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At least 181 records · Page 10

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NASA NTRS1974

Forbidden-line excitation data for certain coronal lines

Some new data pertinent to forbidden transitions for coronal lines of S XII, Ca XIII, K XI, Mn XIII, Fe XIV, Co XV, Ni XVI, N XV, Cr IX, Fe X, and Ni XII are presented. We include additional calculations of collision strengths for a number of ions and then set up and solve the equations of statistical equilibrium in the limiting approximations in which contributions to level population by cascade from higher levels are neglected. We then calculate volume emissivities per ion as a function of density and temperature.

Czyzak, S. J.↗

NASA NTRS2004

Aeolian Sand Transport with Collisional Suspension

Aeolian transport is an important mechanism for the transport of sand on Earth and on Mars. Dust and sand storms are common occurrences on Mars and windblown sand is responsible for many of the observed surface features, such as dune fields. A better understanding of Aeolian transport could also lead to improvements in pneumatic conveying of materials to be mined for life support on the surface of the Moon and Mars. The usual view of aeolian sand transport is that for mild winds, saltation is the dominant mechanism, with particles in the bed being dislodged by the impact of other saltating particles, but without in-flight collisions. As the wind becomes stronger, turbulent suspension keeps the particles in the air, allowing much longer trajectories, with the corresponding increase in transport rate. We show here that an important regime exists between these two extremes: for strong winds, but before turbulent suspension becomes dominant, there is a regime in which in-flight collisions dominate over turbulence as a suspension mechanism, yielding transport rates much higher than those for saltation. The theory presented is based on granular kinetic theory, and includes both turbulent suspension and particle-particle collisions. The wind strengths for which the calculated transport rates are relevant are beyond the published strengths of current wind tunnel experiments, so these theoretical results are an invitation to do experiments in the strong-wind regime. In order to make a connection between the regime of saltation and the regime of collisional suspension, it is necessary to better understand the interaction between the bed and the particles that collide with it. This interaction depends on the agitation of the particles of the bed. In mild winds, collisions with the bed are relatively infrequent and the local disturbance associated with a collision can relax before the next nearby collision. However, as the wind speed increases, collision become more frequent and the agitation need not decay completely. In the regime of collisional suspension, the particles near the surface of the bed are assumed to be in a state of constant agitation. We indicate the conditions at the bed corresponding to the limits of saltation and collisional suspension and outline experiments, simulations, and modeling that have been undertaken to bridge these limits.

Jenkins, James T.↗

NASA NTRS1980

Analysis of solar spectra in the middle ultraviolet and visible for atmospheric trace constituents measurements

Individual spectral line parameters including line positions, strengths, and intensities were generated for the sq Alpha Sigma - sq Chi Pi (0,0) band of OH, applicable to atmospheric and high temperatures. Energy levels and transition frequencies are calculated by numerically diagonalizing the Hamiltonian. Line strengths are calculated using the dipole matrix and eigenvectors derived from energy matrix diagonalization. The line strengths are compared to those calculated from previously published algebraic line strength formulas. Tables of line parameters are presented for 240 K and 4600 K.

Goldman, A.↗

DOE OSTI2020

Ion Conductivity and Correlations in Model Salt-Doped Polymers: Effects of Interaction Strength and Concentration

Correlated anion and cation motion can greatly reduce the overall ion conductivity in electrolytes versus the ideal conductivity calculated based on the diffusion constants alone. Using coarse-grained molecular dynamics simulations, we calculate conductivity and the degree of uncorrelated ion motion in salt-doped homopolymers and block copolymers as a function of concentration and interaction strengths. Calculating conductivity from ion mobility under an applied electric field increases accuracy versus the typical use of fluctuation dissipation relationships in equilibrium simulations. In typical electrolytes, correlation in cation-anion motion is often expected to be reduced at low ion concentrations. Yet, for these polymer electrolytes with strong ion-polymer and ion-ion interactions, we find cation-anion motion is more correlated at lower concentrations when other variables are held constant. We show this phenomenon is related to the slower ion cluster relaxation rate at low concentrations rather than the static spatial state of ion aggregation or the fraction of free ions.

