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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 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
NASA NTRS2007

Space Radiation Organ Doses for Astronauts on Past and Future Missions

We review methods and data used for determining astronaut organ dose equivalents on past space missions including Apollo, Skylab, Space Shuttle, NASA-Mir, and International Space Station (ISS). Expectations for future lunar missions are also described. Physical measurements of space radiation include the absorbed dose, dose equivalent, and linear energy transfer (LET) spectra, or a related quantity, the lineal energy (y) spectra that is measured by a tissue equivalent proportional counter (TEPC). These data are used in conjunction with space radiation transport models to project organ specific doses used in cancer and other risk projection models. Biodosimetry data from Mir, STS, and ISS missions provide an alternative estimate of organ dose equivalents based on chromosome aberrations. The physical environments inside spacecraft are currently well understood with errors in organ dose projections estimated as less than plus or minus 15%, however understanding the biological risks from space radiation remains a difficult problem because of the many radiation types including protons, heavy ions, and secondary neutrons for which there are no human data to estimate risks. The accuracy of projections of organ dose equivalents described here must be supplemented with research on the health risks of space exposure to properly assess crew safety for exploration missions.

Cucinotta, Francis A.↗
DOE OSTI2020

Effective Opacity of the Intergalactic Medium from Galaxy Spectra Analysis

We measure the effective opacity (τ{sub eff}) of the intergalactic medium from the composite spectra of 281 Lyman-break galaxies in the redshift range 2 ≲ z ≲ 3. Our spectra are taken from the COSMOS Lyα Mapping And Tomographic Observations survey derived from the Low Resolution Imaging Spectrometer on the W.M. Keck I telescope. We generate composite spectra in two redshift intervals and fit them with spectral energy distribution (SED) models composed of simple stellar populations. Extrapolating these SED models into the Lyα forest, we measure the effective Lyα opacity (τ{sub eff}) in the 2.02 ≤ z ≤ 2.44 range. At z = 2.22, we estimate τ{sub eff} =0.159±0.001 from a power-law fit to the data. These measurements are consistent with estimates from quasar analyses at z < 2.5 indicating that the systematic errors associated with normalizing quasar continua are not substantial. We provide a Gaussian processes model of our results and previous τ{sub eff} measurements that describes the steep redshift evolution in τ{sub eff} from z = 1.5–4.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1973

Excitation of the W triplet Delta (U), W singlet Delta (U), B prime triplet Sigma (U) (minus), and A prime singlet Epsison (U) (minus) states of N2 by electron impact

Electron energy-loss spectra have been obtained for N2 at 20.6 eV impact energy, and scattering angles of 10-138 deg. The differential cross section for excitation of the W triplet Delta(U) state is the largest triplet-state cross section at all scattering angles, and is the largest inelastic cross section at angles greater than 70 degrees. (Author Modified Abstract)

Cartwright, D. C.↗
NASA NTRS1980

Spectroscopy of compact extragalactic X-ray sources

The X-ray spectra of compact extragalactic sources obtained from the HEAO-1 A-2 experiment and the solid-state spectrometer onboard HEAO-2 (the Einstein Observatory) are reviewed. Seyfert spectra are remarkably consistent with characteristic power-law spectra of energy index alpha = 0.7 + .1 over a dynamic range of almost 100 in both luminosity for the whole sample, and energy for individual members. Radio-quiet quasars have similar spectra, perhaps slightly steeper, for the limited sample available. New solid-state spectrometer results for NGC 4151 yield a consistent picture for the geometry of the broad-line clouds in both these related radio-quiet classes of galactic nuclei. Radio-loud objects, especially BL Lacs, are considerably more variable in spectrum as well as luminosity. Direct synchrotron and synchrotron-self-Compton components are consistent with what we observe from these objects. Finally, the role of spectroscopy in addressing the extent to which compact extragalactic nuclei might contribute to the diffuse X-ray background is discussed.

Holt, S. S.↗
NASA NTRS1991

Spectral signatures of the MeV-PeV radiation from accreting neutron stars

The formation of the high-energy gamma-ray and neutrino spectrum produced by the injection of a power-law proton distribution extending up to Ep = 10 exp 16 eV in the magnetosphere of accreting neutron stars is discussed. The resulting proton spectrum is calculated in the presence of advection and cooling by interactions with the X-ray spectrum of the neutron star, leading to several components of a secondary photon spectrum extending from the MeV to the PeV range, and neutrino spectrum extending from about 0.1 TeV to 0.1 PeV. The shapes of the resulting high-energy secondary spectra depend sensitively on the accreting source's X-ray spectrum and energy density as well as the magnetic field strength involved. Detailed spectra are calculated for the specific examples of Her X-1 and Cyg X-3.

