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Los Alamos National Laboratory

Los Alamos National Laboratory

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Science Highlights, September 30, 2015

Awards and Recognition

Albert Migliori selected for Keithley Award

Albert Miglior

Albert Miglior

The American Physical Society has chosen Albert Migliori (National Security Education Center – Seaborg Institute, NSEC-SI) to receive the 2016 Joseph F. Keithley Award for Advances in Measurement Science. The Keithley Award is the top instrumentation prize of the American Physical Society. The award cited him “For the development of Resonant Ultrasound Spectroscopy, used to study lattice and electronic phenomena in condensed matter physics, solving problems as diverse as the elastic properties of plutonium, to finding that the pseudogap state of cuprates is indeed a thermodynamic phase.”

The work that led to this award began with a Laboratory Directed Research and Development (LDRD) Director’s Reserve project to develop Resonant Ultrasound Spectroscopy (RUS) for thermodynamic studies of high-temperature superconductors. This project required development of theory and electronics and led to important measurements in condensed matter physics including superconductivity. The team recently provided an accurate measurement of important properties of high temperature superconductors. As Migliori and team built on their successes in basic science, it became clear that RUS could also be applied to many compelling problems in support of the Laboratory’s national security mission. An example is the first accurate measurement of the compressibility and shear stiffness of alpha, beta, and gamma plutonium throughout their entire range of existence, and for gallium-stabilized delta plutonium at high temperatures. RUS is also the only measurement that can track and measure the aging of plutonium in real time, which the team does in support of the NNSA Science Campaigns and Plutonium Sustainment.

 

Migliori received a Ph.D. in physics from the University of Illinois. He joined the Lab as a Director’s Funded Postdoctoral Fellow in 1973. Now Migliori is the Director of the Seaborg Institute. He is a co-discoverer of acoustic heat engines and a leading expert in the use of resonant ultrasound spectroscopy as a solid-state physics tool. Recent research interests include elasticity of plutonium, and research and development of new measurement techniques. He holds 25 patents and has authored about 160 publications, six book chapters, and one book. Migliori has won two R&D 100 awards, a Federal Laboratory Consortium Award for Excellence in Technology Transfer, and a Los Alamos National Laboratory Distinguished Performance Award. He is a Fellow of the American Physical Society, American Association for the Advancement of Science, Acoustical Society of America, and Los Alamos National Laboratory. Migliori is Chair, Physical Acoustics Technical Committee, Acoustical Society of America; Vice Chair, General Instrumentation and Measurement Topical Group, American Physical Society; and Chair of the Science Advisory Council for the National High Magnetic Field Laboratory.

The American Physical Society (APS) is a non-profit membership organization working to advance and diffuse the knowledge of physics through its outstanding research journals, scientific meetings,

and education, outreach, advocacy, and international activities. APS represents over 51,000 members, including physicists in academia, national laboratories, and industry in the United States and throughout the world. Keithley Instruments, Inc. and the APS Instrument and Measurement Science Topical Group endowed this award to honor Joseph F. Keithley for his outstanding contributions and numerous accomplishments in area of sensitive and precision instrument development and measurement techniques. This award is presented annually. Technical contact: Albert Migliori

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Yang, Cattaneo, and Mascareñas win Best Paper Award

Yang, Cattaneo, and Mascareñas

Photo. (Left to right): David Mascareñas, Yongchao Yang, and Alessandro Cattaneo with their Best Paper Award.

Yongchao Yang, Alessandro Cattaneo, and David Mascareñas (National Security Education Center -Engineering Institute, NSEC-EI) received the Best Paper Award at the Third Annual International Conference for Sustainable Development. The winning paper is titled, “Potential Structural Health Monitoring Tools to Mitigate Corruption in the Construction Industry Associated with Rapid Urbanization.” This paper builds on more than a decade’s worth of research on the topic of structural health monitoring that has been completed at the Engineering Institute under the Leadership of Charles Farrar, in collaboration with Eric Flynn (NSEC-EI), technical staff in a variety of scientific and engineering disciplines, and the participation of hundreds of students, postdocs and visiting researchers who have taken part in LANL Engineering Institute Student Programs including the Los Alamos Dynamic Summer School and the Advanced Studies Institute.

