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Recent progress at LANSCE (Los Alamos Neutron Scattering Center)

Description: Since the last ICANS meeting in 1986, a new construction project, funded at the level of $17.5 million by the US Department of Energy, has been started at LANSCE. This project comprises an experimental hall with an area of 1700 square metres, a support building which includes both laboratories and offices, and four new spectrometers. The experimental hall was occupied in April of this year and we anticipate the use of the support building within six months. Both of these buildings are essential for the national user program which is described in this paper.
Date: January 1, 1988
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

LANSCE '90: The Manuel Lujan Jr. Neutron Scattering Center

Description: This paper describes progress that has been made at the Manuel Lujan Jr. Neutron Scattering Center (LANSCE) during the past two years. Presently, LANSCE provides a higher peak neutron flux than any other pulsed spallation neutron source. There are seven spectrometers for neutron scattering experiments that are operated for a national user program sponsored by the US Department of Energy. Two more spectrometers are under construction. Plans have been made to raise the number of beam holes available for instrumentation and to improve the efficiency of the target/moderator system. 9 refs., 4 figs.
Date: January 1, 1990
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

Small-angle neutron scattering of DyMo/sub 6/S/sub 8/ at low temperature

Description: The Chevrel phase compound DyMo/sub 6/S/sub 8/ becomes superconducting at 2.05 K and is antiferromagnetic (AFM) T/sub M/ = 0.4 K. Experiments with an applied magnetic field have shown that the AFM state is gradually suppressed and a ferromagnetic component seems to be induced before H/sub c2/ (1200 0e) is reached. The present experiments have been performed at the small-angle scattering facility D11 at the Institut Laue Langevin with a dilution refrigerator and applied fields of up to 2000 0e. Neutron wavelengths of 6.25 and 10A have been used allowing a minimum wavevector Q/sub min/ approx. = 0.005 A/sup -1/ to be achieved. For increasing H we find a sudden increase in the small-angle scattering at H = 180 0e, which we identify as H/sub c1/. With a position sensitive detector we have also investigated the scattering as a function of the relative orientation of Q Vector and H Vector. For a simple paramagnet we would expect more magnetic intensity with Q Vector perpendicular H Vector than with Q Vector parallel H Vector, however, we find the exact reverse with somewhat more intensity for Q Vector parallel H Vector. This is not understood, but we propose that the small-angle scattering is predominantly inelastic. Similar anisotropic effects have been found in other Chevrel phase systems. 7 references.
Date: January 1, 1983
Creator: Lander, G.H.; Pynn, R. & Ishikawa, M.
Partner: UNT Libraries Government Documents Department

The performance of neutron scattering spectrometers at a long-pulse spallation source

Description: The first conclusion the author wants to draw is that comparison of the performance of neutron scattering spectrometers at CW and pulsed sources is simpler for long-pulsed sources than it is for the short-pulse variety. Even though detailed instrument design and assessment will require Monte Carlo simulations (which have already been performed at Los Alamos for SANS and reflectometry), simple arguments are sufficient to assess the approximate performance of spectrometers at an LPSS and to support the contention that a 1 MW long-pulse source can provide attractive performance, especially for instrumentation designed for soft-condensed-matter science. Because coupled moderators can be exploited at such a source, its time average cold flux is equivalent to that of a research reactor with a power of about 15 MW, so only a factor of 4 gain from source pulsing is necessary to obtain performance that is comparable with the ILL. In favorable cases, the gain from pulsing can be even more than this, approaching the limit set by the peak flux, giving about 4 times the performance of the ILL. Because of its low duty factor, an LPSS provides the greatest performance gains for relatively low resolution experiments with cold neutrons. It should thus be considered complementary to short pulse sources which are most effective for high resolution experiments using thermal or epithermal neutrons.
Date: April 1, 1995
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

Spallation sources in support of technology

Description: In this contribution the author summarise a number of recent experiments at the Los Alamos Neutron Science Center (LANSCE) that have contributed to strategic and applied research. A number of new tools have been developed to address these problems, including software that allows materials texture to be obtained during Rietveld refinement, Bragg-edge diffraction, resonant-neutron and proton radiography. These tools have the potential to impact basic as well as applied research. It is clear that a new, more powerful neutron source such as the planned Japanese Hadron Project will be able to use these and other techniques to contribute in a direct way to important industrial technologies.
Date: May 1, 1996
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

