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At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Evaporation and scattering of momentum- and velocity-dependent dark matter in the Sun

Dark matter with momentum- or velocity-dependent interactions with nuclei has shown significant promise for explaining the so-called Solar Abundance Problem, a longstanding discrepancy between solar spectroscopy and helioseismology. The best-fit models are all rather light, typically with masses in the range of 3–5 GeV. This is exactly the mass range where dark matter evaporation from the Sun can be important, but to date no detailed calculation of the evaporation of such models has been performed. Here we carry out this calculation, for the first time including arbitrary velocity- and momentum-dependent interactions, thermal effects, and a completely general treatment valid from the optically thin limit all the way through to the optically thick regime. We find that depending on the dark matter mass, interaction strength and type, the mass below which evaporation is relevant can vary from 1 to 4 GeV. This has the effect of weakening some of the better-fitting solutions to the Solar Abundance Problem, but also improving a number of others. As a by-product, we also provide an improved derivation of the capture rate that takes into account thermal and optical depth effects, allowing the standard result to be smoothly matched to the well-known saturation limit.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Temperature dependence of the Casimir force for bulk lossy media

We discuss the limitations for the applicability of the Lifshitz theory to describe the temperature dependence of the Casimir force between bulk lossy metal slabs of finite sizes. We pay attention to the important fact that Lifshitz's theory is not applicable when the characteristic wavelength of the fluctuating field, responsible for the temperature-dependent terms in the Casimir force, are longer than the size of the sample. As a result, the widely discussed linearly decreasing temperature dependence of the Casimir force can be observed only for dirty and large metal samples at high enough temperatures. Moreover, for the correct description of the Casimir effect at low enough temperatures, a careful consideration of the concrete geometry of the interacting samples is essential.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Time-dependent second-order Born calculations for model atoms and molecules in strong laser fields

Using the finite-element discrete variable representation of the nonequilibrium Green's function (NEGF) we extend previous work [K. Balzer et al., Phys. Rev. A 81, 022510 (2010)] to nonequilibrium situations and compute--from the two-time Schwinger-Keldysh-Kadanoff-Baym equations--the response of the helium atom and the heteronuclear molecule lithium hydride to laser fields in the uv and xuv regimes. In particular, by comparing the one-electron density and the dipole moment to time-dependent Hartree-Fock results on one hand and the full solution of the time-dependent Schroedinger equation on the other hand, we demonstrate that the time-dependent second Born approximation carries valuable information about electron-electron correlation effects. Also, we outline an efficient distributed memory concept which enables a parallel and well-scalable algorithm for computing the NEGF in the two-time domain.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Interpreting quantum states of electromagnetic field in time-dependent linear media

Recently, Pedrosa and Rosas [Phys. Rev. Lett. 103, 010402 (2009)] investigated the quantum states of an electromagnetic field in time-dependent linear media using a Hermitian linear invariant. The wave function obtained by them is represented in terms of an arbitrary weight function g({lambda}{sub l}). Since the type of wave function varies depending on the choice of g({lambda}{sub l}) in their problem, it may be a difficult task to construct a coherent state that resembles the classical state from their theory. We suggest, on the basis of a non-Hermitian linear invariant, another quantum state that is a kind of coherent state. The expectation value of canonical variables in this alternate state follows an exact classical trajectory. For a simple case in which the time dependence of the parameters {epsilon}(t), {mu}(t), and {sigma}(t) disappears, we showed that the quantum energy expectation value in the alternate quantum state recovers exactly to the classical energy in the limit ({h_bar}/2{pi}){yields}0. This alternate state leads to the correspondence between the quantum and the classical behaviors of physical observables in a high-energy limit.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Comparison of the strong-field ionization of N{sub 2} and F{sub 2}: A time-dependent density-functional-theory study

We compare strong-field ionization probabilities of N{sub 2} and F{sub 2} molecules using time-dependent density functional theory calculations. Accurate nuclear potentials and ground vibrational wave functions are incorporated into our study. For both molecules, the effect of molecular vibration is small, while that of the molecular orientation is significant. When compared to the ionization probability of a molecule at the equilibrium geometry, we estimate the effect of the ground state vibration to be within 3% for N{sub 2} and within 6% for F{sub 2} in the intensity range from 1 to 5x10{sup 14} W/cm{sup 2}. The molecular-orientation-dependent ionization probabilities for both molecules at various intensities are presented. They are strongly dependent on the laser intensity, and the anisotropy diminishes when the laser intensity is high. For laser intensities of 1.6 and 2.2x10{sup 14} W/cm{sup 2} we find ionization probability ratios of 5.9 and 4.3, respectively, for the parallel versus perpendicular orientation of N{sub 2}. This is reasonably consistent with experimental measurements. For randomly oriented molecules, the ratio of the probabilities for N{sub 2} and F{sub 2} increases from about 1 at 10{sup 14} W/cm{sup 2} to 2 at 4x10{sup 14} W/cm{sup 2}, which agrees well with experimental results.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Thermionic power-law decay of excited aluminum-cluster anions and its dependence on storage-device temperature

