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At least 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Walker Branch Watershed: Hourly, Daily, and Annual Precipitation

This data set reports hourly, daily, and annual precipitation in Walker Branch Watershed (WBW). The long-term Walker Branch Hydrology Monitoring project was intended to document changes in the water balance (precipitation inputs, stream discharge outputs) for the WBW over time. This data set contains three data files of precipitation amount for hourly, monthly, and annual intervals for the years 1969 through 2012. Five rain gauges were used from 1969-1979, and two rain gauges were used from 1980-2012. WBW is a temperate forest watershed on the U.S. Department of Energy’s Oak Ridge Reservation in east Tennessee, USA, and has been the site of long-term environmental research since the 1960s.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Walker Branch Watershed: Daily Climate and Soil Temperature Data

This data set reports daily climate and soil temperature data for Walker Branch Watershed. As part of the long-term Walker Branch project, daily climate and soil temperature data were collected using instruments on a meteorological tower located in an upper slope of the watershed. This data set contains one data file of daily climate and soil temperature data from 1993 through 2010. Measurement collection began in 1993 as part of the Throughfall Displacement Experiment (TDE) in the control (ambient) plot. Measurements include daily integrated values for above canopy incident photosynthetically active radiation, above canopy incident total radiation, air temperature, relative humidity, rainfall, soil temperature at 10-15 cm, and wind speed. Measurements of incoming rainfall, irradiance, and photosynthetic photon flux density were obtained in a nearby clearing until 1998 when above canopy observations were added to the ambient plot tower. The Walker Branch Watershed is located on the U.S. Department of Energy's Oak Ridge Reservation in eastern Tennessee.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

STUDYING THE INTERSTELLAR MEDIUM AND THE INNER REGION OF NPS/LOOP 1 WITH SHADOW OBSERVATIONS TOWARD MBM36

We analyzed data from a shadow observation of the high density molecular cloud MBM36 (l ∼ 4°, b ∼ 35°) with Suzaku. MBM36 is located in a region that emits relatively weakly in the 3/4 keV band compared to the surrounding North Polar Spur (NPS)/Loop 1 structure and the Galactic Bulge (GB). The contrast between high and low density targets in the MBM36 area allows one to separate the local and distant contributors to the soft diffuse X-ray background, providing a much better characterization of the individual components compared to single pointing observations. We identify two non-local thermal components, one at kT ≈ 0.12 keV and one at kT ≈ 0.29 keV. The colder component matches well with models of emission from the higher latitude region of the GB. The emission of the warmer component is in agreement with models predicting that the NPS is due to a hypershell from the center of the Milky Way. Geometrical and pressure calculations rule out a nearby bubble as responsible for the emission associated with the NPS. Any Galactic Halo/circumgalactic halo emission, if present, is outshined by the other components. We also report an excess emission around 0.9 keV, likely due to an overabundance of Ne ix.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

HST-COS OBSERVATIONS OF AGNs. III. SPECTRAL CONSTRAINTS IN THE LYMAN CONTINUUM FROM COMPOSITE COS/G140L DATA

The rest-frame ultraviolet (UV) spectra of active galactic nuclei (AGNs) are important diagnostics of both accretion disk physics and their contribution to the metagalactic ionizing UV background. Though the mean AGN spectrum is well characterized with composite spectra at wavelengths greater than 912 Å, the shorter-wavelength extreme-UV (EUV) remains poorly studied. In this third paper in a series on the spectra of AGNs, we combine 11 new spectra taken with the Cosmic Origins Spectrograph on the Hubble Space Telescope with archival spectra to characterize the typical EUV spectral slope of AGNs from λ{sub rest} ∼ 850 Å down to λ{sub rest} ∼ 425 Å. Parameterizing this slope as a power law, we obtain F{sub ν} ∝ ν{sup −0.72±0.26}, but we also discuss the limitations and systematic uncertainties of this model. We identify broad emission features in this spectral region, including emission due to ions of O, Ne, Mg, and other species, and we limit the intrinsic He i 504 Å photoelectric absorption edge opacity to τ{sub He} {sub i} < 0.047.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ELECTRON-CAPTURE AND β-DECAY RATES FOR sd-SHELL NUCLEI IN STELLAR ENVIRONMENTS RELEVANT TO HIGH-DENSITY O–NE–MG CORES

