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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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

A spectral radiance comparison of a noise tube and a HgXe arc lamp between 60 GHz and 600 GHz

The relative spectral radiance of a noise tube, model TN-167, designed for the frequency range 90-140 GHz (3.3 mm to 2.1 mm) was compared to that from a 200-watt high pressure HgXe arc lamp over the wavelength region from 0.5 to about 5 mm. A Michelson Fourier transform spectrometer and a lamellar grating instrument were used in conjunction with liquid helium-cooled bolometers of NEP 10 to the -12th to 10 to the -14th watt/(Hz) exp 1/2 to measure relative spectral radiant power. With this instrumental arrangement, the noise tube exhibited a very sharp low frequency cutoff at about 2.2/cm. The HgXe arc lamp emitted more radiant power than the noise tube in the wavelength region below 3 mm (100 GHz) down to 0.5 mm. Above 3 mm, the noise tube had a stronger output. The noise tube spectral radiance shifted to lower frequencies when the input current was lowered from 125 mA to 50 mA.

Heaney, J. B.↗
NASA NTRS1988

Extremely low resistance nonalloyed ohmic contacts on GaAs using InAs/InGaAs and InAs/GaAs strained-layer superlattices

Employing a structure consisting of n(+)-InAs/InGaAs and InAs/GaAs strained-layer superlattices (SLSs) grown by molecular beam epitaxy on GaAs films, nonalloyed contact resistances less than 8.5 x 10 to the -8th ohm sq cm have been obtained. Self-consistent simulations show that these extremely small nonalloyed contact resistances are due to the suppression of the depletion depth in the GaAs channel and tunneling through the SLS layer. Similar structures on InGaAs channels have led to nonalloyed specific contact resistances of about 1.5 x 10 to the -8th ohm sq cm. These results represent the smallest figures reported for these important material systems.

Peng, C. K.↗
NASA NTRS1988

HEAO 1 hard X-ray observation of clusters of galaxies and intracluster magnetic fields

The results of HEAO 1 hard X-ray measurements of three clusters of galaxies, Abell 401, Abell 2255, and Abell 2256 are reported. Nonthermal components were not detected above the level of 10 to the -5th photons/sq cm/s/keV. Comparison of the flux upper limits with theoretical predictions yields lower limits of about 10 to the -7th gauss on the mean value of the intracluster magnetic fields in the central regions of these clusters.

Rephaeli, Yoel↗
NASA NTRS1988

Europium mass balance in polymict samples and implications for plutonic rocks of the lunar crust

The mean concentrations of Sm and Eu in the lunar surface crust were analyzed by correlating the Sm concentration and the Sm/Eu ratio with Th concentration obtained from published data on a large number of polymict samples from various locations in the lunar highlands, and using the value of 0.91 microg/g for the mean Th concentration of the highlands surface crust obtained by the orbiting gamma-ray experiments. The mean concentration of Sm in the lunar surface crust was found to be between 2 and 3 microg/g, and that of Eu between 0.7 and 1.2 microg/g. The results indicate that there is no significant enrichment or depletion of Eu, compared to Sm, relative to chondritic abundances; i.e., there is no significant 'Eu anomaly' in average upper crust, contrary to predictions by some earlier investigators.

Korotev, Randy L.↗
NASA NTRS1989

Time-resolved simplified chemical kinetics modelling using computational singular perturbation

A CO-CH4-air reaction system is used to demonstrate the computational singular perturbation (CSP) method for deriving time-resolved simplified chemical kinetics models. CSP provides a programmable algorithm to group the given collection of elementary reactions into reaction groups which are ordered according to their speed. The concept of Importance Index k(m)exp s is introduced: k(m)exp s is defined to be a number between 0 and 1 which measures the importance of the m-th reaction group to the s-th reactant and can readily be computed from data generated by CSP. It is suggested that the robustness of the solutions of the reaction system can be qualitatively assessed by inspecting the Importance Index data.

Lam, S. H.↗
NASA NTRS1989

Star-formation rates and forbidden O II emission in blue galaxies

Observations of forbidden O II 3727 A and H-beta emission-line strengths are presented from large-aperture spectrophotometric observations of nearby (z less than 0.1) optically blue galaxies covering a wide range in surface brightness and luminosity. The forbidden O II emission-line equivalent widths in these galaxies primarily lie between 30 and 100 A. Emission-line strengths in nearby blue galaxies thus resemble those found in faint galaxies at moderate-to-high redshifts. The most actively star-forming nearby galaxies are characterized by surface brightness in the forbidden O II line of about 10 to the -15th to 10 to the -14th ergs/s per sq cm per sq arcsec over spatial distances of more than 1 kpc. The fluxes of the forbidden O II and H-beta emission lines are well correlated, and thus forbidden O II luminosities can be used to derive approximate star-formation rates in blue galaxies.

