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At least 1,405 records · Page 78

Neutron time-of-flight reflectometer GRAINS with horizontal sample plane at the IBR-2 reactor: Possibilities and prospects

A new time-of-flight neutron reflectometer GRAINS with a horizontal sample plane (vertical scattering plane) has recently been put into operation at the pulsed reactor IBR-2 (JINR, Dubna). The scattering geometry applied makes it possible to carry out complex studies of nanostructured interfaces in solid and liquid states by measuring the specular reflectivity and the intensity of diffuse scattering of thermal neutrons from open surfaces and hidden interlayer boundaries, including solid-liquid interfaces, over a wide momentum transfer range. The classical analysis of specular reflectivity allows one to determine the scattering length density profile in the object under study in a direction perpendicular to the interface for a thickness of ~100 nm with a resolution of 1 nm. The analysis of off-specular (diffuse) neutron scattering makes it possible to characterize lateral correlations on surfaces and interlayer boundaries. The paper summarizes the first experience in the operation of the GRAINS reflectometer. The instrument characteristics are presented together with the possibilities for experiments with solid and liquid samples illustrated with simple systems. Further steps for the development of the reflectometer are discussed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

McStas Analysis of Monoenergetic Thermal Neutron Beam Generation for Total Cross Section Measurements

The thermal neutron scattering law (TSL) of a given material represents data that is essential for assessing the criticality safety of a neutronic system in which this material is utilized. Traditionally, the TSL is developed through elaborate calculations that combine input and models based on condensed matter physics and scattering theory. Nonetheless, measurements of key parameters (e.g., the total cross section at low energies) can help in benchmarking and validating TSL libraries such as those used in the ENDF/B database. In this work, the setup of experiments, at the PULSTAR reactor of North Carolina State University (NCSU), for creating monoenergetic thermal neutron beams in support of total scattering cross section measurements in the thermal energy range is explored using McStas simulations and experimental verification. A McStas model has been established of the NPD facility at the NCSU PULSTAR reactor to explore the production of monoenergetic thermal neutron beams in support of various applications including total cross section measurements. The McStas results clearly demonstrate the viability of six monochromatic thermal neutron beams with substantial intensities that allow future applications at the NPD facility. The experimental results reasonably support the computational model predictions. Based on these results, transmission thermal neutron experiments can be conducted for data validation studies. Furthermore, the model also allows the evaluation of design changes to expand the portfolio of available monoenergetic thermal neutron beams. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗

Metalloprotein catalysis: structural and mechanistic insights into oxidoreductases from neutron protein crystallography

Metalloproteins catalyze a range of reactions, with enhanced chemical functionality due to their metal cofactor. The reaction mechanisms of metalloproteins have been experimentally characterized by spectroscopy, macromolecular crystallography and cryo-electron microscopy. An important caveat in structural studies of metalloproteins remains the artefacts that can be introduced by radiation damage. Photoreduction, radiolysis and ionization deriving from the electromagnetic beam used to probe the structure complicate structural and mechanistic interpretation. Neutron protein diffraction remains the only structural probe that leaves protein samples devoid of radiation damage, even when data are collected at room temperature. Additionally, neutron protein crystallography provides information on the positions of light atoms such as hydrogen and deuterium, allowing the characterization of protonation states and hydrogen-bonding networks. Neutron protein crystallography has further been used in conjunction with experimental and computational techniques to gain insight into the structures and reaction mechanisms of several transition-state metal oxidoreductases with iron, copper and manganese cofactors. Here, the contribution of neutron protein crystallography towards elucidating the reaction mechanism of metalloproteins is reviewed.

