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Science Highlights, August 30, 2017

Awards and Recognition

Joanne Wendelberger receives the William G. Hunter Award

Joanne Wendelberger

Joanne Wendelberger

The American Society for Quality (ASQ), Statistics Division will present the Hunter Award to Joanne Wendelberger (Statistical Sciences, CCS-6). The award is named in honor of the ASQ Statistics Division’s first chair, William G. Hunter. It is given annually to the individual who exemplifies Hunter’s most enduring qualities of excellence in statistics as a leader, a communicator, a consultant, an educator, an innovator, an integrator of statistics with other disciplines, and an implementer who obtains meaningful results.

Wendelberger will receive the award at the 61st Annual Fall Technical Conference in Philadelphia, PA on October 5, 2017. This award is particularly meaningful to Wendelberger because she knew Professor Hunter as an educator and role model when she was a graduate student at the University of Wisconsin – Madison.

After completing her Ph.D. in statistics at University of Wisconsin – Madison, Wendelberger joined the Laboratory as a staff member in 1992. She held multiple R&D management roles in the Statistical Sciences Group and the Computer Computational and Statistical Sciences Division from 2005-2016 before returning to full-time technical work as a senior level Scientist in CCS-6. Throughout her career, Wendelberger’s research has been motivated by the need to develop solutions to complex interdisciplinary problems, with a growing focus on the interface between statistics and computer science. Her research interests include statistical experimental design and test planning, statistical bounding and uncertainty quantification, materials degradation modeling, sampling and analysis for large-scale computation and visualization, probabilistic computing, and education modeling.

Wendelberger is a Fellow of the American Statistical Association (ASA) and a Senior Member of the American Society for Quality. Her awards include the W. J. Youden Memorial Address/Award (ASQ), American Statistical Association, Chapter Service Award, four NNSA Defense Programs Awards of Excellence, Los Alamos Award for Excellence in Industrial Partnerships, and two Los Alamos Achievement Awards. Wendelberger has served as an Associate Editor and Management Committee Member for the journal, Technometrics, as Chair and Program Chair of the ASA Section on Physical & Engineering Sciences, President of the ASA Albuquerque Chapter, and as a member of several conference organization committees and awards committees.

The American Society for Quality is a global community of quality professionals, with nearly 80,000 members dedicated to promoting and advancing quality tools, principles, and practices in their workplaces and communities. ASQ has played a vital role in setting standards on quality management, environmental management, statistics, and social responsibility that facilitate global commerce. Technical contact: Joanne Wendelberger

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Bruce Carlsten, Dinh Nguyen and Richard Sheffield win Free-Electron Laser Prize

The 38th International Free-Electron Laser Conference honored Bruce Carlsten (Engineering Sciences, ADE), Dinh Nguyen (Accelerators and Electrodynamics, AOT-AE), and Richard Sheffield (Experimental Physical Sciences, ADEPS) with the 2017 Free-Electron Laser (FEL) Prize. The prize is given to a person who has contributed significantly to the advancement of the field of Free-Electron Lasers (FEL).

The very brightest sources of x-rays are the latest generation of x-ray light sources called free-electron lasers. The honor is an international recognition of key technologies that originally developed at Los Alamos in the 1980s and 1990s, such as the radio frequency photo-injector and high-brightness electron beams. These significant innovations have enabled the x-ray free-electron laser (XFEL) facilities currently in use worldwide. The developments have paved the way for all of the current ultra-bright fourth-generation light sources that are revolutionizing many fields of science, from biology to materials science.

The prize recognized three specific pioneering contributions:

  • The invention, first practical demonstration, and theoretical understanding of the radio-frequency photo-injector, patented in 1985
  • The first practical demonstration of self-amplified spontaneous emission
  • The design and demonstration of the regenerative amplifier free-electron laser
Free-electron lasers involve techniques and materials central to the Los Alamos National Laboratory mission: x-rays are used to examine the inside and structure of materials, from living tissues to the parts in nuclear weapons. High-energy and very bright sources of x-rays provide the ability to penetrate deep into materials and provide very fast response to changing conditions.
Bruce Carlsten

Bruce Carlsten

Bruce Carlsten is a RF engineer and accelerator physicist. He received a Ph.D. in electrical engineering from Stanford University and joined the Lab in 1982.

Carlsten is a pioneer in the production and use of high-brightness electron beams with applications that span a range of Laboratory programs and which have found widespread usage worldwide. His discovery of techniques that have enabled unprecedented beam brightness has led to a new generation of intense free electron lasers. These ideas are of such fundamental importance that virtually every free-electron laser in the world has embraced them.

Carlsten has overseen a rapid growth in beam-based applications at the Laboratory including microwave tube development and advanced acceleration concepts. He is a Fellow of the American Physical Society, IEEE, and Los Alamos National Laboratory. Carlsten has also received the 1999 U.S. Accelerator Particle School Prize for Achievement in Accelerator Science and Technology.

Dinh Nguyen

Dinh Nguyen

Dinh Nguyen earned Ph.D. in chemistry at the University of Wisconsin – Madison, and then joined the Laboratory in 1984. He has conducted pioneering work in up‑conversion solid-state lasers, RF photoelectron injectors, x-ray generation via Compton backscattering, high-current injectors, and free-electron lasers. Nguyen was the first to detect single molecules and to demonstrate a blue light emitting up‑conversion laser. He participated in the development of the high-brightness RF photoinjector with Richard Sheffield. He also contributed to the development of the RF lasertron and the electron-beam pumped plasma extreme ultraviolet light source. Nguyen led a team that demonstrated the first compact FEL oscillator and developed the rugged cesium telluride photocathode that is now being used in several FELs.

His most important contribution to the x-ray free-electron laser (XFEL) is the high-gain self-amplified spontaneous emission (SASE) experiments in the infrared that he demonstrated independently and in collaboration with University of California – Los Angeles, and the Kurchatov Institute. These demonstrations of very large FEL gains (3 x 105) at the Lab are the first in a series of SASE experiments that culminated in the world’s first XFEL, the Linac Coherent Light Source in 2009. He subsequently demonstrated the Regenerative Amplifier free-electron laser (RAFEL), a new concept of high-gain FEL that utilizes low-reflectivity mirrors to reach saturation in a few passes. The RAFEL concept has been proposed as a new way to achieve fully coherent XFELs. Nguyen has received five patents, six Laboratory Distinguished Performance Awards, and two R&D 100 Awards. He is a Fellow of the American Physical Society.

Richard Sheffield

Richard Sheffield

Richard Sheffield earned a PhD in physics from the Massachusetts Institute of Technology and joined the Lab in 1978. He won the FEL prize for his invention of the radio-frequency photoinjector, and for his subsequent work advancing the technology’s theoretical understanding. He and Dinh Nguyen were also recognized for the first practical demonstration of self-amplified spontaneous emission (SASE), which validated underlying gain theory for fourth generation x-ray sources. This work formed the foundation for these x-ray light sources. Sheffield was the principal physics designer of the advanced free-electron laser initiative (AFELI) a project that built an advanced laser to demonstrate SASE. He also developed electron beam diagnostics for the Lab’s FEL program and was a co-investigator on a proof-of-principle FEL oscillator experiment designed to extend FEL operations into the ultraviolet and vacuum ultraviolet. Stanford University’s Linac Coherent Light Source, as well as several other such lasers, use this technology. The DOE has funded the Linac Coherent Light Source-II, an instrument that will move the technology from 120 pulses per second to 1 million pulses per second. 

