NIST) Center For Neutron Research Research Articles

Preface

Proceedings of the joint 13th and 14th International Conferences on Polarized Neutrons for Condensed-Matter Investigations The international Conference on Polarized Neutrons for Condensed-Matter Investigations (PNCMI) started its long journey in Dubna, Russia in 1996 and has taken place biennially since then. The present volume of proceedings represents the 13th and 14th editions of this series. The 13th PNCMI was originally suggested by the International Advisory Committee (IAC) of PNCMI to be held in Maryland, USA in 2020, and was postponed due to the fast-spreading COVID-19 pandemic. The IAC later suggested, due to the still evolving pandemic, to replace PNCMI 2020 with a virtual PNCMI in 2021 and an in-person PNCMI in 2022.The 13th PNCMI, organized by National Institute of Standards and Technology (NIST), was held online from July 27-30, 2021. One of the main objectives was to keep up the frequency of PNCMI conferences for the community, staying up to date on emerging topics using polarized neutrons and on related instrumentation development. 110 participants from 40 institutions and 15 countries crossing Europe, Asia, Australia, and North America attended the virtual conference. The conference was limited to invited speaker and poster sessions since it was run two- and one-half hours daily to accommodate different time zones for the international attendees.The 14th PNCMI was held in-person from July 25-29, 2022 in Annapolis, Maryland, USA and jointly organized by the University of Maryland and NIST. The number of conference participants was affected by international travel restrictions due to the pandemic. The conference attendees included 62 participants from 30 institutions and 10 countries crossing Europe, Asia, and North America. The scientific program featured a Keynote lunch presentation, titled “Polarization can be good”, given by Dr. Charles Majkrzak (NCNR), invited, and contributed oral talks, and poster presentations.The PNCMI conference covers the latest condensed-matter investigations using polarized neutrons and state-of-the-art methodologies and techniques of polarized-neutron production and utilization for novel instrumentation and experiments. The 13th and 14th editions covered the following traditional topics: multiferroics and chirality, strongly correlated electron systems, frustrated and disordered systems, magnetic nanomaterials, thin films and multilayers, imaging, polarized neutron instrumentation and polarized neutron techniques and methods. Quantum materials as an emerging topic was included in the 13th PNCMI as well. A new session, -- polarized quasi-elastic neutron scattering (QENS) driven by recently growing interest and benefitting from modernized polarized neutron spectroscopies, was also included for the first time in the 14th PNCMI.A two-day Polarized Neutron School, for 12 graduate students and young postdoctoral researchers familiar with scattering techniques, was held at the NIST Center for Neutron Research (NCNR) preceding the 2022 in-person conference.We would like to take this opportunity again to thank all participants for their attendance, and the International Advisory Committee for guidance in developing an excellent scientific program. We’d like to extend a special thank you to the University of Maryland and the NIST conference services for all their hard work and planning to help organize and ensure it ran smoothly. The 14th PNCMI was made possible with the generous support of Cryogenic Limited, HTS-110, Mirrotron, ORNL, and SwissNeutronics.List of International Advisory Committee, Local Organizing Committee are available in this Pdf.

Dielectric RheoSANS - Simultaneous Interrogation of Impedance, Rheology and Small Angle Neutron Scattering of Complex Fluids.

A procedure for the operation of a new dielectric RheoSANS instrument capable of simultaneous interrogation of the electrical, mechanical and microstructural properties of complex fluids is presented. The instrument consists of a Couette geometry contained within a modified forced convection oven mounted on a commercial rheometer. This instrument is available for use on the small angle neutron scattering (SANS) beamlines at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The Couette geometry is machined to be transparent to neutrons and provides for measurement of the electrical properties and microstructural properties of a sample confined between titanium cylinders while the sample undergoes arbitrary deformation. Synchronization of these measurements is enabled through the use of a customizable program that monitors and controls the execution of predetermined experimental protocols. Described here is a protocol to perform a flow sweep experiment where the shear rate is logarithmically stepped from a maximum value to a minimum value holding at each step for a specified period of time while frequency dependent dielectric measurements are made. Representative results are shown from a sample consisting of a gel composed of carbon black aggregates dispersed in propylene carbonate. As the gel undergoes steady shear, the carbon black network is mechanically deformed, which causes an initial decrease in conductivity associated with the breaking of bonds comprising the carbon black network. However, at higher shear rates, the conductivity recovers associated with the onset of shear thickening. Overall, these results demonstrate the utility of the simultaneous measurement of the rheo-electro-microstructural properties of these suspensions using the dielectric RheoSANS geometry.