Gallagher, J. S.↗
NASA NTRS1989

Significant variation from a 1/R-squared potential in the cumulative flux determined from the Giotto Comet Halley Dust Impact Experiment

The Dust Impact Detection System (DIDSY) aboard the Giotto spacecraft provided the information on the dust flux, mass spectrum, and cumulative mass distribution flux in the coma of Comet Halley. Analysis of discrete pulse height data of cometary particles for the mass range of particles between 4.0 x 10 to the -10th g and 6.0 x 10 to the -6th g registered by the Giotto DIDSY detectors 2, 3, and 4 has been completed, and a cumulative flux has been determined for this size range of particles. Inside the cometopause, anomalous peaks have been identified as deviation from a 1/R-squared curve in both pre- and postencounter measurements.

Alexander, W. M.↗
NASA NTRS1989

Infrared fluorescence efficiencies for the nu1 and nu5 bands of formaldehyde in the solar radiation field

Formaldehyde scattering strengths have been determined for equilibrium distributions of 100 K, 50 K, 20 K, and for the the non-LTE case of an essentially fully relaxed distribution. Integrated band g factors of 2.89 x 10 to the -4th photons/s per molecule for nu1 and 3.83 x 10 to the -4th for nu 5 are obtained. The results indicate that the most promising regions to search for cometary H2CO are at about 2782/cm, at 2794.5/cm, and at about 2835/cm.

Reuter, D. C.↗
NASA NTRS1989

The composition of comets

The present discussion of recent studies concerning cometary composition gives attention to the results obtained by in situ measurements of Comet Halley's dust composition by Vega 1's impact mass spectrometer, which discovered a mineral fraction that appears to be CI chondritic, as well as an organic fraction consisting of highly unsaturated hydrocarbons. The mineral fraction of comets appears to form a core that is embedded in essentially organic material; the spectroscopic invisibility of carbon is due to its presence in the cometary dust. The mass of most dust particles is found to be in the 10 to the -12th to 10 to the -14th g range. A considerable fraction of the dust grains serves as an extended source of gas in the inner coma.

Jessberger, E. K.↗
NASA NTRS1989

Analytical expressions for the Rosseland-mean opacity and electron scattering in stellar atmospheres

Simple analytical approximations for the calculations of the electron scattering coefficient, the Rosseland-mean opacity and the equation of state in the temperature range of 4000 to 50,000 K and the density range of 10 to the -13th to 10 to the -7th g/cu cm. These approximations are valid for a gas with 1, 0.1, and 0.01 times the solar metal abundances in LTE. The accuracy of the approximations is better than 5 percent over most of this temperature and density range.

Burger, P.↗
NASA NTRS1990

A scheme for parameterizing cirrus cloud ice water content in general circulation models

Clouds strongly influence th earth's energy budget. They control th amount of solar radiative energy absorbed by the climate system, partitioning the energy between the atmosphere and the earth's surface. They also control the loss of energy to space by their effect on thermal emission. Cirrus and altostratus are the most frequent cloud types, having an annual average global coverage of 35 and 40 percent, respectively. Cirrus is composed almost entirely of ice crystals and the same is frequently true of the upper portions of altostratus since they are often formed by the thickening of cirrostratus and by the spreading of the middle or upper portions of thunderstorms. Thus, since ice clouds cover such a large portion of the earth's surface, they almost certainly have an important effect on climate. With this recognition, researchers developing climate models are seeking largely unavailable methods for specifying the conditions for ice cloud formation, and quantifying the spatial distribution of ice water content, IWC, a necessary step in deriving their radiative characteristics since radiative properties are apparently related to IWC. A method is developed for specifying IWC in climate models, based on theory and measurements in cirrus during FIRE and other experiments.

Heymsfield, Andrew J.↗
NASA NTRS1989

Flight worthy infrared bolometers with high throughput and low NEP

This paper describes the features of flight-worthy IR bolometers that were developed to meet the requirements of the Far Infrared Absolute Spectrometer (FIRAS) and the Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer of a short time constant, high throughput, and low noise equivalent power (NEP). The new bolometers use small chips of doped and compensated silicon as sensing elements, and diamond wafers coated with thin layers of Cr and Au as the absorbing substrate. The throughput values for the FIRAS bolometers and for DIRBE are 1.28 sr-sq cm, and 0.21 sr-sq cm, respectively. At 1.6, the time constants range from 3 to 40 ms, and their NEP range from 4 x 10 to the -15th to 1 x 10 to the -14th W/sq rt of Hz.

