Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Isotopic”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

A mass-independent sulfur isotope effect in the nonthermal formation

A nonmass-dependent sulfur isotope effect is present in the rotationally symmetric S2F10 molecule, produced in an electrical discharge through sulfur tetrafluoride. A similar isotopic fractionation was observed in the product S2F10 from the electrodissociation of SF5Cl and in the reaction between fluorine atoms produced by F2 photolysis and SF2, collectively ruling out the SF5 formation process as the source of the mass-independent fractionation. The secondary dissociation of S2F10 as a source of the mass-independent fractionation is ruled out by control S2F10 dissociation experiments which are shown to produce small mass-dependent fractionations. Mass-dependent effects such as sulfur isotopic exchange and secondary dissociation reactions are significant processes for the system under study, and have been quantitatively accounted for. The role of symmetry in nonmass-dependent isotope effects is strengthened by the present experiments, and the search and characterization of mass-independent effects is extended to sulfur-containing molecules.

Bains-Sahota, Swroop K.↗
NASA NTRS1989

Lead isotopes in trade wind aerosols at Barbados - The influence of European emissions over the North Atlantic

Previous studies have shown that Pb can be used as a transient tracer in the atmosphere and the ocean because of strong time-variability of industrial inputs and because Pb isotopic composition can be used to identify contributions from different sources. Pb isotopic measurements on aerosols collected from the North Atlantic Ocean in the trade wind belt are presented. Aerosols sampled at Barbados during the 1969-1985 period have a Pb isotopic composition different from that observed by previous investigators in Bermuda corals and Sargasso Sea waters. Barbados aerosols appear to contain significant amounts of relatively unradiogenic industrial and automotive Pb that is derived from Europe and carried to Barbados by the trade winds. In contrast, Bermuda corals and Sargasso sea waters are influenced mainly by U.S.-derived emissions, which contain more radiogenic Pb originating from Missouri-type ores. This difference generates a strong latitudinal Europe-U.S.A. isotopic gradient, thus allowing study of trans-Atlantic atmospheric transport and ocean mixing processes.

Hamelin, B.↗
NASA NTRS1990

Helium and neon isotopes in deep Pacific Ocean sediments

Helium and neon concentration measurements, along with isotope ratio determinations, have been made for particles collected in the deep Pacific with a magnetic sled, and they are believed to be of extraterrestrial origin. Analyses were made for samples consisting of composites of many extremely fine particles and for several individual particles large enough to contain sufficient gas for analysis but small enough to escape melting in their passage through the atmosphere. Step-heating was employed to extract the gas. Cosmic-ray spallation products or solar-wind helium and neon, if present, were not abundant enough to account for the isotopic compositions measured. In the case of the samples of magnetic fines, the low temperature extractions provided elemental and isotopic ratios in the general range found for the primordial gas in carbonaceous chondrites and gas-rich meteorites. The isotopic ratios found in the high temperature extractions suggest the presence of solar-flare helium and neon.

Nier, A. O.↗
NASA NTRS1989

The abundances of isotopes in the cosmic radiation

Studies of the isotopic composition of nuclei in the cosmic radiation are reviewed, including abundances of the isotopes of elements from H to Ni, together with their implications for cosmic ray origin, acceleration, and transport in the Galaxy. The review focuses on determinations of the composition of cosmic ray source material, and the extent to which the isotopic composition of this material is different from, or similar to, typical solar system material and other samples of Galactic matter. Theoretical models that have been advanced to explain the observed overabundance of neutron-rich isotopes in cosmic rays are described. Also discussed are studies of various radioactive clocks that record the time-scales associated with the nucleosynthesis, acceleration, and transport of cosmic ray nuclei, and studies of the so-called anomalous cosmic ray component, thought to represent a sample of the neutral interstellar medium.

