NIST Center For Neutron Research Research Articles

Advanced polarization analysis capability on the very small-angle neutron scattering instrument at the NIST Center for Neutron Research

The Very Small Angle Neutron Scattering (VSANS) diffractometer at the National Institute of Standards and Technology has been commissioned and is in the user program. A large available space of nearly 2 m along the beam in the sample area not only enhances the existing SANS polarization analysis capability, but also makes it possible for implementation of other polarization analysis capabilities which would not be easily available on existing SANS instruments, including grazing-incidence small-angle neutron scattering with polarization analysis and spherical neutron polarimetry. We present two polarized setups, one for high magnetic sample fields and the other for low magnetic sample fields, together with a versatile and flexible operational platform for polarized beam experiments. The design of a magnetostatic cavity device that provides better field homogeneity and thus longer 3He polarization relaxation time is discussed. It consists of an end-compensated magnetic shielded solenoid with non-identical holes to accommodate the divergent scattered beam in a constrained distance. Significant improvement in polarized neutronic performance, 3He polarization relaxation time, and an extended momentum transfer range for polarization analysis are presented. Improved neutron polarizing devices, double V-shaped supermirror polarizer, adiabatic radio-frequency spin flipper, and a 3He spin analyzer have yielded an initial instrumental flipping ratio of 100, allowing for higher sensitivity to detection of weak magnetic features in the sample.

Low-background neutron reflectometry from solid/liquid interfaces

Liquid cells are an increasingly common sample environment for neutron reflectometry experiments and are critical for measuring the properties of materials at solid/liquid interfaces. Background scattering determines the maximum useful scattering vector, and hence the spatial resolution, of the neutron reflectometry measurement. The primary sources of background are the liquid in the cell reservoir and the materials forming the liquid cell itself. Thus, characterization and mitigation of these background sources are necessary for improvements in the signal-to-background ratio and resolution of neutron reflectometry measurements employing liquid cells. Single-crystal silicon is a common material used for liquid cells due to its low incoherent scattering cross section for neutrons, and the path lengths of the neutron beam through silicon can be several centimetres in modern cell designs. Here, a liquid cell is constructed with a sub-50 µm thick liquid reservoir encased in single-crystal silicon. It is shown that, at high scattering vectors, inelastic scattering from silicon represents a significant portion of the scattering background and is, moreover, structured, confounding efforts to correct for it by established background subtraction techniques. A significant improvement in the measurement quality is achieved using energy-analyzed detection. Energy-analyzed detection reduces the scattering background from silicon by nearly an order of magnitude, and from fluids such as air and liquids by smaller but significant factors. Combining thin liquid reservoirs with energy-analyzed detection and the high flux of the CANDOR polychromatic reflectometer at the NIST Center for Neutron Research, a background-subtracted neutron reflectivity smaller than 10−8 from a liquid cell sample is reported.

Batteries & Supercaps: In Situ and Operando Methods for Energy Storage and Conversion