Electron-capture and β-decay rates for nuclear pairs in the sd-shell are evaluated at high densities and high temperatures relevant to the final evolution of electron-degenerate O–Ne–Mg cores of stars with initial masses of 8–10 M{sub ⊙}. Electron capture induces a rapid contraction of the electron-degenerate O–Ne–Mg core. The outcome of rapid contraction depends on the evolutionary changes in the central density and temperature, which are determined by the competing processes of contraction, cooling, and heating. The fate of the stars is determined by these competitions, whether they end up with electron-capture supernovae or Fe core-collapse supernovae. Since the competing processes are induced by electron capture and β-decay, the accurate weak rates are crucially important. The rates are obtained for pairs with A = 20, 23, 24, 25, and 27 by shell-model calculations in the sd-shell with the USDB Hamiltonian. Effects of Coulomb corrections on the rates are evaluated. The rates for pairs with A = 23 and 25 are important for nuclear Urca processes that determine the cooling rate of the O–Ne–Mg core, while those for pairs with A = 20 and 24 are important for the core contraction and heat generation rates in the core. We provide these nuclear rates at stellar environments in tables with fine enough meshes at various densities and temperatures for studies of astrophysical processes sensitive to the rates. In particular, the accurate rate tables are crucially important for the final fates of not only O–Ne–Mg cores but also a wider range of stars, such as C–O cores of lower-mass stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The effect of {sup 22}Ne diffusion in the evolution and pulsational properties of white dwarfs with solar metallicity progenitors

Because of the large neutron excess of {sup 22}Ne, sedimentation of this isotope occurs rapidly in the interior of white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of {sup 22}Ne in the inner regions of the star modifies the Brunt–Väisälä frequency, thus altering the pulsational properties of these stars. In this work we discuss the impact of {sup 22}Ne sedimentation in white dwarfs resulting from solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs with masses of 0.528, 0.576, 0.657, and 0.833 M{sub ⊙} derived from full evolutionary computations of their progenitor stars, starting at the zero-age main sequence all the way through the central hydrogen and helium burning, the thermally pulsing asymptotic giant branch (AGB), and post-AGB phases. Our computations show that at low luminosities (log(L/L{sub ⊙})≲−4.25), {sup 22}Ne sedimentation delays the cooling of white dwarfs with solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of {sup 22}Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that {sup 22}Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Application of a theory and simulation-based convective boundary mixing model for AGB star evolution and nucleosynthesis

The s-process nucleosynthesis in Asymptotic giant branch (AGB) stars depends on the modeling of convective boundaries. We present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries. Hydrodynamics simulations suggest the presence of convective boundary mixing (CBM) at the bottom of the thermal pulse-driven convective zone. Similarly, convection-induced mixing processes are proposed for the mixing below the convective envelope during third dredge-up (TDU), where the {sup 13}C pocket for the s process in AGB stars forms. In this work, we apply a CBM model motivated by simulations and theory to models with initial mass M = 2 and M=3 M{sub ⊙}, and with initial metal content Z = 0.01 and Z = 0.02. As reported previously, the He-intershell abundances of {sup 12}C and {sup 16}O are increased by CBM at the bottom of the pulse-driven convection zone. This mixing is affecting the {sup 22}Ne(α, n){sup 25}Mg activation and the s-process efficiency in the {sup 13}C-pocket. In our model, CBM at the bottom of the convective envelope during the TDU represents gravity wave mixing. Furthermore, we take into account the fact that hydrodynamic simulations indicate a declining mixing efficiency that is already about a pressure scale height from the convective boundaries, compared to mixing-length theory. We obtain the formation of the {sup 13}C-pocket with a mass of ≈10{sup −4} M{sub ⊙}. The final s-process abundances are characterized by 0.36<[s/Fe]<0.78 and the heavy-to-light s-process ratio is −0.23<[hs/ls]<0.45. Finally, we compare our results with stellar observations, presolar grain measurements and previous work.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Extended relativistic configuration interaction and many-body perturbation calculations of spectroscopic data for the N ≤ 6 configurations in Ne-like ions between Cr xv and Kr xxvii

Level energies, wavelengths, electric dipole, magnetic dipole, electric quadrupole, and magnetic quadrupole transition rates, oscillator strengths, and line strengths from combined relativistic configuration interaction and many-body perturbation calculations are reported for the 201 fine-structure states of the 2s{sup 2}2p{sup 6}, 2s{sup 2}2p{sup 5}3l, 2s2p{sup 6}3l, 2s{sup 2}2p{sup 5}4l, 2s2p{sup 6}4l, 2s{sup 2}2p{sup 5}5l, and 2s{sup 2}2p{sup 5}6l configurations in all Ne-like ions between Cr xv and Kr xxvii. Calculated level energies and transition data are compared with experiments from the National Institute of Standards and Technology (NIST) and CHIANTI databases, and other recent benchmark calculations. The mean energy difference with the NIST experiments is only 0.05%. The present calculations significantly increase the amount of accurate spectroscopic data for the n > 3 states in a number of Ne-like ions of astrophysical interest. A complete data set should be helpful for analyzing new observations from solar and other astrophysical sources, and is also likely to be useful for modeling and diagnosing a variety of plasmas, including astronomical and fusion plasma.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Joint Ne/O and Fe/O Analysis to Diagnose Large Solar Energetic Particle Events during Solar Cycle 23