Sheffield is the technical director and acting science pillar lead for accelerators and related photon, electron, and proton beam probes for the Lab’s proposed Matter-Radiation Interaction in Extremes (MaRIE) facility. He is a senior advisor to the Experimental Physical Sciences Directorate, a Laboratory Fellow, and a Fellow of the American Physical Society. Sheffield has won an R&D 100 Award, three Laboratory Distinguished Performance Awards, and a Laboratory Distinguished Patent Award for the photoinjector.

The Laboratory sponsored the 38th International Free-Electron Laser Conference, a five-day biennial conference held in Santa Fe. The meeting brought together an international perspective on recent advances in free electron laser theory, experiments, electron beam technology and applications of free-electron lasers. Nguyen and Carlsten served on the conference organizing committee. The FEL Prize Committee was composed of five members who work at institutions other than Los Alamos. Technical contacts: Bruce Carlsten, Dinh Nguyen, and Richard Sheffield

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A. Balatsky and J. Lashley chosen for editorial board of Physical Review Materials

Physical Review Materials has chosen Alexander Balatsky (National Security Education Center, NSEC) and Jason Lashley (Materials Synthesis and Integrated Devices, MPA-11) to serve on the editorial board. The broad-scope international journal was launched recently for the growing multidisciplinary community engaged in materials research. Published by the American Physical Society, it is part of the Physical Review family of journals.

 

Alexander Balatsky

Alexander Balatsky

Alexander Balatsky will develop the journal as a new publication venue that will become a go to publication in the materials community, encourage the community to publish at in the journal, assess the quality of papers submitted, and advise the full-time editors. 

He received a Ph.D. in theoretical physics from the Landau Institute for Theoretical Physics, Moscow and joined the Laboratory as an Oppenheimer Fellow in 1991. Balatsky is the Director of the Institute for Materials Science in the National Security Education Center. His research focuses on the collective states of quantum materials including superconductivity, superfluidity, Dirac materials, multiferroics, nonequilibrium states of matter, spectroscopies and materials informatics. Balatsky developed a theory of impurities in unconventional superconductors, which scanning-tunneling microscope experiments validated. He was instrumental in developing new local spectroscopic techniques for these systems. Balatsky is a Fellow of the American Physical Society, American Association for the Advancement of Science, and Los Alamos National Laboratory. Technical contact: Alexander Balatsky

Jason Lashley

Jason Lashley

Jason Lashley will serve as an associate managing editor. He will referee articles that are at the intersection of physics and chemistry. The journal selected him for his broad scientific background, which ranges from condensed matter physics to synthetic organic chemistry.

Lashley joined the Lab as a student in 1993. He began research for his Ph.D. in condensed matter physics and organic chemistry at Purdue University, and then transferred to Brigham Young University with his advisor for his final year of study. His Laboratory research explores natural product synthesis, critical phenomena, quantum ferroelectrics, narrow band gap semiconductors, superconductivity, Dirac materials, and correlated electron systems. Lashley has published more than 200 papers, and his work has more than 3000 citations. He has received an individual Laboratory Distinguished Performance Award for accomplishments in condensed matter physics and chemistry, a Distinguished Performance Award for accomplishments as part of a team advancing the science of plutonium, and the 62nd Calorimetry Conference’s 2007 Stig Sunner Memorial Award, which recognizes the contributions of young scientists to thermodynamics and thermochemistry. Technical contact: Jason Lashley

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A. Aiken selected to chair American Association for Aerosol Research’s working group

Allison C. Aiken

Allison C. Aiken

The American Association for Aerosol Research (AAAR) Instrumentation’s Working Group has elected Allison C. Aiken (Earth System Observations, EES-14) as chair. The Instrumentation Group focuses on new measurement techniques, instrument design, and analytical methods. Aikin co-chaired the AAAR Atmospheric Aerosol Working Group in 2010, and is completing her second term as the organization’s Early Career Committee Chair. In her new role, Aiken will serve on the Technical Program Committee and will oversee all presentation submissions and organize plenaries and special symposia for the Instrumentation Group.

Aiken first came to the Laboratory as a student in 2000. After earning a Ph.D. degree in analytical/atmospheric chemistry from the University of Colorado and holding a postdoctoral fellowship in Europe, Aiken returned to the Lab as a Director’s Postdoctoral Fellow. Her research focuses on light absorbing carbonaceous aerosols and their impact on global climate. Aiken is the DOE’s Atmospheric Radiation Measurement (ARM) Research Facility’s Aerosol Observing System Operations Manager for the Lab’s Field Instrument Deployments and Operations Team, which customizes, deploys and manages atmospheric research facilities globally. She is also a member of the ARM Aerosol Measurement Science Group, an advisory group for ARM. Thomson Reuters named Aiken one of the World’s Most Influential Scientific Minds for her aerosol research in 2014. Technical contact: Allison Aiken

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John Gordon named Invited Fellow of Japanese Society for the Promotion of Science

John Gordon

John Gordon

The Japanese Society for the Promotion of Science (JSPS) has selected John Gordon (Inorganic, Isotope and Actinide Chemistry, C-IIAC) to be an Invited Fellow. The JSPS was established to contribute to the advancement of science in all fields of the natural and social sciences and the humanities. The JSPS International Fellowships for Research in Japan (Invitational Fellowships) invite overseas researchers to promote international academic exchange. Individual Fellows are competitively selected following the submission of applications by a host researcher in Japan who wishes to invite a particular individual.

Gordon’s selection will allow him to spend up to 60 days in Japan between October 2017 and March 2018. He will interact with researchers at his host institution (Nagoya University) and develop scientific interactions and collaborations centered upon modern catalysis science. Gordon will also travel around the country to present seminars and to visit with researchers at other universities and national laboratories. This highly prestigious honor, designed to promote international scientific cooperation, will afford Gordon the opportunity to interact with some of the most respected scientists in Japan. He plans to establish new, mutually beneficial scientific interactions between Japan and the USA, particularly in the area of chemistry for sustainable and renewable energy applications.

Gordon received a Ph.D. in chemistry from the University of Notre Dame. He has been with the Laboratory for 21 years. Gordon specializes in developing chemocatalytic methods that convert molecules available from renewable and sustainable sources into useful chemicals and fuels. He 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. Gordon has received a Los Alamos Fellows Prize for Leadership in Science. He holds eleven patents. Technical contact: John Gordon

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Bioscience

Distinguishing virulent from harmless bacteria could aid biological surveillance

Fluorescent-labeled Francisella tularensis, 1000x magnification. Credit: CDC.

Fluorescent-labeled Francisella tularensis, 1000x magnification. Credit: CDC.

Los Alamos biologists Jean Challacombe (Biosecurity and Public Health, B-10) and Cheryl Kuske (Bioenergy and Biome Sciences, B-11) have spent many years examining the differences among Francisella bacteria, a few species of which include highly virulent human and animal pathogens. The Francisella genus contains several recognized species, additional potential species, and other representatives that inhabit a range of incredibly diverse ecological niches but are not closely related to the named species. Many of them cause no problems for humans or livestock. However, F. tularensis is a highly virulent zoonotic pathogen (spreads from animals to humans) that causes tularemia. Due to weaponization efforts in past world wars, it is considered a first tier (most serious) biothreat agent.

