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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,549 records · Page 86
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

How bad or good are the external forward shock afterglow models of gamma-ray bursts?

The external forward shock models have been the standard paradigm to interpret the broadband afterglow data of gamma-ray bursts (GRBs). One prediction of the models is that some afterglow temporal breaks at different energy bands should be achromatic; that is, the break times should be the same in different frequencies. Multiwavelength observations in the Swift era have revealed chromatic afterglow behaviors at least in some GRBs, casting doubts on the external forward shock origin of GRB afterglows. In this paper, using a large sample of GRBs with both X-ray and optical afterglow data, we perform a systematic study to address the question: how bad or good are the external forward shock models? Our sample includes 85 GRBs up to 2014 March with well-monitored X-ray and optical light curves. Based on how well the data abide by the external forward shock models, we categorize them into five grades and three samples. The first two grades (Grade I and II) include 45 of 85 GRBs. They show evidence of, or are consistent with having, an achromatic break. The temporal and spectral behaviors in each afterglow segment are consistent with the predictions (the “closure relations”) of the forward shock models. These GRBs are included in the Gold sample. The next two grades (Grade III and IV) include 37 of 85 GRBs. They are also consistent with having an achromatic break, even though one or more afterglow segments do not comply with the closure relations. These GRBs are included in the Silver sample. Finally, Grade V (3/85) shows direct evidence of chromatic behaviors, suggesting that the external shock models are inconsistent with the data. These are included in the Bad sample. We further perform statistical analyses of various observational properties (temporal index α, spectral index β, break time t{sub b}) and model parameters (energy injection index q, electron spectral index p, jet opening angle θ{sub j}, radiative efficiency η{sub γ}, and so on) of the GRBs in the Gold sample, and derive constraints on the magnetization parameter ϵ{sub B} in the forward shock. Overall, we conclude that the simplest external forward shock models can account for the multiwavelength afterglow data of at least half of the GRBs. When more advanced modeling (e.g., long-lasting reverse shock, structured jets, arbitrary circumburst medium density profile) is invoked, up to >90% of the afterglows may be interpreted within the framework of the external shock models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

High temperature superconductivity in the candidate phases of solid hydrogen

As the simplest element in nature, unraveling the phase diagram of hydrogen is a primary task for condensed matter physics. As conjectured many decades ago, in the low-temperature and high-pressure part of the phase diagram, solid hydrogen is expected to become metallic with a high superconducting transition temperature. The metallization may occur via band gap closure in the molecular solid or via a transition to the atomic solid. Recently, a few experimental studies pushed the achievable pressures into the 400–500 GPa range. There are strong indications that at some pressure in this range metallization via either of these mechanisms occurs, although there are disagreements between experimental reports. Furthermore, there are multiple good candidate crystal phases that have emerged from recent computational and experimental studies which may be realized in upcoming experiments. Therefore, it is crucial to determine the superconducting properties of these candidate phases. In a recent study, we reported the superconducting properties of the C2/c-24 phase, which we believe to be a strong candidate for metallization via band gap closure. Here, we report the superconducting properties of the Cmca-12, Cmca-4 and I4 1 /amd-2 phases including the anharmonic effects using a Wannier function-based dense k-point and q-point sampling. Here we find that the Cmca-12 phase has a superconducting transition temperature that rises from 86 K at 400 GPa to 212 K at 500 GPa, whereas the Cmca-4 and I4 1 /amd-2 phases show a less pressure-dependent behavior with their T c in the 74–94 K and 307–343 K ranges, respectively. These properties can be used to distinguish between crystal phases in future experiments. Understanding superconductivity in pure hydrogen is also important in the study of high-T c hydrides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Large-Eddy Simulations of Idealized Atmospheric Boundary Layers Using Nalu-Wind

Accurate prediction of wind-plant performance relies, in part, on properly characterizing the turbulent atmospheric boundary layer (ABL) flow in which wind turbines operate. Large-eddy simulation (LES) is a powerful tool for simulating ABLs because it resolves the largest, most energetic scales of three-dimensional turbulent motions. Yet LES predictions are well known to depend on modeling choices such as grid resolution, numerical discretization schemes, and closures for unresolved scales of turbulence. Here, we evaluate how these choices influence predictions of ABL winds using Nalu-Wind, a wind-specific fork of the open-source, generalized, unstructured, massively parallel flow solver NaluCFD/Nalu.