Take a close look: This Special Collection is devoted to in situ and (in) operando methods developed and used for studying materials applied to electrochemical storage devices. Environmental concerns have pushed energy storage technologies toward higher sustainability and efficiency, and functional materials must evolve to meet these demands. This Special Collection concerns the recent evolution of methods and techniques used to understand the function of such materials, a necessary step towards the rational design of new energy storage technology with improved performance characteristics. Such characterisation methods would ideally give information about energy storage materials and components that are performing as they would in the operating device, as close to real-world applied conditions and with as much detail as possible. Even though it is often necessary to adapt the system being studied to the specific constraints of a characterisation tool to obtain detailed or high quality information, recent advances in instrumentation and methods are bringing us closer to being able to examine materials under representative real-life operating conditions. Such analytical approaches, which are also known as “in situ” and “(in) operando” methods, can provide unique insights on materials function under energy storage and delivery cycling conditions and are becoming increasingly popular in energy materials research. Both the number of available techniques applied to understand function of materials within working devices as well as the energy and spatial resolution gained by these techniques are growing. This Special Collection is devoted to these methods and features works highlighting such approaches as applied to electrochemical storage devices. While some works in this Special Collection highlight recent developments in the in situ and (in) operando characterisation techniques that have been successfully applied for more than a decade, such as X-ray and neutron diffraction and X-ray spectroscopy (Liang et al., Wu et al., Gustafsson et al., Chien et al., Lebens-Higgins), some new rising methods such as acoustic emission methods (Schweidler et al.), online gas analysis (Sångeland et al., Ab-El-Latif et al.), dilatometry (Michael et al.), and terahertz spectroscopy (Krotkov et al.), provide complementary information of the evolution of materials within systems, particularly regarding important insights into materials degradation mechanisms. Because instrumentation and techniques have advanced faster than terms used to describe many of these methods, we do not yet have an agreement within the community on language used to discuss these characterization conditions. The International Union of Pure and Applied Chemistry Committee “Recommendations for terms relating to materials characterization: Latin and other introduced terms” aims to clarify and resolve discrepancies for terms describing materials characterization, particularly for non-ambient conditions, within systems, and during change, as relevant for energy storage materials. The project committee invites you to contribute to this project and seeks input in particular into reviewing the list of terms used to describe sample characterisation conditions. To get involved, please see details at: https://iupac.org/projects/project-details/?project nr=2021-009-2-500. Vanessa Peterson is a Senior Principal Research and Neutron Instrument Scientist for powder diffraction at the Australian Centre for Neutron Scattering and an Honorary Professorial Fellow at the University of Wollongong. She obtained her Ph.D. in 2004 in conjunction with the neutron scattering group at the Australian Nuclear Science and Technology Organisation. She previously worked at the NIST Center for Neutron Research in the USA, and at the University of Sydney. She leads the “Energy Materials” research project from within the ACNS and specializes in characterization methods, particularly using real-time neutron and X-ray scattering to understand materials function. Lorenzo Stievano is Professor at University of Montpellier since September 2009. At ICGM, he is an expert in the spectroscopic characterization of functional inorganic materials and in the development of specific in situ and operando tools for the analysis of their working mechanisms. His research activities are currently focused on the fields of advanced Li-ion and post-lithium battery systems.

Real-Time Signal Processing for Mitigating SiPM Dark Noise Effects in a Scintillating Neutron Detector

A <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> LiF:ZnS(Ag)-based cold neutron detector with wavelength shifting (WLS) fibers and Silicon photomultiplier (SiPM) photodetector was developed at the NIST Center for Neutron Research. For neutron scattering applications at the NCNR, detector false positives severely diminish the quality of very faint neutron scatter patterns. Thermal noise generated by the SiPM significantly increases the likelihood of false positives by the detector/discriminator. This article describes and evaluates a digital real-time algorithm implemented on a field programmable gate array (FPGA) which quickly differentiates SiPM thermal noise and noise pulse pile-up from neutron signals. The algorithm reduces deadtime spent on examining noise pulses as well as reduces the number of false positives.

Measurement of the neutron decay electron-antineutrino angular correlation by the aCORN experiment

The aCORN experiment measures the neutron decay electron-antineutrino correlation ($a$-coefficient) using a novel method based on an asymmetry in proton time-of-flight for events where the beta electron and recoil proton are detected in delayed coincidence. We report the data analysis and result from the second run at the NIST Center for Neutron Research, using the high-flux cold neutron beam on the new NG-C neutron guide end position: $a = -0.10758 \pm 0.00136 (\mbox{stat}) \pm 0.00148 (\mbox{sys})$. This is consistent within uncertainties with the result from the first aCORN run on the NG-6 cold neutron beam. Combining the two aCORN runs we obtain $a = -0.10782 \pm 0.00124 (\mbox{stat}) \pm 0.00133 (\mbox{sys})$, which has an overall relative standard uncertainty of 1.7 \%. The corresponding result for the ratio of weak coupling constants $\lambda = G_A/G_V$ is $\lambda = -1.2796\pm 0.0062$.