36 MATERIALS SCIENCE↗

NASA NTRS1990

Intersubband absorption in Si(1-x)Ge(x/Si superlattices for long wavelength infrared detectors

Researchers calculated the absorption strengths for intersubband transitions in n-type Si(1-x)Ge(x)/Si superlattices. These transitions can be used for the detection of long-wavelength infrared radiation. A significant advantage in Si(1-x)Ge(x)/Si supperlattice detectors is the ability to detect normally incident light; in Ga(1-x)Al(x)As/GaAs superlattices, intersubband absorption is possible only if the incident light contains a polarization component in the growth direction of the superlattice. Researchers present detailed calculation of absorption coefficients, and peak absorption wavelengths for (100), (111) and (110) Si(1-x)Ge(x)/Si superlattices. Peak absorption strengths of about 2000 to 6000 cm(exp -1) were obtained for typical sheet doping concentrations (approx. equals 10(exp 12)cm(exp -2)). Absorption comparable to that in Ga(1-x)Al(x)As/GaAs superlattice detectors, compatibility with existing Si technology, and the ability to detect normally incident light make these devices promising for future applications.

Rajakarunanayake, Yasantha↗

NASA NTRS1979

Negative streamer development in FEP teflon

A computational model is developed which describes the evolution and propagation of an ionizing front (negative streamer) in solid materials. The ionization front consists of drifting avalanching electrons moving self-consistently under the influence of their own space-charge field together with an applied external field. The required input information for the model consists of the functional dependence of the macroscopic transport coefficients on the local electric field, the initial conditions for beginning the calculation, and the strength of the applied field. A computational approach for specifying the transport coefficients and initional conditions is also described. The approach has been implemented by constructing three computer codes which sequentially interface, beginning with single electron scattering, and ending with streamer development. Computational results are presented for model calculations in Teflon. The overall model is perceived to provide a picture of the initiation phase of a propagating discharge in electron-irradiated dielectrics.

Beers, B. L.↗

DOE OSTI2026

Large-scale calculations of 𝛽-decay rates and implications for 𝑟-process nucleosynthesis

Nuclear 𝛽 decay is a key element of the astrophysical rapid neutron capture process (𝑟 process). In this work, we present state-of-the-art global 𝛽-decay calculations based on the quantified relativistic nuclear energy density functional theory and the deformed proton-neutron quasiparticle random-phase approximation. Our analysis considers contributions from allowed and first-forbidden transitions. We used two point-coupling functionals with carefully calibrated time-odd terms and isoscalar pairing strength. The new calculations display consistent results for both employed functionals, especially near the neutron drip line, suggesting slower 𝛽 decays past the 𝑁=126 neutron shell closure than in commonly used 𝛽-decay models. The new rates, along with the existing rates based on the recent nonrelativistic global calculations, are found to slow down the synthesis of heavy elements in the 𝑟 process and significantly reduce the contribution of neutron-induced fission.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

NASA NTRS2007

Sequestration of Single-Walled Carbon Nanotubes in a Polymer

Sequestration of single-walled carbon nanotubes (SWCNs) in a suitably chosen polymer is under investigation as a means of promoting the dissolution of the nanotubes into epoxies. The purpose of this investigation is to make it possible to utilize SWCNs as the reinforcing fibers in strong, lightweight epoxy-matrix/carbon-fiber composite materials. SWCNs are especially attractive for use as reinforcing fibers because of their stiffness and strength-to-weight ratio: Their Young s modulus has been calculated to be 1.2 TPa, their strength has been calculated to be as much as 100 times that of steel, and their mass density is only one-sixth that of steel. Bare SWCNs cannot be incorporated directly into composite materials of the types envisioned because they are not soluble in epoxies. Heretofore, SWCNS have been rendered soluble by chemically attaching various molecular chains to them, but such chemical attachments compromise their structural integrity. In the method now under investigation, carbon nanotubes are sequestered in molecules of poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) [PmPV]. The strength of the carbon nanotubes is preserved because they are not chemically bonded to the PmPV. This method exploits the tendency of PmPV molecules to wrap themselves around carbon nanotubes: the wrapping occurs partly because there exists a favorable interface between the conjugated face of a nanotube and the conjugated backbone of the polymer and partly because of the helical molecular structure of PmPV. The constituents attached to the polymer backbones (the side chains) render the PmPV-wrapped carbon nanotubes PmPV soluble in organic materials that, in turn, could be used to suspend the carbon nanotubes in epoxy precursors. At present, this method is being optimized: The side chains on the currently available form of PmPV are very nonpolar and unable to react with the epoxy resins and/or hardeners; as a consequence, SWCN/PmPV composites have been observed to precipitate out of epoxies while the epoxies were being cured. If the side chains of the PmPV molecules were functionalized to make them capable of reacting with the epoxy matrices, it might be possible to make progress toward practical applications. By bonding the side chains of the PmPV molecules to an epoxy matrix, one would form an PmPV conduit between the epoxy matrix and the carbon nanotubes sequestered in the PmPV. This conduit would transfer stresses from the epoxy matrix to the nanotubes. This proposed load-transfer mode is similar to that of the current practice in which silane groups are chemically bonded to both the epoxy matrices and the fibers (often glass fibers) in epoxymatrix/fiber composites.