Invited Article: miniTimeCube.

We present the development of the miniTimeCube (mTC), a novel compact neutrino detector. The mTC is a multipurpose detector, aiming to detect not only neutrinos but also fast/thermal neutrons. Potential applications include the counterproliferation of nuclear materials and the investigation of antineutrino short-baseline effects. The mTC is a plastic 0.2% (10)B-doped scintillator (13 cm)(3) cube surrounded by 24 Micro-Channel Plate (MCP) photon detectors, each with an 8 × 8 anode totaling 1536 individual channels/pixels viewing the scintillator. It uses custom-made electronics modules which mount on top of the MCPs, making our detector compact and able to both distinguish different types of events and reject noise in real time. The detector is currently deployed and being tested at the National Institute of Standards and Technology Center for Neutron Research nuclear reactor (20 MWth) in Gaithersburg MD. A shield for further tests is being constructed, and calibration and upgrades are ongoing. The mTC's improved spatiotemporal resolution will allow for determination of incident particle directions beyond previous capabilities.

A new polarized neutron interferometry facility at the NCNR

A new monochromatic beamline and facility has been installed at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) devoted to neutron interferometry in the research areas of spin control, spin manipulation, quantum mechanics, quantum information science, spintronics, and material science. This facility is possible in part because of advances in decoherence free subspace interferometer designs that have demonstrated consistent contrast in the presence of vibrational noise; a major environmental constraint that has prevented neutron interferometry from being applied at other neutron facilities. Neutron interferometry measures the phase difference between a neutron wave function propagating along two spatially separated paths. It is a practical example of self interference and due to its modest path separation of a few centimeters allows the insertion of samples and macroscopic neutron spin rotators. Phase shifts can be caused by gravitational, magnetic and nuclear interactions as well as purely quantum mechanical effects making interferometer a robust tool in neutron research. This new facility is located in the guide hall of the NCNR upstream of the existing Neutron Interferometry and Optics Facility (NIOF) and has several advantages over the NIOF including higher incident flux, better neutron polarization, and increased accessibility. The long term goal for the new facility is to be a user supported beamline and makes neutron interferometer more generally available to the scientific community. This paper addresses both the capabilities and characteristics of the new facility.

Experimental characterization of the Advanced Liquid Hydrogen Cold Neutron Source spectrum of the NBSR reactor at the NIST Center for Neutron Research

The recent expansion of the National Institute of Standards and Technology (NIST) Center for Neutron Research facility has offered a rare opportunity to perform an accurate measurement of the cold neutron spectrum at the exit of a newly-installed neutron guide. Using a combination of a neutron time-of-flight measurement, a gold foil activation measurement, and Monte Carlo simulation of the neutron guide transmission, we obtain the most reliable experimental characterization of the Advanced Liquid Hydrogen Cold Neutron Source brightness to date. Time-of-flight measurements were performed at three distinct fuel burnup intervals, including one immediately following reactor startup. Prior to the latter measurement, the hydrogen was maintained in a liquefied state for an extended period in an attempt to observe an initial radiation-induced increase of the ortho (o)-hydrogen fraction. Since para (p)-hydrogen has a small scattering cross-section for neutron energies below 15meV (neutron wavelengths greater than about 2.3Å), changes in the o- p hydrogen ratio and in the void distribution in the boiling hydrogen influence the spectral distribution. The nature of such changes is simulated with a continuous-energy, Monte Carlo radiation-transport code using 20K o and p hydrogen scattering kernels and an estimated hydrogen density distribution derived from an analysis of localized heat loads. A comparison of the transport calculations with the mean brightness function resulting from the three measurements suggests an overall o- p ratio of about 17.5(±1) % o- 82.5% p for neutron energies<15meV, a significantly lower ortho concentration than previously assumed.