17 WIND ENERGY↗
DOE OSTI2018

Dry and hot: the hydraulic consequences of a climate change-type drought for Amazonian trees

How plants respond physiologically to leaf warming and low water availability may determine how they will perform under future climate change. In 2015-2016 an unprecedented drought occurred across Amazonia with record-breaking high temperatures and low soil moisture, offering a unique opportunity to evaluate the performances of Amazonian trees to a severe climatic event. We quantified the responses of leaf water potential, sap velocity, whole-tree hydraulic conductance (Kwt), turgor loss and xylem embolism, during and after the 2015-2016 El Niño for five canopy-tree species. Leaf/xylem safety margins (SM), sap velocity and Kwt showed a sharp drop during warm periods. SMs were negatively correlated with vapor pressure deficit but had no significant relationship with soil water storage. Based on our calculations of canopy stomatal and xylem resistances, the decrease in sap velocity and Kwt was due to a combination of xylem cavitation, and stomatal closure. Our results suggest extremely warm droughts greatly amplify the degree of trees’ physiological stress and can lead to mortality. Given the extreme nature of the 2015-2016 El Niño and that temperatures are predicted to increase, this work can serve as a case study of the possible impact climate warming can have on tropical trees.

Fontes, Clarissa G.↗
DOE OSTI2026

Emergence of vorticity and viscous stress in finite-scale quantum hydrodynamics

The Madelung equations offer a hydrodynamic description of quantum systems, from single particles to quantum fluids. In this formulation, the probability density is mapped onto the fluid density and the phase is treated as a scalar potential generating the velocity field. As examples of potential flows, quantum fluids described in this way are inherently irrotational, but quantum vortices may arise at discrete points where the phase is undefined. In this paper, starting from this irrotational description of a quantum fluid, a coarse-graining procedure is applied to arrive at a macroscopic description of the quantum fluid in terms of a hierarchy of moments in which the role of velocity is played by a Favre average of the microscopic velocity field. This hierarchy is truncated using an explicit closure derived from an expansion in a finite length scale. The resulting coarse-grained fields are shown to allow for finite vorticity at any point in the fluid. Additionally, it is shown that this vorticity obeys a similar equation to the vorticity equation in classical hydrodynamics and includes a vortex-stretching term. The particular closure employed here also gives rise to a novel stress term in the fluid equations, which in the appropriate limit appears analogous to an artificial viscous stress from computational fluid dynamics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Mechanochemical formation of heterogeneous diamond structures during rapid uniaxial compression in graphite

We predict mechanochemical formation of heterogeneous diamond structures from rapid uniaxial compression in graphite using quantum molecular dynamics simulations. Ensembles of simulations reveal the formation of different diamondlike products starting from thermal graphite crystal configurations. Here in this paper, we identify distinct classes of final products with characteristic probabilities of formation, stress states, and electrical properties and show through simulations of rapid quenching that these products are nominally stable and can be recovered at room temperature and pressure. Some of the diamond products exhibit significant disorder and partial closure of the energy gap between the highest-occupied and lowest-unoccupied molecular orbitals (i.e., the HOMO-LUMO gap). Seeding atomic vacancies in graphite significantly biases toward forming products with small HOMO-LUMO gap. We show that a strong correlation between the HOMO-LUMO gap and disorder in tetrahedral bonding configurations informs which kinds of structural defects are associated with gap closure. The rapid diffusionless transformation of graphite is found to lock vacancy defects into the final diamond structure, resulting in configurations that prevent sp 3 bonding and lead to localized HOMO and LUMO states with a small gap.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Valence-shell dependence of the pygmy dipole resonance: E 1 strength difference in 50,54 Cr

The low-lying electric dipole strength provides insights into the parameters of the nuclear equation of state via its connection with the pygmy dipole resonance and nuclear neutron skin thickness. The aim was to complement the systematic of the pygmy dipole resonance and first study its behavior across the N = 28 neutron shell closure. Here, photon-scattering cross sections of states of 50,54 Cr were measured up to an excitation energy of 9.7 MeV via the nuclear resonance fluorescence method using γ-ray beams from bremsstrahlung and Compton backscattering. Transitions strengths, spin and parity quantum number, and average branching ratios for 55 excited states, 44 of which were observed for the first time, were determined. The comparison between the total observed strengths of the isotopes 50,52,54 Cr shows a significant increase above the shell closure. The evolution of the pygmy dipole resonance is heavily influenced by the shell structure.

