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Science Highlights, March 29, 2017

Awards and Recognition

Jaqueline Kiplinger receives International Union of Pure and Applied Chemistry Award

Jaqueline Kiplinger

Jaqueline Kiplinger

The International Union of Pure and Applied Chemistry (IUPAC) selected Jaqueline L. Kiplinger (Inorganic, Isotope and Actinide Chemistry, C-IIAC) as one of 12 women to be honored with a 2017 Distinguished Women in Chemistry or Chemical Engineering Award. She is the first Los Alamos scientist to receive this recognition.

The IUPAC awards program began as part of the 2011 International Year of Chemistry celebrations. It acknowledges and promotes the work of women chemists/chemical engineers worldwide. The IUPAC selected the 12 awardees based on excellence in basic or applied research, distinguished accomplishments in teaching or education, or demonstrated leadership or managerial excellence in the chemical sciences. The Awards Committee is particularly interested in nominees with a history of leadership and/or community service during their careers. The IUPAC announced the selections to coincide with International Women’s Day and will present the awards during the 46th World Chemistry Congress.

The IUPAC Award honors Kiplinger for her groundbreaking work in establishing synthetic routes to novel uranium and thorium compounds that have opened new frontiers in understanding the nature of bonding and reactivity in actinides. She has developed inexpensive, simple, and safe techniques to make thorium and uranium halide starting materials. These materials have been critical to advance the synthetic and mechanistic chemistry of these important elements and to understand their behavior in a variety of applications. Her novel synthetic approaches have led to the systematic isolation of entirely new classes of molecular uranium and thorium complexes. Kiplinger has also pioneered the use of copper and gold reagents as one-electron oxidants for actinide compounds and designed a photochemical synthesis that established the first-ever evidence for the formation of a uranium complex containing a terminal uranium-nitrogen triple bond. The DOE Basic Energy Sciences, Heavy Element Program, the LANL G.T. Seaborg Institute for Transactinium Science, and the Laboratory Directed Research and Development Program (LDRD) have funded much of her research.

Kiplinger came to the Lab as the first Frederick Reines Postdoctoral Fellow in 1999, and became a Technical Staff Member within Chemistry Division in 2002. She is a Fellow of the American Association for the Advancement of Science, the Royal Society of Chemistry, the American Institute of Chemists, and Los Alamos National Laboratory. Kiplinger has published nearly 100 journal articles and has more than 3400 citations. She has received two R&D 100 Awards, was selected as the first woman to receive the F. Albert Cotton Award in Synthetic Inorganic Chemistry from the American Chemical Society (ACS), and is the first Lab scientist to receive two national-level ACS awards (the first being the Nobel Laureate Signature Award in Chemistry.) Los Alamos has honored her with the Fellows Prize, three mentoring awards, and several LANL/NNSA Best-in-Class Pollution Prevention Awards.

The IUPAC fosters worldwide communications in the chemical sciences and unites academic, industrial, and public sector chemistry in a common language. It is the world authority on chemical nomenclature, terminology, standardized methods for measurement, atomic weights and other critically evaluated data. Its conferences and projects promote modern developments in chemistry and assist in chemical education and the public understanding of chemistry. More information about IUPAC: Technical contact: Jaqueline Kiplinger

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Ferrier, Zylstra, and Myers win Postdoctoral Distinguished Performance Awards

The Laboratory established the Postdoctoral Distinguished Performance Awards to honor outstanding and unique contributions by Lab postdocs, which result in a positive and significant impact on LANL’s scientific efforts and status in the scientific community. These awards also recognize outstanding creativity, innovation, and/or dedication and level of performance substantially beyond that which would normally be expected. Technical contact: Mary Anne With

Maryline Ferrier

Maryline Ferrier

Maryline Ferrier (Inorganic, Isotope and Actinide Chemistry, C-IIAC) is a current Postdoctoral Research Associate. The Award recognizes her outstanding research and leadership in actinide chemistry, specifically her investigation of the chelation chemistry of actinium. Worldwide interest in actinium-225 as a potential radiopharmaceutical anti-cancer agent motivated her research.  She conducted the first in a series of unique actinium coordination chemistry studies, which resulted in two papers in Nature Communication and ACS Central Science. Ferrier used extended x-ray absorption fine structure (EXAFS) experiments to make the first measurement of the length of an actinium bond. She overcame significant hurdles to work with extremely radioactive materials and led efforts to analyze her experimental results by combining x-ray absorption fine structure (XAFS) and multidimensional discrete Fourier transform (MDDFT). It is not often that a researcher can examine the macroscopic coordination chemistry of an element for the first time. Ferrier’s accomplishments greatly extend the Lab’s pioneering methods for realistic synthetic and characterization studies of radioactive elements. Ferrier earned a PhD in Radiochemistry from University of Nevada – Las Vegas. Stosh Kozimor (C-IIAC) nominated her.

Alex Zylstra

Alex Zylstra

Alex Zylstra (Plasma Physics, P-24) is a former Reines Distinguished Postdoc Fellow and current Lab scientist. The Award honors him for his outstanding research and leadership in developing a new inertial confinement fusion (ICF) platform with the potential to revolutionize the field. The flagship of the US Inertial Confinement Fusion program, National Ignition Facility (NIF), seeks to achieve a self-sustained fusion nuclear reaction in the deuterium-tritium (DT) fuel by compressing the DT ice with high power lasers. This path requires a high convergence ratio, giving rise to instabilities and preventing the fuel from igniting. Zylstra has led the experimental part of the alternative approach, which utilizes the liquid DT fuel instead. The leads to smaller required convergence ratios, greatly improving the performance of the laser driven fuel implosions.

The campaign led by Zylstra gave the results, which would be considered an extreme success even for a senior scientist. He demonstrated the future prospects of the new concept with only three shots at NIF. The journal Physical Review Letters highlighted the resulting publication as an “Editor’s Suggestion”. Ramon Leeper (P-24) nominated him.

