1 describes the underlying theory of neutron scattering. Part 2 describes the various instruments that exist and the various techniques used to achieve neutron scattering or bombardment. Part 3 discusses data treatment and simulation methods as well as how to assess the environment of the sample (temperature, pressure, shear, and external fields). Part 4 addresses the myriad applications of small and large molecules, biomolecules, and gels. Part 5 describes the various global neutron sources that exist and provides an overview of the different reactors.
The implementation of small-angle (Low-momentum transfer) neutron scattering at pulsed spallation sources, using time of flight methods, has meant the introduction of some new ideas in instrument design, data acquisition, data reduction and computer management of the experiment and the data. Here we recount some of the salient aspects of solutions for implementing time of fight small-angle neutron scattering instruments at pulsed sources, as realized on the Low-Q Diffractometer, LQD, at Los Alamos. We consider, fortlier, some of the problems that are yet to be solved, and take a short excursion into the future of SANS instrumentation at pulsed sources.
Two small-angle neutron scattering instruments have been designed and optimized for installation at a 1 MW long pulse spallation source. The first of these instruments allows access to length scales in materials from 10 to 400 Å, and the second instrument from 40 to 1200 Å. Design characteristics were determined and optimization was done using the MCLIB Monte Carlo instrument simulation package. The code has been {open_quote}benchmarked{close_quote} by simulating the {open_quote}as-built{close_quote} D11 spectrometer at ILL and a performance comparison of the three instruments was made. Comparisons were made by evaluating the scattered intensity for [delta] scatterers at different Q values for various instrument configurations needed to span a Q-range of 0.0007 - 0.44 Å−1.
This book provides a broad survey of the work carried out by scientists at neutron centres around the world, which provide the facilities for generating intense beams of neutrons.These beams are essential in investigating the atomic structures of a wide range of materials such as magnetic alloys, superconductors, polymers, or proteins.