59 BASIC BIOLOGICAL SCIENCES↗

A Catastrophic Charge Density Wave in BaFe 2 Al 9

Charge density waves (CDWs) are modulations of the electron density and the atomic lattice that develop in some crystalline materials at low temperatures. We report an unusual example of a CDW in BaFe 2 Al 9 below 100 K. In contrast to the canonical CDW phase transition, temperature-dependent physical properties of single crystals reveal a first-order phase transition. This is accompanied by a discontinuous change in the size of the crystal lattice. In fact, this large strain has catastrophic consequences for the crystals causing them to physically shatter. Single-crystal X-ray diffraction reveals superlattice peaks in the low-temperature phase signaling the development of a CDW lattice modulation. No similar low-temperature transitions are observed in BaFe 2 Al 9 . Electronic structure calculations provide one hint to the different behavior of these two compounds; the d-orbital states in the Fe compound are not completely filled. Iron compounds are renowned for their magnetism, and partly filled d-states play a key role. Finally, it is therefore surprising that BaFe 2 Al 9 develops a structural modulation at low temperature instead of magnetic order.

36 MATERIALS SCIENCE↗

Irradiation in the Swiss Spallation Neutron Source and its Applications

In high power (∼ 1 MW or higher) spallation targets being operated or developed for different attractive applications such as neutron scattering science and technology or accelerator driven systems (ADS) for nuclear waste transmutation, both target and structural materials are exposed to intensive irradiation of high energy protons and spallation neutrons. Since spallation reactions induced by high energy protons can produce helium (He) and hydrogen (H) and other transmutation elements at very high rates, the behavior of materials used in a spallation irradiation environment can be much different from those observed from materials irradiated with neutrons in fission reactors. Therefore, the damage induced by spallation irradiation cannot be simply simulated by fission neutron irradiation experiments. Seven irradiation experiments have been conducted in the targets of the Swiss Spallation Neutron Source (SINQ) at the Paul Scherrer Institute (PSI) since 1998 within a broad international collaboration with Commissariat a L'energie Atomique Centre d'Etudes de Saclay (CEA), Forschungszentrum Juelich (FZJ), Japan Atomic Energy Agency (JAEA), Los Alamos National Laboratory (LANL), Oak Ridge National Laboratory (ORNL), University California Santa Barbara (UCSB) and several institutes of Chinese Academy of Sciences. More than 7000 samples from various Al-, Ti-, Fe-, Ni-, Zr-, and W-alloys and SiC/SiC, C/SiC and MAX-phase ceramics composites have been irradiated to doses up to ∼25 dpa (in Fe) and ∼2000 appm He at temperatures up to ∼800 deg. C. In addition, quite many specimens were irradiated in contact with stagnant liquid metals such as Hg, PbBi, PbLi etc. Dedicated post-irradiation examinations (PIE) have been performed, together with the international collaborators. Various mechanical tests including tensile, bend, Charpy-impact, small punch and hardness have been carried out to obtain the mechanical properties of the materials after irradiation. The microstructural changes in the irradiated specimens have been investigated by means of transmission electron microscopy, positron annihilation spectroscopy, atom probe tomography etc. Many novel results have been obtained from the PIE conducted by PSI and other STIP partner laboratories. The DBTT-shift of different FM steels increases more or less linearly with dose after irradiation in the SINQ target, while it saturates at a level of few dpa in fission neutron irradiation cases. This phenomenon can be mostly attributed to helium effects. In addition to the strong embrittlement effect, pronounced hardening effect was also detected in high dose specimens, which was contributed by high density helium bubbles.

36 MATERIALS SCIENCE↗

Study of Bronze Statues “John the Baptist” and “Dancing Cupid” from the Collections of the Pushkin State Museum

Bronze statues “John the Baptist” and “Dancing Cupid” from the collections of the Pushkin State Museum of Fine Arts have been investigated at the National Research Centre “Kurchatov Institute.” These statues were previously damaged in fire. Before carrying out restoration, it was necessary to perform their complex study, including identification of statue materials; elemental and phase analysis of contaminations; estimation of the state of surfaces and internal parts; and detection of technological elements, as well as hidden defects, corroded parts, and cracks. To this end, scanning electron microscopy with X-ray microanalysis, synchrotron radiation diffraction, and neutron radiography and tomography have been used.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Implementation of cross correlation for energy discrimination on the time-of-flight spectrometer CORELLI

