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At least 757 records · Page 42

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NASA NTRS1978

Effects of proton irradiation on a gas phase in which condensation takes place. I Negative Mg-26 anomalies and Al-26

In the present paper, isotopic effects in magnesium generated in a proton-irradiated gas phase are examined, taking only (p,n), (p,d), and (p, alpha) reactions in magnesium, aluminum, and silicon into consideration. In the presence of proton radiation, the three elements are 'removed' from the gas phase by condensation. It is required that a value of Al-26/Al-27 greater than 6 times 10 to the -5th must be reached, consistent with the value deduced by Lee Papanastassiou, and Wasserburg (1976) from their studies of the Allende meteorite. The calculations show that fast aluminum condensation reduces the required proton fluence substantially, that a significant fraction of aluminum remains uncondensed when the above value of the Al-26/Al-27 ratio is reached, that a detectable MG-24 excess is very likely to occur, that detectable negative MG-28 anomalies can be generated, and that proton fluxes and irradiation times can be varied simultaneously, and over a wide range of values, without significant changes in the required proton fluence.

Heymann, D.↗

NASA NTRS1978

New evidence on the nature of the soft X-ray source in AM Herculis from HEAO 1

Results are reported for scanning observations of AM Herculis from HEAO 1 during the period September 22-October 12, 1977. The soft X-ray light curve constructed from the observations shows considerable scatter in intensity, together with clear evidence for a residual flux during X-ray minimum. At X-ray maximum the flux in the energy range 0.15-0.5 keV is typically 8 x 10 to the -10th power erg/sq cm per sec. The spectrum of AM Her is characterized by a sharp turnover at 0.2 keV and is well described by a blackbody model with absorption. There is no appreciable change in the spectrum with binary phase. The present observations, when combined with theoretical models, imply that the soft X-rays are produced by the heated surface of the degenerate dwarf component, and that the surface has a temperature of approximately 400,000 K and a luminosity about 30 times that observed at optical or hard X-ray wavelengths. AM Her is predicted to be a spectacular UV source, with an intrinsic UV luminosity comparable to that in soft X-rays.

Tuohy, I. R.↗

NASA NTRS1978

Ultraviolet observations of cool stars. VIII - Interstellar matter toward Procyon

The profile of the chromospheric L-alpha emission line of the F5 IV-V star Procyon (Alpha CMi, d = 3.5 pc) has been measured using the high-resolution Princeton spectrometer aboard NASA's Copernicus satellite. L-alpha absorption lines of interstellar deuterium and hydrogen are distinctly present. The average number density of interstellar hydrogen along the line of sight is found to be 0.11 + or - 0.02 per cu cm, similar to the densities that have been found in the directions of the stars Epsilon Eri, Epsilon Ind, and Alpha Cen A. These stars are all within 3.5 pc of the earth. The ratio of deuterium to hydrogen in the direction of Procyon is found to be 1.3 (+1.2, -0.5) x 10 to the -5th.

Anderson, R. C.↗

NASA NTRS1978

The abundance of boron in B- and A-type stars

Results are reported for Copernicus observations of the 1362.46-A resonance line of B II in 16 normal B and A stars. An average for the stars in which a boron abundance has been determined yields a B/H abundance ratio of 2 x 10 to the -10th; this value is taken to be the cosmic abundance of boron. The observed abundances are compared with those predicted by the galactic-cosmic-ray (GCR) spallation theory for the 'best fit' GCR flux, and remarkably good agreement is obtained. The value derived for the cosmic boron abundance is shown to support the theory that spallation reactions by high-energy GCRs on atoms of C, N, and O in the interstellar gas are responsible for the origin of B and Be as well as at least some Li. It is noted that the meteorite boron abundance is an order of magnitude greater than the cosmic abundance derived from stars, which implies an anomalous local enrichment in boron and possibly in the B-11/B-10 ratio during the formation of carbonaceous chondrites.