Measuring deadtime and double-counts in a non-paralyzable scintillating neutron detector using arrival time statistics

A 6LiF:ZnS(Ag) based cold neutron detector with wavelength shifting (WLS) fibers and SiPM photodetector was developed at the NIST Center for Neutron Research for the CANDoR instrument (Chromatic Analysis Neutron Diffractometer or Reflectometer). The scintillation response of ZnS(Ag) to 6Li fission products is a long and irregular delayed fluorescence lasting more than 50 μs. To wait as long as 60 μs for complete fluorescence decay would impose an unreasonably long deadtime to the system and would severely limit the count rate of this detector. A real-time algorithm was implemented to compensate for the long fluorescence decay, reducing but not eliminating sensitivity for a 60 μs period. This greatly reduces the deadtime to approximately 3 μs compared to imposing a 60 μs hold-off time, while also achieving a double-count fraction (counting the same event multiple times) below 1E−4. Timestamping capabilities of the detector system were used to compile arrival time statistics. Comparisons of the measured arrival time statistics with Poisson arrival statistics illustrates features in the fluorescence compensation algorithm and enables the calculation of deadtime and double-count fraction.

H2 production in the 10B(n,α)7Li reaction in water

We demonstrate a method for measuring the H2 produced in water from the 10B(n,α)7Li fission reaction. Low energy neutrons from the NIST Center for Neutron Research interact with borate-containing water in a temperature-controlled high pressure cell made from titanium. After exposure for one to several hours, the water is extracted and sparged with argon. H2 entrained in the sparging gas is sampled with a small mass spectrometer. To determine the neutron exposure, a small amount of sodium is included in the borate solution. The water is collected and 24Na activation is measured in a counting apparatus on the following day. The G-value for H2 at room temperature is found to be (1.18 ± 0.10) molecules H2/100eV, in good agreement with previous estimates and recent modeling calculations.

Spatially and Time-Resolved Investigation of Lithium Plating on a Graphite Electrode during Fast Charging Using Operando Neutron Depth Profiling (NDP)

Apart from increasing the battery size of electric vehicles (EVs), another approach to reduce range anxiety and to achieve mass market penetration is to increase the charging power and thus significantly lower charging times. To date, commercial lithium-ion battery (LIB) cells are almost exclusively based on graphite as anode material.[1] Here, owing to the low potential of the graphite electrode during charge, the charging rate must be limited in order to avoid lithium plating, which can thermodynamically occur in regions of the electrode where the potential drops below 0 V vs. Li+/Li. Plated lithium is very reactive, leading to irreversible reactions with the electrolyte and therefore strongly reduces lifetime and safety of a battery.[2] Studying lithium plating in a battery cell is very challenging since post-mortem diagnostics do not reflect the electrode state during operation, since plated lithium will reintercalate into the active graphite material on a timescale of minutes at room temperature.[3] , [4] Hence, non-destructive operando techniques[5] , [6] are necessary to elucidate and follow lithium plating mechanisms in an operating cell.In this study, we present data collected via Neutron Depth Profiling (NDP) during lithium plating induced by fast charging of a graphite electrode at a rate of 2 C. NDP is a lithium-sensitive, non-destructive technique utilizing the neutron capture reaction of 6Li atoms to quantitatively measure the lithium distribution over the thickness of an electrode by probing the energy loss of charged particles formed by the neutron capture reaction.[7] We employ a recently developed operando NDP cell that shows a cycling performance comparable to a commercial cell even at high charging rates, and at the same time allows charged particles to exit the cell through a thin Kapton® window.[8] We were able to resolve the lithium depth distribution changes during the open circuit voltage (OCV) phase after a constant current charge with a time resolution of 2 min. This exceptionally high time resolution was achieved by combining the high cold neutron flux of 2.7×109 ncm-2s-1 at the NIST Center for Neutron Research (Washington D.C., USA) with a 6Li enriched electrolyte and a 6LiFePO4 cathode, which enhances the signal by an additional factor of 12. In order to achieve lithium plating already at a rate of 2 C, the LiPF6 salt concentration in the EC/EMC = 3/7 (w/w) electrolyte was reduced to 0.3 M.Figure 1 shows the NDP signal intensity as a function of the energy of the detected tritium particles, which translates into a depth profile: signals at an energy of ≈2400 keV represent lithium at the interface between the graphite electrode and the Cu current collector, and signals at decreasing energies represent lithium from increasing depth into the graphite electrode, with signals at an energy of ≈800 keV deriving from the graphite anode / separator interface. It is clearly visible that lithium plating starts towards the separator facing side of the graphite anode, caused by the lithium-ion concentration gradient forming over the thickness of the electrode during fast charging (dark blue profile); during the subsequent OCV phase, lithium re-distributes over the anode thickness, whereby this equilibration process is observed to be completed within ≈24 min.[1] C. Pillot, “The Rechargeable Battery Market and Main Trends 2014-2025”, 32nd International Battery Seminar and Exhibit, 3/9/2015, Presentation.[2] J. C. Burns, D. A. Stevens, J. R. Dahn, J. Electrochem. Soc. 2015, 162, A959.[3] Z. Y. Tang, J. J. Xue, C. Y. Liu, X. G. Zhuang, Acta Phys. - Chim. Sin. 2001, 17, 388.[4] M. Park, X. Zhang, M. Chung, G. B. Less, A. M. Sastry, J. Power Sources 2010, 195, 7904.[5] J. Wandt, P. Jakes, J. Granwehr, R. A. Eichel, H. A. Gasteiger, Mater. Today 2018, 21, 231.[6] R. Bhattacharyya, B. Key, H. Chen, A. S. Best, A. F. Hollenkamp, C. P. Grey, Nat. Mater. 2010, 9, 504.[7] M. Trunk, M. Wetjen, L. Werner, R. Gernhäuser, B. Märkisch, Z. Révay, H. A. Gasteiger, R. Gilles, Mater. Charact. 2018, 146, 127.[8] F. Linsenmann et al., A Lithium-Ion Battery Cell Design for Operando Neutron Depth Profiling, to be submitted. Figure 1