The effect of surface roughness on the phase of neutrons specularly reflected at grazing incidence

Description: The effect of surface roughness on the specular intensity of a reflected neutron beam can be calculated using the distorted wave Born approximation. For Gaussian roughness the result obtained is the familiar expression of Nevot and Croce. When the distribution of surface heights, z, is proportional to cosh{sup {minus}2}(z/2d), the calculated reflectance is sinh({pi}d(k{sub 0}{minus}k{sub 1})){vert bar}sinh({pi}d(k{sub 0}+k{sub 1})). Here k{sub 0} and k{sub 1} are the components of neutron wavevectors perpendicular to the interface, evaluated outside and inside the reflecting medium, and d is a measure of the amplitude of the roughness. This result for the reflectance differs by a phase factor from that obtained in an exact calculation for an interface with the corresponding graded refractive index. The phase difference between the two expressions is small, scaling as {approximately}d{sup 3}, and usually has little effect on measured reflectivities of multilayered structures. Ways in which the phase could be determined are discussed.
Date: January 1, 1990
Creator: Hamilton, W.A. & Pynn, R.
Partner: UNT Libraries Government Documents Department

Probing fine magnetic particles with neutron scattering

Description: Because thermal neutrons are scattered both by nuclei and by unpaired electrons, they provide an ideal probe for studying the atomic and magnetic structures of fine-grained magnetic materials, including nanocrystalline solids, thin epitaxial layers, and colloidal suspensions of magnetic particles, known as ferrofluids. Diffraction, surface reflection, and small angle neutron scattering (SANS) are the techniques used. With the exception of surface reflection, these methods are described in this article. The combination of SANS with refractive-index matching and neutron polarisation analysis is particularly powerful because it allows the magnetic and atomic structures to be determined independently. This technique has been used to study both dilute and concentrated ferrofluid suspensions of relatively monodisperse cobalt particles, subjected to a series of applied magnetic fields. The size of the cobalt particle core and the surrounding surfactant layer were determined. The measured interparticle structure factor agrees well with a recent theory that allows correlations in binary mixtures of magnetic particles to be calculated in the case of complete magnetic alignment. When one of the species in such a binary mixture is a nonmagnetic, cyclindrical macromolecule, application of a magnetic field leads to some degree of alignment of the nonmagnetic species. This result has been demonstrated with tobacco mosaic virus suspended in a water-based ferrofluid.
Date: January 1, 1991
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

What can we learn from off-specular neutron reflection?

Description: Specular reflection of neutrons or x rays has been developed recently as a tool to probe density variations in the neighborhood of flat interfaces. The techniques have been applied to a variety of materials problems ranging from polymer adsorption to hydrogenation of carbon films and the structure of magnetic multilayers. In addition to the specular scattering, diffuse scattering is often observed, sometimes in strikingly beautiful patterns. This scattering is caused by imperfections such as interfacial roughness or density fluctuations within a layer. As a general rule, the diffuse, off-specular scattering measures the density-density correlation function within and between the interfaces responsible for the specular scattering. Interpretation of diffuse x-ray and neutron scattering from surfaces and interfaces is in its infancy using theoretical schemes that are still being developed. In this talk the author shows examples of diffuse scattering patterns and offers some guidelines for their interpretation.
Date: December 31, 1995
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