The close relationships of some Francisella species make it easy incur false positives when trying to detect them. Therefore, accurate discrimination among the virulent subspecies of F. tularensis and near relatives, such as the environmentally abundant F. novicida, is absolutely critical for the future success of biological surveillance and attribution activities.  

In their paper in PLOS ONE, the team identified several apparently cryptic plasmids – linear or circular structures of double-stranded DNA capable of existing outside the chromosome – in the sequenced genomes of three environmental Francisella species. Because bacterial plasmids can carry traits that enhance the survival of host cells and influence bacterial evolution, cryptic plasmids encode few functions other than those needed to replicate and mobilize. With no obvious benefit to the host cells that carry them, cryptic plasmids are somewhat of an enigma. These plasmids provide additional phylogenetic and genomic features that could differentiate pathogenic F. tularensis strains from clinical and environmental near-neighbor species that are not biothreat agents.

The close relationships of some Francisella species make it easy incur false positives when trying to detect them. Therefore, accurate discrimination among the virulent subspecies of F. tularensis and near relatives, such as the environmentally abundant F. novicida, is absolutely critical for the future success of biological surveillance and attribution activities.  

In their paper in PLOS ONE, the team identified several apparently cryptic plasmids – linear or circular structures of double-stranded DNA capable of existing outside the chromosome – in the sequenced genomes of three environmental Francisella species. Because bacterial plasmids can carry traits that enhance the survival of host cells and influence bacterial evolution, cryptic plasmids encode few functions other than those needed to replicate and mobilize. With no obvious benefit to the host cells that carry them, cryptic plasmids are somewhat of an enigma. These plasmids provide additional phylogenetic and genomic features that could differentiate pathogenic F. tularensis strains from clinical and environmental near-neighbor species that are not biothreat agents.

(Left): Principal investigator Jean Challacombe, assisted by Cheryl Gleasner who runs the sequencing machines, sequence Francisella genomes. The device shown is an Illumina NextSeq 500, used in high-throughput sequencing.

(Left): Principal investigator Jean Challacombe, assisted by Cheryl Gleasner who runs the sequencing machines, sequence Francisella genomes. The device shown is an Illumina NextSeq 500, used in high-throughput sequencing.

The team examined Francisella genomes from samples of seawater in the area of Galveston Bay, Texas (F. novicida-like), some human clinical samples (F. novicida AZ06-7470 and F. opportunistica), water from a warm spring (F. novicida) and a form isolated from an air conditioning system (F. frigiditurris). Los Alamos researchers sequenced several of the plasmid-containing Francisella strains.

The researchers’ results comparing the cryptic plasmids in diverse Francisella genomes demonstrate that cryptic plasmids are also found in clinical isolates. The work showed that of the more than 120 Francisella genomes that have been sequenced, only a few contained plasmids, including several F. novicida strains. The team also found that all of the plasmids were apparently cryptic, encoding functions potentially involved in replication, conjugal transfer and partitioning, a few functions that could be important to environmental survival, and some hypothetical proteins to which a function could not be assigned. These results provide a new understanding of the phenotypic variability and complex taxonomic relationships among the known Francisella species, and also identify new plasmid features to use in characterizing related species groups.

Circular maps of the candidate Francisella TX07-6608 plasmids

Circular maps of the candidate Francisella TX07-6608 plasmids. Restriction sites are indicated on the maps by orange annotation; ori and ter regions are marked in red. Approximate locations of direct repeats are indicated by black Xs, and DnaA boxes by green Ds. Panel A), Plasmid 1. Panel B), Plasmid 2. Panel C), Plasmid 3. Panel D), Plasmid 4.

Reference: “Shared Features of Cryptic Plasmids from Environmental and Pathogenic Francisella Species,” PLOS ONE 12(8): e0183554. (2017); doi: 10.1371/journal.pone.0183554. Authors: Jean F. Challacombe (B-10), Segaran Pillai (US Food and Drug Administration), and Cheryl R. Kuske (B-11).

The Department of Homeland Security funded the research at Los Alamos, which supports the Lab’s Global Security mission area and the Science of Signatures science pillar through the ability to differentiate between benign and pathogenic bacteria. Technical contact: Jean Challacombe

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Capability Enhancement

Sigma Division provides 24/7 manufacturing operations

Working on long-term and classified projects within Sigma Division is nothing new; personnel within the Sigma Facility have been doing just that for nearly six decades. Several recent projects have required the processing of classified parts on a 24/7 basis for weeks at a time. These projects fall into two categories: Additive Manufacturing (AM) and electroplating. These two technologies can build up metal on a substrate, but by very different methods. This capability to add substantial amounts of material to a substrate has drawn increased interest recently.

Additive manufacturing is an agile model for designing, producing, and implementing the process-aware materials of the future. The process enables innovative design options that were once impossible to fabricate. The desired part is first designed in 3-D using SolidWorks or Creo and exported to the Magics build preparation software for rendering into a stack of 2-D layers for the build. Then an AM machine deposits the metal in a step-wise fashion to build up the 3-D part. Sigma houses several metal AM technologies: laser powder bed fusion (EOS), laser directed energy deposition (LENS), and electron beam additive manufacturing (EBAM). The team used the EOS laser powder bed fusion machine to complete a customer’s part that was comprised of 6,000 30-micron layers and required a build time of 300 hours.
(Right): Cameron Knapp (Sigma Division, SIGMA-DO) shows Laboratory Director Charlie McMillan a sample that was created using additive manufacturing techniques.

(Right): Cameron Knapp (Sigma Division, SIGMA-DO) shows Laboratory Director Charlie McMillan a sample that was created using additive manufacturing techniques.

Electroplating deposits metal from a solution phase onto an existing substrate. The method is commonly used to deposit decorative or protecting metal coatings onto an existing part rather than building up bulk materials. Sigma Division owns the last full-scale electroplating shop within the DOE Complex. This unique electroplating facility can deposit metals in bulk form up to several millimeters in thickness.

Several projects use these two technologies to build up metals that could not be accomplished by any other manufacturing method in a timely fashion. These manufacturing technologies are very novel and yield the desired products. However, they require long processing times that can require continuous operation for days to weeks at a time. Because these are new projects and they are producing classified parts, there is a need for continuous staff support on a 24/7 basis. More than a dozen staff members have formed a “watch group” that stay within the Sigma building on a continuous basis to meet the needs of these projects. The watch group alternates their days and nights to maintain the entire facility in normal operations while supporting these new projects. Technical contacts: Cameron Knapp (additive manufacturing) and Justin Tokash (electroplating).

Accurate prompt fission neutron spectrum measurements produced at LANSCE

Hye Young Lee and John O'Donnell (LANSCE Weapons Physics, P-27) check the Chi-Nu instrument at the Weapons Neutron Research Facility.

Hye Young Lee and John O'Donnell (LANSCE Weapons Physics, P-27) check the Chi-Nu instrument at the Weapons Neutron Research Facility.

Accurate nuclear data on neutron-induced fission form the basis of criticality calculations. The spectra of neutrons produced in neutron-induced fission reactions are a significant contributor to reactivity in such fast neutron systems. NNSA supports an effort at the Los Alamos Neutron Science Center (LANSCE) to improve these experimental data. This work is called the Chi-Nu project, after the mathematical symbol for the desired measured quantity, the chi matrix for neutrons. Researchers seek data from the neutron-induced fission of both uranium-235 (235U) and plutonium-239 (239Pu).