Neutron polarimetry using a polarized 3He cell for the aCORN experiment

The neutron polarization of the NG-C beamline at the NIST Center for Neutron Research was measured as part of the aCORN neutron beta decay experiment. Neutron transmission through a polarized 3He spin filter cell was recorded while adiabatic fast passage (AFP) nuclear magnetic resonance (NMR) reversed the polarization direction of the 3He in an eight-step sequence to account for drifts. The dependence of the neutron transmission on the spin filter direction was used to calculate the neutron polarization. The time dependent transmission was fit to a model which included the neutron spectrum, and 3He polarization losses from spin relaxation and AFP-NMR. The neutron polarization averaged over the spectrum of the NG-C beam was found to be ∣Pn∣≤4×10−4 with 90% confidence.

Cold neutron radiation dose effects on a 6LiF:ZnS(Ag) neutron detector with wavelength shifting fibers and SiPM photodetector

A 6LiF:ZnS(Ag) based cold neutron detector with wavelength shifting (WLS) fibers and SiPM photodetector was developed at the NIST Center for Neutron Research for the CANDoR instrument (Chromatic Analysis Neutron Diffractometer or Reflectometer). A series of detectors were irradiated with neutron doses ranging between 1E+11 n/cm2 to 6E+12 n/cm2. It was found that the neutron absorbing 6Li isotope was not measurably depleted, but the photonic yield of the detector deteriorated with increasing neutron dose. Photonic yields were compared before and after neutron exposure by comparing pulse energy spectrum photopeaks before and after exposure. A typical detector used in the CANDoR instrument is expected to withstand a cumulative cold neutron dose of at least 1E+12 n/cm2 before degrading to an unfit state. The component parts of the detector could not be separated, so the degradation of the bulk scintillator and WLS fibers could not be gauged separately.

Neutron grating interferometer with an analyzer grating based on a light blocker.

We study an analyzer grating based on a scintillation light blocker for a Talbot-Lau grating interferometer. This is an alternative way to analyze the Talbot self-image without the need for an often difficult to fabricate absorption grating for the incident radiation. The feasibility of this approach using a neutron beam has been evaluated and experiments have been conducted at the cold neutron imaging facility of the NIST center for Neutron Research. The neutron grating interferometer with the proposed analyzer grating successfully produced attenuation, differential phase, and dark-field contrast images. In addition, numerical simulations were performed to simulate the Talbot pattern and visibility using scintillation screens of different thicknesses and there is good agreement with the experimental measurements. The results show potential for reducing the difficulty of fabricating analyzer grating, and a possibility for the so-called shadow effect to be eliminated and large-area gratings to be produced, especially when applied to X-rays. We report the performance of the analyzer grating based on a light blocker and evaluate its feasibility for the grating interferometer.

