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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 631 records · Page 35
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NASA NTRS1983

Electron-photon coincidence technique for the absolute calibration of VUV detectors

A method is described whereby VUV photon detectors can be accurately calibrated. This method is illustrated by taking the 58.4-nm transition of He as an example. The technique consists of crossing a monoenergetic electron beam with a beam of He atoms. When inelastically scattered electrons which have excited the 2 1P state are detected in coincidence with the 58.4-nm photons emitted in the decay of the excited state, the interaction volume formed by the crossed beams constitutes a standard source of photons. By comparing the number of detected coincidences with the predicted number the calibration can be made. A total detector efficiency of 0.024 + or - 0.003 is obtained for a Galileo 4830 channeltron.

Mcadams, R.↗
NASA NTRS1983

Absolute barotropic instability and monsoon depressions

Monsoon depressions over the Bay of Bengal develop almost entirely in July and August. After studies conducted by Lindzen et al. (1980) and Stevens and Lindzen (1978), only barotropic instability remains as a mechanism for the development of the wave disturbances associated with monsoon depressions. The present investigation has the objective to show that barotropic instability is able to explain the wave aspects of monsoon depressions, but that normal mode analysis is inadequate. It is found that the local barotropically unstable response to regional perturbations in the Bay of Bengal during July and August will be dominated by the lower troposphere. The analysis clearly identifies the features of the mean flow which lead to monsoon depressions in July. The features include the development of an easterly jet as opposed to semijet structure in the mean flow, and the development of a modest easterly flow at the jet center as opposed to westerly flow.

Lindzen, R. S.↗
NASA NTRS1983

The reaction NH2 + PH3 yields NH3 + PH2 - Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter

The rate constant is measured over the temperature interval 218-456 K using the technique of flash photolysis-laser-induced fluorescence. NH2 radicals are produced by the flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons is measured by multiscaling techniques. For each of the five temperatures employed in the study, the results are shown to be indepenent of variations in PH3 concentration, total pressure (argon), and flash intensity. It is found that the rate constant results are best represented for T between 218 and 456 K by the expression k = (1.52 + or - 0.16) x 10 to the -12th exp(-928 + or - 56/T) cu cm per molecule per sec; the error quoted is 1 standard deviation. This is the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to explain results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are then contrasted with those for the corresponding reactions of H and OH with PH3.

Bosco, S. R.↗
NASA NTRS1983

The reaction NH2 + PH3 yields NH3 + PH2: Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter

The rate constant is measured over the temperature interval 218-456 K using the technique of flash photolysis-laser-induced fluorescence. NH2 radicals are produced by the flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons is measured by multiscaling techniques. For each of the five temperatures employed in the study, the results are shown to be independent of variations in PH3 concentration, total pressure (argon), and flash intensity. It is found that the rate constant results are best represented for T between 218 and 456 K by the expression k = (1.52 + or - 0.16) x 10 to the -12th exp(-928 + or - 56/T) cu cm per molecule per sec; the error quoted is 1 standard deviation. This is the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to explain results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are then constrasted with those for the corresponding reactions of H and OH with PH3.

Bosco, S. R.↗
NASA NTRS1984

Absolute two-photon excitation cross-sections in NO

A technique has been developed which allows the determination of a two-photon absorption cross-section to be made relative to the Raman scattering cross-section in nitrogen. Spatial and temporal effects associated with the lasers are ratioed out to give a result independent of laser parameters. The necessary theory to extract a cross-section from the measured ratio of a Coherent Anti-Stokes Raman Scattering (CARS) signal in N2 to a four-wave mixing signal in NO has been developed. The technique has been demonstrated on the R(22) + S(12) (J-double-prime = 9 1/2) line in NO and a cross-section of (2.9 + or - 1.8) x 10 to the -49th (cm to the 4th power)-s was determined. This technique appears to be applicable to a number of other diatomic molecules.

Burris, J.↗
NASA NTRS1985

In-progress Absolute Radiometric Inflight Calibration of the LANDSAT-4 Sensors

Using selected instrumented areas at White Sands Missile Range, New Mexico as reference, radiometric calibration is to be effected on the sensors of LANDSAT 4, particularly the thematic mapper. Optical measurements made during a TM overpass are discussed. The radiances of selected large ground areas are measured in the spectral bandpasses of the TM; the total optical thickness of the atmosphere is measured in nine narrow spectral intervals. Ground truth in the form of reflectances collected for the alkalai flat region of gypsum and for the snow at White Sands is described.

Castle, K. R.↗
NASA NTRS1985

Absolute rigidity spectrum of protons and helium nuclei above 10 GV/c

Proton and helium nuclei differential spectra were gathered with a balloon borne magnet spectrometer. The data were fitted to the assumption that the differential flux can be represented by a power law in rigidity. In the rigidity range 10 to 25 GV/c the spectral indices were found to be -(2.74 plus or minus 0.04) for protons and -(2.71 plus or minus 0.05) for helium nuclei. A brief discussion is given by systematic errors.

Golden, R. L.↗
NASA NTRS1984

Spray characterization with a nonintrusive technique using absolute scattered light

A technique to measure the size and velocity of particles is discussed, and results are presented. In this technique two small laser beams of one color identify the center of a laser beam of a different color. This defines a region of almost uniform intensity where the light scattered by the individual particles can be related to their sizes. A variation of this technique that uses two polarizations of the same color of laser beam is also presented. Results are presented for monodisperse, bimodal, trimodal, and polydisperse sprays produced by the Berglund-Liu droplet generator and a pressure nozzle. Size distributions obtained at three different ranges for the same spray show excellent self-consistency in the overlapping regions. Measurements of a spray of known characteristics exhibit errors in the order of 10 percent.

