High-flux Backscattering Spectrometer Research Articles

Neutron Scattering Studies of Lithium-Ion Diffusion in Ternary Phosphate Glasses

We have studied the diffusion mechanism of lithium ions in glassy oxide-based solid state electrolytes using elastic and quasielastic neutron scattering. Samples of xLi 2 SO 4 -(1-x)(Li 2 O-P 2 O 5 ) were prepared using conventional melt techniques. Elastic and inelastic scattering measurements were performed using the triple-axis spectrometer (TRIAX) at Missouri University Research Reactor at University of Missouri and High Flux Backscattering Spectrometer (HFBS) at NIST Center for Neutron Research, respectively. These compounds have a base glass compound of P 2 O 5 which is modified with Li 2 O. Addition of Li 2 SO 4 leads to the modification of the structure and to an increase lithium ion (Li + ) conduction. We find that an increase of Li 2 SO 4 in the compounds leads to an increase in the Lorentzian width of the fit for the quasielastic data, which corresponds to an increase in Li + diffusion until an over-saturation point is reached ( 2 SO 4 ). We find that the hopping mechanism is best described by the vacancy mediated Chudley-Elliot model. A fundamental understanding of the diffusion process for these glassy compounds can help lead to the development of a highly efficient solid electrolyte and improve the viability of clean energy technologies.

Rotational dynamics and coupling of methyl group rotations in methyl fluoride studied by high resolution inelastic neutron scattering

Methyl group rotations in methyl fluoride were studied using the high flux backscattering spectrometer SPHERES at FRM-II. The asymmetry and width of the low temperature tunneling peak was used to determine if coupled rotations between neighboring methyl fluoride molecules exist. The temperature dependent broadening of the tunneling peak was used to determine the first librational transition and compared to the temperature dependent shift of the position of the tunneling peak. The results obtained by using inelastic neutron scattering confirm previous models that assume rotational coupling. This is the first neutron backscattering experiment with sub-microeV resolution at energy transfers up to 31 microeV.

Direct Analysis of the Ion-Hopping Process Associated with the α-Relaxation in Perfluorosulfonate Ionomers Using Quasielastic Neutron Scattering

This work demonstrates the ability of quasielastic neutron scattering (QENS) to measure the dynamics associated with counterions in perfluorosulfonate ionomers (PFSIs). PFSI membranes were prepared by neutralizing with hydrogenated alkyl ammonium counterions. Counterion dynamics were measured using the High-Flux Backscattering Spectrometer at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). Long-range mobility of the counterions was closely linked with the α-relaxation in these materials measured by dynamic mechanical analysis (DMA). The counterion motions in the membrane were found to follow a mechanism of random jump-diffusion within a confined spatial region with diffusion coefficients on the order of 10−7 cm2 s−1. These data are presented along with variable temperature X-ray scattering investigations of the melting behavior of these materials. Altogether, the data presented here show the link between the onset of long-range counterion mobility and the mechanical properties of these materials. These data provide further fundamental understanding of the link between electrostatic interactions and dynamics in PFSI materials.

A background suppression chopper for the High-Flux Backscattering Spectrometer at NIST

A characteristic of backscattering spectroscopy is the limited flux in achieving high resolution. This means that one requires a very high signal-to-noise ratio by keeping the background to a minimum. The high-flux backscattering spectrometer at the NIST Center for Neutron Research uses a phase-space transformation (PST) chopper to enhance the flux by a factor of 4.2. The chopper has six equal size sectors, alternating between open and pyrolytic graphite. The background in this spectrometer arises from neutrons from the main guide. Measurements show that neutrons which pass through an open sector of the PST chopper at the same time that the monochromatized beam passes through on its way to the sample, contribute to 60% of the background. In order to reduce this background while retaining 80% of the present neutron flux, we propose a second chopper placed between the local shutter and the entrance to the converging guide, synchronized with the PST chopper.

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.

Rotational dynamics of methyl groups in m-xylene

Methyl group dynamics of m-xylene was investigated by using incoherent inelastic and quasi-elastic neutron scattering. Inelastic measurements were carried out at the high flux backscattering spectrometer HFBS at the National Institute of Standards, quasi-elastic measurements at the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institute. Rotational potentials are derived which describe the tunnel splittings, first librational, and activation energies of the two inequivalent CH(3) groups. Indications for coupling of the methyl rotation to low-energy phonons have been found. The finite width of one tunneling transition at He temperature is described by direct methyl-methyl coupling. The combined results of the experiments and the calculations allow a unique assignment of rotor excitations to crystallographic sites.

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 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.

Status of the high-flux backscattering spectrometer RSSM for the FRM-II reactor in Munich

The new backscattering spectrometer RSSM for the FRM-II reactor uses all possibilities to optimize this type of spectrometer. In its high-intensity version it uses unpolished Si(111) crystals. The good energy resolution of δE≤0.9 μeV is achieved by staying as closely as possible at a Bragg angle of 90° not only at the Doppler monochromator in the primary spectrometer but also at the analyzer system of the secondary spectrometer. The large solid angle of the analyzer system compensates the low intensity which is a result of the good energy resolution.

Developments in neutron beam devices and an advanced cold source for the NIST research reactor

The last 5 yr has been a period of steady growth in instrument capabilities and utilization at the National Institute of Standards and Technology Center for Neutron Research. Since the installation of the liquid hydrogen cold source in 1995, all of the instruments originally planned for the Cold Neutron Research Facility have been completed and made available to users, and three new thermal neutron instruments have been installed. Currently, an advanced cold source is being fabricated that will better couple the reactor core and the existing network of neutron guides. Many improvements are also being made in neutron optics to enhance the beam characteristics of certain instruments. For example, optical filters will be installed that will increase the fluxes at the two 30-m SANS instruments by as much as two. Sets of MgF 2 biconcave lenses have been developed for SANS that have demonstrated a significant improvement in resolution over conventional pinhole collimation. The recently commissioned high-flux backscattering spectrometer incorporates a converging guide, a large spherically focusing monochromator and analyzer, and a novel phase space transform chopper, to achieve very high intensity while maintaining excellent energy resolution. Finally, a prototype low background, doubly focusing neutron monochromator is nearing completion that will be the heart of a new cold neutron spectrometer, as well as two new thermal neutron triple axis spectrometers.

Design of a High-Flux Backscattering Spectrometer for Ultra-High Resolution Inelastic Neutron Measurements

ABSTRACTWe discuss the design of a new backscattering spectrometer to be installed at the Cold Neutron Research Facility at the National Institute of Standards and Technology. Si (111) crystals cover both monochromator and analyzer which are spherically bent to a radius of curvature of ~ 2 m to focus the incident and scatterered neutron beams. The bending increases the intrinsic lattice gradient of Si beyond its Darwin limit, resulting in an energy resolution of ~ 0.75 μeV FWHM. The monochromator is Doppler-driven, allowing users access to a dynamic range of ±60 μeV. The elastic Q-range covers 0.15 to 1.8 Å-1. The most novel aspect of this design lies in the incorporation of a phase-space-transform chopper. This device rotates at 4700 rpm while neutrons are Bragg-diffracted from sets of pyrolytic graphite crystals affixed to its periphery. The process enhances the neutron flux at the backscattering energy of 2.08 meV, but at the expense of a larger horizontal divergence. Computer simulations indicate a resultant flux increase of order 3 should be obtained.