Rudak, B.↗
DOE OSTI2020

Tuned and screened range-separated hybrid density functional theory for describing electronic and optical properties of defective gallium nitride

We apply a hybrid density functional theory approach, based on a tuned and screened range-separated hybrid (SRSH) exchange-correlation functional, to describe the optoelectronic properties of defective gallium nitride (GaN). SRSH and time-dependent SRSH (TDSRSH) are tuned to produce accurate energetics for the pristine material and applied to the study of a series of point defects in bulk GaN, a blue light-emitting material that degrades in the presence of defects. We first establish the accuracy of the method by comparing the predicted quasiparticle gap and low-energy excitation spectra of (TD)SRSH and many-body perturbation theory for both pristine GaN and GaN containing a single nitrogen vacancy. Aided by the reduced computational cost of (TD)SRSH, we then report on three additional technologically relevant point defects and defect complexes in GaN: the gallium vacancy, the carbon interstitial, and the carbon-silicon complex. We compute the low-energy optical absorption spectra for these defects and show the presence of defect-centered transitions. Furthermore, by estimating the Stokes shift, we predict, in agreement with previous studies, that the carbon substitutional defect is a candidate for the detrimental yellow luminescence in GaN. Furthermore, this study indicates that TDSRSH is a promising and computationally feasible approach for quantitatively accurate, first-principles modeling of defective semiconductors.

36 MATERIALS SCIENCE↗
NASA NTRS1976

Electron scattering by highly polar molecules. I - KI

Electron-impact energy-loss spectra of KI were studied experimentally in the 15- to 130-deg angular range at impact energies of 6.7, 15.7, and 60 eV. The spectra reveal a number of excitation features which have not been detected previously and indicate that KI is a strong photon absorber in the vacuum-UV region. From the spectra, differential and integral electronically elastic and inelastic cross sections have been obtained by normalizing the experimental data to theoretical results at low scattering angles. Rotational excitation cross sections corresponding to delta j = 0, plus or minus 1, have been calculated using a dipole-plus-repulsive-core interaction potential and the distorted-wave approximation. For comparison, the rotational excitation cross sections have also been calculated in the Born point-dipole approximation.

Rudge, M. R. H.↗
NASA NTRS1973

Primary electron spectrometer, 18:63 UE: Electrostatic analyzer description and energy spectrum determination

The primary electron spectrometer used to detect auroral electrons on sounding rocket 18:63 UE is described. The spectrometer used exponentially decaying positive and negative voltages applied to spherical deflection plates for energy analysis. A method for determining the analyzer response which does not require the assumptions that the ratio of plate separation to mean radius, the entrance or the exit apertures are small is described. By comparison with experiment it is shown that the effect of neither entrance nor exit collimation can be ignored. The experimental and calculated values of the limiting orbits agree well. A non-iterative technique of unfolding the electron differential energy spectrum is described. This method does not require the usual assumption of a flat or histogram-type energy spectrum. The unfolded spectra using both this technique and one which assumes a flat spectrum are compared to actual input spectra. This technique is especially useful in analyzing peaked auroral electron energy spectra.

Pongratz, M. B.↗
NASA NTRS1975

Angular distributions in the electron impact excitation of Xe at 20 eV

Electron impact energy-loss spectra of Xe have been studied at 20 eV incident electron energy at scattering angles from 5 to 135 deg. Differential and integral cross sections for elastic scattering and for the excitation of electronic states up to 10.60 eV energy loss have been determined. The validity of the optical selection rules and the previously established angular behavior associated with direct and exchange processes were examined for this intermediate coupling case.

Williams, W.↗
NASA NTRS1976

Photoabsorption spectrum of UF6 by electron impact

The photoabsorption cross section of UF6 in the wavelength region between 206.7 nm (2.5 eV) and 43.5 nm (28.5 eV) has been derived from the UF6 electron energy loss spectra at a 5-deg scattering angle and at incident electron energies of 75 and 100 eV. The shape of the resulting optical spectrum agrees well with available high-resolution photoabsorption measurements in the 400 nm (3.1 eV) to 200 nm (6.3 eV) region. Below 200 nm, where no photoabsorption data are available, there is strong absorption and appreciable structure present. Absolute values of absorption cross sections have been obtained by normalizing the present relative data to the recently measured photoabsorption value at 225.5 nm.

Srivastava, S. K.↗
NASA NTRS1990

HEAO 1 A-2 low-energy detector X-ray spectra of the Cygnus Loop

The Cygnus Loop supernova remnant was observed by the A-2 low-energy detector (LED) proportional counters on the HEAO 1 satellite. Recent improvements to the non-X-ray background rejection and detector response simulation have allowed production of the most accurate spectra of the Cygnus Loop to date. Three separate regions of the Cygnus Loop were observed. Single-temperature, Raymond-Smith models are inadequate to describe the spectra, but two component model fits are good. Temperature, column density, and emission measure variations across the Cygnus Loop are found. These results are interpreted and compared with previous work.