We have examined 29 large solar energetic particle (SEP) events with the peak proton intensity J {sub pp}(>60 MeV) > 1 pfu during solar cycle 23. The emphasis of our examination is put on a joint analysis of Ne/O and Fe/O data in the energy range (3–40 MeV nucleon{sup −1}) covered by Wind /Low-Energy Matrix Telescope and ACE /Solar Isotope Spectrometer sensors in order to differentiate between the Fe-poor and Fe-rich events that emerged from the coronal mass ejection driven shock acceleration process. An improved ion ratio calculation is carried out by rebinning ion intensity data into the form of equal bin widths in the logarithmic energy scale. Through the analysis we find that the variability of Ne/O and Fe/O ratios can be used to investigate the accelerating shock properties. In particular, the high-energy Ne/O ratio is well correlated with the source plasma temperature of SEPs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

First Measurement of the {sup 19}F(α, p){sup 22}Ne Reaction at Energies of Astrophysical Relevance

The observational {sup 19}F abundance in stellar environments systematically exceeds the predicted one, thus representing one of the unsolved challenges for stellar modeling. It is therefore clear that further investigation is needed in this field. In this work, we focus our attention on the measurement of the {sup 19}F(α,p){sup 22}Ne reaction in the astrophysical energy range, between 0.2 and 0.8 MeV (far below the Coulomb barrier, 3.8 MeV), as it represents the main destruction channel in He-rich environments. The lowest energy at which this reaction has been studied with direct measurements is ∼0.66 MeV, covering only the upper tail of the Gamow window, causing the reaction-rate evaluation to be based on extrapolation. To investigate lower energies, the {sup 19}F(α,p){sup 22}Ne reaction has been studied by means of the Trojan horse method, applied to the quasi-free {sup 6}Li({sup 19}F,p{sup 22}Ne){sup 2}H reaction at E {sub beam} = 6 MeV. The indirect cross section of the {sup 19}F(α,p){sup 22}Ne reaction at energies ≲1 MeV was extracted, fully covering the astrophysical region of interest and overlapping existing direct data for normalization. Several resonances have been detected for the first time inside the Gamow window. The reaction rate has been calculated, showing an increase up to a factor of 4 with respect to the literature at astrophysical temperatures. This might lead to potential major astrophysical implications.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Photoionization of Ne Atoms and Ne{sup +} Ions Near the K Edge: PrecisionSpectroscopy and Absolute Cross-sections

Single, double, and triple photoionization of Ne{sup +} ions by single photons have been investigated at the synchrotron radiation source PETRA III in Hamburg, Germany. Absolute cross-sections were measured by employing the photon–ion merged-beams technique. Photon energies were between about 840 and 930 eV, covering the range from the lowest-energy resonances associated with the excitation of one single K-shell electron up to double excitations involving one K- and one L-shell electron, well beyond the K-shell ionization threshold. Also, photoionization of neutral Ne was investigated just below the K edge. The chosen photon energy bandwidths were between 32 and 500 meV, facilitating the determination of natural line widths. The uncertainty of the energy scale is estimated to be 0.2 eV. For comparison with existing theoretical calculations, astrophysically relevant photoabsorption cross-sections were inferred by summing the measured partial ionization channels. Discussion of the observed resonances in the different final ionization channels reveals the presence of complex Auger-decay mechanisms. The ejection of three electrons from the lowest K-shell-excited Ne{sup +}(1s2s{sup 2}2p{sup 62}S{sub 1/2}) level, for example, requires cooperative interaction of at least four electrons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Energy Spectra of Abundant Cosmic-ray Nuclei in Sources, According to the ATIC Experiment

One of the main results of the ATIC (Advanced Thin Ionization Calorimeter) experiment is a collection of energy spectra of abundant cosmic-ray nuclei: protons, He, C, O, Ne, Mg, Si, and Fe measured in terms of energy per particle in the energy range from 50 GeV to tens of teraelectronvolts. In this paper, the ATIC energy spectra of abundant primary nuclei are back-propagated to the spectra in sources in terms of magnetic rigidity using a leaky-box approximation of three different GALPROP-based diffusion models of propagation that fit the latest B/C data of the AMS-02 experiment. It is shown that the results of a comparison of the slopes of the spectra in sources are weakly model dependent; therefore the differences of spectral indices are reliable data. A regular growth of the steepness of spectra in sources in the range of magnetic rigidity of 50–1350 GV is found for a charge range from helium to iron. This conclusion is statistically reliable with significance better than 3.2 standard deviations. The results are discussed and compared to the data of other modern experiments.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The {sup 95}Zr( n , γ ){sup 96}Zr Cross Section from the Surrogate Ratio Method and Its Effect on s -process Nucleosynthesis