Chi-Nu uses the LANSCE Weapons Neutron Research Facility “white” source of neutrons, created by spallation of the LANSCE 800-MeV proton beam on an unmoderated tungsten target. The resulting neutron beam covers a wide range of energies, hence “white,” and relies on measurements of the time-of-flight of the neutrons to determine their energy on an event-by-event basis. Because Chi-Nu measures both the incoming and outgoing neutron energies, it uses a double time-of-flight technique: the time from the neutron production to fission, and the time from fission to outgoing neutron detection. A parallel-plate avalanche counter 21.5 meters from the neutron production source provides the fission time measurement. Either lithium-6 glass detectors (for lower energy neutrons) or liquid scintillator detectors (for higher energy neutrons) provide the subsequent neutron detection time.

Neutrons scatter off of any material. The possibility of scattering off material in the experimental area complicates the detection and energy measurement of neutrons via time-of-flight. Researchers use an extensive Monte Carlo model to analyze the experimental environment accurately. Backgrounds from neutrons detected from processes other than fission must also be taken into account. Publications by J. M. O’Donnell and K. J. Kelly, et al. (P-27) document methods developed to solve these problems.

The figure, below, shows some of the resulting data for two incident neutron energy ranges and for neutron-induced fission of both 239Pu and 235U and the corresponding Evaluated Nuclear Date File (ENDF/B-VII.1) evaluation for nearby energies. The next ENDF evaluation has incorporated some of the new data, and the complete program will provide a firm foundation of prompt fission neutron data for applications in the future.

A comparison of the recently measured prompt fission neutron spectra for two incident neutron energy ranges for neutron-induced fission of plutonium-239 and uranium-235, and the corresponding ENDF/B-VII.1 evaluations.

A comparison of the recently measured prompt fission neutron spectra for two incident neutron energy ranges for neutron-induced fission of plutonium-239 and uranium-235, and the corresponding ENDF/B-VII.1 evaluations.

Los Alamos researchers: Jaime Gomez, Keegan Kelly, John O’Donnell, Hye Young Lee, Shea Mosby, Bob Haight, and Terry Taddeucci (LANSCE Weapons Physics, P-27); Denise Neudecker and Morgan White (Materials and Physical Data, XCP-5); and Principal Investigator Matt Devlin (P-27). Researchers from Lawrence Livermore National Laboratory: Ching-Yen Wu, Matt Buckner, Brian Bucher, and Roger Henderson.

References:

“A New Method to Reduce the Statistical and Systematic Uncertainty of Chance Coincidence Backgrounds Measured with Waveform Digitizers,” Nuclear Instruments and Methods in Physics Research Section A 805, 87 (2016); doi: 10.1016/j.nima.2015.07.044. Author: J. M. O’Donnell (P-27).

“Numerical Integration of Detector Response Functions via Monte Carlo Simulations,” Nuclear Instruments and Methods in Physics Research Section A 866,182 (2017); doi: /10.1016/j.nima.2017.05.048. Authors: K. J. Kelly, J. M. O’Donnell, J. A. Gomez, T. N. Taddeucci, M. Devlin, R. C. Haight, H. Y. Lee, and S. M. Mosby (P-27); M. C. White and D. Neudecker (Materials and Physical Data, XCP-5); M. Q. Buckner and C. Y. Wu (Lawrence Livermore National Laboratory).

NNSA Science Campaign 1 funded the work, which supports the Lab’s Nuclear Deterrence mission area and the Nuclear and Particle Futures by reducing the uncertainties of key data used to predict nuclear performance. Technical contact: Matt Devlin

Developing a protactinium/uranium radiochronometry capability for nuclear forensics

Nuclear forensics is the science of determining the history and origin of a nuclear material in the context of law enforcement. The age dating of radioactive materials, or radiochronometry, is an essential part of this process. The study of radiochronometry takes advantage of the natural decay of radioactive isotopes for age dating by measuring the ingrowth of decay progeny (daughter atoms) of a radioactive nuclide (parent). From these measurements the material’s model age can be calculated. This model age represents the amount of time that has elapsed since the nuclear material was last processed or purified. Thorium-230/uranium-234 (230Th/234U) (daughter/parent) is the most widely used radiochronometer for uranium materials because there is usually enough 230Th ingrown in nuclear era materials to measure. Increased confidence in the material’s model age (important in a law enforcement setting) can be gained via use of different parent/daughter pairs/radiochronometers to multiple model ages that agree within analytical uncertainty.

(a) The mid-level radiological lab where protactinium separation chemistry occurs, and (b) a ThermoScientific Neptune Plus inductively coupled plasma mass spectrometer used to analyze the purified fractions.

(a) The mid-level radiological lab where protactinium separation chemistry occurs, and (b) a ThermoScientific Neptune Plus inductively coupled plasma mass spectrometer used to analyze the purified fractions.

Nuclear and Radiochemistry (C-NR) researchers have developed a protactinium-231/uranium-235 (231Pa/235U) radiochronometry capability to complement the existing, mature 230Th/234U procedure within the Lab. Obtaining a model age using the 231Pa/235U radiochronometer can be more challenging due to: 1) the lower abundance of 235U and its progeny (compared with 238U and its progeny) in depleted uranium and natural uranium materials, 2) the lack of certified reference materials, and 3) the short half-life of 233Pa used as the isotope dilution tracer.
Multiple radiochronometers for nuclear forensics applications.

Multiple radiochronometers for nuclear forensics applications.

The 231Pa/235U procedure involves 1) material dissolution, 2) taking separate fractions for U and Pa analysis and adding a chemically purified tracer of a minor isotope that is not expected to be in the sample (e.g., 233U and 233Pa respectively), 3) purifying each fraction using ion exchange chromatography (one column for U and three columns for Pa), and 4) analyzing each purified fraction on a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS). This method requires the capability to measure sub-pg quantities of 231Pa. The investigators have tested this method on certified reference materials with a known history and obtained model dates consistent with known purification ages.
Plot of model dates of purification for two reference materials with known production dates (solid black/gray lines) obtained using the protactinium-231/uranium-235 radiochronometer. Previously published data are included.

Plot of model dates of purification for two reference materials with known production dates (solid black/gray lines) obtained using the protactinium-231/uranium-235 radiochronometer. Previously published data are included.

Plot of the concentration of the purified protactinium-231 stock solution calibrated against the most recent protactinium-233 tracer. The researchers conducted the calibration at varying ratios of 231:233. The concentrations agree within analytical uncertainty.

Plot of the concentration of the purified protactinium-231 stock solution calibrated against the most recent protactinium-233 tracer. The researchers conducted the calibration at varying ratios of 231:233. The concentrations agree within analytical uncertainty.

One remaining challenge with the 231Pa/235U radiochronometer is the production and calibration of a 233Pa tracer for isotope dilution mass spectrometry. This tracer is used to determine 231Pa atom concentration. The short half-life (approximately 27 days) of 233Pa renders the tracer obsolete after approximately four months. Consequently, a new tracer has to be produced from a solution of its parent (237Np) and calibrated every six months. One approach to calibrating a new 233Pa tracer is to use geological rock standards. These rocks are old so that the 231Pa is in secular equilibrium with its parent 235U, and the concentration of 231Pa is accurately established.