Bley, Richard A.↗

NASA NTRS1990

Density dependence of the 5 micron infrared spectrum of NH3

Measurements of dilute mixtures of NH3 in H2 were made in the window region 1900-2100/cm of the NH3 spectrum to determine is behavior with increasing pressure of H2. The spectra of pure H2, pure NH3, and mixtures of the two, in the total pressure range from 2.38 to 8.17 atm at 309 K, were obtained with a 975-cm white-cell. Synthetic spectra were calculated using precise line strengths, line positions, and a Lorentz profile. The experimental and calculated spectra are in reasonably good agreement, except that the former is super-imposed on a rather flat background not given by the calculation. A possible mechanism for this background is suggested.

Chapados, Camille↗

NASA NTRS1999

Capella: Structure and Abundances

This grant covers the analysis of ASCA spectra of the cool star binary system Capella. This project has also required the analysis of simultaneous EUVE data. The ASCA spectrum of Capella could not be fit with standard models; by imposing models based on strong lines observed with EUVE, a problem wavelength region was identified. Correcting the problem required calculations of atomic collision strengths of higher principal quantum number than had ever been calculated, resulting in a paper in process by Liedahl and Brickhouse. With these new models applied to the ASCA spectrum, better fits were obtained. While solar abundance ratios are generally consistent with the ASCA data, the ratio of Ne/Fe is three to four times lower than solar photospheric values. Whether there is a general First Ionization Potential (FIP) effect or a specific neon anomaly cannot be determined from these data. Detailed discussion has been provided to NASA in the most recent annual report (1997). Two poster presentations have been made regarding modeling requirements. A substantial paper is in the final revision form, following review by six co-authors. The results of this work have wide implications, since the newly calculated emission lines almost certainly contribute to other problems in fitting not only other stellar spectra, but also composite supernova remnants, galaxies, and cooling flow clusters of galaxies. Furthermore, Liedahl and Brickhouse have identified other species for which lines of a similar nature (high principal quantum number) will contribute significant flux. For moderate resolution X-ray spectra, lines left out of the models in relatively isolated bands, will be attributed to continuum flux by spectral fitting engines, causing errors in line-to-continuum ratios. Thus addressing the general theoretical problem is of crucial importance.

