NIST Center For Neutron Research Research Articles

Dynamics of Hydration Water on Rutile Studied by Backscattering Neutron Spectroscopy and Molecular Dynamics Simulation

The high energy resolution, coupled with the wide dynamic range, of the new backscattering spectrometer (BASIS) at the Spallation Neutron Source, Oak Ridge National Laboratory, has made it possible to investigate the diffusion dynamics of hydration water on the surface of rutile (TiO2) nanopowder down to a temperature of 195 K. The dynamics measured on the BASIS on the time scale of tens of picoseconds to more than a nanosecond can be attributed to the mobility of the outer hydration water layers. The data obtained on the BASIS and in a previous study using the backscattering and disk-chopper spectrometers at the NIST Center for Neutron Research are coupled with molecular dynamics simulations extended to 50 ns. The results suggest that the scattering experiments probe several types of molecular motion in the surface layers, namely a very fast component that involves dynamics of water molecules with unsaturated hydrogen bonds, a somewhat slower component due to localized motions of all water molecules, and...

Characterization of Materials for a Hydrogen-Based Economy by Cold Neutron Prompt Gamma-Ray Activation Analysis

AbstractAn instrument for cold neutron prompt gamma-ray activation analysis (PGAA) at the NIST Center for Neutron Research (NCNR) has proven useful for the chemical characterization of hydrogen storage materials and other materials of importance to a hydrogen-based economy. The detection limit for hydrogen is less than 10 mg/kg for most materials. Potential hydrogen storage materials that have been characterized by PGAA include single-wall carbon nanotubes with and without boron doping, porous carbons, lithium magnesium imides, and ternary hydrides of various elements. The capability to allow in situ hydrogenation and characterization of materials is currently under development. PGAA has also been used to characterize materials used in hydrogen fuel cells, including solid proton conductors, polymer membrane, and proton exchange membranes. Future upgrades to the instrument will improve detection limits and functionality of the instrument.

Neutron Studies of the Iron-based Family of High Tc Magnetic Superconductors

AbstractWe briefly review recent neutron scattering investigations carried out at the NIST Center for Neutron Research on the crystal structures, magnetic structures, and spin dynamics of the iron-based ROFe(As,P) (R=La, Ce, Pr, Nd), and (Ba,Sr,Ca)Fe2As2 systems. All the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic structural transition occurs between ˜100−220 K, below which the systems become antiferromagnets. The magnetic structure within the a-b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than one Bohr magneton. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting and a spin wave bandwidth ˜200 meV. The rare-earth moments order antiferromagnetically at low T like ‘conventional’ magnetic-superconductors. With doping, the structural and magnetic transitions are suppressed in favor of superconductivity, with superconducting transition temperatures up to 56 K, while the Ce crystal field linewidths are affected when superconductivity sets in. The application of pressure in CaFe2As2 transforms the system from a magnetically ordered orthorhombic material to a ‘collapsed’ non-magnetic tetragonal system which is superconducting at lower T when anisotropic pressure is applied. Fe1+xTe shows a transition from a monoclinic to orthorhombic low T structure with increasing x, and a crossover from commensurate to incommensurate magnetic order. Se doping suppresses the magnetic order, while incommensurate magnetic scattering is observed in the superconducting regime.

Cold Neutron Prompt Gamma-Ray Activation Analysis for Characterization of Hydrogen Storage and Related Materials

ABSTRACTPrompt gamma-ray activation analysis (PGAA) is an important technique for analyzing materials that may impact the hydrogen economy. An instrument for cold neutron PGAA, located at the NIST Center for Neutron Research (NCNR), has proven useful for the measurement of hydrogen and other elements in a variety of such materials. The detection limit for hydrogen is less than 10 mg/kg for most materials. PGAA has been used to characterize materials with potential for hydrogen storage (e.g. single wall carbon nanohorns, lithium magnesium imides, and zirconium beryllium hydrides), to measure hydrogen uptake by solid proton conductors, and to characterize stoichiometries of Nafion films used as membranes in fuel cells. Future upgrades to the instrument will improve detection limits, applicability, and user interface.

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) forq≥ 5 × 10−4 Å−1is 4 × 10−7. The instrument uses 2.38 Å 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 Å−1to 5 × 10−3 Å−1in 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 instrument's performance.