Hess, C. F.↗
NASA NTRS1984

Absolute OH absorption cross sections (for lidar measurements)

Experimental data are employed to calculate absorption cross sections for several rotational lines in the OH A-X system. The cross sections are computed as functions of the spectral and line widths and temperature of a laser beam, and account for lifetimes and branching ratios. Detection limits for the P1(2) transition of the (1,0) band were examined. The oscillator strengths and cross-sections obtained are important for quantifying OH concentrations in the stratosphere from lidar return signals.

Mcgee, T. J.↗
NASA NTRS1985

Absolute isotopic abundances of Ti in meteorites

A general formalism is presented for determining possible isotope compositions of exotic Ti, and illustrated with results from Ti isotope abundances measured in Ca-Al-rich inclusions of the Allende and Leoville meteorites and samples of other meteorites. Previously identified correlations between Ti-50 and Ca-48 in FUN (isotope fractionation effects and unknown nucleosynthetic effects) inclusions is shown to hold true, thereby supporting contributions from a neutron-rich equilibrium or nucleosynthetic equilibrium during formation of the meteorites. Extension of the study to carbonaceous meteorites indicated that condensation, volatilization and recondensation occurred during formation, although the isotope fractionation observed cannot yet be explained.

Niederer, F. R.↗
NASA NTRS1985

Pressure dependence of the absolute rate constant for the reaction Cl + C2H2 from 210-361 K

In recent years, considerable attention has been given to the role of chlorine compounds in the catalytic destruction of stratospheric ozone. However, while some reactions have been studied extensively, the kinetic data for the reaction of Cl with C2H2 is sparse with only three known determinations of the rate constant k3. The reactions involved are Cl + C2H2 yields reversibly ClC2H2(asterisk) (3a) and ClC2H2(asterisk) + M yields ClC2H2 + M (3b). In the present study, flash photolysis coupled with chlorine atomic resonance fluorescence have been employed to determine the pressure and temperature dependence of k3 with the third body M = Ar. Room temperature values are also reported for M = N2. The pressure dependence observed in the experiments confirms the expectation that the reaction involves addition of Cl to the unsaturated C2H2 molecule followed by collisional stabilization of the resulting adduct radical.

Brunning, J.↗
NASA NTRS1985

Absolute response and noise equivalent power of cyclotron resonance-assisted InSb detectors at submillimeter wavelengths

Spectra are presented of the responsivity and noise equivalent power (NEP) of liquid-helium-cooled InSb detectors as a function of magnetic field in the range 20-110 per cm. The measurements are all made using a Fourier transform spectrometer with thermal sources. The results show a discernable peak in the detector response at the conduction electron cyclotron resonance (CCR) frequency for magnetic fields as low as 3 kG. The magnitude of responsivity at the resonance peaks is roughly constant with magnetic field and is comparable to the low-frequency hot-electron bolometer response. The NEP at the peaks is found to be comparable to the best long wavelength results previously reported. For example, NEP = 4.5 x 10 to the 13th W/(square root of Hz) at 4.2 K, 6 kG, and 40 per cm was measured. The InSb CCR will provide a much improved detector for laboratory spectroscopy, as compared with hot electron bolometers, in the 20-100 per cm range.

Brown, E. R.↗
NASA NTRS1985

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the Condor rocket campaign conducted from Peru in March 1983. This paper presents density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 x 10 to the 5th per cu cm at 106 km, with large scale fluctuations having amplitudes of roughly 10 percent seen only in the upward gradient in electron density. This is in agreement with plasma instability theory. It is further shown that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.

Baker, K. D.↗
NASA NTRS1984

Asymptotic (h tending to infinity) absolute stability for BDFs applied to stiff differential equations

Methods based on backward differentiation formulas (BDFs) for solving stiff differential equations require iterating to approximate the solution of the corrector equation on each step. One hope for reducing the cost of this is to make do with iteration matrices that are known to have errors and to do no more iterations than are necessary to maintain the stability of the method. This paper, following work by Klopfenstein, examines the effect of errors in the iteration matrix on the stability of the method. Application of the results to an algorithm is discussed briefly.

Krogh, F. T.↗
NASA NTRS1986

Comments on using absolute spectrophotometry of Wolf-Rayet stars

Garmany et al. (1984) have conducted a study involving spectrophotometric scans of 13 Wolf-Rayet stars. They have found that the application of a 'standard' reddening law to the observed data gives spurious results in many cases. They concluded also that previous attempts to determine the intrinsic continua and the effective temperatures of Wolf-Rayet stars are inadequate. In the present study the conclusions of Garmany et al. are evaluated. According to this evaluation, it has not been demonstrated by Garmany et al., beyond a reasonble doubt, that the interstellar extinction law varies greatly from Wolf-Rayet star to Wolf-Rayet star. The procedure followed by Garmany et al. to find the apparent shape of the ultraviolet continuum of a Wolf-Rayet star is unsatisfactory for a number of reasons.

Underhill, A. B.↗
NASA NTRS1986

Convective and absolute instability of a viscous liquid jet

The effect of viscosity on the capillary instability of a liquid jet is examined. The critical Weber number for convective instability is determined as a function of Reynolds number and comparison is made with the inviscid limit. It is shown that certain waves that are neutral in the inviscid case exhibit growth for finite Reynolds numbers.

Leib, S. J.↗