Leahy, D. A.↗
DOE OSTI2022

pnnl/EELSTM

EELSTM: Time Series Prediction on Electron Energy Loss Spectra - long short-term memory model approach to forecasting of electron energy loss spectroscopy, based on a Bayesian deep learning model

Doty, Christina↗
NASA NTRS1986

Electron-impact excitation of the a3Sigma(+)g, B1Sigma(+)u, c3Pi u, and C1Pi u states of H2

Normilized differential and integral cross sections for electron-impact excitation of the dipole-allowed B1Sigma(+)u, C1Pi u and dipole-forbidden a3Sigma(+)g, c3Pi u states of molecular hydrogen have been determined by analysis of energy-loss spectra obtained with a crossed-beam apparatus at electron-impact energies of 20, 30, 40 and 60 eV and scattering angles ranging from 10 to 120 degrees. Normalization of the data was achieved by utilizing elastic differential cross sections measured previously by (Khakoo and Trajmar, 1986). The cross sections are compared with other available theoretical and experimental data.

Khakoo, M. A.↗
NASA NTRS1999

Low-Energy Sputtering Studies of Boron Nitride with Xenon Ions

Sputtering of boron nitride with xenon ions was investigated using secondary ion (SIMS) and secondary neutral (SNMS) mass spectrometry. The ions generated from the ion gun were incident on the target at an angle of 50' with respect to the surface'normal. The energy of ions ranged from 100 eV to 3 keV. A flood electron gun was used to neutralize the positive charge build-up on the target surface. The intensities of sputtered neutral and charged particles, including single atoms, molecules, and clusters, were measured as a function of ion energy. Positive SIMS spectra were dominated by the two boron isotopes whereas BN- and B- were the two major constituents of the negative SIMS spectra. Nitrogen could be detected only in the SNMS spectra. The intensity-energy curves of the sputtered particles were similar in shape. The knees in P-SIMS and SNMS intensity-energy curves appear at around I keV which is significantly higher that 100 to 200 eV energy range at which knees appear in the sputtering of medium and heavy elements by ions of argon and xenon. This difference in the position of the sputter yield knee between boron nitride and heavier targets is due to the reduced ion energy differences. The isotopic composition of secondary ions of boron were measured by bombarding boron nitride with xenon ions at energies ranging from 100 eV to 1.5 keV using a quadrupole mass spectrometer. An ion gun was used to generate the ion beam. A flood electron gun was used to neutralize the positive charge buildup on the target surface. The secondary ion flux was found to be enriched in heavy isotopes at lower incident ion energies. The heavy isotope enrichment was observed to decrease with increasing primary ion energy. Beyond 350 eV, light isotopes were sputtered preferentially with the enrichment increasing to an asymptotic value of 1.27 at 1.5 keV. The trend is similar to that of the isotopic enrichment observed earlier when copper was sputtered with xenon ions in the same energy range.

Ray, P. K.↗
DOE OSTI2021

Application of machine learning for the estimation of electron energy distribution from optical emission spectra

Abstract This paper discusses the use of probabilistic deep neural networks for the prediction of the electron energy probability function in low-temperature non-thermal plasmas. The neural networks are trained using optical emission spectroscopy and Langmuir probe measurements, with the goal of providing a reliable estimate of the electron energy probability function solely from optical emission data. The performance of both non-Bayesian and Bayesian networks is evaluated. It is found that Bayesian models are preferable as they assign a higher level of uncertainty to their prediction especially when the dataset used to train them is small. This work describes one of the many potential applications of machine learning in plasma science and technology.

Physics↗
DOE OSTI2022

Calorimeter with Bayesian unfolding of spectra of high-flux broadband x rays

We report the development of a multipurpose differential x-ray calorimeter with a broad energy bandwidth. The absorber architecture is combined with a Bayesian unfolding algorithm to unfold high energy x-ray spectra generated in high-intensity laser–matter interactions. Particularly, we show how to extract absolute energy spectra and how our unfolding algorithm can reconstruct features not included in the initial guess. The performance of the calorimeter is evaluated via Monte Carlo generated data. The method accuracy to reconstruct electron temperatures from bremsstrahlung is shown to be 5% for electron temperatures from 1 to 50 MeV. We study bremsstrahlung generated in solid target interaction showing an electron temperature of 0.56 ± 0.04 MeV for a 700 μm Ti titanium target and 0.53 ± 0.03 MeV for a 50 μm target. We investigate bremsstrahlung from a target irradiated by laser-wakefield accelerated electrons showing an endpoint energy of 551 ± 5 MeV, inverse Compton generated x rays with a peak energy of 1.1 MeV, and calibrated radioactive sources. The total energy range covered by all these sources ranges from 10 keV to 551 MeV.

47 OTHER INSTRUMENTATION↗