3He spin filter based polarized neutron capability at the NIST Center for Neutron Research

A 3He neutron spin filter (NSF) program for polarized neutron scattering was launched in 2006 as part of the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) Expansion Initiative. The goal of the project was to enhance the NCNR polarized neutron measurement capabilities. Benefitting from more than a decade's development of spin-exchange optical pumping (SEOP) at NIST, we planned to employ SEOP based 3He neutron spin filters for the polarized neutron scattering community. These 3He NSF devices were planned for use on different classes of polarized neutron instrumentation at the NCNR, including triple-axis spectrometers (TAS), small-angle neutron scattering instruments (SANS), reflectometers, and wide-angle polarization analysis. Among them, the BT-7 thermal TAS, NG-3 SANS, and MAGIK reflectometer have already been in the user program for routine polarized beam experiments. Wide-angle polarization analysis on Multi-Axis Crystal Spectrometer (MACS) has been developed for user experiments. We describe briefly the SEOP systems dedicated for polarized beam experiments and polarizing neutron development for each instrument class. We summarize the current status and polarized neutronic performance for each instrument. We present a 3He NSF hardware and software interface to allow for synchronization of 3He polarization inversion (neutron spin flipping) and free-induction decay (FID) nuclear magnetic resonance (NMR) measurements with neutron data collection.

Continuous distribution model for the investigation of complex molecular architectures near interfaces with scattering techniques

Biological membranes are composed of a thermally disordered lipid matrix and therefore require non-crystallographic scattering approaches for structural characterization with x-rays or neutrons. Here we develop a continuous distribution (CD) model to refine neutron or x-ray reflectivity data from complex architectures of organic molecules. The new model is a flexible implementation of the composition-space refinement of interfacial structures to constrain the resulting scattering length density profiles. We show this model increases the precision with which molecular components may be localized within a sample, with a minimal use of free model parameters. We validate the new model by parameterizing all-atom molecular dynamics (MD) simulations of bilayers and by evaluating the neutron reflectivity of a phospholipid bilayer physisorbed to a solid support. The determination of the structural arrangement of a sparsely-tethered bilayer lipid membrane (stBLM) comprised of a multi-component phospholipid bilayer anchored to a gold substrate by a thiolated oligo(ethylene oxide) linker is also demonstrated. From the model we extract the bilayer composition and density of tether points, information which was previously inaccessible for stBLM systems. The new modeling strategy has been implemented into the ga_refl reflectivity data evaluation suite, available through the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR).

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).

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.

3He spin filters for a thermal neutron triple axis spectrometer

We have tested two 3He neutron spin filters (NSF), one for the polarizer and one for the analyzer, in conjunction with a doubly focusing pyrolytic graphite (PG) monochromator on the state-of-the-art BT-7 thermal triple axis spectrometer (TAS) at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). This system will provide significantly better neutronic performance for polarization analysis over a conventional TAS with Heusler crystals. We discuss the scheme for employing NSFs on the TAS instrument, including the 3He cell design, spin-exchange optical pumping (SEOP) of these large 3He cells, and the holding fields on the spectrometer. Using Rb/K hybrid SEOP, we have produced 75% 3He polarization for the 11 cm diameter cells for TAS in less than two days.

Compton suppression for neutron activation analysis applications at the National Institute of Standards and Technology (NIST)

Suppression systems reduce Compton backscattering counts in a gamma ray spectrum. Every system has particular calibration nuances due to differences in electronics, detectors and environment. This study examines several Compton suppression systems at the NIST Center for Neutron Research in an effort to produce a general guide to their setup and use rather than determine specific settings for a certain experiment. Troubleshooting procedures for timing and calibration of these systems were also examined.A central high purity germanium (HPGe) detector is surrounded by guard detectors. Counts picked up by both detectors are Compton or escape events, while those detected only by the HPGe are photopeaks considered useful for analysis. The electronics system accepts counts detected only by the HPGe detector and rejects ones in coincidence with the guard detectors. Analysis of calibrated Cs-137, Na-24 and Co-60 sources demonstrated the effective Compton suppression for the systems. Examination of certified reference materials, activated in the reactor, demonstrated the experimental readiness of the systems.