The use of these standards for calibration is not ideal because the rock powders may not be homogeneous, and the time taken to prepare these solutions is too lengthy for the rapid analytical response required in a nuclear forensics investigation. Therefore, the team is developing an alternative approach to spike calibration using a stock solution of 231Pa purified from a solution of highly enriched uranium. The researchers have calibrated their 231Pa stock solution against the most recent 233Pa tracer that was calibrated using rock standards. The initial results are promising. All the resulting concentrations agreed within analytical uncertainty. The investigators will repeat this procedure for the next two spikes. If successful, this method of 233Pa spike calibration would reduce the time to calibrate each new 233Pa tracer by over 60%. The in-house 231Pa solution will be used until there is a commercially available 231Pa certified reference from the National Institute of Standards and Technology (NIST).

Los Alamos researchers include: Joanna Denton, Theresa Kayzar-Boggs, Allison Wende, William Kinman, Anneliese Cardon, Sean Reilly, Robert Steiner, and Warren Oldham (Nuclear and Radiochemistry, C-NR).

References:

“Developing 226Ra and 227Ac Age-Dating Techniques for Nuclear Forensics to Gain Insight from Concordant and Non-Concordant Radiochronometers,” Journal of Radioanalytical and Nuclear Chemistry 207, 2061 (2016); doi: 10.1007/s10967-015-4435-4. Authors: Teresa M. Kayzar (formerly at Lawrence Livermore National Laboratory, currently at C-NR) and Ross W. Williams (Lawrence Livermore National Laboratory).

“Uranium Radiochronometry Intercomparison Study for Nuclear Forensics Applications,” IAEA International Conference on Nuclear Security, Abstract 494 (2016). Authors: Robert Ernest Steiner, Theresa Marie Kayzar-Boggs, Stephen Philip Lamont, William Scott Kinman (C-NR); Ross Williams, Amy Gaffney, Kerri Schorzman (Lawrence Livermore National Laboratory); Fabien Pointurier and Amelie Hubert (CEA France).

The NNSA Nuclear Smuggling Detection and Deterrence Program and the Nuclear Materials Information Program funded the work, which supports the Laboratory’s Global Security mission area and the Science of Signatures science pillar through the development of measurement methods for nuclear forensics. Technical contact: Joanna Denton

Novel dual-pulsed laser deposition system for improved versatility in thin film research

Los Alamos researchers have used pulsed laser deposition (PLD) to grow and investigate complex functional materials for two decades. They have developed an innovative PLD system, featuring dual-pulsed lasers and novel inventions, which produces exceptionally high quality thin films with finely controlled properties.

Thin films can exhibit exotic properties that differ from their bulk counterparts, which makes them both fundamentally and practically interesting. The films can form materials with well-ordered electrical, magnetic, optical, thermal, and mechanical properties. Advances in thin film deposition have led to breakthroughs in semiconductors and magnetic storage devices. Materials manufactured across multiple length scales are expected to have novel controlled chemical compositions and structures.
Demonstration of CINT’s new dual-pulsed laser deposition system. A green plume of plasma is ejected onto a square substrate, which is mounted to a non-braze heater of Dowden’s invention.

Demonstration of CINT’s new dual-pulsed laser deposition system. A green plume of plasma is ejected onto a square substrate, which is mounted to a non-braze heater of Dowden’s invention.

The new pulsed laser deposition capability is located at the Center for Integrated Nanotechnologies (CINT), a DOE Office of Science national user facility focused on nanoscience integration. CINT researchers and users can customize films using multiple targets, gases, wavelengths, laser pulse rates, and energies.

Paul Dowden (MPA-CINT) designed and built the system, which features the following:

  • A stabilized, patented beamline to overcome the drawbacks of the traditional single-lens focus for repeatable PLD film growth.
  • An ultraclean heating device (patent-pending) reaches 1000 oC and operates without any braze materials, which are common in commercial products and can contaminate films.
  • A novel laser system uses two lasers, each with different wavelengths. An excimer laser that grows matrices works in tandem with an ultrafast, tripled titanium sapphire laser that produces nanoparticles from metals to oxides and nitrides.

Versatility is key to creating diverse thin films. With its new instruments and improved control, this update to the industry-standard PLD technique will enable users to create non-uniform properties such as nanoparticle density, location, and size in films. Dowden and the PLD team are fine-tuning and characterizing the system to evaluate wavelengths and other variables to define their relationship to the produced particles.

The system will advance research vital to the Lab’s quest for controlled functionality – the central concept of its Materials Strategy – by enabling researchers to manufacture precisely tuned thin films that perform in ways beyond their basic properties. The DOE Office of Science and the Laboratory Directed Research and Development (LDRD) program funded different aspects of the work. Los Alamos National Laboratory and Sandia National Laboratories jointly operate CINT as a DOE Office of Science national user facility. The capability to fabricate complex materials of controlled functionality supports Los Alamos’s Energy Security mission area and the Materials for the Future science pillar. Technical contact: Paul Dowden

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Computer, Computational and Statistical Sciences

Three-dimensional upwelling pathways in the Southern Ocean

The Southern Ocean absorbs nearly half of the carbon dioxide and 75% of the total heat that is absorbed by the world’s oceans from the atmosphere. However, the Southern Ocean represents only 30% of the global ocean. One key reason that the Southern Ocean plays such an important role in the regulating the climate system is the fact that the strong winds blowing over the ocean pull cold, deep water from the ocean abyss to the sea surface, forming a connection between the deep ocean and the atmosphere. Despite the importance of the Southern Ocean upwelling, the three-dimensional pathways of deep water masses are largely unknown. A new study published in Nature Communications combines ocean observations with three state-of-the-art ocean models. The research reveals the full 3-D pathway of deep water to the surface of the Southern Ocean for the first time.
The three dimensional upward spiral of North Atlantic Deep Water through the Southern Ocean.

The three dimensional upward spiral of North Atlantic Deep Water through the Southern Ocean. a) Observed warm water (>1.6 °C) on the 28.05 kg m−3 neutral density surface from hydrographic observations, south of 40° S, colored by depth (m). The isoneutral surface is masked in regions with potential temperature below 1.6 °C. 1/4° ocean bathymetry is shown in gray. b) Modeled (CM2.6) particle pathways from the Atlantic Ocean, with particles released in the depth range 1000–3500 m along 30° S. Colored boxes mark 1° latitude × 1° longitude × 100 m depth grid boxes visited by >3.5% of the total upwelling particle-transport from release at 30° S to the mixed layer. Boxes are colored by depth, similar to a). c) Two example upwelling particle trajectories from CM2.6, one originating from the western Atlantic and the other from the eastern Atlantic. Trajectories are colored by depth as in a and b, blue spheres show the particle release locations and red spheres show the location where the particles reach the mixed layer.

A team of researchers, including Wilbert Weijer (Computational Physics and Methods, CCS-2), examined the upwelling the global overturning circulation. This upwelling forms a branch of the global overturning circulation and balances the descent of water into the abyss at high latitudes. This overturning completes the global circulation loop, which is important for the oceanic uptake of carbon and heat, the resupply of nutrients for use in biological production, as well as the understanding of how ice shelves melt.

The team determined the upwelling pathways by releasing virtual water particles in the deep ocean in three high resolution ocean and climate simulation models. The researchers tracked the pathways of the particles toward the surface of the Southern Ocean, and compared the results to hydrographic observations. The investigators found that deep, relatively warm water from the Atlantic, Indian, and Pacific ocean basins enters the Southern Ocean and spirals southeastwards and upwards around Antarctica before reaching the ocean’s mixed layer, where it interacts with the atmosphere. These pathways spiral while being transported eastward by the enormous Antarctic Circumpolar Current. This current flows around the northern edge of the Southern Ocean and is both the world’s strongest current, and also the only major current that circles the globe unimpeded by continent.