Spatially and Time-Resolved Investigation of Lithium Plating on a Graphite Electrode during Fast Charging Using Operando Neutron Depth Profiling (NDP)

Apart from increasing the battery size of electric vehicles (EVs), another approach to reduce range anxiety and to achieve mass market penetration is to increase the charging power and thus significantly lower charging times. To date, commercial lithium-ion battery (LIB) cells are almost exclusively based on graphite as anode material.[1] Here, owing to the low potential of the graphite electrode during charge, the charging rate must be limited in order to avoid lithium plating, which can thermodynamically occur in regions of the electrode where the potential drops below 0 V vs. Li+/Li. Plated lithium is very reactive, leading to irreversible reactions with the electrolyte and therefore strongly reduces lifetime and safety of a battery.[2] Studying lithium plating in a battery cell is very challenging since post mortem diagnostics do not reflect the electrode state during operation, since plated lithium will reintercalate into the active graphite material on a timescale of minutes at room temperature.[3] , [4] Hence, non-destructive operando techniques[5] , [6] are necessary to elucidate and follow lithium plating mechanisms in an operating cell.In this study, we present data collected via Neutron Depth Profiling (NDP) during lithium plating induced by fast charging of a graphite electrode at a rate of 2 C. NDP is a lithium-sensitive, non-destructive technique utilizing the neutron capture reaction of 6Li atoms to quantitatively measure the lithium distribution over the thickness of an electrode by probing the energy loss of charged particles formed by the neutron capture reaction.[7] We employ a recently developed operando NDP cell that shows a cycling performance comparable to a commercial cell even at high charging rates, and at the same time allows charged particles to exit the cell through a thin Kapton® window.[8] We were able to resolve the lithium depth distribution changes during the open circuit voltage (OCV) phase after a constant current charge with a time resolution of 2 min. This exceptionally high time resolution was achieved by combining the high cold neutron flux of 2.7×109 ncm-2s-1 at the NIST Center for Neutron Research (Washington D.C., USA) with a 6Li enriched electrolyte and a 6LiFePO4 cathode, which enhances the signal by an additional factor of 12. In order to achieve lithium plating already at a rate of 2 C, the LiPF6 salt concentration in the EC/EMC = 3/7 (w/w) electrolyte was reduced to 0.3 M.Figure 1 shows the NDP signal intensity as a function of the energy of the detected tritium particles, which translates into a depth profile: signals at an energy of ca. 2400 keV represent lithium at the interface between the graphite electrode and the Cu current collector, and signals at decreasing energies represent lithium from increasing depth into the graphite electrode, with signals at an energy of ca. 800 keV deriving from the graphite anode / separator interface. It is clearly visible that lithium plating starts towards the separator facing side of the graphite anode, caused by the lithium-ion concentration gradient forming over the thickness of the electrode during fast charging (dark blue profile); during the subsequent OCV phase, lithium re-distributes over the anode thickness, whereby this equilibration process is observed to be completed within ca. 24 min.

Progress towards Operation of a Deuterium Cold Neutron Source at the NCNR

The NIST Center for Neutron Research (NCNR) operates a 20 MW research reactor that produces neutrons for a suite of 30 neutron scattering instruments. 70% of these instruments use cold neutrons (E<5 meV), which are moderated by two separate cold neutron sources. The cold moderator for both sources is liquid hydrogen (LH2), which is in turn cooled by a recently commissioned 7 kW, 14K helium refrigerator. NCNR plans to replace the larger cold source with a new one operating with liquid deuterium (LD2). This report focuses on progress towards the upgrade to liquid deuterium, and options to address the particular challenges of designing and operating a cooling system that simultaneously supports operation with both LH2 and LD2.

Heat transfer analysis study for the design of the New Polychromatic Beam Neutron Reflectometer CANDOR (Chromatic Analysis Neutron Diffractometer Or Reflectometer).

A newly developed polychromatic beam neutron reflectometer CANDOR (Chromatic Analysis Neutron Diffractometer Or Reflectometer) on NG-1 at the NIST Center for Neutron research (NCNR) utilizes a wavelength-sensitive neutron detector consisting of 324 analyzing highly-oriented pyrolytic graphite (HOPG) crystals positioned sequentially in rows. Known for having a small thermal diffuse scattering cross section, HOPG crystals can lead to low signal-to-noise ratios in wavelength-sensitive detectors such as CANDOR. Even though it is possible to mathematically separate the desired signal from thermal diffuse scattering; by cooling the detector array of HOPG crystals in order to minimize the Debye Waller effect generates a better solution to this problem. In this heat transfer analysis study we show, within the instrument design constrains and thermodynamic considerations, technical feasibility and test results for the development of the New Polychromatic Beam Neutron Reflectometer CANDOR (Chromatic Analysis Neutron Diffractometer Or Reflectometer) at the NIST Center for Neutron Research.