The theme of the conference was “Implementing the Sustainable Development Goals: Getting Started.” The conference aimed to identify and share practical, evidence-based solutions to support the United Nations Sustainable Development Goals. The conference was held at Columbia University in anticipation of the UN Sustainable Development Summit, which took place immediately after the conference. The event drew more that 1,000 participants and speakers including the Presidents of Liberia, Malta, and Rwanda; the First Lady of Panama; and UN Sustainable Development Solution Network Leadership Council members. 

 

The LANL paper focuses on infrastructure health monitoring technologies developed at the Engineering Institute, which could be applied to mitigate corruption in the construction industry during rapid urbanization in the developing world. This is a particularly important and timely topic because the worldwide investment in infrastructure development in the coming decades is projected to be in the trillions of dollars, with the majority of this development occurring in areas that suffer from high levels of corruption. Therefore, methods to verify that construction is completed to specification and in a safe manner are required. The paper also addresses the need to bring structural monitoring technologies to the city scale to enable resilient infrastructure and ensure that infrastructure is operated in a safe and legal manner.

The Los Alamos work introduced the sustainable development community to structural health monitoring innovations that may help reduce corruption encountered during development activities. To achieve a global impact, the methods must facilitate monitoring and construction verification in an agile, low-cost fashion that goes beyond the individual structure scale to the city scale. The Laboratory researchers presented novel, cross-disciplinary approaches that they are investigating for infrastructure health monitoring. These include imager-based techniques for structural assessment on the city scale, taking aerial robotic structural inspection beyond imaging, remotely readable tamper-evident seals, and haptic interfaces for infrastructure monitoring. The Engineering Institute continues to find opportunities for these technologies to help implement the Sustainable Development goals worldwide.

Reference: “Potential Structural Health Monitoring Tools to Mitigate Corruption in the Construction Industry Associated with Rapid Urbanization,” 2015 International Conference on Sustainable Development, September 23-24, 2015, Columbia University, NY. http://ic-sd.org/  Authors: Yongchao Yang, Alessandro Cattaneo, and David Mascareñas (NSEC-EI). Technical contacts: Y. Yang, A. Cattaneo, and D. Mascareñas

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Chemistry

Using lichens to detect plutonium and neptunium

In work published in the Journal of Radioanalytical and Nuclear Chemistry, researchers in Chemistry Division and the University of Cincinnati used lichens collected in New Mexico to detect residual airborne transuranic radionuclides from the atmospheric nuclear testing that happened as long as 50 years ago. The team conducted this work to define background concentrations of the actinide isotopes of neptunium and plutonium (237Np, 239Pu, and 240Pu) present in samples retrieved from remote locations in New Mexico.

Lichens obtain essential nutrients directly through atmospheric deposition and have evolved highly efficient mechanisms to bioconcentrate trace elements within their tissues. Scientists have used this characteristic in Europe and North America to monitor the distribution of atmospheric pollutants. For example, the pattern of trace, minor, and earth abundant elements measured in lichens in the Four Corners area has distinguished between natural and anthropogenic atmospheric emissions (agriculture, mining, industrial activities and urban traffic) and the distribution of plutonium surrounding the former Rocky Flats nuclear facility in Colorado.

Usnea lichens

Photo. Usnea lichens are common in New Mexico.

Los Alamos researchers determined the trace concentrations of 237Np, 239Pu, and 240Pu in lichen samples. They studied Usnea arizonica lichen, which is commonly called “Western Brushy Beard.” The lichen typically grows on ponderosa and piñon pine trees several meters above the ground surface. This location makes it better for studying actinide atmospheric transport (resuspension) than ground-growing species that would more preferentially absorb contamination from adjacent soils. The team collected samples from ten locations in New Mexico between 2011 and 2013 and analyzed them using isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS).

The observed isotopic ratios for 237Np/239Pu and 240Pu/239Pu indicate trace contamination from global fallout and regional fallout (e.g. Trinity test and atmospheric testing at the Nevada Test Site). Each of these sources is characterized by a unique composition that defines the isotopic pattern measured in a collection of environmental samples. The occurrence of transuranic isotopes in

modern lichen collections reflects the background concentration of nuclear fallout that is actively moving through wind erosion and atmospheric transport. These processes redistribute the actinides.   The concentration of the isotopes in lichen ash samples is comparable or slightly elevated compared with regional soils. Detection of actinide contamination in recent lichen collections suggests continuous re-suspension of fallout radionuclides is occurring even 50 years after ratification of the Limited Test Ban Treaty. Studies of this type could be used in environmental monitoring of programs associated with modern nuclear activities.