Mewaldt, R. A.↗
NASA NTRS1991

Strontium isotopic variations of Neoproterozoic seawater - Implications for crustal evolution

High-precision Sr isotopic data were obtained on carbonate samples from the Neoproterozoic Shaler Group, Victoria Island (Canada). Results indicate that, between ca. 790 and 850 Ma, the Sr-87/Sr-86 ratio of seawater varied betweeen 0.70676 and 0.70561, with the minimum value at about 830 Ma. A curve of the Sr-87/Sr-86 seawater ratio vs. age showed that the new data substantially improve the existing isotopic record of Sr in seawater for the period 790-850 Ma. The Sr isotopic system data were coupled with data for the Nd isotopic system to model changes in the seafloor spreading rates (hydrothermal flux) and the continental erosion for the period 500-900 Ma. Results indicate that hydrothermal flux reached a maximum value at ca. 830 Ma, while a maximum in erosion rate occurred at ca. 570 Ma. These peaks are considered to be related to the developments in the Pan-African and related orogenic events.

Asmerom, Yemane↗
NASA NTRS1990

Experiments to study cosmic ray isotopic composition up to Zr-40

An overview is presented of instruments and techniques being developed with improved mass resolution and large geometry factor for the study of isotopes. The Advanced Composition Explorer and the Large-Isotope Spectrometer for Astromag are presented as two experiment systems that can reduce statistical uncertainty for the iron isotopes to a small percentage. The instruments are capable of studying isotopes with values of Z of 30-40 by means of orbiting or lunar-based platforms when used for long-duration exposure.

Milliken, Barrett↗
NASA NTRS1992

Neodymium and strontium isotopic study of Australasian tektites - New constraints on the provenance and age of target materials

The Nd and Sr isotopic compositions of Australasian tectites (including two flanged Australian tectites, two low-SiO2 Muong Nong-type tectites, and three high-SiO2 Muong Nong-type tectites) and the Nd, Sm, Sr, and Rb concentrations were investigated by isotope-dilution thermal ionization mass spectrometry, and the Sm-Nd and Rb-Sr isotope systematics were used to study the characteristics of the parental material. It is shown that the Nd and Sr isotopic data provide evidence that all Australasian tektites were derived from a single sedimentary formation with a narrow range of stratigraphic ages close to 170 Ma. It is suggested that all of the Australasian tektites were derived from a single impact event and that the australites represent the upper part of a melt sheet ejected at high velocity, whereas the indochinites represent melts formed at a lower level in the target material distributed closer to the area of the impact.

Blum, Joel D.↗
NASA NTRS1992

U-Pb isotopic results for single shocked and polycrystalline zircons record 550-65.5-Ma ages for a K-T target site and 2700-1850-Ma ages for the Sudbury impact event

The refractory mineral zircon develops distinct morphological features during shock metamorphism and retains these features under conditions that would anneal them in other minerals. In addition, weakly shocked zircon grains give primary ages for the impact site, while highly reconstituted (polycrystalline) single grains give ages that approach the age of the impact event. Data for a series of originally coeval grains will define a mixing line that gives both of these ages providing that no subsequent geological disturbances have overprinted the isotopic systematics. In this study, we have shown that the three zircon grain types described by Bohor, from both K-T distal ejecta (Fireball layer, Raton Basin, Colorado) and the Onaping Formation, represent a progressive increase in impact-related morphological change that coincides with a progressive increase in isotopic resetting in zircons from the ejecta and basement rocks. Unshocked grains are least affected by isotopic resetting while polycrystalline grains are most affected. U-Pb isotopic results for 12 of 14 single zircon grains from the Fireball layer plot on or close to a line recording a primary age of 550 +/- 10 Ma and a secondary age of 65.5 +/- 3 Ma. Data for the least and most shocked grains plot closest to the primary and secondary ages respectively. The two other grains each give ages between 300 and 350 Ma. This implies that the target ejecta was dominated by 550-Ma rocks and that the recrystallization features of the zircon were superimposed during the impact event at 65.5 Ma. A predominant age of 550 Ma for zircons from the Fireball layer provides an excellent opportunity to identify the impact site and to test the hypothesis that multiple impacts occurred at this time. A volcanic origin for the Fireball layer is ruled out by shock-related morphological changes in zircon and the fact that the least shocked grains are old. Basement Levack gneisses north of the Sudbury structure have a primary age of 2711 Ma. Data for three single zircons from this rock, which record a progressive increase in shock features, are displaced 24, 36, and 45 percent along a Pb-loss line toward the 1850 +/- 1 Ma minimum age for the impact as defined by the age of the norite. Southeast of the structure three shocked grains from the Murray granite record a primary age of 2468 Ma and are displaced 24, 41, and 56 percent toward the 1853 +/- 4 Ma even as defined by coexisting titanite.

