3
Assessment of the Portfolio of Scientific and Technical Projects and Expertise

This chapter addresses the second issue in the statement of task (see Chapter 1), assessing the portfolio of scientific and technical expertise within the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). That portion of the statement of task asked two sets of questions: (1) Does the organization have world-class expertise in reactor operations and the development and utilization of advanced neutron instrumentation? If not, in what areas should it be improved? (2) How well does the organization’s scientific and technical expertise support the research programs and the organization’s ability to achieve its stated objectives?

The answer to each question is generally yes; some facilities are world class and world leading, owing to recent investments and improvements. However, many key instruments are old and inefficient and need to be upgraded with advanced instrumentation and software or, in some cases, replaced. Challenges associated with the use of the NCNR instruments to investigate condensed matter phenomena are highlighted.

OVERVIEW

The quality of the experimental research at NCNR continues to be of high intellectual value, cutting edge, and world leading in diverse areas. The scientific and technical staff are among the most experienced and very best in the world; this is evident from the experience of panel members who have familiarity with facilities around the world and from interviews with the leadership of the user group. It is also evident from the scientific reputations of the instrument scientists. Of course, the quality of the outcomes that a user can achieve depends not only on the quality of the scientific and technical staff but also on the quality and efficiency of the instruments being used. As noted in Chapter 2, some of the instruments at NCNR are world-leading owing to recent upgrades, whereas others will require improvements if they are to provide reliable, high-resolution data at competitive rates. It should be noted, as discussed earlier, that the older instruments still provide reliable data, although the collection process is slow and time-consuming and requires deep expertise on the part of the NCNR instrument scientists. This is a potential liability in the long run, if critical upgrades are not accomplished in a reasonable time—years, not a decade. If upgrades are not accomplished in a timely manner, the scientific output of these instruments will curtail rapidly. Because of the number of old instruments, and their overall productivities, this will negatively impact the overall productivity of NCNR. This would negatively impact the quality of neutron research in the United States.

During the unplanned shutdown since February 2021, NCNR staff have devoted valuable time toward working with users to provide support for data analysis, exploiting techniques such as machine learning. These efforts have contributed to a deeper understanding of experimental outcomes. The panel is impressed with the expertise of the reactor operators and the scientific and technical support staff. The collaborative design of experiments and sample environments between users and beam scientists led to important new advances, as discussed later in this chapter. While the NCNR facility is understaffed in some areas, causing a stressful situation, the dedication of NCNR staff is impressive, as always. It must

also be noted that the NCNR researchers did a commendable job mentoring the postdoctoral researchers, including helping them to get access to neutrons in other facilities around the world while NCNR has been shut down. This has contributed to the overall productivity of NCNR’s research.

The collaborative efforts between users and beam scientists were highly productive, enabling new advances. In nuclear physics, for example, the research remained focused on meaningful—and important—foundational problems. Experiments on porous inorganic materials, with applications ranging from heat transfer to absorption, were devoted to questions that revolve around structure and dynamics; highly cited publications in high-impact-factor journals, including Science, resulted from these efforts.

Another noteworthy area where the research outcomes are notable is electrochemical batteries. Electrochemical batteries play a key role in the energy transition; they are not only important for electric vehicles, but also for grid applications, enabling the increased integration of renewables. Neutron scattering measurements have provided new insights toward improving the performance and safety of batteries. Specifically, the improved neutron depth profiling instrument and sample environment enhancements provided unprecedented information about lithium-ion transport processes, which are otherwise very difficult to obtain. The improved sensitivity and speed of data measurement enabled unique in operando monitoring of the movement of lithium ions in batteries. This work will contribute to higher-capacity battery systems, increased safety, and longer lifetimes of installed batteries. Additionally, the imaging combining neutrons and X rays is among the very best and enables comparatively rapid measurements of the spatial compositional distributions of material components. Machine learning algorithms were exploited to accomplish these outcomes. The proposed cold neutron station will enable unprecedented in situ information about correlations between bulk transport and nano-structural (compositional) changes that affect performance, lifetimes, and failure. The research in other areas of hard condensed matter, with collaborative NIST staff support, continues to be first rate.