Animation: Atlantic Ocean particle pathways with >2.25% particle-transport

The pathways make several loops around the Antarctic continent as part of the Antarctic Circumpolar Current. This upward spiral is not smooth. It displays distinct jumps where the current meets topographic obstacles, making the circulation more turbulent. Undersea ridges and plateaus create hotspots of swirling eddies that push water southward and up toward the surface. The study identified five major upwelling hotspots associated with large undersea topographic features, which are responsible for most of the upwelling in the Southern Ocean. The upwelling time was surprisingly short (on average less than a century), which is important for the delay in which a change in the North Atlantic can be conveyed to the Southern Ocean.

The researchers also calculated how much water from each ocean basin made it up the spiral staircase. They determined that half of the water that reached the mixed layer originated from the Atlantic Ocean, while the Indian and Pacific oceans each contributed approximately a fourth.

 The team concluded that structure of the upwelling is controlled by the under-sea topography and the eddy field, rather that by the winds. This study identified critical regions for upwelling of deep water, which can now be targeted by future research expeditions and modeling studies. To predict whether the Southern Ocean will continue to absorb heat and carbon dioxide from the atmosphere at its current rate, it is necessary to understand the processes that make these hotspots so important for the upwelling of the ocean’s deepest waters.

 The overturning circulation is an important part of the climate system because it has the capacity to sequester heat and carbon from the atmosphere, mitigating anthropogenic carbon emissions and its resulting warming. It also brings relatively warm water close to the Antarctic continent, where it has the potential to interact with the ice shelves, potentially destabilizing them.

 Reference: “Spiraling Pathways of Global Deep Waters to the Surface of the Southern Ocean, Nature Communications 8, 172 (2017); doi: 10.1038/s41467-017-00197-0. Authors: Veronica Tamsitt, Lynne D. Talley, and Matthew R. Mazloff (Scripps Institution of Oceanography); Henri F. Drake (Princeton University, currently Massachusetts Institute of Technology and Woods Hole Oceanographic Institution); Adele K. Morrison (Princeton University, currently Australian National University); Carolina O. Dufour, Alison R. Gray, Jorge L. Sarmiento, and Stephen M. Griffies (Princeton University); Jinbo Wang (California Institute of Technology); and Wilbert Weijer (Computational Physics and Methods, CCS-2).

The DOE Office of Science Regional and Global Climate Modeling Program funded the Los Alamos portion of the work, which supports the Lab’s Global Security and Energy Security mission areas and the Information, Science and Technology science pillar. Technical contact: Wilbert Weijer

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Earth and Environmental Sciences

Robust pressure management strategies for geologic carbon dioxide sequestration

Making good decisions with limited information is difficult. Some scenarios lack the quantity and quality of information necessary to inform decisions using traditional, probabilistic approaches. This has led researchers to develop non-probabilistic approaches that can more appropriately utilize the available information in these cases. Los Alamos researchers and collaborators have demonstrated such an approach to support decisions related to pressure management strategy selection for carbon dioxide (CO2) sequestration operations when the available information regarding reservoir permeabilities is limited. The International Journal of Greenhouse Gas Control published the research.

A lack of information to support decisions is a common issue with energy related applications deep below the ground surface. The collection of data at depth is expensive, and properties of interest can often only be measured indirectly with low resolution. Ensuring that subsurface energy related applications do not induce seismicity is a current technical challenge.

The team developed an approach to select from alternative pressure management strategies during CO2 injection operations. The pressure management strategies involve extracting brine from the injection reservoir to reduce injection-induced pressures at a nearby fault. The alternative strategies include the location of the extraction well(s), the rate of extraction, the number of extraction wells, and pre-injection brine-extraction. The researchers’ method quantifies the robustness of alternative strategies to 1) not induce seismicity in a nearby fault, 2) inject a desired quantity of CO2, 3) not extract more brine than feasible, and 4) achieve a desired extraction efficiency.

The researchers ran ensembles of the Lab’s Finite Element Heat and Mass Trans Code (FEHM, fehm.lanl.gov) simulations to quantify the robustness of meeting criteria for the alternative pressure management strategies based on the lack of information about the reservoir permeability. The robustness of achieving the desired criteria is quantified as the amount that the nominal permeability field (determined using the available information including a geophysical survey and velocity/porosity and porosity/permeability relationships derived in the laboratory from well core samples) can be incorrect and the criteria are still met. If the nominal permeability field can be incorrect by a large amount, the strategy has high robustness. If a small deviation in the nominal permeability field results in failing to meet the criteria, the strategy has low robustness. If the strategy fails to meet criteria with the nominal permeability field, it has zero robustness.

The figure presents FEHM numerical simulation results for a pressure management strategy using data and site characteristics from the Rock Springs Uplift Carbon Storage Site in southwestern Wyoming. The plots contain horizontal pressure contours in the injection reservoir as colored lines and the CO2 plume in the injection reservoir as gray shading. The strategy involves three extraction wells that are pumped sequentially starting with the closest extraction well to the injection well and finishing with the extraction well closest to the fault (dashed line). Extraction is switched to the next well when CO2 arrives at the current extraction well. The extraction at the wells reduces the pressure along the fault, thereby reducing the risk of reactivating the fault and inducing seismicity.
Plan view of overpressure contours (colored lines) and carbon dioxide plume extent (gray fill) for a triple-extraction well pressure management FEHM simulation. The carbon dioxide Rock Springs Uplift injection well (RSU #1) is indicated by a circle, brine extraction wells are indicated by x's, and the Jim Bridger Fault by a dashed line. The time and cumulative brine extracted (mb) are indicated in the upper right corner of each plot. Because carbon dioxide injection is 1 Mt/year, the amount injected in Mt is equal to the time in years. Axes are in the State Plane Coordinate System S27-4903.

Plan view of overpressure contours (colored lines) and carbon dioxide plume extent (gray fill) for a triple-extraction well pressure management FEHM simulation. The carbon dioxide Rock Springs Uplift injection well (RSU #1) is indicated by a circle, brine extraction wells are indicated by x's, and the Jim Bridger Fault by a dashed line. The time and cumulative brine extracted (mb) are indicated in the upper right corner of each plot. Because carbon dioxide injection is 1 Mt/year, the amount injected in Mt is equal to the time in years. Axes are in the State Plane Coordinate System S27-4903.

This new approach to selecting pressure management strategies based on concepts from information gap theory focuses on evaluating the robustness against multiple performance criteria. The method enables a detailed evaluation and comparison of competing and

complementary performance criteria while taking into account uncertainties. The authors presented results that consider uncertainty in permeability. However, the approach also could be used to assess the effect of uncertainties in multiple parameters. This strategy provides an alternative to optimization strategies, which identify solutions that exactly meet specified

criteria without a detailed consideration of strategy robustness. Considering performance criteria robustness is critically important for applications like CO2 sequestration

Reference: “Development of Robust Pressure Management Strategies for Geologic CO2 Sequestration,” International Journal of Greenhouse Gas Control 64, 43 (2017); doi: 10.1016/j.ijggc.2017.06.012. Authors: Dylan R Harp, Philip H Stauffer, Daniel O’Malley, Terry A Miller, Daniella Martinez, Richard S Middleton and Rajesh Pawar (Computational Earth Science, EES-16); Zunsheng Jiao and Evan P. Egenolf (University of Wyoming); Kelsey A. Hunter and Jeffrey M. Bielicki (The Ohio State University).