Sources of uncertainties in prompt gamma activation analysis

Two prompt gamma-ray activation analysis (PGAA) facilities at the NIST Center for Neutron Research have been used routinely to perform elemental analyses of a variety of materials. Results from these analyses are usually expressed as mass fraction values with expanded uncertainties. The expanded uncertainty consists of the combined uncertainty multiplied by the appropriate coverage factor (k) required to achieve a 95% confidence interval. The combined uncertainty includes the uncertainties associated with preparation, irradiation, and γ-ray spectrometry of samples and standards, and corrections for γ-rays from the background or blanks where necessary. To determine the combined uncertainty, each component of uncertainty associated with each variable and constant in the basic measurement equation is evaluated. In this paper we present the PGAA measurement equation, a description of the potential sources of uncertainty for each component of the equation, and three examples of uncertainty evaluation. The examples are for determination of H in standard reference material (SRM) 2454, hydrogen in titanium alloy using the cold neutron PGAA facility, Cd in SRM 2702 Inorganics in Marine Sediment using the original thermal neutron PGAA facility, and N in SRM 3244 Ephedra-Containing Protein Powder using the recently designed thermal neutron PGAA facility.

Determination of boron in materials by cold neutron prompt gamma-ray activation analysis

An instrument for cold neutron prompt gamma-ray activation analysis (PGAA), located at the NIST Center for Neutron Research (NCNR), has proven useful for the measurement of boron in a variety of materials. Neutrons, moderated by passage through liquid hydrogen at 20 K, pass through a (58)Ni coated guide to the PGAA station in the cold neutron guide hall of the NCNR. The thermal equivalent neutron fluence rate at the sample position is 9 x 10(8) cm(-2) s(-1). Prompt gamma rays are measured by a cadmium- and lead-shielded high-purity germanium detector. The instrument has been used to measure boron mass fractions in minerals, in NIST SRM 2175 (Refractory Alloy MP-35-N) for certification of boron, and most recently in semiconductor-grade silicon. The limit of detection for boron in many materials is <10 ng g(-1).

New thermal neutron prompt γ-ray activation analysis instrument at the National Institute of Standards and Technology Center for Neutron Research

A new thermal neutron prompt γ-ray activation analysis (PGAA) instrument was designed and built to replace the original PGAA system at the NIST Center for Neutron Research. By placing a sapphire filter in the neutron beam shutter assembly, the fast neutron fluence rate was reduced by a factor of 5 and low-energy (50–200 keV) γ-ray intensities were reduced by factors of 5–10. The thermal neutron fluence rate was reduced by only a factor of 1.13. A new external beam tube, sample chamber, beam stop, and support structure were built and a new detection system installed. The new beam tube is made of two cylindrical aluminum sections lined with a lithiated polymer. Both sections are kept under vacuum to reduce the number of neutrons scattered by air into the beam tube walls. The sample chamber is also fabricated from aluminum and lined with lithiated polymer, and may be evacuated to minimize the number of neutrons scattered and absorbed by air. The beam tube and sample chamber assembly is suspended from the aluminum support structure. The detection system consists of a 40% efficient (relative) germanium detector (resolution 2.0 at 1332.5 keV) and a bismuth germanate Compton suppressor. The detection system is shielded by lead, surrounded by borated and lithiated polyethylene, and placed on a table attached to the support structure. The new, more compact beam stop is welded to the support structure. Capture γ-ray photopeaks from H, B, C, N, Na, Al, Fe, Ge, I and Pb in the background spectrum were either of lower intensity or eliminated with the new PGAA instrument. The more efficient detection system, positioned closer to the sample, yielded element sensitivity increases of 5–50%. Limits of detection have been greatly reduced compared with those of the original instrument due to reduced Compton and scattered γ-ray backgrounds (especially in the low-energy region), increased sensitivities, and reduction of background γ-ray photopeak intensities.

Simulations and measurements of the performance of a channeled neutron guide for a time-of-flight spectrometer at the NIST Center for Neutron Research

We describe the identification and analysis of the principal sources of intensity loss within the five-channeled neutron guide tube that was originally installed in the chopper section of the Disk Chopper Spectrometer at the National Institute of Standards and Technology Center for Neutron Research. (The purpose of the five channels was to optimize intensity and resolution in three different modes of operation known as “resolution modes.”) By combining measurements, Monte Carlo simulations, and analytical calculations, we have developed a model that successfully explains performance losses in the original guide. We have used this model to quantify expected returns in performance using a replacement guide in which the principal contributions to the intensity loss are reduced to the minimum achievable with current technology. We have also estimated the intensity gains that would be achieved if one of the limited number of options were adopted for modifying the original guide in a manner likely to produce such gains. We describe factors that affect the performance of the original guide and compare the measured and predicted performance of the modified guide against predictions for the optimal replacement guide. The simulations indicate that the modified guide (which has three channels rather than the original five) produces greater intensity gains over a large incident wavelength band for the low and medium resolution modes, whereas a high quality replacement guide greatly improves performance in the high resolution mode of operation. Because the low and medium resolution modes are most heavily demanded, we opted to modify the guide rather than replace it. We describe the nature of this modification and present intensity measurements that meet or exceed predictions in all resolution modes with no detectable change in the energy resolution nor increase in the instrumental background.