39 ≤ A ≤ 58↗
DOE OSTI2019

Contribution of chiral three-body forces to the monopole component of the effective shell-model Hamiltonian

We present a study of the role played by realistic three-body forces in providing a reliable monopole component of the effective shell-model Hamiltonian. To this end, starting from a nuclear potential built up within the chiral perturbation theory, we derive effective shell-model Hamiltonians with and without the contribution of the three-body potential and compare the results of shell-model calculations with a set of observables that evidence shell-evolution properties. The testing ground of our investigation is nuclei belonging to the fp shell, since the shell-evolution towards shell closures in 48 Ca and 56 Ni provides a paradigm for shell-model Hamiltonians. Furthermore, our analysis shows that only by including contributions of the three-body force is the monopole component of the effective shell-model Hamiltonian then able to reproduce the experimental shell evolution towards and beyond the closure at N = 28.

39 ≤ A ≤ 58↗
DOE OSTI2020

Prompt-delayed γ-ray spectroscopy of neutron-rich 119.121 In isotopes

The Z = 50 shell closure, near N = 82, is unique in the sense that it is the only shell closure with the spin-orbit partner orbitals, π g 9 / 2 and π g 7 / 2 , enclosing the magic gap. The interaction of the proton hole/particle in the above-mentioned orbitals with neutrons in the νh 11/2 orbital is an important prerequisite to the understanding of the nuclear structure near N = 82 and the νπ interaction. To explore the structural similarity between the high-spin isomeric states in In ( Z = 49), Sn ( Z = 50), and Sb ( Z = 51) isotopes from a microscopic point of view. In addition, to understand the role of a proton hole or particle in the spin-orbit partner orbitals, π g 9 / 2 and π g 7 / 2 , respectively, with neutron holes in the ν h 11 / 2 orbital on these aforementioned isomers. The fusion and transfer induced fission reaction Be 9 ( U 238 , f ) with 6.2 MeV/u beam energy, using a unique setup consisting of AGATA, VAMOS++, and EXOGAM detectors, was used to populate through the fission process and study the neutron-rich In 119 , 121 isotopes. This setup enabled the prompt-delayed γ -ray spectroscopy of isotopes in the time range of 100 ns – 200 μ s . In the odd- A In 119 , 121 isotopes, indications of a short half-life 19/2 - isomeric state, in addition to the previously known 25/2 + isomeric state, were observed from the present data. Further, new prompt transitions above the 25/2 + isomer in In 121 were identified along with reevaluation of its half-life. The experimental data were compared with the theoretical results obtained in the framework of large-scale shell-model calculations in a restricted model space. Furthermore, the ( π g 9 / 2 ν h 11 / 2 ; I | H ^ | π g 9 / 2 ν h 11 / 2 ; I ) two-body matrix elements of residual interaction were modified to explain the excitation energies and the B ( E 2) transition probabilities in the neutron-rich In isotopes. The (i) decreasing trend of E (29/2 + ) - E (25/2 + ) in odd-In (with dominant configuration π g 9 / 2 - 1 ν h 11 / 2 - 2 and maximum aligned spin of 29/2 + ) and (ii) increasing trend of E (27/2 + ) - E (23/2 + ) in odd-Sb (with dominant configuration π g 7 / 2 + 1 ν h 11 / 2 - 2 and maximum aligned spin of 27/2 + ) with increasing neutron number could be understood as a consequence of hole-hole and particle-hole interactions, respectively.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Spectroscopic quadrupole moments in 124 $\mathrm{Xe}$

The Xe isotopic chain with four valence protons above the Z=50 shell closure is an ideal laboratory for the study of the evolution of nuclear deformation. At the N=82 shell closure, 136 Xe presents all characteristics of a doubly closed shell nucleus with a spherical shape. In the very neutron-deficient isotopes close to N=50, the α-decay chain of Xe was investigated to probe the radioactive decay properties near the drip-line and the magicity of 100 Sn. Additionally, the Xe isotopes present higher order symmetries in the nuclear deformation such as the octupole degree of freedom near N=60 and N=90 or O(6) symmetry in stable isotopes. The relevance of the O(6) symmetry has been investigated by measuring the spectroscopic quadrupole moment of the first excited states in 124 Xe. In the O(6) symmetry limit, the spectroscopic quadrupole moment of collective states is expected to be null. A stable 124 Xe beam with energies of 4.03A MeV and 4.11 A MeV was used to bombard a nat W target at the GANIL facility. Excited states were populated via the safe Coulomb excitation reaction. The collision of the heavy ions with a large Z at low energy make this reaction sensitive to the diagonal E2 matrix element of the excited states. The recoils were detected in the VAMOS++ magnetic spectrometer and the γ rays in the AGATA tracking array. The least squares fitting code gosia was used for the analysis to extract both E2 and M1 transitional and E2 diagonal matrix elements. The rotational ground state band was populated up to the 8$^{+}_{1}$ state as well as the 2$^{+}_{2}$ and 4$^{+}_{2}$ states. Using high precision spectroscopic data to constrain the gosia fit, the spectroscopic quadrupole moments of the 2$^{+}_{1}$, 4$^{+}_{1}$, and 6$^{+}_{1}$ states were determined for the first time. The spectroscopic quadrupole moments were found to be negative, large, and constant in the ground state band underlining the prolate axially deformed ground state band of 124 Xe. Here, the present experimental data confirm that the O(6) symmetry is substantially broken in 124 Xe .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Nucleonic shells and nuclear masses