Thomas Myers

Thomas Myers

Thomas Myers (High Explosive Science and Technology, M-7) is a former Director’s Postdoc Fellow and current Lab scientist. The Honorable Mention Award recognized him for his outstanding research and leadership in the general field of energetic materials, in particular for the design and synthesis of photoactive energetic materials. The latter work has the potential to make a profound impact in the field of detonator design, where materials that have significant stability with respect to mechanical, thermal, and electrostatic insult can be selectively initiated using an infrared laser pulse. The Journal of the American Chemical Society published this research.

During Myers’s tenure as a postdoc, he synthesized more than 50 new energetic materials, published or submitted ten manuscripts in top-tier journals describing these new materials, and given three invited talks and submitted one patent application. In addition, he developed and received approval for a new safety test at LANL, which enables the rapid and efficient screening of these photoactive materials by using samples that are less than 50 mg. Myers also supported the instrumentation needs of his group and coworkers. His work has attracted considerable interest, funding, and collaborations across the defense community.Jacqueline Veauthier (C-IIAC) nominated him.

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Inkret, Junghans, Lookman, Thompson win Postdoctoral Distinguished Mentor Awards

The Lab’s Postdoctoral Distinguished Mentors Awards recognize the positive contributions that a mentor makes during a postdoctoral researcher’s appointment. The mentor’s accomplishments often make a difference in the postdoc’s life, going beyond science, such as career development, and make a positive impact on a number of postdocs. Technical contact: Mary Anne With
Bill Inkret

Bill Inkret

Bill Inkret (Nuclear and Radiochemistry, C-NR) was an exceptional mentor to students, postdocs, and early career scientists over many years at the Lab before announcing his retirement in September 2016. As the longtime leader of the Radiochemistry Data Evaluation Team, he recognized the hidden talents of very different individuals and brought them together to build a strong and unified multidisciplinary team. Throughout his career, he strived to instill a passion in young scientists for work that serves the Laboratory’s core missions of Nuclear Deterrence and Global Security. His investment in young people ensures that the nation has an enduring technical capability in these critical areas.

The national labs are now staffed with dozens of people who were positively impacted by Inkret. As a leader, he was fiercely loyal and fought tirelessly to help his people achieve their goals. He strongly supported and advocated for the personal wellbeing of his team members. Inkret’s welcoming and encouraging mentoring style has left a positive legacy for the young scientists and postdocs fortunate enough to work on the Data Evaluation Team. A previous postdoc, Nick Travia, nominated him.

Christoph Junghans

Christoph Junghans

Christoph Junghans (Applied Computer Science, CCS-7) was nominated for his dedication to fostering success amongst his postdocs. He understands that success is the culmination of individual talent, dedication, hard work, knowledge, passion, and opportunities to demonstrate these skills. Junghans recognizes that working with a skilled team at the Laboratory is the ideal place to demonstrate these qualities. He gives his postdocs the confidence to push themselves and take leading roles on projects and proposals. He ensures their valuable opinions and inputs are heard at the highest levels. His postdocs have been delighted to interact with Junghans outside of the lab setting, quickly being able to settle into the local community and given insights into the wonderful opportunities that New Mexico has to offer.

Thanks to his excellent mentorship and personal leadership, the majority of his postdocs proudly aspire to continue their carriers and research as scientists here in the LANL community. All of the postdocs who have had the pleasure of working with Junghans are delighted to see his hard work and dedication be rewarded, especially for the recognition that he consistently goes above and beyond in his role as mentor. Robert Bird, a current postdoc in CCS-7, nominated him.

Turab Lookman

Turab Lookman

Turab Lookman (Physics of Condensed Matter and Complex Systems, T-4) was recognized for providing postdocs an environment for scientific collaborations and constantly encouraging them to expand their professional and scientific skills critical to become independent and successful scientists. He is a very kind and gifted mentor, who leads by example and sets a high standard for mentorship. Some of his exceptional traits include a passion for science, hard work, professionalism in working with his peers and mentees, undiminishing curiosity to learn new concepts/ideas from his peers (including postdocs and graduate students), attention to technical details, patience, and creating an environment that fosters open communication and constructive criticism.

Lookman encourages his postdocs to engage in discourse with colleagues outside of their domain expertise, which he believes is an important trait for professional development. He has a rare ability to inspire his mentees by nurturing their inner creativity and setting high expectations, reminding them to not lose sight of the hard work and avoiding compromise on the quality of work. Lookman takes their career and future with a level of seriousness that underscores his commitment to mentoring graduate students and postdocs. He is a very humble, generous and supportive mentor who goes out of his way to support his postdocs both personally and professionally. Current postdocs Prasanna Balachandran and Anil Kumar (T-4) nominated Lookman.

Joe Thompson

Joe Thompson

Joe Thompson (Condensed Matter and Magnet Science, MPA-CMMS) was nominated for being an example of leadership, kindness, and exceptional dedication to his postdocs and to MPA-CMMS. Over the last 40 years, his outstanding mentorship has guided 25 postdocs to successful careers, and the condensed matter community speaks very highly of him. Thompson is an exceptional role model due to his scientific integrity, hard work, broad technical knowledge, and ability to guide his mentees into becoming independent scientists. He is very generous with his time and knowledge makes himself available both to his direct mentees and other postdocs in the group, whether on Monday morning or Sunday afternoon.

Thompson also stands out for his effort on career development and personal well being of his postdocs. For instance, his efforts recently culminated in the conversion of one of his postdocs to staff. The transition from being a postdoc to becoming an independent scientist is far from trivial, and Thompson’s example serves as a template for future mentoring efforts. Quoting one of his postdocs “I would consider myself lucky if I become as successful as he is in guiding my future postdocs to successful careers in science.” Priscila Rosa (MPA-CMMS, previous postdoc) nominated Thompson.