The DOE’s Brine Extraction Storage Test Project, DOE Fossil Energy Office’s National Risk Assessment Partnership (NRAP, managed by the National Energy Technology Laboratory); and the US-China Advanced Coal Technology Consortium (managed by West Virginia University) funded different aspects of the work. The research used computer clusters at Los Alamos supported by the Los Alamos National Laboratory High Performance Computing Environments Group (HPC-ENV). The work supports the Lab’s Energy Security and Global Security mission areas and in the Information, Science and Technology science pillar through support for the decision-making process for subsurface energy applications. Technical contact: Dylan Harp

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Materials Physics and Applications

Quasi-phase shift in nanoconfined water observed

Confinement of molecules determines the properties and functions of many systems in biology, geology, catalysis, and nanofluidics. Nanoconfined water plays a vital role in selective single-file water molecule and ion transport through biological membranes, and it has potential for technological applications. Center of Integrated Nanotechnologies (MPA-CINT) and University of Antwerp researchers examined temperature-induced photoluminescence spectral changes occurring in a single-file chain of water molecules encapsulated in single-wall carbon nanotubes. The team interpreted the shift as a change from a disordered arrangement of the water molecules to one with ordered orientation. Physical Review Letters published the research.

There is growing interest in nanoporous materials for potential applications in nanofluidics and filtration. Nanomaterial-based targeted drug delivery schemes could also rely on developing selective transport of active agents through nanoscale membrane channels. Many of these envisioned applications will depend on understanding and control of molecular transport through highly constrained channels. In these scenarios, water would be the transport medium, and carbon nanotubes could serve as nanoscale channels in synthetic membrane assemblies. Encapsulation of optically active molecules inside carbon nanotubes might also lead to new optical behaviors that are attractive for use in sensing and imaging.  To advance these applications, it is essential to understand the behavior of molecules in nanoscale channels and the limits imposed by the channel dimensions. Well-defined model systems (such as this study of water in a specific type of nanotube) could serve to advance theoretical models for predictive capability.

Researchers encapsulated water inside ultrathin carbon nanotubes with a diameter so narrow that it fit a single row of water molecules. As the molecules cooled to cryogenic temperatures, the team observed an unexpected temperature-induced transition in the orientation of the water dipoles. The investigators detected the transition as a change in the emission spectrum of the filled nanotubes originating from the subtle interaction of the water dipoles with the nanotube walls. Molecular dynamics simulations indicated three different regimes of water under the experimental conditions: classical hydrogen-bonded, predominantly bifurcated hydrogen-bonded (i.e., hydrogen bond from a single oxygen atom distributed over both hydrogen atoms of a single neighboring water molecule), and disordered chains. The simulations supported the finding that qualitative changes in orientational order should occur within the measured temperature range.
(Left panel) depicts how the data can be understood as a consequence of the structure of the water network changing within the tube. The top section shows random orientation of the water dipoles.

(Left panel) depicts how the data can be understood as a consequence of the structure of the water network changing within the tube. The top section shows random orientation of the water dipoles. The middle section shows a transition to a more ordered orientation (called bifurcated hydrogen bonding), which caused a sudden shift in the emission color, as shown in the black data points in the right panel. The emission color of an empty nanotube, visible in the empty data points in the right panel, only shows a gradual shift. The bottom section completes the transition to a classical hydrogen bonding structure at lowest temperatures. (Right panel) depicts a plot of how photoluminescence (PL) emission energy (or color, x-axis) changes as the temperature (T) is changed. This energy change displays different behavior depending on if the nanotube is empty (open circles) or filled with water (black circles). The abrupt shift in emission energy midway through the temperature variation for water filled tubes indicates the water goes through a phase transition at that temperature.

The exact nature of the phase transition has important implications for understanding the physics of one-dimensional systems and the behavior of confined fluids. This research, a step closer to controlling fluid transport at the nanoscale, could also benefit a variety of applications, from targeted drug delivery to water desalination.

CINT capabilities included the processing and separation of carbon nanotubes, as well as single nanoelement optical spectroscopy (e.g., single nanotube photoluminescence spectroscopy in a variable temperature cryostat).

Reference:

“Quasiphase Transition in a Single File of Water Molecules Encapsulated in (6, 5) Carbon Nanotubes Observed by Temperature-Dependent Photoluminescence Spectroscopy,” Physical Review Letters 118, 027402 (2017); doi: 10.1103/PhysRevLett.118.027402. Authors: Xuedan Ma, Stephen K. Doorn, and Han Htoon (MPA-CINT); Sofie Cambré and Wim Wenseleers (University of Antwerp, Belgium).

The work supports the Lab’s Energy Security mission area and the Materials for the Future science pillar as an example of the quest for controlled functionality, the ability to tailor materials to perform in ways beyond their basic properties and for specific purposes. The Laboratory Directed Research and Development (LDRD) program funded the Los Alamos work, which was performed in part at CINT, a DOE Office of Science User Facility operated jointly by Los Alamos and Sandia national laboratories. Technical contact: Steve Doorn

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Materials Science and Technology

Using density functional theory to examine defects in unalloyed delta-plutonium

Understanding the aging of face-centered cubic (fcc) delta (δ)-phase plutonium (Pu) stabilized with gallium is a key issue in stockpile stewardship, as questions of the effects of plutonium self-irradiation on phase stability remain unanswered. To further understand self-irradiation-induced defect formations, Los Alamos researchers conducted a comprehensive study of various classical point defects that include gallium (Ga, an alloying impurity) and uranium (U, a radioactive daughter decay product) in unalloyed δ-plutonium. The journal Scripta Materialia published their findings.

In many metals, self-irradiation causes damaging effects in the lattice while inducing void swelling, but aged plutonium does not void swell like other fcc alloys, such as austenitic steel. The mechanisms that inhibit void swelling in plutonium have not been fully investigated, and the fundamental physical role of gallium during radiation damage is unclear. Increasing the gallium concentration has been shown to increase or have no influence on the relative lattice swelling. Although experimental results can provide insight into plutonium aging, first principles and molecular dynamics may guide research into the existence of certain point defects in the lattice that are induced from radiation damage, such as vacancies, interstitials, and〈100〉split-interstitials.

Density functional theory (DFT), a modeling method for simulating material properties, is a useful technique to study the influence of these point defects on the geometric and electronic properties of unalloyed δ-plutonium. Classification of defects by energetics calculated by DFT could identify possible candidates of defects in unalloyed δ-plutonium that affect the structural stability.

The researchers determined that for plutonium-only defects, the most energetically stable defect is a distorted split-interstitial. Gallium, the δ-phase stabilizer, is thermodynamically stable as a substitutional defect, but becomes unstable when participating in a complex defect configuration. Complex uranium defects may thermodynamically exist as uranium substitutional with neighboring plutonium interstitial. Stabilization of uranium within the lattice is shown via partial density of states and charge density difference plots to be 5f hybridization between uranium and plutonium
Charge density difference distribution of uranium substitutional (left) and of uranium substitutional with nearby plutonium interstitial (right) in fcc lattice. Red regions indicate charge accumulation, while blue regions indicate charge depletion.

