Disk Chopper Spectrometer Research Articles

Self-diffusion of liquid deuterium hydride and liquid tritium.

We present a quasi-elastic neutron scattering study of liquid deuterium hydride carried out using the Disk Chopper Spectrometer at the National Institute of Standards and Technology. Under saturated vapor pressure, the self-diffusion constant of deuterium hydride obeys an Arrhenius law D=D0exp-EA/kBT, where the prefactor D0 is given by D0=9.5±1.2Å2/ps and the activation energy is given by EA = 58 ± 2K. We apply the quantum law of corresponding states to the known diffusion constants of the hydrogen isotopologues. From this application, we estimate that D0≈9.1Å2/ps and EA ≈ 75K in liquid tritium. Young's theory of quantum-mechanical effects in van der Waals fluids [Young, Phys. Rev. A 23, 1498 (1981)] is shown to apply to the diffusion constants of the liquid hydrogens. Our results underscore the importance of nuclear quantum effects in shaping the properties and behavior of the hydrogen isotopologues.

Probing Magnetic Excitations in CoII Single-Molecule Magnets by Inelastic Neutron Scattering.

Co(acac)2(H2O)2 (1, acac = acetylacetonate), a transition metal complex ( ), displays field-induced slow magnetic relaxation as a single-molecule magnet. For 1 and its isotopologues Co(acac)2(D2O)2 (1-d 4 ) and Co(acac-d 7)2(D2O)2 (1-d 18 ) in approximately symmetry, zero-field splitting of the ground electronic state leads to two Kramers doublets (KDs): lower energy and higher energy states. This work employs inelastic neutron scattering (INS), a unique method to probe magnetic transitions, to probe different magnetic excitations in 1-d 4 and 1-d 18 . Direct-geometry, time-of-flight Disk-Chopper Spectrometer (DCS), with applied magnetic fields up to 10 T, has been used to study the intra-KD transition as a result of Zeeman splitting, , in 1-d 18 . This is a rare study of the excitation in transition metal complexes by INS. Indirect-geometry INS spectrometer VISION has been used to probe the inter-KD, ZFS transition, in both 1-d 4 and 1-d 18 , by variable-temperature (VT) properties of this excitation. The INS spectra measured on VISION also give phonon features of the complexes that are well described by periodic DFT phonon calculations.

Probing Magnetic Excitations in CoII Single‐Molecule Magnets by Inelastic Neutron Scattering

Co(acac)2(H2O)2 (1, acac = acetylacetonate), a transition metal complex (S = 3/2), displays field‐induced slow magnetic relaxation as a single‐molecule magnet. For 1 and its isotopologues Co(acac)2(D2O)2 (1‐d4) and Co(acac‐d7)2(D2O)2 (1‐d18) in approximately D4h symmetry, zero‐field splitting of the ground electronic state leads to two Kramers doublets (KDs): lower energy MS = ±1/2 (ϕ1,2) and higher energy MS = ±3/2 (ϕ3,4) states. This work employs inelastic neutron scattering (INS), a unique method to probe magnetic transitions, to probe different magnetic excitations in 1‐d4 and 1‐d18. Direct‐geometry, time‐of‐flight Disk‐Chopper Spectrometer (DCS), with applied magnetic fields up to 10 T, has been used to study the intra‐KD transition as a result of Zeeman splitting, MS = –1/2 (ϕ1) → MS = +1/2 (ϕ2), in 1‐d18. This is a rare study of the MS = –1/2 → MS = +1/2 excitation in transition metal complexes by INS. Indirect‐geometry INS spectrometer VISION has been used to probe the inter‐KD, ZFS transition, MS = ±1/2 (ϕ1,2) → MS = ±3/2 (ϕ3,4) in both 1‐d4 and 1‐d18, by variable‐temperature (VT) properties of this excitation. The INS spectra measured on VISION also give phonon features of the complexes that are well described by periodic DFT phonon calculations.

Beam-transport optimization for cold-neutron spectrometer

We report the design of the beam-transport system (especially the vertical geometry) for a cold-neutron disk-chopper spectrometer AMATERAS at J-PARC. Based on the elliptical shape, which is one of the most effective geometries for a ballistic mirror, the design was optimized to obtain, at the sample position, a neutron beam with high flux without serious degrading in divergence and spacial homogeneity within the boundary conditions required from actual spectrometer construction. The optimum focal point was examined. An ideal elliptical shape was modified to reduce its height without serious loss of transmission. The final result was adapted to the construction requirements of AMATERAS. Although the ideas studied in this paper are considered for the AMATERAS case, they can be useful also to other spectrometers in similar situations.

Diffusion processes in water on oxide surfaces: Quasielastic neutron scattering study of hydration water in rutile nanopowder

Quasielastic neutron scattering (QENS) was used to investigate the diffusion dynamics of hydration water on the surface of rutile (TiO(2)) nanopowder. The dynamics measurements utilizing two inelastic instruments, a backscattering spectrometer and a disk chopper spectrometer, probed the fast, intermediate, and slow motions of the water molecules on the time scale of picoseconds to more than a nanosecond. We employed a model-independent analysis of the data collected at each value of the scattering momentum transfer to investigate the temperature dependence of several diffusion components. All of the probed components were present in the studied temperature range of 230-320 K, providing, at a first sight, no evidence of discontinuity in the hydration water dynamics. However, a qualitative change in the elastic scattering between 240 and 250 K suggested a surface freezing-melting transition, when the motions that were localized at lower temperatures became delocalized at higher temperatures. On the basis of our previous molecular dynamics simulations of this system, we argue that interpretation of QENS data from such a complex interfacial system requires at least qualitative input from simulations, particularly when comparing results from spectrometers with very different energy resolutions and dynamic ranges.