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X-ray crystal protein structure helps elucidate function

Sequencing, finishing, analysis in the future

Acetyl-coenzyme A (acetyl-CoA) is a key component of metabolism. It contributes an acetyl group to the Krebs cycle so that it can be oxidized for energy production in a cell. Lab researchers seek to understand the generation of acetyl-CoA for energy applications. The team used the Stanford Synchrotron Radiation Light Source to examine the structure of the malyl-coenzyme A lyase (malyl-CoA lyase) enzyme, which catalyzes a reaction to create acetyl-CoA and glyoxylate. These enzymes are found in a variety of bacteria, where they are involved in the assimilation of one- and two carbon compounds. Acta Crystallographica published their findings of the structure and understanding of how the structure contributes to the enzyme’s function. The journal featured the x-ray crystal structure of malyl-CoA lyase on its cover.

The team reported a 1.56 Å resolution structure of Methylobacterium extorquens AM1 malyl-CoA lyase (MexMCL) crystallized in the presence of magnesium (2+). Based on recently reported malyl coenzyme A lyase (MCL) structures and its close sequence identity to that from other organisms (such as R. sphaeroides and C. aurantiacus), the authors inferred the domain movements that are likely to occur upon the binding of malyl-CoA or acetyl-CoA to MexMCL.

With this new understanding of the mechanism of the malyl-CoA enzyme, the team has begun experimenting with its use as a carbon-neutral way to produce acetyl-CoA. Normally, an acetyl-CoA-producing reaction releases carbon dioxide (CO2). However, the reaction uses the malyl-CoA enzyme to produces a 2-carbon molecule, glyoxylate, which can be further metabolized to other products. This pathway could potentially improve lipid biosynthesis in micro-algae, a biofuel feedstock.

Reference: “Structure of Methylobacterium extorquens Malyl-CoA lyase: CoA Substrate Binding Correlates with Domain Shift,” Acta Crystallographica F73, 79 (2017); doi: 10.1107/S2053230X17001029. Authors: Javier M. González, Ricardo Marti-Arbona, Julian C. -H. Chen, and Clifford J. Unkefer (Bioenergy and Biome Sciences, B-11).

The Laboratory Directed Research and Development (LDRD) program funded the work, and a Director’s Postdoctoral Fellowship sponsored González. The research supports the Lab’s Energy Security mission area and the Science of Signatures and Materials for the Future science pillars through the development of methods to produce biofuels. Technical contact: Julian Chen

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Capability Enhancement

Fuel kernel coating technology developed for self-regulating nuclear reactors

Optical cross-sectional

extruded graphite composite nuclear fuel rod surrogate

Figure 2. Optical cross-sectional (top) and 3-D x-ray computed tomography (bottom) images of an extruded graphite composite nuclear fuel rod surrogate with high loading (estimated 44 vol%) of PyC-coated spherical zirconium oxide (ZrO2) kernels. The graphite matrix represents the non-fissile phase, and PyC-coated ZrO2 is a surrogate for the fissile (low-enriched UC) phase of the composite nuclear fuel concept.

Laboratory researchers are developing a compact nuclear reactor design capable of self-regulation. Self-regulating reactors are attractive because they can match varying operational needs while protecting the reactor core from exceeding thermal limits during abnormal events. This power source is ideal for unattended operations, and for applications in space and in remote areas. One of the promising directions in the development of advanced nuclear fuel forms is a transition from traditional single-phase fuels to composites consisting of fissile and non-fissile phases. The Los Alamos investigators have reestablished a fuel kernel coating technology last used decades ago that enables this development.

The Nuclear Fuel Design Team in Systems Design and Analysis (NEN-5) has proposed a composite fuel constructed of closely packed pyrolitic carbon (PyC)-coated low-enriched uranium carbide (UC) kernels embedded in a graphite matrix (Figure 2). The PyC layer would act as a thermal barrier to control heat release from a kernel into the matrix. The kernel and the matrix would be in thermal equilibrium during steady state operation. If the coating performs according to the design, it will retain heat in the kernel during a reactivity spike. Therefore, high fuel kernel temperatures would trigger strong negative feedback by absorption of thermal and epithermal neutrons, mitigating an accident.
Optical cross-sectional

3D X-ray computed tomography

Figure 3. Optical cross-sectional (top) and 3D X-ray computed tomography (bottom) images of a PyC-coated spherical ZrO2 kernel embedded in a graphite matrix and without matrix, respectively.

The timing of the self-regulation feedback is controlled by the heat release from the kernel to the matrix, which is related to the PyC layer thickness, microstructure, and the two interfaces separating the kernel from the matrix (Figure 3).

One of the most challenging aspects of this concept is the PyC coating, with finely tuned and anisotropic thermal transport impedance. This is important because it controls heat release from the fuel kernels to the fuel matrix. Expertise in this coating technology disappeared from U.S. industry decades ago, but Lab researchers have now been reestablished it within the Materials Science and Technology (MST) Division. The team created a unique capability for fabrication of such coatings on low-enriched UC kernels in Engineered Materials (MST-7). Laboratory organizations have collaborated to rebuild the fluidized bed chemical vapor deposition technology (FBCVD) used at the Lab in the 1960s.  The team brought the technology up to current safety and efficiency standards. The researchers have applied modern characterization

tools (such as x-ray computed tomography) to evaluate the properties of the coated kernels. The Thin Films and Coatings team within MST-7 optimized FBCVD conditions to produce approximately 1.5 kg of PyC-coated fuel surrogate kernels of zirconium oxide, hafnium oxide, and tungsten carbide, as well as approximately 0.3 kg of naturally enriched UC kernels.

Researchers: Miles Beaux, Kevin Hubbard, Brian Patterson, Kevin Henderson, Bryan Bennett, Douglas Vodnik, Reuben Peterson, and Igor Usov (MST-7); Erik Luther and Eric Tegtmeier (Sigma Division, Sigma-DO); Graham King (Materials Science in Radiation and Dynamics Extremes, MST-8); Alice Smith (Nuclear Materials Science, MST-16); Jeffrey Goettee (Advanced Nuclear Technology, NEN-2); and James Jurney (Manufacturing Engineering and Technology, MET-DO).