Krogh, T. E.↗
NASA NTRS1993

Research relative to the heavy isotope spectrometer telescope experiment

The Heavy Isotope Spectrometer Telescope (HIST) was launched during August 1978 on ISEE-3 (ICE). HIST was designed to measure the isotopic composition of solar, galactic, and interplanetary cosmic ray nuclei for the elements from H to Ni (1 less than or equal to Z less than or equal to 28) in the energy range from approximately 5 to approximately 200 MeV/nucleon. The results of these measurements have been used in studies of the composition of solar matter and galactic cosmic ray sources, the study of nucleosynthesis processes, studies of particle acceleration and propagation, and studies of the life-history of cosmic rays in the heliosphere and in the galaxy. On December 1, 1978, after 110 days in orbit, HIST suffered an electronic failure in its readout system. After that point, only one-half of the telemetry bits associated with the pulse heights measured by HIST were transmitted to Earth. As a result, the resolution of HIST was significantly degraded, and it served as an element rather than an isotope spectrometer. Fortunately, HIST was able to measure the isotopic composition of heavy nuclei in the 9/23/78 solar event (the largest solar energetic particle event since 1972) during the brief period that it operated at full resolution. This grant funded the analysis of data from the HIST instrument over the period from 12/1/85 to 11/30/92. In section 2 of this final report, we summarize the scientific accomplishments that have resulted from HIST measurements during this time period. A bibliography of tasks and papers that resulted is attached.

Stone, Edward C.↗
NASA NTRS1993

Mass Spectrometric Measurement of Martian Krypton and Xenon Isotopic Abundance

The Viking gas chromatograph mass spectrometer experiment provided significant data on the atmospheric composition at the surface of Mars, including measurements of several isotope ratios. However, the limited dynamic range of this mass spectrometer resulted in marginal measurements for the important Kr and Xe isotopic abundance. The Xe-129 to Xe-132 ratio was measured with an uncertainty of 70%, but none of the other isotope ratios for these species were obtained. Accurate measurement of the Xe and Kr isotopic abundance in this atmosphere provides an important data point in testing theories of planetary formation and atmospheric evolution. The measurement is also essential for a stringent test for the Martian origin of the SNC meteorites, since the Kr and Xe fractionation pattern seen in gas trapped in glassy nodules of an SNC (EETA 79001) is unlike any other known solar system resevoir. Current flight mass spectrometer designs combined with the new technology of a high-performance vacuum pumping system show promise for a substantial increase in gas throughput and the dynamic range required to accurately measure these trace species. Various aspects of this new technology are discussed.

Mahaffy, P.↗
NASA NTRS1992

Methane transport mechanisms and isotopic fractionation in emergent macrophytes of an Alaskan tundra lake

The stable carbon isotopic composition of methane associated with and emitted by the two dominant emergent macrophytes abundant in the many Alaskan tundra lakes, Carex rostrata and Arctophila fulva, is determined. The carbon isotopic composition of the methane was -58.6 +/- 0.5 (n=2) for Arctophila and -66.6 +/- 2.5 (n=6) for Carex. The methane emitted by these species is depleted in C-13 by 12 per mil for Arctophila and 18 per mil for Carex relative to methane withdrawn from plant stems 1-2 cm below the waterline. The results suggest more rapid transport of (C-12)H4 relative to (C-13)H4 through plants to the atmosphere. Plant stem methane concentrations ranged from 0.2 to 4.0 percent in Arctophila, with an isotopic composition of -46.1 +/- 4.3 percent (n=8). Carex stem methane concentrations ranged from 150 to 1200 ppm, with an isotopic composition of -48.3 +/- 1.4 per mil (n=3).