Boesgaard, A. M.↗

NASA NTRS1978

Abundances and spectra for cosmic-ray nuclei from lithium to iron for 2 to 150 GeV per nucleon

Measurements of primary cosmic-ray composition and energy spectra made with a balloon-borne superconducting magnetic spectrometer are described. Results for both 6.7 g/sq cm equivalent vertical atmospheric depth and the top of the atmosphere are presented for the absolute and relative integral abundances, the differential energy spectra, and the spectral indices for cosmic-ray nuclei from Li to Fe and energies of 2 to 50 GeV/n. It is found that propagation effects can explain essentially all the elemental abundances, that the abundances of Li, Be, and B for rigidities below 10 GV/c are consistent with an energy-independent mean interstellar path length of 4.5 + or - 0.5 g/sq cm for the 'leaky box' propagation model, that the abundances of all elements above 10 GV/c are consistent with a path length that decreases as the inverse n-th power of rigidity, and that n equals 0.6 (+0.4, -0.3) for the simplest assumptions made in fitting the source spectra.

Orth, C. D.↗

NASA NTRS1978

Hard X-rays from U Geminorum

The A-2 experiment on HEAO 1 has discovered that the dwarf nova U Geminorum can be a weak source of hard X-rays at optical maximum. During the outburst in October 1977, when a strong soft X-ray flux was discovered, a 2-10-keV flux was also observed. Evidence is presented that this flux appeared with the rise to optical maximum, about half a day before the soft X-rays appeared, and persisted about 2 days. During these first 2 days of the outburst the average flux was 2.4 x 10 to the -11th erg/sq cm per sec, corresponding to a luminosity of 2.8 x 10 to the 31st erg/s for a distance of 100 pc and an absorbed thin thermal spectrum with kT of about 5 keV. During optical minimum an OSO 8 upper limit implies a luminosity less than 10 to the 31st erg/s. Restrictions on models of U Geminorum are discussed.

Swank, J. H.↗

NASA NTRS1979

Electrotransport purification of thorium under low pressure conditions

It was demonstrated that the ultrapurification of refractory metals by electrotransport refining could best be accomplished in the noncontaminating environment of an orbiting low density materials laboratory such as the Molecular Shield Device. Refining experiments were performed at 2 x 10 to the -12th Torr which resulted in the preparation of small quantities of the World's purest thorium metal. The microgravity occurring in orbit was simulated electromagnetically and shown to be advantageous in eliminating grain-sliding caused by the plastic deformation of the sample at high temperature. The electrotransport sample assembly was tested in several environments including simulated solar irradiation, coldness and darkness and under various pressure conditions. Ultrapure single crystals of alpha thorium were also prepared and characterized. Laboratory electronics for the experiment were developed and a totally automatic control system was used to heat the specimens.

Schmidt, F. A.↗

NASA NTRS1979

Properties and performance of fine-filament bronze-process Nb3Sn conductors

Fine-filament bronze-process Nb3Sn superconductors were fabricated to fulfill stringent requirements on critical current under tension in a one-inch bend diameter, current density, stability, and conductor losses. A large fraction of niobium filaments was incorporated in a bronze matrix, and a cable containing 4453 2-micron diameter filaments in each of six strands provided a current density of 85,000 A/sq cm at 10 to the -11th ohm-cm, 10 T, and 4.2 K. The next step was to incorporate copper stabilizer in a conductor with 20,538 filaments. These conductors, fabricated so that bend strains of about 1% would be experienced by the filaments in the one-inch bend, stably carried currents in the 200-500 A range. A quench current of 540 A in a 5-cm bend corresponds to 158,000 A/sq cm in the bronze and filament area, believed to be the highest critical current density reported for multifilamentary Nb3Sn.