The Laboratory Directed Research and Development (LDRD) project “Multi-Scale Kinetics of Self-Regulating Nuclear Reactors” (Principal Investigator DV Rao) funded the work, which supports the Lab’s Energy Security mission area and Materials for the Future science pillar through development of materials for nuclear energy generation. Technical contact: Igor Usov

LANSCE cold neutron source enables first neutron phase-contrast imaging

Neutrons can non-destructively image a variety of samples, including nuclear and high explosive components, in a manner similar to x-rays. However, neutrons have advantages compared with x-rays. Lower neutron energies can image light elements, such as hydrogen-rich polymers, organic compounds, lithium, or penetrate many materials such as aluminum and lead. The ability of higher energy neutrons to penetrate dense, thick objects of materials such as steel, uranium and plutonium enables the study of materials buried inside thick casings. Because neutrons interact with the nucleus rather than with the electron shell, they can also distinguish between different isotopes of the same element.

Typical neutron imaging performed at nuclear reactors or spallation neutron sources, such as LANSCE, requires 2-D neutron detectors in which the transmitted neutrons are converted into optical or electronic signals with high spatial resolution (typically from 50 to 200 microns). In the usual case, the resulting image is based on the neutron attenuation of the imaged object, showing regions of high vs. low absorption. However, neutron imaging becomes more challenging if the studied objects have low neutron attenuation cross sections. The quantum mechanical, wave-like, neutron properties of thermal and cold neutrons can be used to advantage to perform phase-contrast or refraction-enhanced neutron imaging in such cases.
Photo of lead sinkers

Cold neutron attenuation

Cold neutron phase

Figure 4. (Top): Photo of lead sinkers held together with aluminum wires. (Middle): Cold neutron attenuation image of nearly transparent lead and aluminum. (Bottom): Cold neutron phase contrast images clearly shows the lead and aluminum outlines.

In quantum mechanics, neutrons are described by de Broglie wave packets whose spatial extent (or coherency) may be large enough to show refraction and interference effects similar to what can be observed with visible laser light or highly brilliant x-rays from synchrotron sources. Differing interaction potentials of the neutron wave packets in various materials result in measurable refraction and phase shifts. With intense neutron sources and sensitive detectors, sufficiently short exposure times enable phase-sensitive neutron computed tomography that provides 3-D reconstructions of objects of interest.

The Lujan Center’s ASTERIX and SPEAR beamlines at LANSCE provide cold neutrons (neutrons with de Broglie wavelengths of approximately 5-15 Å), which are especially suitable

for performing phase-contrast imaging. Preliminary experiments demonstrated that neutron phase contrast conditions could be realized using the high flux and highly coherent cold neutron beams produced at the 1L target at LANSCE. The investigated sample (Figure 4 left) consisted of lead fishing weights suspended by an aluminum wire on a thin aluminum plate. Both lead and aluminum have very low neutron attenuation, with greater than 98% transmission through 1 cm of lead and 96% through 1 cm of aluminum. Figure 4 center presents a conventional attenuation radiograph using cold neutrons. The lead and aluminum pieces are difficult to see. Figure 4 right shows a cold neutron radiograph where the phase contrast is visible at the interfaces as white and black shades produced by refraction and interference of neutron wave packets.

The ability to perform phase-contrast imaging and cold neutron attenuation radiography at LANSCE has important applications to issues that arise in explosives fabrication, as well as other programmatic and research areas. Cold neutron imaging is ideal for materials science studies. Examples include examining welds and properties of material bonds, revealing hydrogen distributions and processes occurring in fuel cells, examining interfaces between materials, and studying issues in the application of adhesives that are often difficult or impossible to study with other techniques. The new phase contrast imaging capability can image interior features of solid objects (e.g., defects or cracks in low atomic number organic materials) and can detect these features inside metal casings, which are difficult to penetrate with x-rays. Cold-neutron attenuation imaging and dark-field imaging techniques could be applied using the same flight path and detectors. In addition, the thermal and high-energy neutron beams at LANSCE are available and have been developed to image items requiring greater penetrating power.

Team: Jarek Majewski (Center for Integrated Nanotechnologies, CINT and present address National Science Foundation), Erik Watkins (Materials Synthesis and Integrated Devices MPA-11), Adrian Losko and Sven Vogel (Materials Science in Radiation and Dynamics Extremes, MST-8), Ron Nelson (LANSCE Weapons Physics, P-27), David Montgomery (Plasma Physics, P-24), Kyle Ramos (High Explosives Science and Technology, M-7), and James Hunter (Non-Destructive Testing and Evaluation, AET-6).

The NNSA W76 Stockpile Stewardship, Science Campaigns 1 and 2 funded the work, which benefited from use of equipment and expertise from NNSA Enhanced Surveillance. The research supports the Laboratory’s Nuclear Deterrence and Energy Security mission areas and the Materials for the Future, Science of Signatures, and Nuclear and Particle Futures pillars. It also supports the Integrating Information, Science and Technology for Prediction science pillar through improved CT reconstruction and analysis algorithms for large and multi-probe data sets. Technical contacts: Erik Watkins and Jarek Majewski

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Photophysical pathways in open-shell actinide molecules

Renewed interest in nuclear power generation and concerns about nuclear safeguards and security have increased interest in actinide-containing materials. Photoluminescence spectroscopy has been useful in a multitude of disciplines both for detection of small quantities of materials in diverse environments and for studying molecular behavior in complex, multicomponent chemical systems. In particular, photoluminescence spectroscopy is a valuable tool for understanding fundamental photophysical properties of the uranyl ions (U(VI)O22+) in the environment, aqueous solutions, and organic media. The vast majority of literature on actinide photoluminescence is confined to studies of the uranyl ion. However, it

is essential to characterize the fundamental properties of the electronic structure of more electronically complex actinides, such as neptunium (Np). A Lab team has investigated these properties and published them in The Journal of Physical Chemistry A.