Time-of-Flight Spectroscopy at the NIST Center for Neutron Research

The time-of-flight method has a long and distinguished history. In one form or another it has been in use at the National Institute of Standards and Technology (NIST), formerly known as the National Bureau of Standards, since 1970. The current incumbent is the Disk Chopper Spectrometer (DCS) [1], a direct geometry machine with an optical filter and cooled graphite filter, seven disk choppers, a sample stage, and 913 non-position-sensitive detectors. Slots of different widths in the pulsing and monochromating choppers enable the user to choose among three distinct intensity/resolution conditions without having to change the wavelength or chopper speeds. In practice the high resolution option is not used because the beam does not have sufficient intensity. On the other hand we have used the narrowest (1.35°) slots in the choppers closest to the sample, with the remaining choppers stopped, for pulsed white beam transmission measurements (e.g. ref. [2]) and for Laue diffraction measurements (to determine the orientations of single crystals of helium grown in situ).

Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. II. Dynamics

The dynamics of monolayer films of the n-alkane tetracosane (n-C24H52) and the branched alkane squalane (C30H62) adsorbed on graphite have been studied by quasielastic and inelastic neutron scattering and molecular dynamics (MD) simulations. Both molecules have 24 carbon atoms along their carbon backbone, and squalane has an additional six methyl side groups symmetrically placed along its length. The authors' principal objective has been to determine the influence of the side groups on the dynamics of the squalane monolayer and thereby assess its potential as a nanoscale lubricant. To investigate the dynamics of these monolayers they used both the disk chopper spectrometer (DCS) and the high flux backscattering spectrometer (HFBS) at the National Institute of Standards and Technology. These instruments made it possible to study dynamical processes such as molecular diffusive motions and vibrations on very different time scales: 1-40 ps (DCS) and 0.1-4 ns (HFBS). The MD simulations were done on corresponding time scales and were used to interpret the neutron spectra. The authors found that the dynamics of the two monolayers are qualitatively similar on the respective time scales and that there are only small quantitative differences that can be understood in terms of the different masses and moments of inertia of the two molecules. In the course of this study, the authors developed a procedure to separate out the low-frequency vibrational modes in the spectra, thereby facilitating an analysis of the quasielastic scattering. They conclude that there are no major differences in the monolayer dynamics caused by intramolecular branching. It remains to be seen whether this similarity in monolayer dynamics also holds for the lubricating properties of these molecules in confined geometries.

A Molecular View of Melting in Anhydrous Phospholipidic Membranes

A high-flux backscattering spectrometer and a time-of-flight disk chopper spectrometer are used to probe the molecular mobility of model freeze-dried phospholipid liposomes at a range of temperatures surrounding the main melting transition. Using specific deuteration, quasielastic neutron scattering provides evidence that, in contrast to the hydrocarbon chains, the headgroups of the phospholipid molecules do not exhibit a sharp melting transition. The onset of motion in the tails is located at temperatures far below the calorimetric transition. Long-range motion is achieved through the onset of whole-lipid translation at the melting temperature. Atomistic simulations are performed on a multibilayer model at conditions corresponding to the scattering experiments. The model provides a good description of the dynamics of the system, with predictions of the scattering functions that agree with experimental results. The analysis of both experimental data and results of simulations supports a picture of a gradual melting of the heterogeneous hydrophobic domain, with part of the chains spanning increasingly larger volumes and part of them remaining effectively immobile until the thermodynamic phase transition occurs.

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.

Investigation of the Temporal Evolution of Translational Dynamics of Water Molecules in Hydrated Calcium Aluminate Pastes

We investigated the translational dynamics of water molecules in the hydrated aluminate and aluminum-ferrite pastes (C 3 A and C 4 AF), two main components of the Ordinary Portland Cement (OPC), using the incoherent quasi-elastic neutron scattering (QENS) technique. We used a high-resolution disk chopper spectrometer (DCS) at the Center For Neutron Research (NCNR) in NIST, with incident monochromatic neutrons of wavelength, λ = 9 A, resulting in an energy resolution of 20 μeV, covering a Q-range from 0.31 to 1.22 A - 1 . QENS spectra have been analyzed with a translational relaxing cage model previously developed by us. 1 We were able to extract time evolution of three characteristic parameters of the setting pastes: p, β, and τ, describing the fraction of the bound water (p) and relaxation behavior of the glassy water (β and τ) present in the aluminate pastes during the last stage of the reaction kinetics. These parameters, obtained for the first time for C 3 A and C 4 AF, show striking difference from that obtained in previous experiments for calcium silicate pastes (C 2 S and C 3 S).

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).

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 optical filter of the disk chopper spectrometer at NIST

The Disk Chopper Spectrometer at the NIST Center for Neutron Research (NCNR) uses a “neutron optical filter” to remove high-energy neutrons and gamma rays. Nevertheless the measured background at the sample position is time-dependent because of short-wavelength neutrons that get through the optical filter and the choppers. We explain how this happens.