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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Fragmentation of Fe nuclei on carbon, hydrogen and CH2 targets. I - Individual charge changing and total cross sections. II - Isotopic cross sections

The fragmentation of Fe nuclei in carbon and CH2 targets at energies of 650, 800, and 950 MeV/nuc has been studied. Direct measurements of the interaction between Fe nuclei and the carbon and CH2 targets were carried out. The Fe to H charge changing cross sections derived for selected nuclei showed a clear energy dependence, consistent with the results of Westfall et al. (1979). Values for the total inelastic cross section Fe to H charge changing were obtained by two methods: the first method involved subtracting the total CH2 and C cross sections (values of 703 plus or minus 9 mb and 65 plus or minus 10 mb at 950 MeV/nuc and 650 MeV/nuc, respectively were obtained). The same cross sections were obtained by adding the individual charge changing cross sections. A cross section for Fe to fragment into lower mass Fe isotopes of 65 plus or minus 5 mb was also observed, giving total inelastic mass changing cross sections of 768 plus or minus 11 mb and 716 plus or minus 12 mb, respectively. In addition, the individual isotopic cross sections for Z equals 16-25 are reported.

Brautigam, D. A.↗
NASA NTRS1982

Abundances of cosmic ray nuclei heavier than 50 Sn

Preliminary results are reported from 430 days of exposure of the heavy nuclei experiment on the HEAO-3 spacecraft. These results are confined to the heavy nuclei with Z equal to or greater than 50 and emphasize the conclusions obtained on the relative numbers of actinides and heavy stable elements in the lead-platinum region. The extreme paucity of actinides found is inconsistent with the predictions of a cosmic ray source that is highly enriched in r-process material, but quite consistent with a source whose composition is similar to that of normal solar system material. An upper limit, at the 95 percent confidence level, is placed in the ratio of nuclei with Z equal to or greater than 88/(Z in the range from 74 to 87) of 0.03.

Waddington, C. J.↗
NASA NTRS1983

The study of the physics of cometary nuclei

On the basis of the icy conglometate model of cometary nuclei various observations demonstrate the spotted nature of many or most nuclei, i.e., regions of unusual activity, either high or low. Rotation periods, spin axes and even precession of the axes have been determined. Narrow dust jets near the nuclei of some bright comets require that small sources be embedded in larger active areas. Certain evidence suggests that very dusty areas and very dusty comets may be less active, respectively, than surrounding areas or other comets.

Whipple, F. L.↗
NASA NTRS1984

Ultraviolet and X-ray observations of active galactic nuclei: Constraints on models of the broad emission line region

Many active galactic nuclei, in particular quasars and Seyfert I's, are characterized by strong, broad emission lines from permitted transitions in abundant elements. In the simplest models, these lines ofriginate in clouds located at some characteristic distance from the central ionizing source in a quasi-spherical distribution. Ultraviolet and X-ray spectroscopic data gathered from the International Ultraviolet Explorer (IUE), HEAO 1, and HEAO 2 are utilized herein to further define and refine the photoionization models of the broad emission line region (BLR) of these galactic nuclei. Parameters including X-ray spectra, X-ray absorption, ultraviolet spectra, hydrogen cloud geometry, cosmic dust, cloud metallicity, and continuous spectra are considered. The luminosity of galactic nuclei is also discussed.

Mushotzky, R. F.↗
NASA NTRS1985

On the retention of high-energy protons and nuclei with charges Z or equal to 2 in large solar flares after the process of their acceleration

Data which suggest that the protons with energies of up to several GeV should be retained on the Sun after the process of their acceleration are presented. The protons are on the average retained for 15 min, irrespectively of the solar flare heliolatitude and of the accelerated particle energy ranging from 100 MeV to several GeV. It is suggested that the particles are retained in a magnetic trap formed in a solar active region. No Z or = 2 nuclei of solar origin during large solar flares. The absence of the 500 MeV/nucleon nuclei with Z or = 2 may be due to their retention in the magnetic trap which also retains the high-energy protons. During the trapping time the approx. 500 MeV/nucleon nuclei with Z or = 2 may escape due to nuclear interactions and ionization loss.

Volodichev, N. N.↗
NASA NTRS1985

Cross sections for the production of fragments with Z greater than or equal to 8 by fragmentation of Z greater than or equal to 9 and less than or equal to 26 nuclei

Charge changing nuclear collisions in plastic nuclear track detectors were studied using a new experimental technique of automatic track measurement for etched tracks in plastic detectors. Partial cross sections for the production of fragments of charge Z approximately 8 were measured for projectile nuclei of charge 9 approximately Z approximately 26 in the detector material CR39 and in silver. for this purpose three independent experiments were performed using Bevalac beams. The first one was an exposure of a stack of CR39 plastic plates to 1.8 GeV/nucl. Ar-40 nuclei. The second one was an exposure of another CR39 stack of 1.7 GeV/nucl. Fe-56 projectiles. In the third experiment a mixed stack of CR39 plates and silver foils was irradiated with 1.7 GeV/nucl. Fe-56 nuclei. Thus the measurement of nuclear cross sections in a light target (CR39 = C12H18O7) and as well in a heavy target (silver) was possible.