The Nuclear and Radiochemistry group (C-NR) has focused on the photophysical dynamics and relaxation pathways of ligand-to-metal charge-transfer states in the 5f1 [Np(VI)O2Cl4]2- anion.  Reports of spectroscopic measurements that detail the photophysical dynamics of open-shell actinide molecules and ions are limited in number. In prior studies by others, an experimental approach based upon time-resolved photoluminescence characterized the photophysics of the electronic structures of a variety of molecular and ionic systems, including gaseous plutonium hexafluoride and a number of uranyl compounds. The team has investigated the electronic structure of solid-state, open-shell 5fx (x ≥ 0) systems, but this published work has been limited to a few photoluminescence studies of intra-5f transitions of the 5f1 neptunyl tetrachloride ion and unassigned photoluminescence from the 5f2 plutonyl tetrachloride ion.

Photoluminescence emission spectrum of intra-5f and LMCT transitions

Figure 5

Figure 5 shows the photoluminescence emission spectrum of intra-5f and LMCT transitions from 3% Cs2U(Np)O2Cl4 at 75 K following pulsed excitation of the neptunyl state IX + nIX3 at ~19 600 cm-1. Inset shows the orbital structures of the NpO22+ ion.

The researchers have expanded the understanding of the open-shell neptunyl ion beyond identification of excited states, allowing characterization of photophysical properties and evidence for the electronic character of the ground state. The team measured photoemission of ligand-to-metal charge-transfer transitions following visible excitation of Cs2U(Np)O2Cl4. The investigators selected neptunyl tetrachloride (Np(VI)O2Cl4) for study due to its high symmetry.

The researchers gained two primary insights. One is a contribution to the understanding of the symmetries of the 5f electronic states, in which population of an anti-bonding metal-chloride orbital influences the metal-oxo bond in a way that is consistent with the observed changes in the metal-oxo vibrational frequency. The second is the deduction of a relaxation pathway for energy deposited into higher energy electronic states based on the observed emission lifetimes for several excitation experiments.

Reference: “Photophysical Dynamics and Relaxation Pathways of Ligand-to-Metal Charge-Transfer States in the 5f1 [Np(VI)O2Cl4]2- Anion,” The Journal of Physical Chemistry A 121, 2353 (2017); doi: 10.1021/acs.jpca/7b01029. Authors: Beau J. Barker (Nuclear and Radiochemistry, C-NR), John M. Berg (Defense and Space Power Systems, MET-5), Stosh A. Kozimor (Inorganic, Isotope, and Actinide Chemistry, C-IIAC), Nicholas R. Wozniak (C-NR and University of Nevada – Las Vegas), and Marianne P. Wilkerson (C-NR).

The DOE, Office of Science, Office of Basic Energy Science; a Los Alamos National Laboratory Seaborg Institute Fellowship; and the Laboratory Directed Research and Development (LDRD) program funded different aspects of the research. The work supports the Lab’s Global Security and Energy Security mission areas and the Materials for the Future science pillar through the investigation and understanding of actinide reactions. Technical contact: Marianne P. Wilkerson

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Earth and Environmental Sciences

Understanding the multi-scale problem of hydraulic fracturing

Comparison of the results of the Lab’s triaxial direct-shear coreflood experiment

Figure 6. Comparison of the results of the Lab’s triaxial direct-shear coreflood experiment (left) compared with LANL’s combined finite-discrete element simulation (right). In the experiment, Utica shale is subject to shear from two-offset platens that create fractures and enhanced permeability at 1.4 MPa injection pressure through the platens and 3.5 MPa confining pressure. The fractures that cross and propagate along horizontal bedding planes were successfully simulated using the experimental stress conditions and injected fluid pressure within the fractures. The simulation captures the primary fracture network (outlined in red on the experiment) as well as bedding plane fractures and densely fractured regions. The simulation illustrates the calculated stress intensity within the specimen.

Hydraulic fracturing (also known as fracking) has had a substantial impact on the energy sector by providing access to hydrocarbons in low permeability formations that were previously inaccessible. However, the physical mechanisms that control its efficiency and environmental impacts remain poorly understood in part because the relevant length scales range from nanometers to kilometers. Hari Viswanathan (Computational Earth Science, EES-16) has led a team performing an integrated experimental and computational assessment of the key phenomena in hydraulic fracturing that control the production of hydrocarbons. Philosophical Transactions of the Royal Society A published the work in the “Energy and the Subsurface” theme issue and featured it on the journal’s cover. Ivan Christov (Purdue University, formerly at LANL) and Viswanathan co-edited the issue. The theme for the journal issue arose from an international Geological Fluid Mechanics Conference sponsored by the Lab’s Center for Nonlinear Science (CNLS) and Center for Space and Earth Science (CSES). Issue contributors were selected from attendees of the Conference. National Academy of Science members, a National Academy of Engineering member, former CNLS director Robert Ecke, editors from leading technical journals, and prominent oil and gas industry scientists also contributed to the issue.

Hypothesized breakdown of physical mechanisms governing shale gas production

Figure 7. Hypothesized breakdown of physical mechanisms governing shale gas production. The key features of the curve are a high initial peak followed by a rapid decline that develops into a sustained low level of production. Plotted along with the curve are regimes corresponding to the hypothesis that different pieces of the production curve are governed by different length scales.

The team characterized the processes governing hydrocarbon production and described the methodology and techniques used to better understand flow and transport in shale formations across length-scales that range from nanometers to kilometers. Figure 6 shows the researcher’s hypothesized breakdown of the physical mechanisms that govern shale gas production

Key features of the production curve include a high initial peak followed by a rapid decline that develops into a sustained low level of production. At early times, large background fractures in the connected network full of gas are flushed, which leads to the rapid initial decline in production. Next, smaller tributary fracture zones around these natural fractures induced by hydraulic fracturing increase the surface area within the matrix and sustain production for intermediate times. Matrix processes, such as diffusion and desorption, contribute to and control late production.