Brickhouse, Nancy S.↗

NASA NTRS1999

Capella: Structure and Abundances

This grant covers the analysis of EUVE spectra of the cool star binary system Capella. This project has also required the analysis of simultaneous Advanced Satellite for Cosmology and Astrophysics (ASCA) data. The ASCA spectrum of Capella could not be fit with standard models; by imposing models based on strong lines observed with EUVE, a problem wavelength region was identified. Correcting the problem required calculations of atomic collision strengths of higher principal quantum number than had ever been calculated. With these new models applied to the ASCA spectrum, better fits were obtained. Findings are that: (1) ASCA and EUVE spectra are both dominated by a region at 6 x 10(exp 6) K. (2) The high energy cut-off of the ASCA spectrum is consistent with emission from the highest ionization stages of EUVE, namely Fe XXIV. (3) EUVE requires a continuous emission measure distribution with more than two temperatures. (4) The ASCA spectra are of such high statistical significance that systematic uncertainties dominate, including atomic physics issues and calibration issues. (5) While the ASCA spectral fits achieve lower Chi(exp 2 with two-temperature fits, the EUVE-derived emission measure distribution models are also consistent with the spectra. (6) The Fe/H ratio obtained from the ASCA fit is within 20 % of the Fe/H abundance obtained from the summed spectra of Capella over 5 EUVE pointings, as well as the 1996 EUVE data. This result confirms our claims that quasi-continua composed of weak emission lines in the short wavelength spectrometer of EUVE are not major contributors to the measured Capella continuum. Other abundance ratios are also determined from the ASCA data, using models derived with EUVE. Si, Si, and Mg appear to be close to solar photospheric values, while the ratio of Ne/Fe is three to four times lower than solar photospheric values. Whether there is a general First Ionization Potential (FIP) effect or a specific neon anomaly cannot be determined from these data. (8) EUVE line ratios indicating low optical depth place limits on the extent to which the X-ray lines are optically, thick. While the brightest lines may be marginally thick, the ASCA spectrum is not of sufficient resolution to determine the optical depth in these lines.

Brickhouse, Nancy S.↗

DOE OSTI2020

E 2 collectivity in shell-model calculations for odd-mass nuclei near 132 Sn

Shell-model calculations for 127,129In and 129,131Sb are presented, and interpreted in the context of the particle-core coupling scheme, wherein proton g9/2 holes or g7/2 particles are added to semimagic 128,130Sn cores. These results indicate that the particle-core coupling scheme is appropriate for the Sb isotopes, whilst less so for the In isotopes. B(E2) excitation strengths are also calculated, and show evidence of enhanced collectivity in both Sb isotopes, especially 131Sb. This observation suggests that 131Sb would be an excellent case for an experimental study seeking to investigate the early onset of collectivity near 132Sn.

Gray, Tim↗

DOE OSTI2022

Evaluation of Gamma Ray Production - Quarterly Report FY22Q2

The LLNL team has performed the first systematic study of the effect of two-body weak currents in β decay rates across the entire mass table. In light and medium-mass nuclei, two-body weak currents from chiral effective field theory account for a significant portion of the phenomenological quenching of Gamow-Teller transition matrix elements. We examined the systematic effects of two-body axial currents on Gamow-Teller strength and β-decay rates in heavy nuclei within energy-density functional theory. We extended the charge-changing finiteamplitude pnFAM code to incorporate the contributions of two-body currents to the usual onebody linear response in the Gamow-Teller channel. We found that the two-body currents, as expected, usually quench both summed Gamow-Teller strength and decay rates, but by an amount that decreases as the neutron excess grows; see Fig. 1. In addition, they can enhance individual low-lying transitions, leading to decay rates that are quite different from those that an energy-independent quenching would produce, particularly in neutron-rich nuclei. This new physics capability is unique and provides the highest-fidelity description of β decay on the market. The underlying computational implementation can be reused to calculate γ-strength functions for finite-range nuclear potentials such as the Gogny force, which will allow us to benchmark some of our future developments against published results. The paper is in print in Phys. Rev. C.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2023

Defect-specific strength factors and superposition model for predicting strengthening of ion irradiated Fe18Cr alloy

Here, a high-purity binary alloy Fe18Cr was subjected to heavy-ion irradiation in order to provide improved understanding of the irradiation effect on radiation hardening associated with dislocation loop and network formation, α’ precipitation. The specimens were irradiated with 8 MeV Fe ions (~2 μm ion range) to midrange doses of 0.37 and 3.7 dpa at 300, 350 and 450 °C using dose rates of ~10 –5 –10 –3 dpa/s. Nanoindentation testing was performed to extract the bulk equivalent hardness at low depth from these ion irradiated specimens. High-quality transmission electron microscopy (TEM) images were acquired by utilizing a flash electropolishing method on the Focused Ion Beam (FIB) prepared liftouts. An accurate calculation of the strength factor was provided based on detailed TEM characterization of the irradiated microstructures. A newly refined hardening superposition method was applied to combine the strengthening components from each microstructure. The good agreement between microstructure predicted strength and measured strength further demonstrated the fidelity to use hardening model to quantify the mechanical properties of ion irradiated materials.