The decay of excited aluminum-cluster anions (Al{sub n}{sup -}, n=4 and 5) has been investigated in a cryogenic linear ion-beam trap. A power-law is found to accurately reproduce the time dependence of the observed decay rates at early storage times, although the exponents are significantly larger than the typically observed 1/t decay. It is shown that the power-law exponent is, at most, weakly dependent on the cluster electron affinity and heat capacity. A previous power-law exponent model for small clusters is also shown to be in disagreement with both investigated species. The attribution of a drop in the decay rate at later times to radiative cooling as observed in larger molecules also does not appear justified in our case. A strong dependence of the power-law exponent on the ambient temperature was observed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Equivalence between Redfield- and master-equation approaches for a time-dependent quantum system and coherence control

We present a derivation of the Redfield formalism for treating the dissipative dynamics of a time-dependent quantum system coupled to a classical environment. We compare such a formalism with the master equation approach where the environments are treated quantum mechanically. Focusing on a time-dependent spin-1/2 system we demonstrate the equivalence between both approaches by showing that they lead to the same Bloch equations and, as a consequence, to the same characteristic times T{sub 1} and T{sub 2} (associated with the longitudinal and transverse relaxations, respectively). These characteristic times are shown to be related to the operator-sum representation and the equivalent phenomenological-operator approach. Finally, we present a protocol to circumvent the decoherence processes due to the loss of energy (and thus, associated with T{sub 1}). To this end, we simply associate the time dependence of the quantum system to an easily achieved modulated frequency. A possible implementation of the protocol is also proposed in the context of nuclear magnetic resonance.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

First-principles wave-vector- and frequency-dependent exchange-correlation kernel for jellium at all densities

Here we propose a spatially and temporally nonlocal exchange correlation (XC) kernel for the spin-unpolarized fluid phase of ground-state jellium for use in time-dependent density functional and linear response calculations. The kernel is constructed to satisfy known properties of the exact XC kernel to accurately describe the correlation energies of bulk jellium and to satisfy frequency-moment sum rules at a wide range of bulk jellium densities, including those low densities that display strong correlation and symmetry breaking. These effects are easier to understand in the simple jellium model than in real systems. All exact constraints satisfied by the recent MCP07 kernel are maintained in the revised MCP07 (rMCP07) kernel, while others are added. The revision $f^{rMCP07}_{XC}$ (q, ω) differs from MCP07 only for nonzero frequencies ω. Only at densities much lower than those of real bulk metals is the frequency dependence of the kernel important for the correlation energy of jellium. As the wave vector q tends to zero, the kernel has a -4$πα(ω)/q^2$ divergence whose frequency-dependent ultranonlocality coefficient $α(ω)$ vanishes in jellium, and is predicted by rMCP07 to be extremely small for the real metals Al and Na.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Non-monotonic temperature dependence of thermopower in strongly correlated electron systems

We examine the temperature dependence of thermopower in the single-band Hubbard model using dynamical mean-field theory. The strong Coulomb interaction brings about the coherent-to-incoherent crossover as temperature increases. As a result, the thermopower exhibits nonmonotonic temperature dependence and asymptotically approaches values given by the Mott-Heikes formula. In the light of our theoretical result, we discuss the thermopower in some transition metal oxides. The magnetic field dependence of the thermopower is also discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Stress and temperature dependence of screw dislocation mobility in {alpha}-Fe by molecular dynamics