The soft condensed matter efforts continue to be highly productive, with important scientific and technological impact. There is a suite of instruments that have either recently undergone upgrades or are scheduled for upgrades, as discussed below. The nSOFT consortium continues to be highly successful because of the unique suite of instruments that provide new and complimentary information. The newly upgraded very-small-angle neutron scattering (VSANS) instrument, for example, enables measurements over wider spatial and temporal scales. The panel continues to be pleased with the scientific support and innovation in this area. In the following sections, additional details of activities and accomplishments in the areas mentioned above are presented.

Fundamental Neutron Physics

The basic neutron physics program at NCNR has a rich history dating back to the early 1990s, when a pioneering neutron interferometry capability was implemented. Neutron physics experiments are carried out by the Neutron Physics Group, which is part of the Radiation Physics Division of the NIST Physical Measurement Laboratory (PML). While not formally part of NCNR, the Neutron Physics Group nevertheless operates eight beam lines beam instruments at NCNR and is fully committed to the goal of delivering world class science using the unique cold neutron beam facilities.

Many of the experiments discussed in this report are still addressing important questions in nuclear physics, particle physics, and astrophysics, such as the neutron lifetime (a key parameter in Big Bang nucleosynthesis), whether the radiative decay of a neutron can be observed (a prediction of quantum electrodynamics), whether time reversal violation can be observed in neutron decay (a possible window into the matter-antimatter asymmetry in the universe), and whether the precession of the spin of a transversely polarized neutron through liquid helium can be observed (a measure of the weak force between two nucleons). The work is often published in Science, Nature, and Physical Review Letters, the major high-impact journals.

All experiments require meticulous attention to detail, a distinctive characteristic of the work of the Neutron Physics Group. Students and postdocs in the program spend large amounts of time at NCNR

and become intimately familiar with what is required to execute experiments at a world-class level. The training is invaluable, and many go on to faculty and research positions, at NIST or elsewhere, staying in the neutron field and contributing to the next generation of leaders in neutron research. Recent successes include the following:

• Pendellösung interferometry measurements that set new limits on the existence of so-called “fifth forces” and set new bounds on the neutron charge radius,
• The first image of a physically isolated electric field using neutrons,
• Achievement of neutron helical wavefronts carrying quantized orbital angular momentum values,
• A confirmation that residual gas in the neutron lifetime proton trap is not causing an overestimate of the lifetime in a beam measurement,
• Development of the capability to measure an absolute neutron flux at the 0.1 percent level,
• A new precision result from Electron-Antineutrino Correlation experiments (aCORN³) for the value of the antineutrino–electron correlation “little a,”
• A precision measurement of the antineutrino spectrum at High-Flux Isotope Reactor, setting new limits on sterile antineutrinos and confirming that the anomaly seen by others at around 6 MeV is definitely present.

Future plans include performing an interferometric measurement of the gravitational constant, G, which could lead to an outcome with a relative uncertainty of 50 parts per million (ppm) in a year; the development of thermal kinetic inductance detectors that could affect a new series of beta decay studies; and the completion of the ongoing neutron beam lifetime measurement.

Porous Inorganic Materials

There is a broad portfolio of world-class experiments at NCNR investigating the structure and dynamics of host materials and adsorbed molecules, including systems for hydrogen storage; carbon capture; and molecular separation, sieving, or catalysis. This is enabled through a combination of neutron instrumentation that is particularly suited to exploring the structure and dynamics of materials and molecules, particularly when the latter contain hydrogen, in combination with the complementary development of sample environments and in-house scientists and collaborators of strong international standing.

Despite the age and modest technical performance of the powder diffractometer BT-1,⁴ as mentioned earlier, it is an essential research instrument in this area, responsible for several high-impact publications. The share of time on this instrument has increased steadily in this field in recent years and is likely to continue to increase. This poses an inherent challenge.

The High-Flux Backscattering Spectrometer (HFBS) also plays a key role for developing a deeper understanding of diffusional dynamics of molecules in porous inorganic materials, particularly for hydrogenous species. An illustration of the critical insights this instrument can provide in this field is provided by work on CO₂ diffusion in mesoporous hosts modified by adsorbed polyethyleneimine molecules in which the HFBS revealed the influence of polyethyleneimine chain motion on the efficiency of the host as a material for CO₂ adsorption.

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³ aCORN converts the angular correlation into a proton time-of-flight asymmetry. This is counted directly, and thus avoids the need for proton spectroscopy (Collett et al. 2017).

⁴ BT stands for beam tube. But the instruments are referred to as “BT” and that is the nomenclature used in this report.