. A plot of 237Np/239Pu versus 240Pu/239Pu for Usnea lichen collected in New
Mexico

Figure 1. A plot of 237Np/239Pu versus 240Pu/239Pu for Usnea lichen collected in New
Mexico, compared with the
isotopic composition of 
global fallout and regional fallout due to testing at the Nevada Test Site and the Trinity Test.

Reference: “Distribution of Neptunium and Plutonium in New Mexico Lichen Samples (Usnea arizonica) Contaminated by Atmospheric Fallout,” Journal of Radioanalytical and Nuclear Chemistry (published online 30 August 2015); doi: 10.1007/s10967-015-4402-0. Authors: Warren J. Oldham, Susan K. Hanson, and Jeffrey L. Miller (Nuclear and Radiochemistry, C-NR); and Kevin B. Lavelle (University of Cincinnati).

The Laboratory Directed Research and Development (LDRD) program funded the Los Alamos work, and the Department of Homeland Security sponsored Kevin Lavelle as a Nuclear Forensics Graduate Fellow. The research supports the Laboratory’s Global Security mission area and the Science of Signatures science pillar through the analysis of actinides in the environment. Technical contact: Warren J. Oldham

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Earth and Environmental Sciences

Mixed fossil and biofuel aerosols in the UK enhance light absorption

Smoke from biomass burning and fossil fuel combustion alter climate by changing the solar energy balance of earth. Current climate models consider two types of carbonaceous particles – organic carbon (OC) and black carbon (BC) – that scatter and absorb sunlight, respectively to cool and warm climate. However, the net effect of the components is not additive due to photochemical reactions between them and with other atmospheric constituents. Moreover, there is evidence for an organic carbon called brown carbon (BrC), which is produced from residential wood combustion and forest fires. Current climate models due not include of effect of the short wavelength light absorption of sunlight by brown carbon. Recent assessments suggest that black carbon could be the second most important anthropogenic-warming agent, but this is very uncertain. Therefore, climate models need to capture the complex and dynamic evolution of optical properties of smoke to determine their impact on climate. The Laboratory led an international research team, which provided observational evidence that complex regional processes increase the light absorption by smoke particles and exacerbate their warming effect. The journal in Nature Communications published the results.

The team used using state-of-the-art instrumentation to perform an in-depth field study of optical, chemical and microphysical properties of particles emitted by fossil and residential fuel burning in the UK in winter. The measurements were part of the 2012 Clean Air for London (ClearfLo) project at Detling, a rural site 45 km southeast and downwind of London.
A measurement station in Detling, UK, is one of several deployed in the UK during the study

Manvendra Dubey, Allison Aiken, and Kyle Gorkowski

Photos. (Top panel): A measurement station in Detling, UK, is one of several deployed in the UK during the study. (Bottom panel): Manvendra Dubey, Allison Aiken, and Kyle Gorkowski (Earth System Observations, EES-14) work inside the station.

Scientists have recognized that coatings on black carbon focus sunlight (the lensing effect), which can enhance the light absorption cross-section. However, the effect had not been observed in the field previously. The team directly measured this lensing enhancement factor for black carbon by removing the coatings via heating the particles to 250 C. They determined that the ratio of the light absorption by ambient black carbon to the heated bare black carbon (termed the enhancement factor) has a mean value of about 1.4 at 780 nm. The enhancement factor increases with the coating thickness. The data reveal that coatings are secondary products of oxidized organics and nitrate that increase with age. The aged emissions contain significant amounts of brown carbon that are stable to

heat and absorb light at 405 nm. The directly measured enhancement factor is suppressed at 405 nm because they are low volatility. The study successfully matched the observed enhanced absorption using optical models from climate models and the chemical data. Single-particle morphological analysis provided mechanistic insight of the enhancement factor in comparison with previous studies. The researchers conclude that the enhancement factor from the carbon particles is source and regionally dependent.
Mass fraction of the non-refractory components internally mixed with black carbon

Representative electron microscopy images of black carbon-containing particles collected at the Detling site for embedded

Figure 2. (Top panel): Mass fraction of the non-refractory components internally mixed with black carbon (shaded areas) and black carbon core median volume-weighted diameter (open circles) as a function of refractory black carbon (RBC). The colors represent nitrate (blue), ammonium (orange), sulfate (red), chloride (purple), oxygenated organic aerosol factor (pink), solid fuel organic aerosol factor (brown), and hydrocarbon-like organic aerosol factor (grey). The error bars depict standard deviation of the values for each RBC interval. (Bottom panel): Representative electron microscopy images of black carbon-containing particles collected at the Detling site for embedded (top left), partly coated (top right), thinly coated (bottom left) and partially encapsulated and/or surface attached (bottom right) black carbon particle types. The size of each panel is 1 m by 1m.