Probing fine magnetic particles with neutron scattering

Description: Because thermal neutrons are scattered both by nuclei and by unpaired electrons, they provide an ideal probe for studying the atomic and magnetic structures of fine-grained magnetic materials, including nanocrystalline solids, thin epitaxial layers, and colloidal suspensions of magnetic particles, known as ferrofluids. Diffraction, surface reflection, and small angle neutron scattering (SANS) are the techniques used. With the exception of surface reflection, these methods are described in this article. The combination of SANS with refractive-index matching and neutron polarisation analysis is particularly powerful because it allows the magnetic and atomic structures to be determined independently. This technique has been used to study both dilute and concentrated ferrofluid suspensions of relatively monodisperse cobalt particles, subjected to a series of applied magnetic fields. The size of the cobalt particle core and the surrounding surfactant layer were determined. The measured interparticle structure factor agrees well with a recent theory that allows correlations in binary mixtures of magnetic particles to be calculated in the case of complete magnetic alignment. When one of the species in such a binary mixture is a nonmagnetic, cyclindrical macromolecule, application of a magnetic field leads to some degree of alignment of the nonmagnetic species. This result has been demonstrated with tobacco mosaic virus suspended in a water-based ferrofluid.
Date: December 31, 1991
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

Fabrication of boron-phosphide neutron detectors

Description: Boron phosphide is a potentially viable candidate for high neutron flux neutron detectors. The authors have explored chemical vapor deposition methods to produce such detectors and have not been able to produce good boron phosphide coatings on silicon carbide substrates. However, semi-conducting quality films have been produced. Further testing is required.
Date: July 1, 1997
Creator: Fitzsimmons, M. & Pynn, R.
Partner: UNT Libraries Government Documents Department

The performance of reflectometers at continuous wave and pulsed-neutron sources

Description: To quantify gains from time-of-flight (TOF) methods, identical reflectometers viewing a continuous wave (CW) neutron source and a variety of pulsed-neutron sources were simulated using a Monte Carlo technique. Reflectivity profiles obtained for a simple thin-film, reflecting,sample were nearly identical in all simulations, and models fitted to the simulated data yielded parameters (film thickness, surface roughness, and scattering length density) that were equally accurate and precise in all cases. The simulations confirm the power of the TOF method and demonstrate that the performance of pulsed sources for reflectometry does not scale simply as the inverse duty factor of the source. In the case of long-pulse sources, the simulations suggest that pulse tails have little effect on results obtained from specular reflectometry and that maximum brightness of the neutron source should be the primary design criterion.
Date: December 1995
Creator: Fritzsimmons, M. R. & Pynn, R.
Partner: UNT Libraries Government Documents Department

The performance of neutron spectrometers AR a long-pulse spallation source

Description: At a recent workshop at Lawrence Berkeley National Laboratory members of the international neutron scattering community discussed the performance to be anticipated from neutron scattering instruments installed at a 1 MW long-pulse spallation source (LPSS). Although the report of this workshop is long, its principal conclusions can be easily summarised and almost as easily understood. This article presents such a synthesis for a 60 Hz LPSS with 1 msec proton pulses. We discuss some of the limitations of the workshop conclusions and suggest a simple analysis of the performance differences that might be expected between short- and long-pulse sources both of which exploit coupled moderators.
Date: December 1, 1995
Creator: Pynn, R. & Daemen, L.L.
Partner: UNT Libraries Government Documents Department

A proposal for a long-pulse spallation source at Los Alamos National Laboratory

Description: Los Alamos National Laboratory is proposing a new spallation neutron source that will provide the US with an internationally competitive facility for neutron science and technology that can be built in approximately three years for less than $100 million. The establishment of a 1-MW, long-pulse spallation source (LPSS) at the Los Alamos Neutron Science Center (LANSCE) will meet many of the present needs of scientists in the neutron scattering community and provide a significant boost to neutron research in the US. The new facility will support the development of a future, more intense spallation neutron source, that is planned by DOE`s Office of Energy Research. Together with the existing short pulse spallation source (SPSS) at the Manual Lujan, Jr. Neutron Scattering Center (MLNSC) at Los Alamos, the new LPSS will provide US scientists with a complementary pair of high-performance neutron sources to rival the world`s leading facilities in Europe.
Date: December 1, 1995
Creator: Pynn, R. & Weinacht, D.
Partner: UNT Libraries Government Documents Department