Preface

The 12th International Conference on Polarised Neutrons for Condensed Matter Investigations (PNCMI 2018) was held from 3rd-6th July 2018 at Milton Hill near Abingdon, UK. 105 registered participants attended the conference which benefitted from unusually glorious summer weather in the peaceful surroundings of the house. This biennial meeting was inaugurated in Dubna (Russia) in 1996 and since then has been the main forum for condensed matter researchers and polarized neutron instrument scientists to get together to discuss current scientific and technical challenges in the field of polarized neutron scattering, as well as highlighting recent achievements and challenges.The PNCMI 2018 conference programme continued this tradition of providing a broad overview of both cutting edge science enabled by polarised neutron methods and technical developments that push the boundaries of neutron science. Of the many scientific highlights from the conference presented in this volume of proceedings, the paper of Prof. Stephen Lovesey (Rutherford Appleton Laboratory, UK) et al. on the discovery of unusual time- and parity-odd multipoles (anapoles) in a long-studied actinide intermetallic deserves particular mention, as does the fine work of Dr. Stephane Raymond (CEA-Grenoble, France) et al., which shows an unusual cancellation of the magnetic and nuclear intensities in the excitation spectrum of a magnetocaloric material. A particular standout technical achievement is presented in a paper by Dr Stephan Sponar (TU Wien, Austria) and co-workers on the ongoing development of a compact interferometer to study a previously contentious “which-way” measurement of neutron trajectories, highlighting the strength of polarised neutrons as a quantum mechanical probe.PNCMI 2018 was preceded by a polarised neutron summer school on the two days prior to the conference, organised by staff at the ISIS pulsed Neutron and Muon source of the STFC-Rutherford Appleton Laboratory. 15 PhD students attended lectures presented by Prof. Ken Andersen (European Spallation Source, Sweden), Dr. Jörg Voigt (FZ-Jülich, Germany), Dr. Joe Paddison (University of Cambridge, UK) and ISIS polarized neutron instrument scientists. Practical sessions on ISIS instruments included; polarised neutron reflection, 3He spin filters, and polarised single crystal diffraction. One of the great successes of PNCMI 2018 was the large attendance by an international cohort of PhD students and their very impressive contributions to the conference programme. The PhD poster prizes (sponsored by Oxford Instruments, UK) were awarded to Niels Geerits (TU Wien, Austria / TU Delft, Netherlands) presenting a novel time-of-flight-MIEZE spectrometer, and Gesara Bimashofer (Paul Scherrer Institut, Switzerland) for a polarized neutron reflectometry study of lithium-intercalated manganite thin-films. The future of polarized neutron scattering is in very safe and capable hands.The organisers would like to take this opportunity to gratefully acknowledge the support of the international advisory committee and the conference sponsors; ISIS (UK), Oxford Instruments (UK), The Institute of Physics (UK), The European Spallation Source (Sweden), Swiss Neutronics (Switzerland), Ansto (Australia) and the Institut Laue-Langevin (France). Special thanks are due to staff at Milton Hill for their hard work during the conference, and to Claire Garland of the Institute of Physics and her team for their excellent organizational support.The next International Conference on Polarised Neutrons for Condensed Matter Investigations will take place in Maryland in 2020 and will hosted by the NIST Center for Neutron Research (NCNR).Ross Stewart (Conference Chair)Gøran Nilsen (Editor)Rob Dalgliesh (Editor)

Improved Methods for Simultaneous Neutron and X-Ray Tomography of Operating Fuel Cells

Neutrons and X-rays provide complementary, non-destructive information on the operating conditions inside fuel cells. As previously reported, NIST has developed a fuel cell to facilitate simultaneous, multimodal tomography on the Neutron and X-ray Tomography (NeXT) system at the NIST Center for Neutron Research. The original cell design served as a developmental testbed but suffered several drawbacks such as CT artifacts caused by cell design and scan parameter choices and long, 18 hour tomography scans where the cell would need to operate at steady-state. Multiple improvements to the NeXT system and the tomography reconstruction workflow has resulted in a reduction of scan times down to as low as 4 hours with a reduction in CT artifacts. The improvements to reconstruction quality and the development of registration and segmentation tools that work on the multimodal datasets allows for the tracking of the gas diffusion layer – catalyst layer interfaces. This will improve the conversion of measured water thickness to saturation as local compression effects and MEA waviness can be accounted for. A new cell design to further reduce reconstruction artifacts and a new method for hybrid high resolution radiography and tomography will be discussed.