Serlemitsos, Aristides T.↗
NASA NTRS1989

Latitudinal variations of ammonia in the atmosphere of Uranus - An analysis of microwave observations

A 2-cm wavelength VLA map of Uranus is found upon analysis to display strong limb darkening and a lack of symmetry about the subearth point, perhaps due to a latitudinally varying NH3 mixing ratio. The change in NH3 abundance occurs near -45 deg latitude, and the calculated NH3 molar mixing ratios in the 5-20 bar region average 4.9 + or - 0.7 x 10 to the -7th at -90 to -45 deg latitudes, and 1.3 + or - 0.4 x 10 to the -6th at -45 to -15 deg latitudes. The model is in agreement both with previous work indicating the depletion of ammonia relative to solar abundance and a preliminary analysis of 6-cm data.

Hofstadter, Mark D.↗
NASA NTRS1990

50-500 MeV observations of LMC supernova 1987A

Since the discovery of the supernova outburst in the LMC in 1987, two attempts (on day 55 and day 407) have been made to measure the high energy gamma-ray flux in the range 50-500 MeV, by using a balloon-borne spark chamber telescope. On day 55, no positive signal was seen from the source. A 3 sigma upper limit of 2.9 x 10 to the -5th ph/sq cm s was obtained after the analysis of the spark chamber data. Preliminary analysis of the quick look data obtained in the second flight shows that the gamma-ray flux even on day 407 was less than 9 x 10 to the -4th ph/sq cm s (3 sigma).

Sumner, T. J.↗
NASA NTRS1989

Pre-biotic organic matter from comets and asteroids

Only meteoritic fragments small enough to be gently decelerated by the atmosphere (10 to the -12th g to 10 to the -6th g) can deliver organic matter intact. The amount of such 'soft-landed' organic carbon can be estimated from data for the infall rate of meteoritic matter. At present rates, only about 0.0006 g/sq cm intact organic carbon would accumulate in 100 million years, but at the higher rates of about four billion yr ago, about 20 g/sq cm may have accumulated in the few hundred million years between the last cataclysmic impact and the beginning of life. It may have included some biologically important compounds that did not form by abiotic synthesis on earth.

Anders, Edward↗
NASA NTRS1989

Tracer versus trace element diffusion - Diffusional decoupling of Sr concentration from Sr isotope composition

Chemical diffusivities for nonalkali major, minor, and selected trace elements have been measured at 10 kbar, 1300 C during interdiffusion of dacitic and rhyolitic melts. Sr tracer diffusivities were measured at the same conditions in melts of dacitic and rhyolitic composition and in a diffusion couple of these two melts undergoing simultaneous chemical diffusion. Chemical diffusivities of all elements studied are functions of the silica content of the melt, but at any one silica content the chemical diffusivities of all elements studied are within a factor of five of the Si and Al diffusivities. Si diffusivity varies from 2.8 x 10 to the -9th sq cm/s at 65 percent SiO2 to 5.1 x 10 to the -10th sq cm/s at 75 percent SiO2. The diffusivities of minor and trace elements are proposed to be controlled by the ability of the local melt structure to accommodate these elements while maintaining LTE. The measured Sr tracer diffusivities are an order of magnitude above Sr chemical diffusivities.

Baker, Don R.↗
NASA NTRS1990

Limits on the antiproton/proton ratio in the cosmic radiation from 100 MeV to 1580 MeV

A search for antiprotons (p-bars) in the cosmic radiation with energies below 1580 MeV at the top of the atmosphere was performed using the PBAR balloon-borne magnetic spectrometer. No antiprotons were observed in 124,000 proton events. For the energy interval 100-640 MeV, an upper limit is reported to the p-bar/p ratio of 2.8 x 10 to the -5th at the top of the atmosphere, after correcting for instrumental efficiencies and contributions from secondary particles. No antiproton was observed in the energy interval 640-1580 MeV, which yields an upper limit to the p-bar/p ratio of 6.1 x 10. By combining both data sets, the limits on the p-bar/p ratio can be improved to 2.0 x 10 to the -5th. The detector performance and instrumental efficiencies of the individual detector components are discussed. A detail Monte Carlo calculation was used to evaluate the instrumental efficiency for both antiprotons and protons as a function of momentum.

Salamon, M. H.↗
NASA NTRS1990

A search for anisotropy in the cosmic microwave radiation at medium angular scales

The results are reported from a search for anisotropy in the cosmic microwave radiation (CMR) at angular scales of 0.5 to 5 deg. No temperature fluctuations are detected, and limits are placed on intrinsic CMR anisotropy. These limits are expressed in terms of two models which describe the temperature fluctuations. For models specified by a Gaussian-shaped correlation function, the fluctuation amplitude is found to be 1.1 x 10 to the -4th or less at 1.1 deg; for models with monochromatic fluctuations it is 7.5 x 10 to the -5th or less at 1.7 deg. These limits satisfy the likelihood ratio test at the 95 percent confidence level. The implications of the results for large-scale structure information scenarios are discussed.

Timbie, Peter T.↗