The CORELLI instrument at Oak Ridge National Laboratory is a statistical chopper spectrometer designed and optimized to probe complex disorder in crystalline materials through diffuse scattering experiments. On CORELLI, the high efficiency of white-beam Laue diffraction combined with elastic discrimination have enabled an unprecedented data collection rate to obtain both the total and the elastic-only scattering over a large volume of reciprocal space from a single measurement. To achieve this, CORELLI is equipped with a statistical chopper to modulate the incoming neutron beam quasi-randomly, and then the cross-correlation method is applied to reconstruct the elastic component from the scattering data. Details of the implementation of the cross-correlation method on CORELLI are given and its performance is discussed.

36 MATERIALS SCIENCE↗

Magnetic moments and spin structure in single-phase B20 Co 1+x Si 1-x (x = 0.043)

Neutron powder diffraction (NPD) and x-ray magnetic circular dichroism (XMCD) spectroscopy are employed to investigate the magnetism and spin structure in single-phase B20 Co 1.043 Si 0.957 . The magnetic contributions to the NPD data measured in zero fields are consistent with the helical order among the allowed spin structures derived from group theory. The magnitude of the magnetic moment is (0.3 ± 0.1) μ B /Co according to NPD, while the surface magnetization probed by XMCD at 3 kOe is (0.18–0.31) μ B /Co. Both values are substantially larger than the bulk magnetization of 0.11 μ B /Co determined from magnetometry at 70 kOe and 2 K. Finally, these experimental data indicate the formation of a helical spin phase and the associated conical states in high magnetic fields.

36 MATERIALS SCIENCE↗

The thermal neutron scattering law for hydrogen bound in plutonium dihydride and predicted critical mass for several configurations

The thermal neutron scattering law (TSL) for hydrogen bound in plutonium dihydride (H-PuH{sub 2}) is developed using first-principles density functional theory and lattice dynamics calculations. The GGA+U exchange correlation with spin-polarized magnetism is used to simulate the electronic structure of the material with a Hubbard U parameter that is selected to reproduce experimental lattice constant measurements. The partial phonon density of states from these calculations is used to develop a thermal neutron scattering law evaluation for H-PuH{sub 2} in the incoherent approximation. This thermal scattering law is used to predict the critical mass of {sup 239}PuH{sub 2} in various bare, reflected, and moderated spherical configurations with MC21 using ENDF/B-VII.1 cross sections. The H-PuH{sub 2} TSL is designed to be suitable for the analysis of bulk PuH{sub x} compositions with 0.0 < x ≤ 2.0. Water volume fractions from 0.0 to 0.95 are analyzed for the homogenized slurry. A small positive difference in k{sub eff} is observed for the water reflected configuration when comparing calculations using the {sup 1}H free-gas treatment instead of the H-PuH{sub 2} thermal scattering law.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

Improvements to Thermal Neutron Scattering Law of CH{sub 2}

MCNP, as a widely accepted simulation tool, depends heavily on evaluated nuclear data found in current ENDF libraries. Modeling CH{sub 2} (polyethylene), as one of the prominent moderators, is of high importance for many applications such as spent fuel storage or shipping systems. Current thermal scattering S (α, β) evaluations are based on theoretical calculations of the phonon spectrum that are usually adjusted to match the energy dependent total scattering cross section. Double differential scattering data is not always available and thus rarely used in the evaluation. In this work double differential scattering measurements were used to obtain a phonon spectrum that can be used to generate a thermal scattering evaluation. This procedure resulted in better agreement with the experimental total cross section of CH{sub 2} and better agreement with double differential scattering experiments. (authors)