36 MATERIALS SCIENCE↗

NASA NTRS1992

Atomic data and spectral analysis of carbon, nitrogen, oxygen and silicon ions observed with the International Ultraviolet Explorer

According to the plan presented in the original proposal we have now completed most of the atomic calculations involving collision strengths and rate coefficients for electron impact excitation of C II, N III, and O IV ions. These have been reported in the first two publications appended with this report. We have now moved into the applications phase of the project with the new data being used to analyze the International Ultraviolet Explorer (IUE) observations of a variety of objects, as described in the third publication recently submitted (also appended). The analysis and interpretation of archival data will continue well into the next year with several collaborators that the PI and Co-PI are involved with. In addition, the atomic calculations on Si II have been started.

Pradhan, Anil K.↗

NASA NTRS1994

Residual strength of thin panels with cracks

The previous design philosophies involving safe life, fail-safe and damage tolerance concepts become inadequate for assuring the safety of aging aircraft structures. For example, the failure mechanism for the Aloha Airline accident involved the coalescence of undetected small cracks at the rivet holes causing a section of the fuselage to peel open during flight. Therefore, the fuselage structure should be designed to have sufficient residual strength under worst case crack configurations and in-flight load conditions. Residual strength is interpreted as the maximum load carrying capacity prior to unstable crack growth. Internal pressure and bending moment constitute the two major components of the external loads on the fuselage section during flight. Although the stiffeners in the form of stringers, frames and tear straps sustain part of the external loads, the significant portion of the load is taken up by the skin. In the presence of a large crack in the skin, the crack lips bulge out with considerable yielding; thus, the geometric and material nonlinearities must be included in the analysis for predicting residual strength. Also, these nonlinearities do not permit the decoupling of in-plane and out-of-plane bending deformations. The failure criterion combining the concepts of absorbed specific energy and strain energy density addresses the aforementioned concerns. The critical absorbed specific energy (local toughness) for the material is determined from the global specimen response and deformation geometry based on the uniaxial tensile test data and detailed finite element modeling of the specimen response. The use of the local toughness and stress-strain response at the continuum level eliminates the size effect. With this critical parameter and stress-strain response, the finite element analysis of the component by using STAGS along with the application of this failure criterion provides the stable crack growth calculations for residual strength predictions.

Madenci, Erdogan↗

NASA NTRS1995

Theoretical Studies of the Quasi-Bound Vibrational Levels in the B State of CH3

Consider the calculation of the oscillator strength of th B yielded by X electronic transition in CH3. The X state is strongly bound, but in the B state, CH3 is pre-dissociated by H atom tunneling through a potential energy barrier to form CH2+H. If we turn off pre-dissociation by making the potential outside of the barrier repulsive, the calculated 0-0 oscillator strength for CD3 from high quality ab initio potential and transition moment data along with accurate ro-vibrational wavefunctions is 0.0212, which is a factor of two greater than experiment. This discrepancy is largely due to the neglect of pre-dissociation. In this poster we explore techniques for including pre-dissociation in the calculations.

Schwenke, David W.↗

DOE OSTI2020

Lattice Dynamics of Sb 2 Se 3 from Inelastic Neutron and X-Ray Scattering

The lattice dynamics of orthorhombic Sb 2 Se 3 is studied by a combination of inelastic neutron and 121 Sb nuclear inelastic scattering giving access to the total and Sb partial density of phonon states (DPS). The Se partial DPS is determined from the difference between the total and Sb partial DPS. The total DPS is determined at 39, 150, and 300 K, and an analysis of the temperature-induced mode shifts in combination with low-temperature powder diffraction data is provided. Using an earlier reported theoretical approach, the corresponding total and partial DPS of Sb 2 Se 3 are calculated by first-principles calculations. Herein, a detailed analysis of the Grüneisen parameter, element-specific and bulk Debye temperatures, and the mean force constants as derived from the experimental data and discrete Fourier transform calculations is provided. In general, the calculations underestimate the strength of the covalent Sb-Se bonds.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