The low-temperature plastic yield of {alpha}-Fe single crystals is known to display a strong temperature dependence and to be controlled by the thermally activated motion of screw dislocations. In this paper, we present molecular dynamics simulations of (1/2)<111>{l_brace}112{r_brace} screw dislocation motion as a function of temperature and stress in order to extract mobility relations that describe the general dynamic behavior of screw dislocations in pure {alpha}-Fe. We find two dynamic regimes in the stress-velocity space governed by different mechanisms of motion. Consistent with experimental evidence, at low stresses and temperatures, the dislocations move by thermally activated nucleation and propagation of kink pairs. Then, at a critical stress, a temperature-dependent transition to a viscous linear regime is observed. Critical output from the simulations, such as threshold stresses and the stress dependence of the kink activation energy, are compared to experimental data and other atomistic works with generally very good agreement. Contrary to some experimental interpretations, we find that glide on {l_brace}112{r_brace} planes is only apparent, as slip always occurs by elementary kink-pair nucleation/propagation events on {l_brace}110{r_brace} planes. Additionally, a dislocation core transformation from compact to dissociated has been identified above room temperature, although its impact on the general mobility is seen to be limited. This and other observations expose the limitations of inferring or presuming dynamic behavior on the basis of only static calculations. We discuss the relevance and applicability of our results and provide a closed-form functional mobility law suitable for mesoscale computational techniques.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Time-dependent orbital-free density functional theory for electronic stopping power: Comparison to the Mermin-Kohn-Sham theory at high temperatures

Electronic stopping power in warm dense matter can affect energy transport and heating in astrophysical processes and internal confinement fusion. For cold condensed matter systems, stopping power can be modeled from first-principles using real-time time-dependent density functional theory (DFT). However, high temperatures (10's to 100's of eV) may be computationally prohibitive for traditional Mermin-Kohn-Sham DFT. New experimental measurements in the warm dense regime motivates the development of first-principles approaches, which can reach these temperatures. Here, we have developed a time-dependent orbital-free density functional theory, which includes a novel nonadiabatic and temperature-dependent kinetic energy density functional, for the simulation of stopping power at any temperature. The approach is nonlinear with respect to the projectile perturbation, includes all ions and electrons, and does not require a priori determination of screened interaction potentials. Finally, our results compare favorably with Kohn-Sham for temperatures in the WDM regime, especially nearing 100 eV.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Energy dependence of nonlocal optical potentials

Recently, a variety of studies have shown the importance of including nonlocality in the description of reactions. The goal of this work is to revisit the phenomenological approach to determining nonlocal optical potentials from elastic scattering. We perform a χ 2 analysis of neutron elastic scattering data off 40 Ca, 90 Zr, and 208 Pb at energies E ≈ 5–40 MeV, assuming a Perey and Buck [Nucl. Phys. 32, 353 (1962)] or Tian et al. [Int. J. Mod. Phys. E 24, 1550006 (2015)] nonlocal form for the optical potential. We introduce energy and asymmetry dependencies in the imaginary part of the potential and refit the data to obtain a global parametrization. Independently of the starting point in the minimization procedure, an energy dependence in the imaginary depth is required for a good description of the data across the included energy range. We then present two parametrizations, both of which represent an improvement over the original potentials for the fitted nuclei as well as for other nuclei not included in our fit. Our results show that, even when including the standard Gaussian nonlocality in optical potential, a significant energy dependence is required to describe elastic-scattering data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

Glass transition in thaumatin crystals revealed through temperature-dependent radiation-sensitivity measurements

Radiation damage to protein crystals exhibits two regimes of temperature-activated behavior between T = 300 and 100 K, with a crossover at the protein glass transition near 200 K. These results have implications for mechanistic studies of proteins and for structure determination when cooling to T = 100 K creates excessive disorder. The temperature-dependence of radiation damage to thaumatin crystals between T = 300 and 100 K is reported. The amount of damage for a given dose decreases sharply as the temperature decreases from 300 to 220 K and then decreases more gradually on further cooling below the protein-solvent glass transition. Two regimes of temperature-activated behavior were observed. At temperatures above ∼200 K the activation energy of 18.0 kJ mol{sup −1} indicates that radiation damage is dominated by diffusive motions in the protein and solvent. At temperatures below ∼200 K the activation energy is only 1.00 kJ mol{sup −1}, which is of the order of the thermal energy. Similar activation energies describe the temperature-dependence of radiation damage to a variety of solvent-free small-molecule organic crystals over the temperature range T = 300–80 K. It is suggested that radiation damage in this regime is vibrationally assisted and that the freezing-out of amino-acid scale vibrations contributes to the very weak temperature-dependence of radiation damage below ∼80 K. Analysis using the radiation-damage model of Blake and Phillips [Blake & Phillips (1962 ▶), Biological Effects of Ionizing Radiation at the Molecular Level, pp. 183–191] indicates that large-scale conformational and molecular motions are frozen out below T = 200 K but become increasingly prevalent and make an increasing contribution to damage at higher temperatures. Possible alternative mechanisms for radiation damage involving the formation of hydrogen-gas bubbles are discussed and discounted. These results have implications for mechanistic studies of proteins and for studies of the protein glass transition. They also suggest that data collection at T ≃ 220 K may provide a viable alternative for structure determination when cooling-induced disorder at T = 100 is excessive.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Load-Dependent Soft-Switching Method of Half-Bridge Current Doubler for High-Voltage Point-of-Load Converter in Data Center Power Supplies