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Deposition and characterization of α-Fe 2 O 3 /Pd thin films for neutron reflectometry studies

Here, we report deposition of hematite/Pd thin films on silicon wafers via radio frequency (RF) magnetron sputtering and subsequent characterization for future in situ neutron reflectometry studies. Following deposition, the hematite/Pd thin films were characterized as prepared and after annealing in air for 2h at 400, 500, and 600 °C, respectively. Raman spectroscopy, grazing incidence x-ray diffraction, and neutron reflectometry (NR) were used to characterize the structure and chemical compositions of the thin films. The results indicate that pure α-Fe 2 O 3 (hematite) films were produced, free from other iron oxide phases and impurities. NR data reveal that one intermediate layer between the Pd layer and the hematite layer was formed during sputtering deposition processes. The fitted scattering length density (SLD) of the as-deposited hematite layer is 70% of the theoretical SLD value, indicating that the grains are loosely packed in the RF-deposited hematite films. After annealing at elevated temperatures, the hematite films show increased SLD values but remain comparable to that of preannealed.

36 MATERIALS SCIENCE↗

Testing of graphite thermal neutron scattering law data in support of treat neutronic analysis

The Transient Reactor Test Facility (TREAT) has historically provided the capability for transient fuel performance and safety testing in the United States. The operation of this facility was suspended in 1994. However, in recent years preparations for the restart of TREAT were initiated. TREAT is a graphite moderated and air cooled reactor. Its core is composed of a 19 x 19 array of fuel assemblies that contain UO{sub 2} (HEU) fuel particles dispersed in a graphite matrix, along with reflector assemblies filled with graphite only. Each fuel assembly is 8 feet tall with a fueled region that is 4 feet tall and upper and lower regions of graphite that are 2 feet tall. In addition, a 2 feet thick radial graphite reflector region is present. Finally, a 5 feet thick concrete biological shield surrounds the core. Previous work showed that the graphite thermal scattering cross sections can introduce variations exceeding 2000 pcm in the predicted eigenvalue of the TREAT core. Recognizing that the graphite in the assembly (and thereby the core) is a porous mixture of crystalline and amorphous carbon (with a lower density in comparison to ideal crystalline graphite), it becomes clear that TREAT specific thermal neutron scattering cross section libraries will be needed (at all temperatures of interest) for use within neutronic codes seeking to simulate TREAT. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗

High-pressure polymorphism in pyridine

Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P 2 1 2 1 2 1 with Z ′ = 1 and phase III in P 4 1 2 1 2 with Z ′ = ½. Neutron powder diffraction experiments using pyridine-d 5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C 2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH...π and CH...N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified.

36 MATERIALS SCIENCE↗

Axisymmetric and Focusing Analyzers to Enable Efficient Powder and Residual Stress Neutron Diffractometers