The binding energy of an isotope is a sensitive indicator of the underlying shell structure as it reflects the net energy content of a nucleus. Since magic nuclei are significantly lighter, or more bound, compared to their neighbors, the presence of nucleonic shell structure makes an imprint on nuclear masses. In this study, using a carefully designed binding-energy indicator, we catalog the appearance of spherical and deformed shell and subshell closures throughout the nuclear landscape. After presenting experimental evidence for shell and subshell closures as seen through the lens of nuclear masses, we study the ability of global nuclear mass models to predict local binding-energy variations related to shell effects.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Evolution of shell gaps in the neutron-poor calcium region from invariant-mass spectroscopy of 37,38 Sc, 35 Ca, 34 K

A fast secondary beam of 37 Ca impinged on a 9 Be target resulting in a set of reactions populating proton-rich nuclei including 35 Ca and the first observations of 37,38 Sc and 34 K. Invariant-mass spectroscopy, used to reconstruct proton decays for these nuclei, yielded three new ground-state masses and information on their low-lying structures. The newly measured mass excesses are: ΔM( 37 Sc) = 3500(410) keV, ΔM( 38 Sc) = –4656(14) keV, and ΔM( 34 K) = –1487(17) keV. These nuclei straddle the well-known Z = 20 shell closure as well as the N = 16 subshell closure. Furthermore, trends in separation energies help elucidate how nuclear structure evolves showing a fading of the Z = 20 shell gap for N ≥ 18 and indications of a N = 16 subshell gap.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Variations in the charge radii of indium isotopes between 𝑁=52 and 82

Measurements of the 5⁢𝑝 2 𝑃 3/2 → 9⁢𝑠 2 𝑆 1/2 and 5⁢𝑝 2 𝑃 1/2 → 8⁢𝑠 2 𝑆 1/2 transitions in the indium atom, combined with new atomic physics calculations, were used to extract the changes in mean-square nuclear charge radii, 𝛿⁡⟨𝑟 2 ⟩, of the indium (𝑍 = 49) isotopes 101–111 In , 113–123 In , and 125–131 In . With a proton hole in the closed nuclear shell of 𝑍 = 50, indium provides a detailed study of the effect of unpaired nucleons adjacent to the proton-shell closure, allowing investigation into the charge radii for isotopes between the two major neutron-shell closures at 𝑁 = 50 and 𝑁 = 82. A study of the variations in charge radii between neighboring isotopes with neutron number (the ‘odd-even staggering') is presented and provides further insight and challenges for the theoretical description of the size of proton-hole nuclei. Two nuclear theories, density functional theory and the valence-space in-medium similarity renormalization group method, were employed to interpret the data. The new information obtained in this work provides valuable insights into the successes and shortcomings of the theoretical approaches employed.

ab initio calculations↗
DOE OSTI2015

Analogous intruder behavior near Ni, Sn, and Pb isotopes

Near shell closures, the presence of unexpected states at low energies provides a critical test of our understanding of the atomic nucleus. New measurements for the N = 42 isotones $69\atop{27}$Co and $71\atop{29}$Cu, along with recent data and calculations in the Ni isotopes, establish a full set of complementary, deformed, intruder states astride the closed-shell 28 Ni isotopes. In addition, nuclei with a one-proton hole or one-proton particle adjacent to Z = 28 were populated in β-decay experiments and in multinucleon transfer reactions. A β-decaying isomer, with a 750(250)-ms half-life, has been identified in $69\atop{27}$Co 42 . It likely has low spin and accompanies the previously established 7/2 - state. Complementary data for the levels of isotonic $71\atop{29}$Cu 42 support the presence of a deformed, ΔJ = 1 band built on the proton intruder 7/2 - level at 981 keV. Finally, these data, together with recent studies of lower-mass Co and Cu isotopes and extensive work near 68 Ni , support the view that intruder states based on particle-hole excitations accompany all closed proton shells with Z ≥ 28 .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Dissipative magnetohydrodynamics for nonresistive relativistic plasmas: An implicit second-order flux-conservative formulation with stiff relaxation