The team’s integrated approach underscores the need for a mechanistic description of these multiple scales to make accurate predictions and eventually optimize hydrocarbon extraction from these unconventional reservoirs. The investigators characterized flow and transport in shale formations using integrated computational, theoretical, and experimental methods. The researchers’ experimental work included microfluidics and triaxial coreflood experiments at high pressures, temperatures and stresses that mimic subsurface reservoir conditions. Lattice Boltzmann models simulated pore-scale multi-phase flow and were benchmarked by microfluidic experiments in both synthetic and shale rock micromodels. The investigators benchmarked a finite-discrete element fracture propagation model against core-scale triaxial fracture experiments. The team employed the Lab’s dfnWorks software suite at the field scale to simulate hydrocarbon production using discrete fracture networks based on the site characterization of a shale gas reservoir.

Reference: “Understanding Hydraulic Fracturing: A Multi-scale Problem,” Philosophical Transactions of the Royal Society A 374, 20150426 (2016); doi: 10.1098/rsta.2015.0426. Authors: J. D. Hyman, L. Frash, S. Karra, Q. Kang, D. O’Malley, N. Makedonska, and H. S. Viswanathan (EES-16); J. W. Carey, J. Jiménez-Martínez, and M. L. Porter (Earth System Observations, EES-14); E. Rougier, L. Chen, and Z. Lei (Geophysics, EES-17).

The Laboratory Directed Research and Development (LDRD) program, Laboratory Director’s Funded Postdoctoral Fellowships (Hyman, Frash, Chen, and O’Malley), and DOE Fossil Energy-National Energy Technology Laboratory Unconventional Oil and Gas funded different aspects of the work. The research supports the Lab’s Energy Security mission area and the Information, Science, and Technology science pillar by providing the scientific underpinning and predictive capabilities to support decision and policy makers. Technical contact: Jeffrey De’Haven Hyman

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Materials Physics and Applications

Exploiting self-assembling copolymers for light-harvesting and energy transfer

A team of Center for Integrated Nanotechnologies (CINT) and Sandia National Laboratories researchers has demonstrated the ability of a short-chain amphiphilic block copolymer to self assemble, forming a supramolecular light-harvesting system. The work is a step towards mimicking nature’s best examples of optimizing both light harvesting and energy transfer, a great scientific challenge. From an application perspective, the goal is to mimic biological function in a manner conducive to device design. Nanoscale published the research.

The investigators exploited a short chain bio-inspired block copolymer to generate a synthetic analog of a green bacterial antenna system. Green bacteria use a supramolecular light-harvesting complex (called the chlorosome), which contains tens of thousands of self-organized pigments to create an optically dense light-harvesting structure. A self-contained nanocomposite material that replicates such organization and functional response had not been achieved previously.

The research showed that the inherent flexibility and propensities of the block copolymer dictates the system assembly. The team prepared all materials through self-assembly and non-covalent interactions of chromophores. The resulting polymer chlorosome nanocomposites were highly modular with physical and optical properties that can be tuned by switching or adding different chromophores and functionalized polymers. Unlike native green bacterial systems that use multiple lipids as a matrix to generate the appropriate environment for chlorosome assembly and function, the artificial system matrix is comprised entirely of a single type of polymer amphiphile (molecule having a polar water-soluble group attached to a water-insoluble hydrocarbon chain). The results suggest that replicating and modulating light harvesting and energy transfer in a materials system predicated upon green bacteria is possible entirely through self-assembly. This work demonstrates the potential of short-chain amphiphilic block copolymers to generate scalable, self-assembled, biomimetic membrane architectures. This discovery represents a potential shift in the construction of artificial photosynthetic systems for materials applications.

Self assembly process of polymer chlorosome nano composites (PCN) and atomic force microscopy image

Figure 8. Self assembly process of polymer chlorosome nano composites (PCN) and atomic force microscopy image of resultant PCNs (far right). Bchl c is bacteriochlorophyll c, and pEO-b-pBD is a 2.5 kDa amphiphilic block copolymer [poly(ethylene oxide)-block-poly(butadiene)].

Reference: “Amphiphilic Block Copolymers as Flexible Membrane Materials Generating Structural and Functional Mimics of Green Bacterial Antenna Complexes,” Nanoscale 8, 15056 (2016); doi: 10.1039/c6nr02497a. Authors: A. M. Collins and G. A. Montaño (Center for Integrated Nanotechnologies, MPA-CINT), and J. A. Timlin and S. M. Anthony (Sandia National Laboratories).

The Laboratory Directed Research and Development (LDRD) program sponsored the synthesis and characterization work. The Center for Integrated Nanotechnologies, an Office of Science User Facility operated jointly by Los Alamos and Sandia national laboratories for the U.S. Department of Energy Office of Science, funded the atomic force microscopy. The research supports LANL’s Energy Security mission area and the Materials for the Future science pillar by developing materials for solar energy harvesting. Technical contact: Gabriel A. Montaño

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Materials Science and Technology

Machine learning provides insights into amorphization of irradiated pyrochlores

Structure-property relationships are a key materials science concept enabling the design of new materials. Correlating radiation tolerance with fundamental structural features of a material enables discovery of materials that can be used in radiation environments. However, predictive physics models of complex phenomena are extremely difficult to build. In a study published in the “Computational Design of Functional Materials” special issue of Chemistry of Materials, researchers from Materials Science and Technology (MST) Division, Idaho National Laboratory, University of Tennessee, and UK colleagues used a machine learning model to quantify factors governing radiation response in the complex oxide pyrochlore (A2B2O7) in a regime wherein amorphization occurs due to defect accumulation. Pyrochlores have potential applications as nuclear waste forms and have been incorporated into some compositions of the Synroc (synthetic rock) waste form.

A key performance metric in nuclear energy materials is tolerance against radiation damage. The team used the experimentally determined critical amorphization temperature (Tc) as an indicator of radiation tolerance. Machine learning correlated fundamental properties to Tc. The research highlights how an inherently kinetic property like Tc can be correlated with fundamental thermodynamic properties of the material. The team used electronic structure calculations as input to machine learning for these studies.