Heinrich, W.↗
NASA NTRS1983

Electronic detection of ultra-heavy nuclei by pyroelectric materials

A recent prediction by the authors that pyroelectric materials may be capable of detecting ultra-heavy nuclei has been confirmed. Charge pulse signals from pyroelectric crystals of lithium tantalate exposed to Au ions and a pulsed beam of Ni-58 ions, and from pyroelectric films of polyvinylidene fluoride exposed to a pulsed beam of Ni-58 ions, have been measured using pulse electronics with time constants in the microsecond range. These studies show that pyroelectric materials, in general, are capable of detecting incident nuclei having very high mass and charge. In particular, pyroelectric polymers, such as polyvinylidene fluoride, are readily available as inexpensive flexible films. This new class of charged particle detector could eventually find applications in large-area experiments for detection and trajectory determination of low-energy, ultra-heavy nuclei.

Simpson, J. A.↗
NASA NTRS1983

The non-Z-squared response of the heavy nuclei cosmic ray detector on HEAO-3

A combination of ion chambers and Cerenkov radiators similar to the Heavy Nuclei Experiment flown on HEAO-3 was calibrated at the Bevalac heavy-ion accelerator using beams of Mn-25 nuclei at kinetic energies up to about 1700 MeV/nucleon and Au-79 nuclei up to about 1000 MeV/nucleon. The data show only a small deviation (about 2-3 charge units at Au) from the Z-squared scaling used previously (Binns et al., 1981, 1982, 1983) to analyze the HNE data. Although at lower energy, the calibration indicates that the published relative abundances of the Sn-50/Ba-56 group and the published upper-limit actinide abundances are not likely to be significantly affected by non-Z-squared effects.

Garrard, T. L.↗
NASA NTRS1985

Short-lived nuclei in the early solar system

An overview is given of the existence and abundance of short-lived nuclei that were present in the early solar system. It is shown that there are excesses of certain isotopes that may be assigned to the decay of short-lived radioactive nuclei corresponding to an abundance of about 0.0001 relative to a neighboring stable isotope. This abundance is roughly similar to the level of general nonlinear isotopic anomalies present in meteoritic material (excluding oxygen) which are not attributable to short-lived nuclei. If these results are representative of the bulk solar system, they indicate that about 0.0001 solar mass of freshly synthesized material of peculiar composition was added to the protosolar nebula as it was formed from the interstellar medium. This material must have been synthesized about three million yr before the formation of the solar system.

Wasserburg, G. J.↗
NASA NTRS1985

UH-nuclei in the cosmic radiation, present and future

A brief survey of the present state of knowledge on the composition of the heaviest nuclei in the cosmic radiation, with emphasis on the actinides, is followed by a similarly brief description of expected near term improvements. A description is given of a large modular array which could be built and serviced from the Space Platform and would be capable of making critical tests on the various models of the origin and acceleration of these nuclei. This array would be composed of plastic Cerenkov detectors and would also measure time-of-flight across the array for each particle. In the configuration proposed it would observe two or more orders of magnitude more nuclei than any current or near future array.

Waddington, C. J.↗
NASA NTRS1986

Observational determination of the masses of active galactic nuclei

The masses of 24 active galactic nuclei are determined on the basis of published observations, using data on both the X-ray variability and the width of the narrow forbidden lines of O III. The results are presented in tables and graphs, and it is found that the mass estimates based on the X-ray variability are in good agreement with the dynamic masses determined from the O III linewidth when reasonable values are selected for the radius of the region of X-ray origin and when a standard photoionization model is adopted for the line-emitting gas. The O III dynamic masses of Seyfert-2 nuclei are shown to be systematically lower than those predicted by the (delta t)/M(O III) correlation for Seyfert-1 nuclei.

Wandel, A.↗
NASA NTRS1986

Near-ultraviolet spectroscopy of Seyfert nuclei - Reddening and Bowen fluorescence

Spectra with high signal-to-noise ratios extending down to the atmospheric ultraviolet cut-off have been obtained for 11 Seyfert nuclei with narrow emission lines. In all objects the He II lines at 3204 A and 4686 A were measured simultaneously. Their ratio gives an estimate of reddening accurate to 0.14 mag in E(B-V). The redshifts of nine of the Seyfert galaxies are sufficient to allow detection of the O III 3133 line A, produced by Bowen fluorescence of He II Ly-alpha photons. The inferred efficiency of the Bowen mechanism ranges from 15 to 50 percent. The lower values, which are considerably less than the efficiencies found in planetary nebulae, are observed in Seyfert 2 nuclei with systematically weaker emission lines from high-excitation, high-density gas. In contrast, the Seyfert 1 and high-excitation Seyfert 2 nuclei have more 'normal' Bowen efficiencies. Several possible explanations are considered.