With the increased cloud computing and digital information storage, the energy requirement of data centers keeps increasing. A high-voltage point of load (HV POL) with an input series output parallel structure is proposed to convert 400 to 1 VDC within a single stage to increase the power conversion efficiency. The symmetrical controlled half-bridge current doubler is selected as the converter topology in the HV POL. A load-dependent soft-switching method has been proposed with an auxiliary circuit that includes inductor, diode, and MOSFETs so that the hard-switching issue of typical symmetrical controlled half-bridge converters is resolved. The operation principles of the proposed soft-switching half-bridge current doubler have been analyzed in detail. Then, the necessity of adjusting the timing with the loading in the proposed method is analyzed based on losses, and a controller is designed to realize the load-dependent operation. A lossless RCD current sensing method is used to sense the output inductor current value in the proposed load-dependent operation. In conclusion, experimental efficiency of a hardware prototype is provided to show that the proposed method can increase the converter's efficiency in both heavy- and light-load conditions.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2023

A Distributionally Robust Resilience Enhancement Strategy for Distribution Networks Considering Decision-Dependent Contingencies

When performing the resilience enhancement for distribution networks, there are two obstacles to reliably model the uncertain contingencies: 1) decision-dependent uncertainty (DDU) due to various line hardening decisions, and 2) distributional ambiguity due to limited outage information during extreme weather events (EWEs). Here, to address these two challenges, this paper develops scenario-wise decision-dependent ambiguity sets (SWDD-ASs), where the DDU and distributional ambiguity inherent in EWE-induced contingencies are simultaneously captured for each possible EWE scenario. Then, a two-stage tri-level decision-dependent distributionally robust resilient enhancement (DD-DRRE) model is formulated, whose outputs include the optimal line hardening, distributed generation (DG) allocation, and proactive network reconfiguration strategy under the worst-case distributions in SWDD-ASs. Subsequently, the DD-DRRE model is equivalently recast to a mixed-integer linear programming (MILP)-based master problem and multiple scenario-wise subproblems, facilitating the adoption of a customized column-and-constraint generation (C&CG) algorithm. Finally, case studies demonstrate a remarkable improvement in the out-of-sample performance of our model, compared to its prevailing stochastic and robust counterparts. Moreover, the potential values of incorporating the ambiguity and distributional information are quantitatively estimated, providing a useful reference for planners with different budgets and risk-aversion levels.

decision-dependent uncertainty↗
DOE OSTI2010

Dependence of Yb-169 absorbed dose energy correction factors on self-attenuation in source material and photon buildup in water

Purpose: Absorbed dose energy correction factors, used to convert the absorbed dose deposited in a LiF thermoluminescent dosimeter (TLD) into the clinically relevant absorbed dose to water, were obtained for both spherical volumetric sources and for the model 4140 HDR Yb-169 source. These correction factors have a strong energy dependence below 200 keV; therefore, spectral changes were quantified as Yb-169 photons traveled through both source material (Yb{sub 2}O{sub 3}) and water with the corresponding absorbed dose energy correction factors, f(r,{theta}), calculated as a function of location in a phantom. Methods: Using the MCNP5 Monte Carlo radiation transport simulation program, the Yb-169 spectrum emerging from spherical Yb{sub 2}O{sub 3} sources (density 6.9 g/cm{sup 3}) with radii between 0.2 and 0.9 mm were analyzed and their behavior compared against those for a point-source. The absorbed dose deposited to both LiF and H{sub 2}O materials was analyzed at phantom depths of 0.1-10 cm for each source radius and the absorbed dose energy correction factor calculated as the ratio of the absorbed dose to water to that of LiF. Absorbed dose energy correction factors for the Model 4140 Yb-169 HDR brachytherapy source similarly were obtained and compared against those calculated for the Model M-19 Ir-192 HDR source. Results: The Yb-169 average spectral energy, emerging from Yb{sub 2}O{sub 3} spherical sources 0.2-0.9 mm in radius, was observed to harden from 7% to 29%; as these photons traveled through the water phantom, the photon average energy softened by as much as 28% at a depth of 10 cm. Spectral softening was dependent on the measurement depth in the phantom. Energy correction factors were found to vary both as a function of source radius and phantom depth by as much as 10% for spherical Yb{sub 2}O{sub 3} sources. The Model 4140 Yb-169 energy correction factors depended on both phantom depth and reference angle and were found to vary by more than 10% between depths of 1 and 10 cm and angles of 0 deg. and 180 deg. This was in contrast to that of the Model M-19 Ir-192 source which exhibited approximately 3.5%-4.4% variation in its energy correction factors from phantom depths of 0.5-10 cm. The absorbed dose energy correction factor for the Ir-192 source, on the other hand, was independent of angle to within 1%. Conclusions: The application of a single energy correction factor for Yb-169 TLD based dosimetry would introduce a high degree of measurement uncertainty that may not be reasonable for the clinical characterization of a brachytherapy source; rather, an absorbed dose energy correction function will need to be developed for these sources. This correction function should be specific to each source model, type of TLD used, and to the experimental setup to obtain accurate and precise dosimetric measurements.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2014