The researchers examined the fidelity of predictions based on cation radii and electronegativities, the oxygen positional parameter, and the energetics of disordering and amorphizing the material. The investigators determined that no one factor adequately predicted amorphization resistance. When they considered multiple families of pyrochlores (with different B cations), radii and electronegativities provided the best prediction. However, when the team restricted the machine learning model to only the B = Ti pyrochlores, the energetics of disordering and amorphization were the critical factors.


Electronic structure

Electronic structure results of the energies to disorder and amorphize pyrochlore as a function of chemistry are used as input to a machine learning model (bottom right), which reveals that these properties best predict the critical amorphization temperature TC as a function of pyrochlore chemistry.

This research highlights how an inherently kinetic property like Tc can be correlated to fundamental thermodynamic properties of the material. The researchers determined that the energetics of disordering and amorphization are key factors in radiation response. This work also demonstrated how machine learning, complemented by domain knowledge, can lead to predictive understanding.

Reference: “Using Machine Learning to Identify Factors that Govern Amorphization of Irradiated Pyrochlores,” Chemistry of Materials 29, 2574 (2017) doi: 10.1021/acs.chemmater.6b04666. Researchers: Ghanshyam Pilania, Blas Pedro Uberuaga, and Christopher R. Stanek (Materials Science in Radiation and Dynamics Extremes, MST-8); Karl R. Whittle (University of Liverpool, UK); Chao Jiang (Idaho National Laboratory); Robin W. Grimes (Imperial College, UK); and Kurt E. Sickafus (University of Tennessee – Knoxville).

The DOE Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, funded the research, which supports the Laboratory’s Energy Security mission area and Materials for the Future science pillar. Los Alamos pursues the science and engineering needed to establish design principles, synthesis pathways, and manufacturing processes for advanced and new materials that control functionality relevant to Lab missions. Technical contact: Blas Uberuaga

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First test of an aerogel Cherenkov detector to characterize the Cygnus x-ray source

A dual-module ACD/C

Photo. A dual-module ACD/C installed at the U1a facility in Nevada National Security Site.

A Physics Division team collaborated with other Lab organizations and National Security Technologies, LLC to lead a proof-of-concept test of an aerogel Cherenkov detector for Cygnus (ACD/C). The development of the detector is part of a broader effort by the NNSA Science Program to improve overall understanding of the physics of dense object radiography including detectors, converters, cameras and data recorders, and detailed analysis techniques, complementing large Science Program investments in radiation sources to support NNSA. The ACD/C measurements characterize the time-dependent x-ray energy spectrum from Cygnus – an intense flash x-ray source operated at the Nevada National Security Site (NNSS). The ACD/C effort provides an in-situ, off-axis, time-dependent, x-ray spectral detector, and complements measurements made by a “MiniMe” Compton Spectrometer, also developed by the Physics Division team. Understanding the spectrum from Cygnus helps reduce uncertainties in density determination for subcritical experiments such as the Gemini/Leda/Lyra series. The journal Review of Scientific Instruments published their detector development and calibration.

The ACD/C employs an array of silicon dioxide aerogels (synthetic porous ultralight material) and solids (i.e., quartz) at different and hence varying Cherenkov energy thresholds (greater than 2.8 MeV X-ray energy for aerogel with 50 mg/cc down to less 0.32 MeV X-ray energy for quartz with 2500 mg/cc). Aerogels allow this detector to access thresholds unobtainable with solids and liquids (below 0.5 MeV) and gases (above 2 MeV). The energy range of ACD/C is adequate to characterize the Cygnus spectrum, where the maximum energy of the spectrum is nominally around 2.25 MeV. The ACD/C also has a fast-time response, on the order of 1 ns, which could provide the temporal resolution needed to characterize the approximately 50 ns radiation pulse of Cygnus.

The photo, above, shows the prototype of the ACD/C system fielded at the Nevada National Security Site’s U1a facility. The prototype ACD/C is a 2-channel detector, made of aluminum instead of tungsten. The prototype detector allows Cherenkov radiators such as quartz or aerogels to be exchanged shot by shot for the proof-of-principle test.  The full ACD/C would include many more channels.

Temporal evolutions of Cygnus X-ray signals

Temporal evolutions of Cygnus X-ray signals. ACD/C can track Cygnus spectral variation as a function of time.

The team simultaneously examined two energy thresholds (0.32 MeV by quartz and 1.11 MeV by aerogel of 260 mg/cc) for the initial proof-of-concept test. For a 50 ns full width at half maximum (FWHM) Cygnus electrical power pulse, the ACD/C signal from the quartz detector at 0.32 MeV threshold was approximately 32 ns FWHM and the aerogel signal of 1.11 MeV threshold was approximately 18 ns FWHM (Figure 10).

The data qualitatively suggest that the Cygnus X-ray spectrum is evolving in time, and the high-energy x-ray peak exists on a shorter timescale than the Cygnus voltage or current pulse. The team expects this to occur because the voltage on the diode requires approximately 10 ns to rise to full value. The ACD/C signal ratio of the 1.11 MeV threshold to the 0.32 MeV threshold responded to variations in diode voltage intended to vary the spectral end point energy. Researchers will quantify this measurement through comparison with Compton Spectrometer data. The team performed the initial test downstream of the Cygnus bulkhead, in the “zero room” where the bulkhead provides good background shielding. Demonstrating a tungsten ACD/C and extracting adequate Cherenkov signals above background when the ACD/C is installed in the unshielded, Cygnus source room will be challenging. In the future, a multichannel ACD/C may support a Neutron-Diagnosed Subcritical Experiment by measuring prompt-fission gamma rays.        