A previous study did not observed significant lensing enhancement in Sacramento, CA during a field campaign in the summer. The key difference is the widespread use of biofuels for residential heating in winter in UK. This paper is the first field demonstration of biofuel burning emitting organic species that coat BC particles produced by diesel combustion, resulting in an increase in light absorption. This is a twofold effect: 1) the organics amplify BC warming by lensing, and 2) stable BrC causes additional warming. The results underscore the importance of understanding and treating mixed emissions that are region specific and dynamic in climate assessments, and also provide a framework to implement this information.

Reference: “Enhanced Light Absorption by Mixed Source Black and Brown Carbon Particles in UK Winter,” Nature Communications 6, 8435 (2015); doi: 10.1038/ncomms9435. Manvendra Dubey (Earth System Observations, EES-14) led the project. The LANL team included Shang Liu (now at University of Colorado – Boulder), Allison Aiken (EES-14), Kyle Gorkowski (now a graduate student at Carnegie Mellon University), and Dubey. Collaborators: Aerodyne Research Inc., Georgia Institute of Technology, Michigan Technological University, University of California – Davis, and researchers from the UK, Switzerland, and Germany.

The DOE Office of Science, Office of Biological and Environmental Research, Atmospheric Science Program funded the Los Alamos work. Aiken received a Los Alamos Director’s Postdoctoral Fellowship, funded by the Laboratory Directed Research and Development (LDRD) program. The work supports the Lab’s Energy and Global Security mission areas and the Science of Signatures science pillar by improving the capability to quantify past, present, and future contributions of these particles to climate change models. Technical contact: Manvendra Dubey

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LANSCE

LANSCE User Group solicits input on neutron science needs

During the next LANSCE User Group Meeting, members of the scientific community are invited to provide feedback on capabilities needed at the Los Alamos Neutron Science Center (LANSCE) over the next five years. LANSCE is a national user facility providing the scientific community with intense sources of neutrons and protons for experiments supporting civilian and national security research.

The LANSCE User Group Meeting will be held November 2-3 at La Posada de Santa Fe, NM. The meeting will include presentations and discussions covering the main areas of research at LANSCE: imaging, proton and neutron radiography, materials science, nuclear science supporting national security, fundamental nuclear and particle physics, and industrial applications. Presenters from the LANSCE user community will showcase the current status of LANSCE, the needs of the scientific community, and the future of LANSCE on the path to MaRIE (Matter-Radiation Interactions in Extremes). MaRIE is Los Alamos’s proposed experimental facility for time-dependent control of dynamic properties of materials for national security science missions. The meeting will feature presentations and discussions that cover the principle areas of research at LANSCE (imaging, materials science, industrial applications, and fundamental nuclear and particle physics) with poster sessions.

The LANSCE accelerator accelerates protons to high energies

Photo. The LANSCE accelerator accelerates protons to high energies. The protons are used to produce medical isotopes, radiograph dynamic events, and generate both high- and low-energy neutrons. The neutrons enable researchers to study atomic nuclei, the properties and behavior of materials, industrial applications, and neutron radiography.

The LANSCE User Group is an association of LANSCE users who share information about their neutron and proton related science and advise LANSCE management on beam time allocation, facility upgrades and new beam lines; experimental programs; and the Rosen Prize for doctoral theses. Albert Young (North Carolina State University), chair of the User Group Executive Committee, is working with LANSCE management to help invigorate and increase user community involvement. Registration ends October 20. Link to register, view the agenda, or reserve accommodations in Santa Fe: http://lansce.lanl.gov/lug2015.

The DOE, National Nuclear Security Administration, Office of Science, and Office of Nuclear Energy, Science, and Technology are the principal sponsors of LANSCE, which supports the Lab’s national security missions and the Materials for the Future, Nuclear and Particle Futures, and the Science of Signatures science pillars. Technical contact: Gus Sinnis

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Materials Physics and Applications

Discovery of novel topological states

Madhab Neupane, a 2015 Director’s Postdoctoral Fellow with the Condensed Matter and Magnet Science group (MPA-CMMS), is seeking bulk-insulating material with metallic surface properties that could revolutionize the electronics industry, including enabling applications in spintronics and quantum computing. Publications in Physical Review Letters and Physical Review B report his findings.