SU-E-T-354: Peak Temperature Ratio of TLD Glow Curves to Investigate the Spatial Dependence of LET in a Clinical Proton Beam

Purpose: Use the ratio of the two high temperature peaks (HTR) in TLD 700 glow curves to investigate spatial dependence of the linear energy transfer (LET) in proton beams. Studies show that the relative biological effectiveness (RBE) depends upon the physical dose as well as its spatial distribution. Although proton therapy uses a spatially invariant RBE of 1.1, studies suggest that the RBE increases in the distal edge of a spread out Bragg peak (SOBP) due to the increased LET. Methods: Glow curve studies in TLD 700 show that the 280 C temperature peak is more sensitive to LET radiation than the 210 C temperature peak. Therefore, the areas under the individual temperature peaks for TLDs irradiated in a proton beam normalized to the peak ratio for 6 MV photons are used to determine the HTR to obtain information on its LET. TLD 700 chips with dimensions 0.31×0.31×0.038 cc are irradiated with 90 MeV protons at varying depths in a specially designed blue wax phantom to investigate LET spatial dependence. Results: Five TLDs were placed at five different depths of the percent depth dose curve (PDD) of range 16.2 cm: center of the SOPB and approximately at the 99% distal edge, 90%, 75% and 25% of the PDD, respectively. HTR was 1.3 at the center of the SOBP and varied from 2.2 to 3.9 which can be related to an LET variation from 0.5 to 18 KeV/μ via calibration with radiation beams of varying LET. Conclusion: HTR data show a spatially invariant LET slightly greater than the 6 MV radiations in the SOBP, but a rapidly increasing LET at the end of the proton range. These results indicate a spatial variation in RBE with potential treatment consequences when selecting treatment margins to minimize the uncertainties in proton RBE.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

SU-E-T-526: On the Linearity, Stability and Beam Energy Dependence of CdSe Quantum Dots as Scintillating Probes

Purpose: Structure and energy transfer mechanisms confer colloidal quantum dots (cQDs) interesting properties, among them their potential as scintillators. CdSe multi-shell cQDs in powder were investigated under photons irradiation. The purpose of this work is to characterize signal to dose linearity, stability with time and to quantify the dependence of their light output with beam energy. Methods: The cQDs are placed at the extremity of a non-scintillating plastic collecting fiber, with the other extremity connected to an Apogee U2000C CCD camera. The CCD camera collects the fluorescence light from irradiated cQDs from which the delivered dose is extracted. This signal is corrected for Cerenkov contamination at MV energies using the chromatic technique. The detector was irradiated with two devices: Xstrahl 200 orthovoltage unit for 120, 180 and 220 kVp and a Varian Clinac iX for 6 and 23 MV. Results: Linear output response with varying dose is observed for all beam energies with R2 factors > 0,999. Reproducibility measurements were performed at 120 kVp: the same set-up was irradiated at different time intervals (one week and three months). The results showed only a small relative decrease of light output of 3,2 % after a combine deposited dose of approximately 95 Gy. CdSe nanocrystals response has been studied as a function of beam energy. The output increases with decreasing energy from 120 kVp to 6 MV and increase again for 23 MV. This behavior could be explained in part by the cQDs high-Z composition. Conclusion: The fluorescence light output of CdSe cQDs was found to be linear as a function of dose. The results suggest stability of the scintillation output of cQDs over time. The specific composition of cQDs is the main cause of the observed energy dependence. We will further look into particle beam dependence of the cQDs. Bourse d'excellence aux etudes graduees du CRC (Centre de Recherche sur le Cancer, Universite Laval) Bourse d'excellence aux etudes graduees du CRC (Centre de Recherche sur le Cancer, Universite Laval)

60 APPLIED LIFE SCIENCES↗