Reference: “Aerogel Cherenkov Detector for Characterizing the Intense Flash X-ray Source, Cygnus, Spectrum”, Review of Scientific Instruments 87, 11E723 (2016); doi: 10.1063/1.4960541. Authors: Y. Kim, H. W. Herrmann, A. M. McEvoy, and C. S. Young (Plasma Physics, P-24); C. E. Hamilton (Engineered Materials, MST-7); D. D. Schwellenbach, R. M. Malone, M. I. Kaufman, and A. S. Smith (National Security Technologies, LLC).  Other collaborators include: T. J. Haines, J. R. Smith, and M. E. Gehring (Neutron Science and Technology, P-23); G. W. Pokorny (J Division-Nevada Operations, J-NV); M. A. Espy (Non-Destructive Testing and Evaluation, AET-6); R. A. Howe, R. Owens, S. E. Mitchell, J. A. Green, I. Garza, M. Larson, and N. Prock (National Security Technologies, LLC).

NNSA Science Campaign 3 (managed by Bob Reinovsky) funded the work, which supports the Laboratory’s Nuclear Deterrence mission area and the Nuclear and Particle Futures science pillar. Technical contact: Yongho Kim

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Evaluating the risk for local Zika transmission across the eastern United States

Zika and chikungunya arboviruses are transmitted by Aedes mosquitoes, including the Asian tiger mosquito (Ae. albopictus). Asian tiger mosquitoes are abundant in cities located in temperate climates. While disease risk is lower in temperate regions where viral amplification cannot build across years, there is significant potential for localized disease outbreaks in urban populations. Mosquito-borne disease occurs when specific combinations of conditions maximize virus-to-mosquito and mosquito-to-human contact rates. Carrie Manore (Theoretical Biology and Biophysics, T-6) and a team used a model informed by field data to assess the conditions likely to facilitate local transmission of virus from an infected traveler to Ae. albopictus and then to other people in U.S. cities with variable human densities and seasonality. The journal PLOS Neglected Tropical Diseases published their findings. Their model demonstrated that up to 50% of Zika-infected travelers returning to the U.S. could initiate local transmission in temperate cities if they are infectious and are exposed to high mosquito densities early in the mosquito season. Moreover, 10% of the introductions could result in 100 or more people infected. Despite the propensity for Ae. albopictus to bite non-human vertebrates, the study also showed that local virus transmission and human outbreaks may occur when vectors feed from humans even just 40% of the time. Inclusion of human behavioral changes and mitigations were not incorporated into the models and would likely reduce the predicted number of infections.
Global distribution of Aedes albopictus

Fig 11. Global distribution of Aedes albopictus (orange dots) with superimposed major urban areas (blue triangles).

Distribution of the basic reproduction number, R0,

Figure 12: Distribution of the basic reproduction number, R0, for Zika virus across ranges of human feeding rates, Ph, for New York City. The average number of times a human was bitten per day in the model ranges from 0 to 4 bites. Even for number of bites per person per day below 1, there were several scenarios with significant onward transmission. With P ≥0.4 probability of an outbreak increases significantly, resulting in 62.7% of runs with R >1. However, when P < 0.4, the percent of runs with R >1 decreases to 10.1% (for P ≥0.8, 76.3% of runs have R >1). When P < 0.4, the mean value of R is 0.46, while for P ≥0.4, the mean value of R is 1.55 and if P ≥0.8, the mean value of R jumps to 1.97.

The team evaluated how the duration of the active mosquito season following the arrival of an infectious traveler and propensity for biting diverse vertebrate species, where every non-human bite slows the transmission process, could influence outbreak potential for different urban densities. Higher probability of human host-use is associated with greater reproduction number, R. For a given seasonal duration and human population density, increasing the proportion of bites on humans in the mosquito population above 40% resulted in more model runs that returned R >1, signifying increased potential for local transmission and human disease even when a significant proportion of blood meals are from non-human animals. The average number of times a human was bitten per day in the model ranges from 0 to 4 bites. Even for the number of bites per person per day below 1, the team found several scenarios with significant onward transmission.

This work demonstrates how a conditional series of non-average but realistic events can result in local arbovirus transmission and outbreaks of human disease, even in temperate cities. This study highlights the need for high-resolution spatial data on Ae. albopictus density, biting behavior, and seasonality to better understand, predict and manage arboviral transmission risk in temperate cities.

Scientists and public health officials involved with arbovirus transmission have had limited ability to make credible predictions, in part based on limited information about conditions that permit an outbreak and the likelihood those conditions will be met. The new model provides quantitative assessments of the probability of an outbreak (R0) and the potential numbers of human victims when key parameter values can be specified. Guided by published data on virus and mosquito vital rates, the model indicates that outbreaks can plausibly occur in major cities in the eastern United States, with hundreds of potential victims in localized areas, under conditions that are not atypical. The model suggests that outbreaks are more likely in urban areas with higher human and mosquito population densities, in years and cities with longer growing seasons, when infected travelers arrive early in the growing season, and when Ae. albopictus have fewer non-human hosts that result in wasted bites. These conditions are most likely met in

urban landscapes where social, structural, and environmental inequities facilitate human-mosquito contact and potentially limit early detection and mitigation of local transmission. The model’s results indicate that climate change, urban wildlife ecology, and human behavior all could strongly influence the probability of new outbreaks in major U.S. cities.

Reference, “Defining the Risk of Zika and Chikungunya Virus Transmission in Human Population Centers of the Eastern United States,” PLOS Neglected Tropical Diseases (2017); doi: 10.1371/journal.pntd.0005255. The journal publishes research devoted to the pathology, epidemiology, prevention, treatment and control of the neglected tropical diseases. Authors: Carrie A. Manore (T-6), Richard S. Ostfeld and Shannon L. LaDeau (Cary Institute of Ecosystem Studies), Folashade B. Agusto (University of Kansas), and Holly Gaff (Old Dominion University).

The Laboratory Directed Research and Development (LDRD) program, National Institute for Mathematical and Biological Synthesis, the National Science Foundation, and the National Institutes of Health funded different aspects of the research. A Laboratory Director’s Funded Postdoctoral Fellowship sponsored Manore. The work supports the Lab’s Global Security mission area and the Information, Science, and Technology science pillar through the capability to model and predict detect infectious disease outbreaks. Technical contact: Carrie Manore

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