Battery Materials

NCNR has a long-standing program of research activities supporting the development and analysis of battery technology. Neutrons provide an invaluable and unique probe of the location and movement of lithium within batteries. The work performed by the NIST Material Measurement Laboratory (MML) staff at NCNR using neutron depth profiling is unique worldwide, and the recent upgrade of the neutron depth profiling station operated by MML at NCNR with a dedicated neutron guide and endstation has enhanced the sensitivity of lithium quantification and increased the speed of measurement to enable in operando monitoring of the movement of lithium in batteries. This unique tool provides crucial information in support of developing higher capacity battery systems, enabling more reliable operation under extreme conditions, and ensuring longer lifetimes of installed batteries. All these aspects are key to advancing the electrification and de-carbonization of personal transport.

The Neutron Physics Group from the NIST PML operates the two neutron imaging stations at NCNR, with a world-leading program of neutron imaging. While the development of imaging stations at other facilities, for example, the neutron and X-ray tomography station at the Institut Laue-Langevin, have now surpassed NCNR in terms of flux, the expertise in the development and application of neutron imaging that exists at NCNR is second to none.

The thermal beam Neutron Imaging Facility has been in operation as a national user facility since 2006 and recently underwent upgrades, including a new detector and detector optics and the addition of an X-ray tomography station enabling simultaneous X-ray and neutron tomography. This facility supports imaging of battery systems, providing information on the spatial arrangement of battery chemistry on the several micrometer-length scale and enabling examination of the underlying mechanisms behind the effects that cycling, aging, and storage conditions have on battery performance. During the unplanned outage, the team has worked to compare various machine learning methods available in the tomography community. The primary goal has been to enable faster measurements by permitting the reconstruction of tomographs using fewer source images. This work to understand the limits of such algorithms not only allows for more and faster measurements, but also ensures that the results of such measurements are comparable to standard tomographic reconstructions.

The Cold Neutron Imaging station, which is constructed but still under development, will enable new studies of materials through the application of neutron focusing techniques (Wolter optics) providing for neutron microscopy, and through dark field imaging (the INFER Innovations in Measurement Science project) giving access to spatially resolved information on nanometer-scale structure. These methods will enable, in situ, the correlation of bulk transport of chemical components within batteries with nanostructural changes that can lead to battery lifetime decrease and failure.

Engineering Materials Research

NCNR excels in the domain of condensed matter research, which explores the structures and dynamics of materials such as metals, alloys, polymers, and complex fluids. The primary research revolves around investigating residual stresses arising from manufacturing and processing, measuring applied stresses in multiaxial deformation routes, studying preferred orientation phenomena, and identifying phase compositions. The hard matter research in NCNR has been carried out successfully with industrial partners in transportation sectors for their continuous interests in manufacturing and forming lightweight metals.

As noted in Chapter 2, the BT-8 diffractometer, which is used to perform stress and texture analysis, received significant enhancements during the 2021–2023 outage. Recent research results at the BT-8 diffractometer highlighted the instrument setup of octo-strain deformation with visualization of strain development using an advanced high-strength steel, showing the well-prepared instruments for concurrent neutron diffraction texture measurements. It is also interesting to note that the sample environments team plays a very important role in making modifications to the different experimental

setups, and their contributions not only improved the structure analysis of materials described in this section but have also vastly contributed to many improvements on different beamlines, ranging from controlling temperature to controlling the flow of liquids as well as obtaining a time stamp for time-sensitive experiments.

HARD CONDENSED MATTER

NCNR has a very strong hard condensed matter physics program, with research enabled by neutron scattering instruments at NCNR and facilities at other institutions. The program focuses on several very important topics in areas such as conventional superconductivity, van der Waals magnetic materials, and topological magnetic materials. Understanding the physics of these materials, broadly termed as quantum materials, is important to quantum computation, sustainable energy, and dissipation-less transport measurements. Compared with soft condensed matter physics groups at NCNR, the hard condensed matter physics groups have fewer resources and fewer staff members. In spite of this, the hard condensed matter physics groups still manage to make large impacts on quantum materials research, including the discovery of the unconventional spin triplet superconductivity candidate Ute₂, topological spin excitations in van der Waals materials, quantum spin liquid candidates, and frustrated magnetic materials. Because NCNR was not operational during the period of this assessment, most of the published work was based either on data taken before the shutdown or on data collected at other facilities.