Diffuse neutron scattering signatures of rough films

Description: Patterns of diffuse neutron scattering from thin films are calculated from a perturbation expansion based on the distorted-wave Born approximation. Diffuse fringes can be categorised into three types: those that occur at constant values of the incident or scattered neutron wavevectors, and those for which the neutron wavevector transfer perpendicular to the film is constant. The variation of intensity along these fringes can be used to deduce the spectrum of surface roughness for the film and the degree of correlation between the film's rough surfaces.
Date: January 1, 1992
Creator: Pynn, R. & Lujan, M. Jr.
Partner: UNT Libraries Government Documents Department

Neutron scattering methods for materials science

Description: This article describes some of the neutron scattering techniques which can be used to probe structures of materials on length scales which range from less than 1 {Angstrom} to 1 {mu}m. The intent is to remove the veils of secrecy and mystery which have apparently prevented the wide application of these techniques to problems in materials science. 28 refs., 5 figs.
Date: January 1, 1989
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

Los Alamos constant-Q spectrometer

Description: A constant-Q spectrometer, which has been installed on the pulsed source of the Los Alamos Neutron Scattering Center, is described. It features a number of innovations that ease sample and analyser alignment and a novel composite crystal analyzer which exploits the mosaic properties of plastically deformed germanium in a new way. Observations of phonon dispersion in aluminium and of incoherent scattering in ZrH/sub 2/ are reported, along with background measurements. 11 refs., 5 figs., 1 tab.
Date: January 1, 1985
Creator: Robinson, R.A.; Pynn, R.; Eckert, J. & Goldstone, J.A.
Partner: UNT Libraries Government Documents Department

Information content of lineshapes

Description: We examine the question of figures-of-merit for optimizing the lineshapes of neutron scattering sources, instruments and experiments. Using maximum entropy deconvolution of simulated data, we test the effects of various features of lineshapes including intensity, resolution (FWHM), shape, and background. We demonstrate that conventional figures-of-merit are of limited validity, and we suggest that bandwidth is an important criterion for optimization. 9 refs., 7 figs.
Date: January 1, 1988
Creator: Silver, R.N.; Sivia, D.S. & Pynn, R.
Partner: UNT Libraries Government Documents Department

Using spallation neutron sources for defense research

Description: Advanced characterization techniques and accelerated simulation are the cornerstones of the Energy Department`s science-based program to maintain confidence in the safety, reliability, and performance of the US nuclear deterrent in an era of no nuclear testing. Neutrons and protons provided by an accelerator-based facility have an important role to play in this program, impacting several of the key stockpile stewardship and management issues identified by the Department of Defense. Many of the techniques used for defense research at a spallation source have been used for many years for the basic research community, and to a lesser extent by industrial scientists. By providing access to a broad spectrum of researchers with different backgrounds, a spallation source such as the Los Alamos Neutron Science Center is able to promote synergistic interaction between defense, basic and industrial researchers. This broadens the scientific basis of the stockpile stewardship program in the short term and will provide spin-off to industrial and basic research in the longer term.
Date: December 31, 1996
Creator: Pynn, R.; Sterbenz, S.M. & Weinacht, D.J.
Partner: UNT Libraries Government Documents Department

The performance of neutron scattering spectrometers at a long-pulse spallation source

Description: In this document the author considers the performance of a long pulse spallation source for those neutron scattering experiments that are usually performed with a monochromatic beam at a continuous wave (CW) source such as a nuclear reactor. The first conclusion drawn is that comparison of the performance of neutron scattering spectrometers at CW and pulsed sources is simpler for long-pulsed sources than it is for the short-pulse variety. Even though detailed instrument design and assessment will require Monte Carlo simulations (which have already been performed at Los Alamos for SANS and reflectometry), simple arguments are sufficient to assess the approximate performance of spectrometers at an LPSS and to support the contention that a 1 MW long-pulse source can provide attractive performance, especially for instrumentation designed for soft-condensed-matter science. Because coupled moderators can be exploited at such a source, its time average cold flux is equivalent to that of a research reactor with a power of about 15 MW, so only a factor of 4 gain from source pulsing is necessary to obtain performance that is comparable with the ILL. In favorable cases, the gain from pulsing can be even more than this, approaching the limit set by the peak flux, giving about 4 times the performance of the ILL. Because of its low duty factor, an LPSS provides the greatest performance gains for relatively low resolution experiments with cold neutrons. It should thus be considered complementary to short pulse sources which are most effective for high resolution experiments using thermal or epithermal neutrons.
Date: June 1, 1997
Creator: Pynn, R.
Partner: UNT Libraries Government Documents Department