The fundamental neutron physics facilities at NIST

The National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) is a national center that provides thermal and cold neutrons beams for activities such as condensed matter physics, materials science, nuclear chemistry, and biological science. Four beams are currently in use for fundamental physics experiments. These include a high-intensity polychromatic beam, a 0.496nm monochromatic beam, a 0.89nm monochromatic beam, and a neutron interferometer and optics facility. Experiments focus primarily on studies of the weak interaction using neutrons. This paper provides a general overview of the facilities and highlights some current experiments.

Detecting the Radiative Decay Mode of the Neutron.

Beta decay of the neutron into a proton, electron, and electron antineutrino is occasionally accompanied by the emission of a photon. Despite decades of detailed experimental studies of neutron beta-decay, this rare branch of a fundamental weak decay has never been observed. An experiment to study the radiative beta-decay of the neutron is currently being developed for the NG-6 fundamental physics endstation at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The experiment will make use of the existing apparatus for the NIST proton-trap lifetime experiment, which can provide substantial background reduction by providing an electron-proton coincidence trigger. Tests and design of a detector for gamma-rays in the 10 keV to 200 keV range are under development. The need for a large solid-angle gamma-ray detector that can operate in a strong magnetic field and at low temperature has led us to consider scintillating crystals in conjunction with avalanche photodiodes. The motivation and experimental technique will be discussed.

The Fundamental Neutron Physics Facilities at NIST.

The program in fundamental neutron physics at the National Institute of Standards and Technology (NIST) began nearly two decades ago. The Neutron Interactions and Dosimetry Group currently maintains four neutron beam lines dedicated to studies of fundamental neutron interactions. The neutrons are provided by the NIST Center for Neutron Research, a national user facility for studies that include condensed matter physics, materials science, nuclear chemistry, and biological science. The beam lines for fundamental physics experiments include a high-intensity polychromatic beam, a 0.496 nm monochromatic beam, a 0.89 nm monochromatic beam, and a neutron interferometer and optics facility. This paper discusses some of the parameters of the beam lines along with brief presentations of some of the experiments performed at the facilities.

Polarized [formula omitted] spin filters in neutron scattering

We report progress in the development and application of neutron spin filters based on transmission through polarized 3 He gas. Tests and application of these devices are underway at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) and the Intense Pulsed Neutron Source (IPNS). The results of application to polarization analysis for diffuse reflectometry is discussed, along with an example of the issues in choosing the flipping ratio for such applications. The status of our development of polarized gas production using both spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping will be presented. For SEOP, we currently obtain 70–75% 3 He polarization in cells up to 500 cm 3 in volume.

Polarized [formula omitted]He analyzers for neutron reflectometry

At the National Institute of Standards and Technology (NIST) and Indiana University, we are developing polarized 3 He analyzers for neutron reflectometry. We have employed 3 He analyzers at two polarized neutron reflectometers, NG1 at the NIST Center for Neutron Research (NCNR) and POSY I at the Intense Pulsed Neutron Source (IPNS), Argonne National Laboratory. The long-term goal for both efforts is to perform efficient studies of magnetic diffuse scattering. At the NCNR, we tested a 3 He analyzer by comparing measurements of specular scattering obtained with a supermirror analyzer to those obtained with a 3 He analyzer. For this test, we measured the spin-flip and nonspin-flip scattering from a Mn 0.52Pd 0.48/Fe thin film. The results with the 3 He analyzer show very good agreement with those obtained with the supermirror analyzer. We have also carried out tests of the 3 He analyzer for application to magnetic diffuse scattering experiments. We discuss the development of a 3 He analyzer for IPNS that will be employed for studies of patterned magnetic arrays.

Summer school on methods and applications of neutron spectroscopy held at NIST

Neutron scatterers from the National Institute of Standards and Technology (NIST) for the second time organized a workshop-style school on advanced spectroscopy methods and tools. The school, held from August 9–13, 1999 at the NIST Center for Neutron Research (NCNR) in Gaithersburg, Maryland, was attended by more than thirty participantsmostly graduate students and young researchers-from the United States and elsewhere.