Accomplishments

During the period covered by this assessment, NCNR staff members continued to work on the physics of Ute₂, an unconventional spin triplet superconductivity candidate. By collaborating with outside groups, they have provided evidence for an exotic superconducting state of Ute₂ through scanning tunneling microscopy, transport, and Kerr effect measurements. These measurements provide strong evidence for spin triplet superconductivity in this candidate material, have had a large impact on the condensed matter physics community, and will stimulate future work in this area. An additional important contribution from the group is the work on kagome lattice material Ymn₆Sn₆ and doped materials, where the microscopic origin of the topological Hall effect is found. A third important contribution is the work on Weyl-mediated helical magnetism in NaAlSi and magnetic exchange interactions in Kitaev interaction candidate BaCo₂(AsO₄)₂. These accomplishments, carried out despite the fact that NCNR was not running during this assessment period, are indicative of the quality of the staff and associated postdocs and students at the facility.

The future upgrade for the cold neutron source should considerably improve the competitiveness of the available spectrometers for hard condensed matter physics. The detailed design and improvement of the Spin Polarized Inelastic Neutron Spectrometer (SPINS) triplet axis spectrometer in the future cold source are also noted.

Challenges and Opportunities

The hard condensed matter program is producing world-leading science. The Multi-Axis Crystal Spectrometer (MACS)-II spectrometer offers unique experimental capability (e.g., event mode measurements accessing unique time scales with different sample environments) that is already delivering outstanding science with the promise of much more to come, assuming NCNR will operate normally in the near future. As discussed in Chapter 2, while the MACS at the cold source will continue to be competitive with the best cold neutron spectrometers and deliver world-leading science for years to come, the same cannot be said of the BT-4, the SPINS spectrometer, and the BT-1 powder diffractometer. To make NCNR relevant and be able to compete with the best spectrometers and diffractometers at other neutron facilities, these instruments will need to be upgraded to make them relevant again. It was noted

that the hard condensed matter physics community has not used other instruments, such as small-angle neutron scattering (SANS) and the spin-echo spectrometer at NCNR. The use of these instruments by the hard condensed matter physics community is important for the long-term health of these instruments, particularly when they consume the major part of the NCNR budget.

MANUFACTURING, ROBOTICS, AND ARTIFICIAL INTELLIGENCE–DRIVEN ANALYSIS

NCNR’s advances in support of manufacturing, robotics, and artificial intelligence-driven analysis are impressive. In particular, the work being done to enhance the Q-range accessible to experiments and the speed of data acquisition and analysis are especially welcome. There are a number of customers that NCNR collaborates with, including the National Science Foundation (NSF)-funded Center for High Resolution Neutron Scattering (CHRNS), which has been very successful in providing excellent access to NCNR to the academic community via the various beamlines that have been supported and upgraded; nSoft, which is an industrial consortium designed to help industrial members achieve specific goals via an annual paid membership; and the usual NIST collaborative programs and work individually funded by industrial companies.

Three projects stand out as being at the cutting edge, having no match as yet around the world:

1. Modification of the neutron reflectometry specifications at the Chromatic Analysis Neutron Diffractometer or Refractometer (CANDOR) beamline where, instead of collecting data for a single film and a single wavelength, the objective is to use up to 54 multiple wavelengths simultaneously and do the analysis on the backend.
2. The automatic dispensing equipment, referred to as the autonomous formulation laboratory, which allows the deposition of multiple material compositions and analysis in tandem and can be used with a variety of beamlines; this laboratory has traveled around the world to demonstrate its usefulness to a variety of applications.
3. The VSANS extension to lower Q is an excellent use that will make acquisition of information about the volume and size of scatterers quicker and more accurate over a wide range of Q values.

Because many of the cutting-edge improvements were instituted just before the reactor shutdown in 2021, not many papers have as yet been published. For the newly proposed reactor, the INFER neutron interferometer—with goals to develop new software, sample environments, and analysis techniques such as machine learning data analysis—is also noteworthy. It is expected that after the reactor is back online there will be many experiments that will be completed and published.

Additionally, for the beamlines that are designed to collect many more spectra than are often currently acquired, it will be important to ensure that NCNR improves its accessibility to computational capability that will allow processing of multiple versions of the machine learning models that will be generated with the goal of more quickly arriving at the best fitted model.