The new concept of topological states in condensed matter and material science predicts the existence of surface states that are insensitive to environmental conditions, like contamination, due to protection by fundamental symmetries. Neupane’s research focuses on strongly correlated topological insulators – unusual materials that host both strong correlations in the bulk material and topologically protected metallic surface states. Tomasz Durakiewicz (MPA-CMMS) mentors him.

Fermi surface and dispersion map

Figure 3. Fermi surface and dispersion map. (a) ARPES measured Fermi surface of YbB6. Circular-shaped pockets are observed at the Γ¯ point (center of the Brillouin zone) and X¯ point (center of the face). The Fermi surface is measured with a photon energy of 50 eV at a temperature of 15 K. (b) ARPES dispersion maps measured with different photon energies. Researchers collected the data at the Advanced Light Source, Lawrence Berkeley National Laboratory.

Scientists had predicted rare-earth ytterbium hexaboride (YbB6) to be a topological insulator, where hybridization of localized f electrons with conduction electrons leads to opening of the gap and inhibits the metal’s ability to carry an electric current. However, Neupane’s angle-resolved photoemission studies (ARPES) showed instead that YbB6 exhibits a novel topological insulator state in the absence of a Kondo mechanism (Figure 3). Through experiment and theory, Neupane and collaborators provide a promising approach for realizing a topological insulator in rare-earth materials through the newly discovered phenomenon.

Neupane has investigated other possible topological insulators. He has discovered a unique example of a topological surface state in a metallic compound. Experiments and calculations revealed the first topological surface state in the noncentrosymmetric material (BiPd). The magnetic susceptibility as a function of temperatures displays a sharp superconducting transition temperature at approximately 3.7 K in a field of 20 Gs. In other research, high-resolution ARPES resolved the electronic band structure of CeB6, which shows the presence of 4f flat bands and dispersive 5d bands in the vicinity of the Fermi level. Pump-probe photoemission spectroscopy studies of the optically excited Dirac surface states in the bulk-insulating topological insulator Bi2Te2Se reveal optical properties that are in sharp contrast to those of bulk-metallic topological insulators. Bi2Te2Se displays a gigantic optical lifetime exceeding 4 μs for the surface states, providing evidence for a power-law charge relaxation that is unique to 2-D electrodynamics. It also offers a direct optical signature of low bulk conductivity in a topological insulator.

Crystal structure of BiPd

Figure 4. Crystal structure of BiPd with the b axis shown as its unique axis. It crystallizes in a monoclinic structure at low temperature.

References:

  • “Non-Kondo-like Electronic Structure in the Correlated Rare-Earth Hexaboride YbB6,” Physical Review Letters 114, 016403 (2015); doi: 10.1103/PhysRevLett.114.016403. Authors: Neupane and Durakiewicz, and collaborators from Princeton University, University of California – Irvine, National Tsing Hua University (Taiwan), Academia Sinica (Taiwan), National University of Singapore, and Northeastern University.
  • “Gigantic Surface Life-time of an Intrinsic Topological Insulator,” Physical Review Letters 115, 116801 (2015); doi: 10.1103/PhysRevLett.115.116801. Authors: Neupane and Durakiewicz, and collaborators from Princeton University, ISSP (Japan), Peking University (China), University of California – Berkeley, University of Maryland – College Park, Monash University (Australia).
  • “Fermi Surface Topology and Hotspots Distribution in Kondo Lattice System CeB6,” Physical Review B92, 104420 (2015); doi: 10.1103/PhysRevB.92.104420. Authors: Neupane and Durakiewicz, and collaborators from Princeton University, University of California – Irvine, Tsing Hua University (Taiwan), Academia Sinica (Taiwan), Temple University, National University of Singapore, and Northeastern University.
  • “Discovery of the Topological Surface State in a Noncentrosymmetric Superconductor BiPd,” arXiv preprint arXiv:1505.03466 (May 14, 2015). Authors: Neupane and Durakiewicz, and co-authors from Princeton University, National Tsing Hua University (Taiwan), Academia Sinica (Taiwan), National University of Singapore, Northeastern University, Polish Academy of Sciences.