Characterization of the phase sensitivity, visibility, and resolution in a symmetric neutron grating interferometer.

A symmetric Talbot-Lau neutron grating interferometer has been developed for achieving high phase sensitivity. The gratings for the interferometer have been designed by maximizing the intergrating distances available at the experimental facility and optimizing the period of the gratings. The phase sensitivity in a Talbot-Lau grating interferometer has been mathematically modeled and analyzed and compared with experimental data. Evaluation experiments have been performed at the cold neutron imaging facility of the NIST Center for Neutron Research. The symmetric Talbot-Lau neutron grating interferometer exhibits distinct advantages in terms of high sensitivity and wide range for dark-field contrast imaging. Also, the fabrication of gratings is made easier, simpler, and more economical by the Gadox powder filling method, which gives them excellent phase contrast compared with other geometric Talbot-Lau neutron grating interferometers.

Visualization of Magnetic Domains in Electrical Steel Using High-Resolution Dark-Field Imaging

Electrical steel is a soft magnetic steel material used in electric devices such as transformers and motors. The performance of these electric devices is primarily related to the magnetic properties of electrical steel, and the assessment of the magnetic properties of electrical steel has been considered an important topic. We use neutron grating interferometry, which is an imaging technique for visualizing the magnetic domain of electrical steel as the evaluation of magnetic properties. The dark-field image provided by neutron grating interferometry shows a sensitive contrast with respect to the magnetic domain of electrical steel due to the small angle neutron scattering generated at the domain wall. The Talbot-Lau interferometer was installed, and the feasibility test of high-resolution dark-field imaging was conducted at cold neutron imaging beamline of the NIST Center for Neutron Research. The dark-field image of electrical steel was compared with the magnetic domain image observed by the Bitter pattern based on the magnetic powder method to prove the validity of neutron grating interferometry. The dark-field image visualizes the magnetic domains of electrical steel, more detailed domain walls regardless of laser-irradiated lines than Bitter pattern result. (Received January 25, 2019; Accepted April 22, 2019)

Small-angle neutron polarimetry apparatus (SANPA): Development at the NIST Center for Neutron Research

Spherical neutron polarimetry directly measures the rotation of the neutron polarization after scattering from materials with magnetic structure. It is an under utilized measurement technique that is capable of measuring all nine elements of the polarization tensor of a material. In this article, we describe our new cryogen-free small-angle neutron polarimetry apparatus and infrastructure at the NIST Center for Neutron Research. The resulting apparatus is capable of continuous operation and is designed for measurements at low temperatures (4-8 K) using niobium Meissner shielding and mu-metal shielding to produce a zero-field (≤1 μT) cooling sample environment.

Wide-angle polarization analysis on the multi-axis crystal spectrometer for the study of collective and single particle dynamics of methanol at its prepeak

Neutron polarization analysis capability with a large scattering angle and momentum transfer coverage has not been widely implemented for the separation of coherent and incoherent quasi-elastic neutron scattering (QENS) spectra. A feasibility test for soft condensed matter applications was performed on the Multi-Axis Crystal Spectrometer (MACS) at the NIST Center for Neutron Research (NCNR). This capability benefitted from the high flux wide angle polarization analysis capability recently developed on MACS for hard condensed matter investigations. The experimental procedure is thoroughly described on how to take data and perform polarization efficiency correction before further data analysis is done. This capability was employed to unambiguously separate the coherent and incoherent signal from the QENS spectra for the investigation of both the collective and the single particle dynamics in partially deuterated (CH3OD, D 99.5% purity) methanol. By studying a Q range encompassing the prepeak position, it was possible to directly measure the dynamics of the h-bonding associates which originate the prepeak signal, without contamination from the incoherent scattering, in spite of the relative weakness of the coherent scattering.