CONDENSED MATTER PHYSICS: SOFT MATTER

In the area of soft matter, the scientific and technical expertise is world-leading and well suited to support the research programs and external user community. NCNR has maintained a world-leading research portfolio in soft matter, particularly in the areas of flow-induced structure, phase behavior, and

dynamics of self-assembled systems, pharmaceuticals, and membrane biophysics. In addition to a strong academic user community, NCNR continues to be successful in engaging industry to solve commercially relevant problems. This is clearly evident in the continued publications in leading journals, including Science and Nature (more than 100 per year). The partnership with NSF via CHRNS supporting the operation of these instruments is crucial for the long-term viability and success of the user program.

Protein Interactions

The majority of all Food and Drug Administration–approved small molecule drugs that target membrane proteins are particularly challenging to study because of the atomic compositions and noncrystalline structures in commercial drugs. Molecular interactions become increasingly important to shelf stability, dose, injection, lipid membrane interaction, and efficacy. Neutron scattering is ideally suited to study the structure and dynamics of these systems. NCNR has done a commendable job in engaging industry to help understand the critical problems and where neutron imaging approaches can be helpful. Continued studies on monoclonal antibodies have led to the development of models that can better represent the interaction potential. In addition, the combination of scattering, reflectometry, and neutron spin echo can drive the fundamental understanding of small molecule and membrane interactions. The development of CANDOR and neutron scattering with time resolution, combined with neutron spin echo, will drive the next generation of targeted drug development.

Flow and Rheo-SANS

The mechanical properties of a soft matter system are directly related to the structure across all length scales within the system. Historically, systems were independently characterized where the structure was studied at rest, and researchers speculated about how the structure changed under shear forces. NCNR has established itself as a world leader in the capability to measure soft matter under flow with the implementation of rheo-SANS, 1-2 shear cell, and capillary rheo-SANS. More recently, these techniques have been combined with time-stamp resolution. These techniques have revolutionized the scientific field of rheology with direct application in consumer goods and pharmaceuticals.

Phase Behavior

The phase behavior of complex systems is a primary research focus area for many industrial segments, including consumer goods and pharmaceuticals. While neutrons are particularly suited to study these systems through deuterium isotope contrast matching, mapping out a phase diagram for a system can be time intensive and laborious. NCNR has developed world-leading facilities to efficiently study these systems with the development of temperature-controlled, automated, and artificial intelligence–directed sample environments. This is accomplished in part through the nSoft consortium, highlighting the importance of taking advantage of industrial expertise and capability.

Time Resolution

Soft matter systems are dynamic, even at rest. When perturbed by an external field, these systems will deform or flow. The response is controlled by the internal structure and interactions that constitute the material. Historically, neutron data were collected at equilibrium conditions and averaged over the data-collection time period. This limited the opportunity to study the dynamics of a system. The ability to associate a time stamp with the neutron information has been proven, and NCNR is in the process of expanding this capability across all its sample environments. This is a significant task for the sample environment team but will significantly expand the capability across all areas of soft matter, especially in

the areas of rheology, membrane biophysics, and phase behavior of complex systems. Time-resolved measurements offer a significant improvement across nearly all sample environments. In addition to the added effort in bringing forward this capability, equal effort will need to be taken to educate the user community to integrate this into their experimental plans.

Software

Data reduction and analysis directly complements data collection. Historically, the approach has been extremely fractured, with each facility—and, in many cases, individual research groups—developing their own software without independent verification of accuracy. The NCNR SANS team has led a global effort to develop software, called SasView, which is capable of reducing data universally across all neutron scattering facilities and leads the world in data analysis capability. The hurdle to gaining global alignment and support should not be understated, and this effort is viewed as monumental for the community as a whole. SasView will standardize data reduction, limiting errors and accelerating improvements in data analysis. It also will be a platform for continuing improvements in data analysis as new modeling capability continues to be developed.

Opportunities and Challenges

The most significant challenge for soft matter work is the technician and instrument scientist staffing needed to support the instruments at NCNR, both for startup after the unplanned shutdown and to support the instrument user community. The panel believes that funding needs to be secured urgently and open positions filled to ensure program readiness and instrument proficiency upon reactor startup. The integration of CANDOR, VSANS, and the new Neutron Spin Echo Spectrometer (NSE)-II into the user community is a significant opportunity for NCNR and is a priority with respect to staffing and sustainable access to the user community.

REFERENCE