Determination of Hydrogen in Semiconductors and Related Materials by Cold Neutron Prompt Gamma-ray Activation Analysis

ABSTRACTAn instrument for prompt gamma-ray activation analysis (PGAA) at the NIST Center for Neutron Research has proven useful for the measurement of hydrogen and other elements in a variety of materials. The sample is irradiated by a beam of low energy neutrons. Gamma-rays emitted by atomic nuclei upon neutron capture are measured and elemental concentrations determined by comparison with appropriate standards. The detection limit for hydrogen is &lt; 5 mg/kg in most materials, and 2 mg/kg for hydrogen measured in silicon. The instrument has been used to measure hydrogen mass fractions of &lt; 100 mg/kg in high purity germanium, and &lt; 10 mg/kg in quartz. More recently PGAA has been used to measure hydrogen in 1 μm thick porous thin films on a silicon substrate, and in crystals of silicon carbide and cerium aluminate.

An acceptance diagram analysis of the contaminant pulse removal problem with direct geometry neutron chopper spectrometers

Phased choppers are used to produce pulsed beams of monochromatic neutrons at research reactors and spallation neutron sources. Depending on the design of the instrument, it is very possible that the choppers will transmit neutrons with wavelengths other than those within the desired band of wavelengths. One or more additional choppers are typically needed to remove these contaminant pulses. We describe a method of determining the wavelength- and time-dependent transmission of a system of choppers using acceptance diagrams. The method is illustrated with calculations for the Disk Chopper Spectrometer at the NIST Center for Neutron Research and the proposed Cold Neutron Chopper Spectrometer at the Spallation Neutron Source (Oak Ridge, TN).

Polarized neutron reflectivity studies of magnetic semiconductor superlattices

Polarized neutron reflectivity studies of EuS/PbS, EuS/YbSe and GaMnAs/GaAs superlattices performed at the NIST Center for Neutron Research are presented. Pronounced antiferromagnetic (AFM) interlayer coupling has been found in EuS/PbS superlattices for a very broad range of PbS spacer thicknesses. Similar, but weaker, AFM coupling is also present in EuS/YbSe, although only for relatively thin YbSe layers. Neutron polarization analysis shows distinct in-plane asymmetry of the magnetization directions of EuS layers in both systems under investigation. For GaMnAs/GaAs superlattices, ferromagnetic (FM) interlayer correlations have been observed. Polarized neutron reflectometry investigations of several GaMnAs/GaAs superlattices have revealed that the manganese magnetic moments in individual GaMnAs layers, in spite of low Mn concentration, form a truly long range, that is in certain cases a single domain, ferromagnetic state.

The high-flux backscattering spectrometer at the NIST Center for Neutron Research

We describe the design and current performance of the high-flux backscattering spectrometer located at the NIST Center for Neutron Research. The design incorporates several state-of-the-art neutron optical devices to achieve the highest flux on sample possible while maintaining an energy resolution of less than 1 μeV. Foremost among these is a novel phase-space transformation chopper that significantly reduces the mismatch between the beam divergences of the primary and secondary parts of the instrument. This resolves a long-standing problem of backscattering spectrometers, and produces a relative gain in neutron flux of 4.2. A high-speed Doppler-driven monochromator system has been built that is capable of achieving energy transfers of up to ±50 μeV, thereby extending the dynamic range of this type of spectrometer by more than a factor of 2 over that of other reactor-based backscattering instruments.

The Disk Chopper Spectrometer at NIST: a new instrument for quasielastic neutron scattering studies

We describe the Disk Chopper Spectrometer (DCS) at the NIST Center for Neutron Research (NCNR). The maximum measured flux at the sample, averaged over the 3 cm width and 10 cm height of the beam, with all choppers turning at 20,000 revolutions per minute and phased to transmit neutrons through the widest slots on each disk, is approximately 3 � 10 4 nc m � 2 s � 1 . In this configuration the elastic resolution FWHM (full width at half maximum height) with an incident wavelength of 6 � s 67leV. In an alternative configuration, achieved by changing the chopper phases so that neutrons pass through somewhat narrower slots, the elastic resolution FWHM at 6 � s 34leV. Useful wavelengths range from about 1.5 � to at least 12 � The DCS has been used for a variety of measurements, including quasielastic neutron scattering (QENS) studies of systems such as water in confined geometries and biological molecules in solution. 2003 Elsevier Science B.V. All rights reserved.