The DOE, Office of Basic Energy Sciences, Division of Material Sciences, and the Los Alamos Laboratory Directed Research and Development (LDRD) program funded different aspects of the LANL work. The research supports the Laboratory’s Energy Security mission area and Materials for the Future science pillar by establishing the scientific foundations to design the functionality of strongly correlated materials with non-trivial topology. Technical contact: Madhab Neupane

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Materials Science and Technology

Radiation-induced spectroscopic changes in Parylene-C

Parylene-C has a long history of use in medical devices, and several studies have evaluated the effects of radiation sterilization procedures on the polymer. Sterilization procedures expose a material to a single large radiation dose (approximately Mrad). Then the material is evaluated for performance after the exposure. However, there has been a general lack of systematic radiation damage studies on Parylene-C that cover a wide range of doses. Therefore, a Laboratory team of researchers initiated a study of the mechanical and spectroscopic material response of Parylene-C to a wide range of radiation doses.

The LANL researchers used a thermomechanical analyzer under compression mode to measure the stress-strain behavior of Parylene-C films. This method minimizes the sample geometry dependence observed in previous studies that used dynamic mechanical analysis and tensile measurements.

Structure of Parylene-C

Figure 5. Structure of Parylene-C.

Preliminary results from scoping studies provide thermomechanical analyzer curves and indicate the appropriate dose levels for a more detailed study. The full stress-strain curves show dynamic differences as a function of radiation dose (Figure 6). The graph on the right depicts the more interesting linear viscoelastic region for the samples where the slope reveals the modulus of the material. Parylene-C becomes softer after even mild radiation doses (5 Gy), but the trend is not clear. The material is significantly harder after a 500 Gy dose, but then becomes significantly softer after a 5000 Gy dose. This result implies that different mechanisms (i.e., chain scission and cross-linking) become dominant in different dose regimes. The team’s future studies will determine the dose ranges where competing mechanisms are dominant.
Thermomechanical analyzer stress-strain curves for Parylene-

Figure 6. Thermomechanical analyzer stress-strain curves for Parylene-C after various radiation doses. (Left): Full scale. (Right): Linear viscoelastic region.

The researchers also want to understand the specific chemical and molecular changes that lead to the mechanical effects. They used a two-dimensional Fourier transform infrared (FTIR) spectroscopy technique to examine a large number of spectra (200 or more) that are evenly spaced over a variable range (dose range in this case). Statistical analysis enables identification of subtle changes in the spectroscopic signature.

FTIR spectra

Figure 7. FTIR spectra of Parylene-C for various doses.

The team measured FTIR spectra after a wide range of radiation exposures to establish the dose range. Figure 7 ranges near 1750 and 1300 cm-1 (likely carbonyl groups) and 3000-3500 cm-1 (typical of general oxidation). Based on these results, the team plans to measure FTIR spectra after 500 Gy dose increments up to 100 kGy and analyze the results with specially designed algorithms.

Researchers include: Joseph Torres, Matthew Herman, Michael Blair, Robert Gilbertson, and Nicholas Parra-Vasquez (Engineered Materials, MST-7); Nickolaus Smith (Production Liaison, W-8); and David Ceman (Detonator Technology, W-6).

The NNSA B61-12 Life Extension Program (LANL Program Manager Patti Buntain) sponsored the work. The findings support the Nuclear Deterrence mission area and the Materials for the Future science pillar through an increased understanding of how radiation-induced defects are manifested and affect material performance. Technical contact: Michael Blair

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Physics

Reducing the uncertainty of backgrounds for Chi-Nu fission measurements

Measurement of background is important for good statistical design of experiments. John O’Donnell (LANSCE Weapons Physics, P-27) has developed a method to measure backgrounds associated with coincidence as part of the Chi-Nu project at the Los Alamos Neutron Science Center (LANSCE). The method enables true in situ background measurements and a dramatic reduction in the statistical uncertainty of the background. Nuclear Instruments and Methods A published the research.