Malkan, Matthew A.↗
NASA NTRS1988

Production of the diffuse X-ray background spectrum by active galactic nuclei

This paper proposes a class of models in which the intensity and spectrum of the X-ray background radiation between 0.2 and 160 keV can be produced by active galactic nuclei. The key assumption of these models is that the spectra of active galactic nuclei are not described by a single power law. No evolution of the X-ray spectra of active galactic nuclei is required.

Schwartz, D. A.↗
NASA NTRS1988

Relativistic beaming and the nuclei of double-lobed radio quasars

A complete sample of double-lobed 3CR quasars with minimal orientation bias is defined. Properties of these objects on the greater than kpc scale are consistent with the simple beaming model for their nuclei. VLBI maps of six nuclei reveal 'core-jet' structures. Both nuclei mapped at multiple epochs are modestly superluminal. The distribution of apparent velocities in this sample will provide a stringent test of beaming.

Hough, D. H.↗
NASA NTRS1989

Balloon-borne measurements of aerosol, condensation nuclei, and cloud particles in the stratosphere at McMurdo Station, Antarctica, during the spring of 1987

Measurements of the vertical profile of particles with condensation nuclei counters and eight channel aerosol detectors at McMurdo Station, Antarctica, in 1987 verified observations made in 1986 concerning the absence of upwelling in the polar vortex and the presence of a condensation nuclei layer in conjunction with the ozone hole region. New observations of a bimodal aerosol size distribution, consisting of a large-particle mode mixed in with the small-particle sulfate mode, at temperatures below -79 C are consistent with the presence of nitric acid-water particles at low concentrations. Higher concentrations of large particles were observed in association with nacreous clouds. An unusual particle layer which contained enhanced concentrations of both the small-particle (sulfate) mode and the large-particle (nitric acid) mode was detected at temperatures below -85 C, suggesting simultaneous nucleation and growth phenomena. The vortex condensation nuclei layer was observed to form at the same time as the ozone hole, indicating that formation of the layer is triggered by photochemical processes and may be important in controlling ozone depletion above 22 km.

Hofmann, D. J.↗
NASA NTRS1990

Comet nuclei and Trojan asteroids - A new link and a possible mechanism for comet splittings

Relatively elongated shapes, implied by recent evidence of a greater incidence of high amplitude lightcurves for comet nuclei and Trojan asteroids than for similarly scaled main belt asteroids, are suggested to have evolved among comet nuclei and Trojans due to volatile loss. It is further suggested that such an evolutionary course may account for observed comet splitting; rotational splitting may specifically occur as a result of evolution in the direction of an elongated shape through sublimation. Supporting these hypotheses, the few m/sec separation velocities projected for rotationally splitting elongated nuclei are precisely in the observed range.

Hartmann, William K.↗
NASA NTRS1992

Modelling, simulation and sounding of cometary nuclei; Proceedings of the International Workshop, Cagliari, Italy, May 28-30, 1991

The present workshop on the modeling, simulation, and sounding of cometary nuclei discusses the Rosetta comet-nucleus sample return mission, the importance of comet models in the design of the Rosetta mission, the challenge of sample acquisition in a cometary environment, and a simulation of Rosetta on-comet operations. Attention is given to HST observations of the inner coma of Comet Levy 1990c, cometary coma particulate modeling for the Rosetta mission aphelion rendezvous, recent gas flux modeling and applications of cometary nuclei, and the 3D time-dependent modeling of comet nucleus surface layers. Topics addressed include models of cometary nuclei and future space mission, the composition of cometary materials, experiments with cometary analogs at the DLR, and crust formation in cometary analogs. Also considered are gaseous products from VUV photolysis of cometary solids, cosmic ray modification of organic cometary matter as simulated by cyclotron irradiation, and KOSI comet simulation experiments.

Source record↗
NASA NTRS1992

Rotation of split cometary nuclei

A simple model for the rotational motion of split cometary nuclei is studied. A large-amplitude precession is easily excited due to the change of the moments of inertia even if the perturbation is small at the splitting. The damping timescale of the excited precession is widely ranged because of the uncertainty of the physical parameter of cometary nuclei. Another possibility for clarifying the evolution of the short period comets by studying the split cometary nuclei is also discussed.

Watanabe, Jun-Ichi↗