The NIST high-flux backscattering spectrometer

The neutron-backscattering technique extends the dynamic range of neutron time-of-flight and conventional triple-axis spectrometers down to μeV energies and is ideal for the study of slow motions in complex liquids, jump diffusion, and quantum rotational tunneling. We report on the performance of the new high-flux backscattering spectrometer (HFBS) at the NIST Center for Neutron Research. Compared to other backscattering spectrometers, the HFBS achieves a higher neutron intensity through the use of a device called a phase space transformation chopper and a large analyzer array. A custom-designed Doppler drive for the monochromator provides a triangular velocity profile capable of high-frequency operation, thus extending the dynamic range of the spectrometer by more than a factor of two over similar instruments.

Correction of intensities for preferred orientation in neutron-diffraction data of NiTi shape-memory alloy using the generalized spherical-harmonic description

Analysis of quantitative texture (crystallographic preferred orientation, PO) in polycrystalline materials is of interest not only because the PO gives errors in quantitative phase analysis, but also because it can affect the results of structure determination from powder diffraction data. In the present study, texture characterization of the polycrystalline Ni-rich NiTi shape-memory alloy (SMA) of nominal composition 50.14 atomic percent nickel has been carried out using the BT-1 high-resolution, fixed-wavelength, 32-detector powder diffractometer at the NIST Center for Neutron Research. Data were collected along the differential scanning calorimeter (DSC) heating curve. The results obtained from Rietveld refinement with the generalized spherical harmonic (GSH) description for all neutron diffraction data sets show that the weight percentages for monoclinic and cubic phases during the phase transition are consistent with the DSC heating curve.

MACS low-background doubly focusing neutron monochromator

A novel doubly focusing neutron monochromator has been developed as part of the Multi-Analyzer Crystal Spectrometer (MACS) at the NIST Center for Neutron Research. The instrument utilizes a unique vertical focusing element that enables active vertical and horizontal focusing with a large, 357-crystal (1428 cm2), array. The design significantly reduces the amount of structural material in the beam path as compared to similar instruments. Optical measurements verify the excellent focal performance of the device. Analytical and Monte Carlo simulations predict that, when mounted at the NIST cold-neutron source, the device should produce a monochromatic beam (ΔE=0.2 meV) with flux φ>108 n/cm2 s.

Assessment of preferred orientation with neutron powder diffraction data

Crystallographic texture (or preferred orientation) characterization of uniaxially pressed molybdite (MoO3) and calcite (CaCO3) powders has been carried out using the BT-1 high-resolution fixed-wavelength 32-detector powder diffractometer at the NIST Center for Neutron Research. Initially, each pattern was analysed assuming a random orientation of the crystallites. Subsequently, the March model and the generalized spherical harmonic description were used independently to extract the texture description directly from a refinement with neutron diffraction data of molybdite and calcite. The results indicate that the generalized spherical harmonic description provided a better Rietveld fit than the March model for the molybdite sample that had been subjected to the highest pressure.

Summer school on SANS and reflectometry held at NIST

The National Institute of Standards and Technology's (NIST) Center for Neutron Research (NCNR) held its eighth annual Summer School on Neutron Scattering at the reactor on the NIST campus in Gaithersburg, Maryland from June 3–7, 2002. The course this year focused, as it does every two years, on the complementary techniques of small-angle neutron scattering (SANS) and neutron reflectometry (NR). An enthusiastic group of forty graduate students and postdocs, predominantly from university chemical engineering and materials science departments, attended, which is the largest class thus far. Some of the participants had attended the NCNR's course on cold neutron spectroscopy that was given last year and alternates with the SANS/NR course.

Magnetic Structure Determinations at NBS/NIST.

Magnetic neutron scattering plays a central role in determining and understanding the microscopic properties of a vast variety of magnetic systems, from the fundamental nature, symmetry, and dynamics of magnetically ordered materials to elucidating the magnetic characteristics essential in technological applications. From the early days of neutron scattering measurements at NBS/NIST, magnetic diffraction studies have been a central theme involving many universities, industrial and government labs from around the United States and worldwide. Such measurements have been used to determine the spatial arrangement and directions of the atomic magnetic moments, the atomic magnetization density of the individual atoms in the material, and the value of the ordered moments as a function of thermodynamic parameters such as temperature, pressure, and applied magnetic field. These types of measurements have been carried out on single crystals, powders, thin films, and artificially grown multilayers, and often the information collected can be obtained by no other experimental technique. This article presents, in an historical perspective, a few examples of work carried out at the NIST Center for Neutron Research (NCNR), and discusses the key role that the Center can expect to play in future magnetism research.