Monte Carlo study of the performance of a time-of-flight multichopper spectrometer

Description: The Monte Carlo method is a powerful technique for neutron transport studies. While it has been applied for many years to the study of nuclear systems, there are few codes available for neutron transport in the optical regime. The recent surge of interest in so-called next generation spallation neutron sources and the desire to design new and optimized instruments for these facilities has led us to develop a Monte Carlo code geared toward the simulation of neutron scattering instruments. The time-of-flight multichopper spectrometer, of which IN5 at the ILL is the prototypical example, is the first spectrometer studied with the code. Some of the results of a comparison between the IN5 performance at a reactor and at a Long Pulse Spallation Source (LPSS) are summarized here.
Date: December 1, 1995
Creator: Daemen, L.L.; Eckert, J. & Pynn, R.
Partner: UNT Libraries Government Documents Department

Polarized neutrons and their applications

Description: The role of polarized neutrons in the study of magnetic and nonmagnetic materials is discussed. magnetic structure, spin density, nuclear magnetism and spin dynamics are listed as magnetic properties which may be accessed by polarized neutrons. Low dimensional magnets, disordered magnetic systems, surface magnetism are discussed as well as topography and depolarization. Polarization techniques are discussed for both study state and pulsed modes. 4 refs. (WRF)
Date: January 1, 1985
Creator: Moon, R.; Endoh, Y.; Felcher, G.; Majkrzak, C.; Mezei, F.; Pynn, R. et al.
Partner: UNT Libraries Government Documents Department

Off-specular scattering in neutron reflectometry

Description: When neutrons are scattered at small angles from planar, laterally homogeneous, stratified media, only specular (mirror like) reflection is observed. Sample inhomogeneities, such as interfacial roughness or voids, give rise to off-specular scattering which has been observed in many experiments with neutrons and x-rays. The easiest way to describe this scattering theoretically is based on the distorted-wave Born approximation (DWBA), which uses the neutron wavefunctions that describe reflection from a smooth surface as the basis functions for perturbation theory. From the DWBA one may obtain a number of qualitative results which are supported by experiment. Examples include the Yoneda fringes observed in reflection experiments with microscopically rough surfaces and the constant-q{sub z} fringes observed for multilayers with correlated, rough interfaces. One must, however, use the DWBA with care. When the correlation range within the reflecting interfaces is large--for example, when a surface is composed of misoriented facets--the approximation breaks down. Some authors have also reported a lack of quantitative agreement between versions of the DWBA calculations and the scattering observed with microscopically rough surfaces. A remarkable feature of neutron (or x-ray) reflectometry is the length scales that are probed within reflecting surfaces. These range from a few hundred Angstroms up to several microns, allowing neutron scattering to probe objects of a size normally visible by optical microscopy! The intent of this paper is to provide a simple description of scattering from rough surfaces that is accessible to a wide audience. Mathematical completeness is sacrificed in favor of intuitive arguments and experimental examples.
Date: March 1, 1995
Creator: Pynn, R.; Baker, S. M.; Smith, G. & Fitzsimmons, M.
Partner: UNT Libraries Government Documents Department

Low energy magnetic fluctuations in the TSDW phase of chromium

Description: A polarized neutron study of chromium carried out in a field of 6T applied to a single-domain single-Q crystal indicates that the inelastic intensity observed close to the transverse spin density wave (TSDW) satellite positions (1 {+-} {delta}, 0,0) does not behave as expected for spin-wave scattering. In particular, the signal corresponds to magnetization fluctuations of almost equal magnitude both parallel and perpendicular to the ordered moments in the TSDW phase.
Date: December 1, 1997
Creator: Azuah, R.T.; Kulda, J.; Pynn, R. & Stirling, W.G.
Partner: UNT Libraries Government Documents Department