Charge density difference distribution of uranium substitutional (left) and of uranium substitutional with nearby plutonium interstitial (right) in fcc lattice. Red regions indicate charge accumulation, while blue regions indicate charge depletion.

Reference: “Density Functional Theory Study of Defects in Unalloyed δ-Pu,” Scripta Materialia 134, 57 (2017); doi: 10.1016/j.scriptamat.2017.02.025. Authors: Sarah C. Hernandez and Franz J. Freibert (Nuclear Materials Science, MST-16), and John M. Wills (Physics and Chemistry of Materials, T-1).

The Laboratory Directed Research and Development (LDRD) program funded the work, and Laboratory Institutional Computing provided computational support. The research benefitted from Los Alamos’s expertise and capabilities in actinide science research and computing, including the High Performance Computing (HPC) platforms on the Open Collaborative Network. The research supports the Laboratory’s Nuclear Deterrence mission area and the Materials for the Future science pillar. Technical contact: Sarah Hernandez

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Physics

Multi-laboratory assessment verifies reaction history accuracy from past nuclear events

In 2015 the Reaction History Working Group formed to facilitate direct working exchanges and joint projects between Lawrence Livermore National Laboratory (LLNL), Los Alamos National Laboratory (LANL), and National Security Technologies (NSTec) including Nevada National Security Site (NNSS).

An archival photo shows transportation of a rack in preparation for the Auk nuclear event at the Nevada Test Site in 1964. Gamma and neutron detectors were placed on such racks.

An archival photo shows transportation of a rack in preparation for the Auk nuclear event at the Nevada Test Site in 1964. Gamma and neutron detectors were placed on such racks.

LANL and LLNL recorded gamma reaction history data from about 870 underground nuclear events during the underground testing era, 1963-1992. During a nuclear event, gamma detectors of various kinds, including photodiodes and detectors based on the Compton effect, provided data as a function of time (called reaction history). In the subsequent decades, independent reaction history analysis software was developed and used by LANL personnel and a combination of researchers from LLNL and NNSS/NSTec. Computing capabilities now available in the Advanced Simulations and Computing (ASC) era, are very different than anything available during the time the tests took place. Therefore, current researchers are able to extract much more information from the recorded data than was possible immediately after a nuclear event.

The two laboratories compared the output from the independent software. They exchanged raw data from three nuclear events and compared analyzed results. The results agreed within the quoted uncertainty for all the events. This level of agreement gives the laboratories additional confidence that the analyses are being performed correctly and are free of systematic errors. The LANL and LLNL/NNSS teams plan to extend the study by adding additional events to analyze, and the Atomic Weapons Establishment (AWE) UK has expressed interest in joining the study using their independently developed software.

This knowledge is particularly important, because the comparisons of analyzed reaction history results to simulations are a key metric in characterizing the accuracy of weapon modeling, such as in baseline development for the Annual Assessment. NNSA’s Verification and Validation, ASC, Science Campaigns, Weapons Annual Assessments, and the Life Extension Projects benefit from this comparison.

Fred Wysocki [LANL Verification and Validation Program Manager, under the Advanced Simulations and Computing Program] led the Los Alamos participants. Dave Anderson (LLNL), Hanna Makaruk (Applied Modern Physics, P-21) and Ding Yuan (NNSS) performed the comparisons. Other Reaction History Working Group participants include: Tanim Islam, Dave Stevens, Mark May, Matt Buckner, and Don Smith (LLNL); Jerome Blair (NSTec); Bob Berglin (NNSS); Mike Ham, Michael Malone, and Yvette Maes (P-21); and Doug Thayer (P-21 and NSTec).

The NSSA funded the work, which supports Los Alamos’s Nuclear Deterrence mission area and the Nuclear and Particle Futures science pillar. Technical contact: Hanna Makaruk

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Theoretical

Evaluating the role of coherent delocalized phonon-like modes in DNA cyclization

The flexibility of the DNA molecule plays an important role in a multitude of biological functions as well as in the compact storage of the genetic material of cells. For example, the genome in each eukaryotic cell is tightly packed by sharply bending DNA around nucleosome, and DNA bending by transcription factors is a common cellular mechanism that participates in the regulation of gene expression. In the majority of cases, bending DNA results in the formation of highly curved loops. The existence of these loops challenges the classical polymer physics view, in which double-stranded DNA is virtually unbendable at scales below its persistence length. Experimentally, the innate flexibility of the DNA molecule, is quantified by the Jacobson-Stockmayer's J-factor, that represent the probability for cyclization. Recent experimental studies of ultra-short DNA sequences revealed a discrepancy of up to six orders of magnitude between the measured and predicted J-factors. These large differences suggested that, in addition to the elastic moduli, other factors, such as intrinsic curvature and propensity for local melting (aka DNA bubbles) can contribute to the loop formations. In a study published in Scientific Reports, researchers reported a new computational approach to evaluate DNA cyclization by linking the physics of intrinsic curvature and 3-D structure with the physics of local melting.

The team suggested that thermal fluctuations induce local strand separation resulting in short single-stranded DNA regions of the otherwise doubled-stranded rigid molecule. These regions act as effective hinges in the rigid molecule and significantly enhance DNA flexibility. This new model quantified these effects in an explicit sequence dependent manner. The researchers applied the model to all DNA sequences whose J-factors have previously been experimentally characterized. Their analysis demonstrated that the model accurately determined the J-factors of ultra-short DNA sequences. Most predictions were within an order of magnitude of experimental measurements. Moreover, the model accurately described the J-factors of longer sequences, and it was applicable to experimental DNA sequences containing a base pair mismatch.

The model yields more than 82% of the calculated J-factors within an order of magnitude of experimental measurements, while the analysis of J-factors for sequences longer than 100 base pairs shows that it gives results indistinguishable from the results obtained by previous models.
Estimated J-factors for all examined DNA sequences.

Estimated J-factors for all examined DNA sequences. The two different curves with colors correspond to the Czapla-Swigon-Olson (CSO) and CSO - extended Peyrard-Bishop-Dauxois (EPBD) model calculations. The y-axis reflects the orders of magnitude difference between experimentally measured and computationally derived J-factors. The x-axis corresponds to the percentage of sequences for a given order of magnitude difference.

The research is the first report of a direct link between sequence-dependent bubbles and an accurate calculation of J-factors for short DNA segments. Because recent experimental observations demonstrate that DNA bubbles are in the terahertz range, this model suggests an avenue for future exploration of J-factors in the presence of a strong, pulsed terahertz field. Such future studies would allow elucidation of the connection between terahertz irradiation and DNA functionality.

Reference: “Evaluating the Role of Coherent Delocalized Phonon-like Modes in DNA Cyclization,” Scientific Reports 7, 9731 (2017); doi:10.1038/s41598-017-09537-y. Authors: Ludmil B. Alexandrov (Theoretical Biology and Biophysics, T-6 and University of New Mexico Comprehensive Cancer Center, currently University of California – San Diego), Kim Ø. Rasmussen (Fluid Dynamics and Solid Mechanics, T-3), Alan R. Bishop (Science, Technology and Engineering, PADSTE), and Boian S. Alexandrov (Physics and Chemistry of Materials, T-1 and University of New Mexico Comprehensive Cancer Center).

The Laboratory Directed Research and Development (LDRD) funded the Los Alamos work, which supports the Lab’s Energy Security mission area and the Information, Science and Technology science pillar. Technical contact: Boian Alexandrov

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