Based on a 14-moment closure for nonresistive (general-) relativistic viscous plasmas, here we describe a new numerical scheme that is able to handle all first-order dissipative effects (heat conduction, bulk and shear viscosities), as well the anisotropies induced by the presence of magnetic fields. The latter is parametrized in terms of a thermal gyrofrequency or, equivalently, a thermal Larmor radius and allows to correctly capture the thermal Hall effect. By solving an extended Israel-Stewart-like system for the dissipative quantities that enforces algebraic constraints via stiff-relaxation, we are able to cast all first-order dissipative terms in flux-divergence form. This allows us to apply traditional high-resolution shock capturing methods to the equations, making the system suitable for the numerical study of highly turbulent flows. We present several numerical tests to assess the robustness of our numerical scheme in flat spacetime. The 14-moment closure can seamlessly interpolate between the highly collisional limit found in neutron star mergers, and the highly anisotropic limit of relativistic Braginskii magnetohydrodynamics appropriate for weakly collisional plasmas in black-hole accretion problems. We believe that this new formulation and numerical scheme will be useful for a broad class of relativistic magnetized flows.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Neutrino fast flavor oscillations with moments: Linear stability analysis and application to neutron star mergers

Providing an accurate modeling of neutrino physics in dense astrophysical environments such as binary neutron star mergers presents a challenge for hydrodynamic simulations. Nevertheless, understanding how flavor transformation can occur and affect the dynamics, the mass ejection, and the nucleosynthesis will need to be achieved in the future. Computationally expensive, large-scale simulations frequently evolve the first classical angular moments of the neutrino distributions. By promoting these quantities to matrices in flavor space, we develop a linear stability analysis of fast flavor oscillations using only the first two “quantum” moments, which notably requires generalizing the classical closure relations that appropriately truncate the hierarchy of moment equations in order to treat quantum flavor coherence. After showing the efficiency of this method on a well-understood test situation, we perform a systematic search of the occurrence of fast flavor instabilities in a neutron star merger simulation. Here, we discuss the successes and shortcomings of moment linear stability analysis, as this framework provides a time-efficient way to design and study better closure prescriptions in the future.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Realizability-preserving discontinuous Galerkin method for spectral two-moment radiation transport in special relativity

Here we present a realizability-preserving numerical method for solving a spectral two-moment model to simulate the transport of massless, neutral particles interacting with a steady background material moving with relativistic velocities. The model is obtained as the special relativistic limit of a four-momentum-conservative general relativistic two-moment model. Using a maximum-entropy closure, we solve for the Eulerian-frame energy and momentum. The proposed numerical method is designed to preserve moment realizability, which corresponds to moments defined by a nonnegative phase-space density. The realizability-preserving method is achieved with the following key components: (i) a discontinuous Galerkin phase-space discretization with specially constructed numerical fluxes in the spatial and energy dimensions; (ii) a strong stability-preserving implicit-explicit time-integration method; (iii) a realizability-preserving conserved to primitive moment solver; (iv) a realizability-preserving implicit collision solver; and (v) a realizability-enforcing limiter. Component (iii) is necessitated by the closure procedure, which closes higher order moments nonlinearly in terms of primitive moments. The nonlinear conserved to primitive and the implicit collision solves are formulated as fixed-point problems, which are solved with custom iterative solvers designed to preserve the realizability of each iterate. With a series of numerical tests, we demonstrate the accuracy and robustness of this discontinuous-Galerkin-implicit-explicit method.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Unitarity and the three flavor neutrino mixing matrix

Unitarity is a fundamental property of any theory required to ensure we work in a theoretically consistent framework. In comparison with the quark sector, experimental tests of unitarity for the 3x3 neutrino mixing matrix are considerably weaker. It must be remembered that the vast majority of our information on the neutrino mixing angles originates from v - e and v μ disappearance experiments, with the assumption of unitarity being invoked to constrain the remaining elements. New physics can invalidate this assumption for the 3x3 subset and thus modify our precision measurements. We also perform a reanalysis to see how global knowledge is altered when one refits oscillation results without assuming unitarity, and present 3σ ranges for allowed U PMNS elements consistent with all observed phenomena. We calculate the bounds on the closure of the six neutrino unitarity triangles, with the closure of the v - e and v μ triangle being constrained to be ≤0.03, while the remaining triangles are significantly less constrained to be ≤ 0.1 - 0.2. Similarly for the row and column normalization, we find their deviation from unity is constrained to be ≤ 0.2 - 0.4, for four out of six such normalizations, while for the v μ and v e row normalization the deviations are constrained to be ≤0.07, all at the 3σCL. Additionally, we emphasize that there is significant room for new low energy physics, especially in the v τ sector which very few current experiments constrain directly.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