The {sup 95}Zr( n , γ ){sup 96}Zr reaction cross section is crucial in the modeling of s -process nucleosynthesis in asymptotic giant branch stars because it controls the operation of the branching point at the unstable {sup 95}Zr and the subsequent production of {sup 96}Zr. We have carried out the measurement of the {sup 94}Zr({sup 18}O, {sup 16}O) and {sup 90}Zr({sup 18}O, {sup 16}O) reactions and obtained the γ -decay probability ratio of {sup 96}Zr* and {sup 92}Zr* to determine the {sup 95}Zr( n , γ ){sup 96}Zr reaction cross sections with the surrogate ratio method. Our deduced Maxwellian-averaged cross section of 66 ± 16 mb at 30 keV is close to the value recommended by Bao et al., but 30% and more than a factor of two larger than the values proposed by Toukan and Käppeler and Lugaro et al., respectively, and routinely used in s -process models. We tested the new rate in stellar models with masses between 2 and 6 M {sub ⊙} and metallicities of 0.014 and 0.03. The largest changes—up to 80% variations in {sup 96}Zr—are seen in models of mass 3–4 M {sub ⊙}, where the {sup 22}Ne neutron source is mildly activated. The new rate can still provide a match to data from meteoritic stardust silicon carbide grains, provided that the maximum mass of the parent stars is below 4 M {sub ⊙}, for a metallicity of 0.03.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Non-relativistic Free–Free Emission due to the n -distribution of Electrons—Radiative Cooling and Thermally Averaged and Total Gaunt Factors

Tracking the thermal evolution of plasmas, characterized by an n -distribution, using numerical simulations, requires the determination of the emission spectra and of the radiative losses due to free–free emission from the corresponding temperature-averaged and total Gaunt factors. Detailed calculations of the latter are presented and associated with n -distributed electrons with the parameter n ranging from 1 (corresponding to the Maxwell–Boltzmann distribution) to 100. The temperature-averaged and total Gaunt factors with decreasing n tend toward those obtained with the Maxwell–Boltzmann distribution. Radiative losses due to free–free emission in a plasma evolving under collisional ionization equilibrium conditions and composed by H, He, C, N, O, Ne, Mg, Si, S, and Fe ions, are presented. These losses decrease with a decrease in the parameter n , reaching a minimum when n = 1, and thus converge with the loss of thermal plasma. Tables of the thermal-averaged and total Gaunt factors calculated for n -distributions, and a wide range of electron and photon energies, are presented.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Charge Exchange of Highly Charged Ne and Mg Ions with H and He

Cross sections for single electron capture (SEC), or charge exchange (CX), in collisions of Ne{sup (8–10)+} and Mg{sup (8–12)+} with H and He, are computed using an approximate multichannel Landau–Zener (MCLZ) formalism. Final-state-resolved cross sections for the principal (n), orbital angular momentum (ℓ), and where appropriate, total spin angular momentum (S) quantum numbers are explicitly computed, except for the incident bare ions Ne{sup 10+} and Mg{sup 12+}. In the latter two cases, nℓ-resolution is obtained from analytical ℓ-distribution functions applied to n-resolved MCLZ cross sections. In all cases, the cross sections are computed over the collision energy range 1 meV/u to 50 keV/u with LZ parameters estimated from atomic energies obtained from experiment, theory, or, in the case of high-lying Rydberg levels, estimated with a quantum defect approach. Errors in the energy differences in the adiabatic potentials at the avoided crossing distances give the largest contribution to the uncertainties in the cross sections, which are expected to increase with decreasing cross section magnitude. The energy differences are deduced here with the Olson–Salop–Tauljberg radial coupling model. Proper selection of an ℓ-distribution function for bare ion collisions introduces another level of uncertainty into the results. Comparison is made to existing experimental or theoretical results when available, but such data are absent for most considered collision systems. The nℓS-resolved SEC cross sections are used in an optically thin cascade simulation to predict X-ray spectra and line ratios that will aid in modeling the X-ray emission in environments where CX is an important mechanism. Details on a MCLZ computational package, Stueckelberg, are also provided.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1999

Fourier-transform optical microsystems

The design, fabrication, and initial characterization of a miniature single-pass Fourier-transform spectrometer (FTS) that has an optical bench that measures 1 cm x 5 cm x 10 cm is presented. The FTS is predicated on the classic Michelson interferometer design with a moving mirror. Precision translation of the mirror is accomplished by microfabrication of dovetailed bearing surfaces along single-crystal planes in silicon. Although it is miniaturized, the FTS maintains a relatively high spectral resolution, 0.1 cm-1, with adequate optical throughput.

NASA Discipline Environmental Health↗