The work aims to determine the background under the true coincidence events without assumptions for the shape of the background or determination of arbitrary parameters. The measurement method uses flash waveform digitizers with on-board processing capabilities to acquire all the singles data for each detector, with near-zero dead time, and then record all the data on a computer. Later analysis includes a simple software search for coincidences. The analysis preserves the high statistical accuracy of the singles data; thus the statistical uncertainty on the background extracted can be very small. Data acquisition for the background measurement occurs simultaneously with the foreground measurement to provide two immediate benefits: 1) this new method is four times more effective at using beam time, in contrast with methods that dedicate a fraction of the beam time to measuring the background; and 2) it reduces systematic uncertainties (e.g., no sources of background are changed between the two phases of measurement, and the background normalization is absolute).

Researchers expect that the method could be used for a large class of coincidence experiments, in which the backgrounds cannot be fully eliminated. Prompt fission spectra experiments performed at Weapons Neutron Research (WNR) Facility/LANSCE as part of the Chi-Nu project are an example.

Researchers developed the Chi-Nu detector array for experiments at LANSCE to accurately measure the spectrum of neutrons emitted in neutron-induced fission of plutonium-239 (239Pu) and uranium-235 (235U). The prompt fission neutron spectrum is an important quantity, and NNSA Science Campaign 1 is funding an experimental campaign to reduce the uncertainties associated with this nuclear data. Chi-Nu uses a two-arm time-of-flight technique, detecting fission fragments to infer the incoming neutron time of flight and also to start an outgoing neutron time-of-flight measurement. In addition to detecting fission-fragment fission-neutron coincidences, there is an opportunity to observe coincidences involving alpha particles misidentified as fission fragments, neutrons from other fission events, beam neutrons scattered in the experimental area, and fission fragments generated from these scattered neutrons. The background from these chance coincidences must be measured accurately.
Measured coincidences

Figure 8. Measured coincidences (left) and background (right) for outgoing neutrons from neutron-induced fission of 239Pu as a function of the fission time (incoming neutron time of flight), tf − t0, and the outgoing neutron time, tn− t0.

Figure 8 represents the 2-D background obtained for the preliminary 239Pu fission data from the Chi-Nu project. Red diagonal lines depict boundaries that researchers used to project Figure 9.

Figure 9 shows the projected 1-D backgrounds for four different regions of incoming neutron energies. Features of the background shapes can be better understood from the full 2-D backgrounds. These backgrounds are not flat, and the background shape varies with the incident neutron energy. The new background analysis method appears to account for all the significant background components and has very small uncertainties.

Time of flight, tn-tf, spectra

Figure 9. Time of flight, tn-tf, spectra (black) for outgoing neutrons following neutron-induced fission of 239Pu in coincidence with fission fragments for different regions of incoming neutron time of flight, tf-t0. The measured background (red) has error bars smaller than the line width.

Reference: “A New Method to Reduce the Statistical and Systematic Uncertainty of Chance Coincidence Backgrounds Measure with Waveform Digitizers,” Nuclear Instruments and Methods A (2015), article in press; doi: 10.1016/j.nima.2015.07.044. J. M. O’Donnell (LANSCE Weapons Physics, P-27) authored the paper.

This work benefited from the use of the LANSCE accelerator facility and was performed under the auspices of the NNSA. The work supports the Laboratory’s Nuclear Deterrence mission area and the Nuclear and Particle Futures science pillar by reducing the uncertainties of key data used to predict nuclear performance. Technical contact: John O’Donnell (LANSCE Weapons Physics, P-27)

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Science on the Roadmap to MaRIE

Advanced lasers examine extreme environment of a meteorite impact

Los Alamos scientists and collaborators used the high-brightness, short-pulse Linac Coherent Light Source x-ray free electron laser at the SLAC National Accelerator Laboratory to simulate the extreme environment of a meteorite impact and its effects in silica (SiO2). Nature Communications published the research.

Silica is one of the most abundant materials in the Earth’s crust. The research revealed its unexpectedly swift transformation to rare stishovite – a hard and dense mineral that is found at bolide-impact craters on the Earth’s surface. This study demonstrates the first-ever shock-induced crystallization of an amorphous material observed via femtosecond (10-15 second) x-ray diffraction. These results will lead to a greater understanding of important problems in shock physics and materials science, potentially enabling refined planetary models to provide insight for the impact history of the Earth and solar system, and techniques to design new materials with improved functionality.
Arianna Gleason makes final adjustments to detector positions inside the Matter in Extreme Conditions

Photo. Arianna Gleason makes final adjustments to detector positions inside the Matter in Extreme Conditions (MEC) target chamber at the Stanford Liner Accelerator facility (SLAC) in California. The MEC facility combines SLAC’s Linac Coherent Light Source with high power optical laser beams, and a suite of dedicated diagnostics tailored for warm dense matter physics, high pressure studies, shock physics, and high energy density physics.

