P.A. Seeger

Double-difference method to improve the resolution of an eV neutron spectrometer

Abstract Epithermal (eV) neutron spectrometers designed to measure the dynamic properties of condensed matter have been developed at several pulsed neutron sources, using nuclear resonances to define neutron energy. The energy resolution of such a spectrometer is limited by the width of the nuclear resonance. The paper describes how a linear combination of measurements with two thicknesses of foils of the resonance material yields better energy resolution than either foil alone. With proper selection of foil thicknesses, there is no penalty in counting statistics to obtain the improved resolution. The double-difference method also suppresses the wings of the resonance and yields a function with a finite second moment so that uncertainties may be properly propagated.

Numerical solution of Bloch's equation for neutron spin precession

Abstract The increasing importance of polarization in neutron scattering instrumentation for condensed matter research means that Monte Carlo design tools must be able to track neutron spin during neutron transport. In particular, we must be able to solve Blochs precession equation for arbitrary magnetic induction configurations, including time-dependence. Since Monte Carlo simulations require averaging a large number of neutron histories, the computational procedure must be fast, as well as accurate and precise. A suitable algorithm is presented here, in the context of the Neutron Instrument Simulation Package (NISP), a Monte Carlo package developed at Los Alamos National Laboratory for neutron scattering instrument design. Accuracy is assessed by the comparison to simple cases for which analytical expressions are known, and precision and execution time are shown for a case with a non-uniform magnetic induction field.

The neutron instrument simulation package, NISP

The Neutron Instrument Simulation Package (NISP) performs complete source-to-detector simulations of neutron instruments, including neutrons that do not follow the expected path. The original user interface (MC_Web) is a web-based application, http://strider.lansce.lanl.gov/NISP/Welcome.html. This report describes in detail the newer stand-alone Windows version, NISP_Win. Instruments are assembled from menu-selected elements, including neutron sources, collimation and transport elements, samples, analyzers, and detectors. Magnetic field regions may also be specified for the propagation of polarized neutrons including spin precession. Either interface writes a geometry file that is used as input to the Monte Carlo engine (MC_Run) in the users computer. Both the interface and the engine rely on a subroutine library, MCLIB. The package is completely open source. New features include capillary optics, temperature dependence of Al and Be, revised source files for ISIS, and visualization of neutron trajectories at run time. Also, a single-crystal sample type has been successfully imported from McStas (with more generalized geometry), demonstrating the capability of including algorithms from other sources, and NISP_Win may render the instrument in a virtual reality file. Results are shown for two instruments under development.

Resolution of pulsed-source small-angle neutron scattering

Abstract An analytic form is found for resolution of small-angle scattering in a plane, at a pulsed source with a white neutron spectrum. The function is found to be asymmetric at low values of Q and to have broad tails if data recorded over the entire wavelength range are combined. Monte Carlo calculations in three dimensions and including realistic spectra and collimator geometry confirm these features and provide “data” for studying the question of what regions of scattering angle and wavelength should be retained in data reduction. Comparisons are made with a spectrometer at a reactor, based on the accuracy, statistical precision, and time required to collect data for (simulated) monodisperse hard spheres of various radii.

Optimization of geometric resolution in small-angle scattering

Abstract Analytic forms have been derived for optimization of geometric resolution, with the result that the incident flight path should be twice as long as the scattered flight path in some cases. Although the results differ from commonly accepted conditions, they have been found to agree with Monte-Carlo calculations in all respects. The results apply equally to X-rays or neutrons, for steady-state or pulsed sources, and for position-sensitive or energy-dependent detectors.

Neutron instrument simulations in the next millennium

Abstract The Neutron Instrument Simulation Package (NISP) is available on the world-wide web at URL http://strider.lansce.lanl.gov/NISP/Welcome.html. With more than 20 years of development, there is a certain amount of maturity in the code, but also the potential for a great deal of growth. As requirements for more sophisticated simulations grow, NISP can be expanded or modified to meet those needs. This report describes features of the NISP structures that make it possible for users to contribute algorithms. Any interaction that can be coded as a Fortran-callable subroutine can be included as a region type in the simulations. New ideas are always solicited, and may be sent by e-mail to

Structure of a binary colloidal suspension under shear

Neutron scattering intensities from an aqueous mixture suspension of 91 nm polystyrene latex particles and 54 nm silica particles are reported in the range 0.02 < Q/nm–1 < 0.2, where Q is the momentum transfer. The suspension was dense at a mixture volume fraction of 0.15, and the polystyrene/silica particle ratio was ca. 1.7 : 1. Results are given for the suspension at rest and under shear. The sheared data were obtained with a concentric cylinder shearing apparatus constructed and tested at the SANS facility of the National Institute of Standards and Technology and the pulsed neutron facility, LANSCE, of the Los Alamos National Laboratory. The design and operation of the cell is described. The shear-influenced behaviour of the mixture is compared with and contrasted to that of a pure polystyrene suspension that can form a crystal lattice in equilibrium, but which melts to a liquid-like structure under shear.A method is proposed to measure, by contrast matching or variation, the polystyrene and silica partial scattered intensities from the mixture suspension in H2O–D2O solvents of different scattering-length densities. Estimates of the partial structure factors are given.

Monte Carlo tool for neutron optics and neutron scattering instrument design

Unlike x-ray generators, neutron source have inherently low brightness, and care must be exerted in the design of neutron scattering instruments and their coupling to the source to ensure optimal use of the beam. We present a general, versatile Monte Carlo tool for the computer simulation of neutron optics and neutron scattering instruments that allows a user to produce computer models of an instrument and study its performance quantitatively. The Neutron Instrument Simulation Package (NISP) implements a wide range of neutron optics models to describe neutron transport (including gravity) and scattering in the elements making up the instrument. The program is freely available on the world-wide web at http://strider.lansce.lanl.gov/NISP/Welcome.html

NISP: The neutron instrument simulation package at Los Alamos National Laboratory

Abstract We describe the philosophy and structure of the Neutron Instrument Simulation Package (NISP), which has been developed to create computer models of neutron scattering instruments and to estimate their performance using Monte Carlo techniques. The package includes a Web-based application for the assembly of elements into instruments (http://strider:lansce.lanl.gov/NISP/Wlcome.html), a Monte Carlo engine to run the instrument simulation (MC-Run), and a library of Fortran subroutines to support the element types (MCLIB). Additional programs are provided for visualization of the output. The latest release of the source code and documentation may be obtained from ftp://strider:lunsce.lunl.gov/pub/NISP/ or through the Web site.

Comparison of collimation systems for small-angel neutron scattering

It is shown by simple first-order geometric arguments that for a given resolution the flux on a sample in a small-angle scattering instrument is independent of the form of the collimator or of the length of the instrument. Count rate may be increased by increasing the sample size, through the use of multi-aperture systems. In second order, it is shown to be advantageous to place the beam defining elements close to the source and the sample. The multiple-pinhole system gives maximum flux on small samples but has non-uniform illumination so that the intensity increases only about half as fast as the sample area. Soller slits and continuous tubes from source to sample were also considered, but neutron scattering and reflection from surfaces generate a large halo. Monte Carlo simulations confirm these results, with the conclusion that the optimum collimator configuration is the multiple-pinhole system.