Walker, M. S.↗

NASA NTRS1978

The center-to-limb

Center-to-limb measurements of the Ca I 6573 intercombination line and the Ca II 7324 forbidden line are compared with synthetic profiles based on a simple representation of the non-LTE Ca-Ca(+) ionization equilibrium. The effects of photoionizations from low lying excited states of neutral calcium are found to reduce the sensitivity of the 6573 center-to-limb behavior as a thermal structure diagnostic. The synthetic center-to-limb behavior is also sensitive to uncertainties in the nonthermal broadening. Nevertheless, the measured center-to-limb behavior of 6573 favors a 'cool' photospheric model over hotter models based on the Ca II K wings. The non-LTE calcium abundance obtained from the disk center equivalent widths of 6573 and 7324 using the best fit model is A(Ca) = 2.1 plus or minus 0.2 x 10 to the -6th (by number relative to hydrogen).

Source record↗

NASA NTRS1978

Low frequency spectra of type III solar radio bursts

Flux density spectra have been determined for 91 simple type III solar bursts observed by the Goddard Space Flight Center radio astronomy experiment on the IMP-6 spacecraft during 1971 and 1972. Spectral peaks were found to occur at frequencies ranging from 44 kHz up to 2500 kHz. Half of the bursts peaked between 250 kHz and 900 kHz, corresponding to emission at solar distances of about 0.3 to 0.1 AU. Maximum burst flux density sometimes exceeds 10 to the -14th W/sq m/Hz. The primary factor controlling the spectral peak frequency of these bursts appears to be a variation in intrinsic power radiated by the source as the exciter moves outward from the sun, rather than radio propagation effects between the source and IMP-6. Thus, a burst spectrum strongly reflects the evolution of the properties of the exciting electron beam, and according to current theory, beam deceleration could help account for the observations.

Weber, R. R.↗

NASA NTRS1979

Further observational support for the lossy radial diffusion model of the inner Jovian magnetosphere

A mathematical model describing radial diffusion, with violation of the third adiabatic invariant and with local losses, is applied to the Pioneer 10 and 11 observations of omnidirectional integral intensities of electrons with energy exceeding 21 MeV. Local losses outside L = 3 are much stronger than synchrotron losses but weaker than the losses one would expect for strong pitch angle diffusion. If radial diffusion is driven by ionospheric winds, as was suggested by Brice and McDonough (1973), it is found that the theoretical solution which best fits the data requires that the radial diffusion coefficient equals 3 x 10 to the -9th L-cubed/s and the lifetime against local losses is approximately a few million seconds.

Goertz, C. K.↗

NASA NTRS1979

Rotationally excited HD toward Zeta Ophiuchi

Copernicus satellite measurements of HD in J-double prime = 1 and J-double prime = 0 toward Zeta Oph are reported. The ratio of the number densities of HD in the J = 0 and J = 1 states is determined to be 0.15 + or - 0.02 at the 1-sigma level. A value of approximately 24 x 10 to the -17th erg/cu cm per A at 1000 A is obtained for the UV energy density at the Zeta Oph cloud, and the mechanisms for excitation of HD are examined. A tight upper limit is derived for the abundance of HCl, which has been predicted to be present due to the interaction of ionized chlorine with neutral hydrogen. A calculation is performed which indicates that the cloud is 28 pc from the star. It is shown that the two-component cloud model of Black and Dalgarno (1977) with densities of 500 and 2500 H nuclei per cu cm for the outer regions and core, respectively, is in excellent agreement with the observations.

Wright, E. L.↗

NASA NTRS1979

Stark broadening of Fe I 5383 A

Results are reported for a laboratory measurement of the electron broadening of the Fe I line at 5383 A as a function of electron density over the temperature range from 8000 to 12,000 K. A linear Z-pinch electrical discharge served as the spectroscopic light source in the experiment, and special precautions were taken to allow for optical-depth effects, which have been a major source of error in previous investigations. The results obtained include a Stark-broadening constant of 0.106 + or - 0.004 x 10 to the -16th A-cu cm at a mean temperature of 9500 K, an estimated Stark width/electron-broadened width ratio of 1.09 + or - 0.02, and a normalized electron-broadened width of 0.097 + or - 0.004 x 10 to the 16th A-cu cm. Comparison of the last result with a calculated value and with a previous measurement indicates that the present measurement is approximately 38% larger than the value measured earlier.