Project Summary: Company: Adelphi Technology, Inc., Redwood City, CA 94063-4121 DOE Phase 1 STTR PI: Dr. Jay Theodore Cremer, Jr. Chief Scientist at Adelphi Technology, Inc. DOE Phase 1 STTR with: Dr. Boris Khaykovich of Massachusetts Institute of Technology (MIT) MIT Nuclear Reactor Laboratory DOE Phase 1 STTR Title: Axisymmetric and Focusing Analyzers to Enable Efficient Powder and Residual Stress Neutron Diffractometers Statement of the problem or situation that is being addressed: Although thermal and cold neutron scattering is widely used, and is critical for success in many areas of materials science and engineering, relatively low neutron fluxes severely limit applications of not only laboratory neutrons generators, but also large national neutron facilities. State-of-the-art thermal and cold neutron sources are large expensive national facilities, which serve diverse community of scientific and industrial users. The constant need to improve the instruments performance, stems from the fact that neutron methods are gaining in popularity, and becoming more and more powerful, while new neutron sources are not being constructed, so as to keep pace with the developments and needs of the scientific community. Small research reactors at universities and National Labs, and laboratory-based neutron generators, are necessary not only for education and training, but also when samples cannot be transported to other facilities. However, the standard neutron techniques, which were developed for high-flux facilities, require much higher efficiencies to be used effectively with the low fluxes of small sources. Thus, the efficient use of neutron sources, such as with our proposed analyzer, is important for the progress and broader use of these neutron techniques. General statement of how this problem is being addressed: We strive to achieve the goal of efficient use of available thermal and cold neutrons, by introducing here, novel analyzers for powder diffraction and stress-strain measurements. These analyzers will enable polychromatic cold and thermal neutron diffractometers, which will have much higher throughput than existing instruments. At large national facilities, such as at Oak Ridge National Laboratory, these analyzers would enable very fast instruments for screening of multiple samples, for measuring kinetic processes, or for measuring residual stress tensors. Commercial Applications and Other Benefits: By combining the novel analyzers with the Adelphi thermal neutron generator, we will open a new market, which includes the testing of samples that cannot be transported, such as nuclear fuels, or the testing of parts during production. Given the reasonable cost and size of our proposed analyzer systems, along with the continuing development of more powerful thermal neutron generators, the proposed analyzers will be suitable for weak neutrons sources, which are installed at small to large university, industrial, and government laboratories. Key Words: Neutron powder diffraction, residual stress measurements, thermal neutron generators, TRIGA reactors, low flux thermal and cold neutron sources, neutron diffraction. Summary for Members of Congress: Thermal and cold neutron beams are a powerful materials science probe, which provide unique information about the structure of matter. The proposed innovations expand the reach of neutron-based investigations to new materials and industries by enabling new instrumentation capabilities, thereby greatly enhancing and expanding the role of small, laboratory-based neutron instrumentation, and improving education and training of neutron users.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Neutron interferometry detection of early crack formation caused by bending fatigue in additively manufactured SS316 dogbones

In our research, fatigue in selective laser melted (SLM) and conventionally manufactured stainless steel (SS) 316 dogbones was studied with neutron imaging methods for attenuation, diffraction, and scattering. Neutron attenuation tomography and Bragg edge imaging did not reveal crack formation. Conversely, to efficiently detect the microcrack evolution, two methods of grating-based neutron interferometry, Talbot-Lau and far-field, were employed. Both interferometry methods detect early crack formation via the dark-field (scattering) images, especially for porous microstructures in the range of 0.6 μm to 2 μm. The dark-field image combines sensitivity to micrometer-sized scattering centers at crack formation with sub-millimeter imaging resolution. The crack formation observed with neutron interferometry dark-field was validated post-imaging with additional fatigue cycles to fracture. Further inspection was performed by scanning electron microscopy (SEM) and optical photography. In the two fatigued dogbones, SLM and conventional crack formation was determined to within 1 mm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Pyrochlore NaYbO2: A Potential Quantum Spin Liquid Candidate

The search for quantum spin liquids (QSL) and chemical doping in such materials to explore superconductivity have continuously attracted intense interest. Here, we report the discovery of a potential QSL candidate, pyrochlore-lattice β-NaYbO2. Colorless and transparent NaYbO2 single crystals, layered α-NaYbO2 (∼250 μm on edge) and octahedral β-NaYbO2 (∼50 μm on edge), were grown for the first time. Synchrotron X-ray single-crystal diffraction unambiguously determined that the newfound β-NaYbO2 belongs to the three-dimensional pyrochlore structure characterized by the R3̅m space group, corroborated by synchrotron X-ray and neutron powder diffraction and pair distribution function. Magnetic measurements revealed no long-range magnetic order or spin glass behavior down to 0.4 K with a low boundary spin frustration factor of 17.5, suggesting a potential QSL ground state. Under high magnetic fields, the potential QSL state was broken and spins order. Our findings reveal that NaYbO2 is a fertile playground for studying novel quantum states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