Pressure- and temperature-induced phase transitions have been studied for more than a century. However, little is known about the non-equilibrium processes by which the atoms rearrange. Shock compression, the fastest mechanical loading that can be applied to a material, generates a nearly instantaneous propagating high-pressure/temperature condition. In situ x-ray diffraction (XRD) can probe the time-dependent atomic rearrangement that occurs.

This method resolved the growth of nanocrystalline stishovite on the nanosecond timescale just after shock compression. The functional form of this grain growth suggests homogeneous nucleation and attachment as the growth mechanism, rather than a diffusion-based mechanism. These are the first observations of crystalline grain growth in the shock front between low- and high-pressure states via XRD.

In situ XRD, such as available at the Linac Coherent Light Source and planned at the center of MaRIE (Matter-Radiation Interactions in Extremes), LANL’s proposed experimental facility for time-dependent materials science at the mesoscale, provides a unique tool to study materials under extreme conditions. MaRIE would take this research further by allowing structural determination of non-crystalline materials and by performing the time-dependent measurements on a single shock event. Additional dynamic drivers could enable longer time measurements of the crystallization dynamics near the phase boundaries.

. Cartoon (for 33.6 GPa at 10-ns delay, grey box)

Figure 10. Cartoon (for 33.6 GPa at 10-ns delay, grey box) illustrates the researchers’ interpretation of grain growth behind the shock front (black dashed line, propagation direction is grey arrow). The distribution of grain size increases with distance from the shock front.

Reference: “Ultrafast Visualization of Crystallization and Grain Growth in Shock-compressed SiO2,” Nature Communications (2015); doi: 10.1038/ncomms919. Researchers: Arianna Gleason (Shock and Detonation Physics, M-9 and SLAC National Accelerator Laboratory), Cindy Bolme (M-9), Richard Sandberg (Center for Integrated Nanotechnologies, MPA-CINT), and collaborators from the Linac Coherent Light Source, the Stanford Institute for Materials and Energy Sciences at SLAC, Stanford University; Washington State University; Lawrence Livermore National Laboratory; Carnegie Institution of Washington; and the Center for High Pressure Science and Technology Advanced Research in Shanghai.

The Laboratory Directed Research and Development (LDRD) program sponsored the Los Alamos work, and a Reines Distinguished Postdoctoral Fellowship funded Gleason. The research supports the Lab’s Nuclear Deterrence mission area and the Materials for the Future science pillar by demonstrating a strategy and methodology to extract phase transition kinetics using time-resolved x-ray diffraction data. Technical contact: Arianna Gleason

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Scientific Society Meetings

Town Hall meetings to refresh Materials Strategy

The Materials science community is invited to participate in upcoming Materials Strategy Town Hall meetings on October 26 (2 – 4 PM, NSSB Auditorium, open to all badge holders) and 29 (8:30 – 10:30 AM, MSL Auditorium). The meetings are intended to help the Laboratory refresh its Materials Strategy in light of evolving national security missions. The meetings will be focused on establishing a current baseline, engaging program offices to understand future mission needs, and soliciting feedback and comments from the broader materials science community and programs on how to improve the Lab’s materials strategy. The two Town Hall meetings are open to anyone in the materials science community at the Laboratory.

Los Alamos developed current version of the Materials Strategy in 2010. The Materials community will re-examine LANL’s materials strategy in light of the evolving Laboratory and National Security missions. The goals of the strategy are the following:

Color metallography, palladium

Figure 11. Color metallography, palladium.

  • Connect the LANL Materials Strategy to National Security Mission
  • Drive future LANL and National Strategy
  • Position LANL as a world-changing organization to provide transformational materials science
  • Articulate a vision that speaks to the breadth of the Materials Community
  • Guide institutional investments for capability development
  • Ensure it is clear, understandable and actionable to staff (Early Career, Mid-Career, Management), customers and external community

The Town Hall is intended to help refresh the Laboratory’s 2010 Materials Strategy: http://www.lanl.gov/discover/publications/_assets/docs/VISTAS-Winter2010-LALP-09-036.pdf. More information: http://int.lanl.gov/science/science-pillars/materials-for-future/materials-town-hall.shtml Technical contacts: Dave Teter and Rick Martineau

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