Freudenstein, S.↗

NASA NTRS1979

Evidence for a massive X-ray halo around Markarian 541

Mkn 541, a Type I Seyfert galaxy, has been detected as an X-ray source with the rotating modulation collimator on SAS 3. The galaxy, which is identified as the optical counterpart of 4U 2351+06, has an X-ray flux of 3 x 10 to the -11th erg/sq cm per sec, which corresponds to a luminosity of 2 x 10 to the 44th erg/s at 240 Mpc. Evidence is presented that the X-rays are emitted in a volume of diameter 150 kpc. A variety of possible emission mechanisms are investigated, and it is suggested that the X-rays are caused by thermal bremsstrahlung from 500 billion solar masses of hot gas trapped in the gravitational well of Mkn 541, which implies that the mass of the galaxy exceeds 5 trillion solar masses. The hot gas can be explained as the accumulated ejecta of the Seyfert nucleus.

Cash, W.↗

NASA NTRS1979

Polycrystalline silicon on tungsten substrates

Thin films of electron-beam-vaporized silicon were deposited on fine-grained tungsten substrates under a pressure of about 1 x 10 to the -10th torr. Mass spectra from a quadrupole residual-gas analyzer were used to determine the partial pressure of 13 residual gases during each processing step. During separate silicon depositions, the atomically clean substrates were maintained at various temperatures between 400 and 780 C, and deposition rates were between 20 and 630 A min. Surface contamination and interdiffusion were monitored by in situ Auger electron spectrometry before and after cleaning, deposition, and annealing. Auger depth profiling, X-ray analysis, and SEM in the topographic and channeling modes were utilized to characterize the samples with respect to silicon-metal interface, interdiffusion, silicide formation, and grain size of silicon. The onset of silicide formation was found to occur at approximately 625 C. Above this temperature tungsten silicides were formed at a rate faster than the silicon deposition. Fine-grain silicon films were obtained at lower temperatures.

Bevolo, A. J.↗

NASA NTRS1978

H Lyman-alpha emission from photodissociation of H2O

Lyman-alpha fluorescence measurements were used to study the photodissociation of water vapor to produce H(n equals 2) atoms from threshold to 500 A. The experimental threshold of 813.0 A nearly coincides with the thermodynamic limit. Excitation cross sections rise slowly from threshold to a maximum value of (1.55 plus or minus 0.12) x 10 to the -18th sq cm at about 709 A and decrease to a negligible value below 550 A. Structure in the excitation function is attributed to predissociation of Rydberg series converging to the B(overbar)B2 state of H2O(plus).

Mentall, J. E.↗

NASA NTRS1979

Interstellar carbon dioxide

The possibility is considered that the reaction CO + OH yields CO2 + H may lead to a significant concentration of CO2 in dense clouds where both CO and OH have been found to be present. Experimental measurements and observational data are discussed which indicate a formation rate of 8 x 10 to the -15th per cu cm/s for CO2 by the cited reaction under conditions typical of dense interstellar clouds. Other CO2 formation mechanisms and some loss processes for CO2 are examined. It is concluded that an appreciable concentration of CO2 produced by the cited reaction is possible in dense clouds.

Prasad, S. S.↗

NASA NTRS1979

Implications of general covariance and maximum four-dimensional Yang-Mills gauge symmetry

General covariance and maximum four-dimensional Yang-Mills gauge symmetry lead to these results: (1) gravity is characterized by a dimensionless constant F of the order of 10 to the -19th; (2) the Newtonian force is always attractive; (3) space-time has a torsion; and (4) gravitational spin-force between two protons is about 10 to the 19th times stronger than the corresponding Newtonian force. A possible experimental test is discussed.

Hsu, J. P.↗