Chanton, Jeffrey P.↗
NASA NTRS1992

Silicon isotopic composition in large meteoritic SiC particles and Na-22 origin of Ne-22

Large SiC particles extracted from acid-insoluble residues of carbonaceous chondrites are isotopically anomalous in both silicon and carbon and contain isotopically extreme noble gases. These particles are thought to have originated in mass outflows from red giant stars and to have existed in the interstellar medium at the time the solar system formed from an interstellar cloud. Calculations show that the silicon isotope correlations in those large SiC particles can be generated only in the most massive carbon stars. Consequently, the almost pure Ne-22 in those particles must be interpreted as the condensation of radioactive Na-22 in the particles as they flowed away from the stars. The Na-22 is produced through proton capture by Ne-21 at the base of the surface convection zone. Ne-22 does not exist abundantly in helium shells hot enough to burn magnesium, which is necessary to establish the measured silicon isotopic composition.

Brown, Lawrence E.↗
NASA NTRS1993

Rb-Sr and Sm-Nd isotopic variations in dissected crustal xenoliths

The effect of magma-xenolith interaction on the Rb-Sr and Sm-Nd isotopic systematics was investigated by studying the Rb-Sr and Sm-Nd variations in dissected crustal xenoliths sampled from different localities across Scotland. The Nd isotopic compositions were found to be virtually uniform across each xenolith, but significant variations were found in Rb, Sr, and REE concentrations, as well as in Rb/Sr and Sm/Nd ratios and Sr isotopic composition. Most of these variations appear to be inherited from the protolith, but, in one case, they have been modified by melt infiltration from the host magma. The results lend confidence to the interpretation of the isotopic and chemical compositions of xenoliths transported in basaltic magmas as reflecting their source regions, but they also highlight the potential problems of interpreting Sm-Nd model ages from metamorphic rocks.

Lee, Der-Chuen↗
NASA NTRS1993

Ca-,Al-rich inclusions in the unique chondrite ALH85085 - Petrology, chemistry, and isotopic compositions

A comprehensive study is performed for the Ca-,Al-rich inclusions (CAIs) in the unique chondrite ALH85085. The ALH85085 inclusions are smaller (5-80 microns) and more refractory than their counterparts in carbonaceous chondrites. The study includes 42 inclusions for petrography and mineralogy, 15 for bulk major and minor element chemical composition, six for Mg-Al isotopic systematics, 10 for Ca isotopes, nine for Ti isotopes, and six for trace element abundances. In addition, oxygen-isotopic compositions were determined in minerals from a single inclusion. No correlation is found between mineralogy, major element chemistry, and trace element abundances. It is further shown that the high-temperature geochemical behavior of ultrarefractory trace elements is decoupled from that of the major elements Ca and Ti (Ti is correlated with the relatively volatile elements Nb and Yb) implying that perovskite is of only minor importance as carrier of ultrarefractories.

Kimura, Makoto↗
NASA NTRS1993

Isotopic composition and concentration of sulfur in carbonaceous chondrites

New sulfur isotopic ratio measurements are reported for seven carbonaceous chondrites. Newly developed procedures permit measurement of delta S-33, delta S-34, and delta S-36 at precisions significantly greater than previously reported. A search for S-36 nucleosynthetic anomalies coproduced with anomalies in, for example, Ti-50 and Ca-48 was negative. The high endemic sulfur concentration probably dilutes any S-36 anomaly, and separation of individual sulfur phases may be needed to identify S-36 carrier phases. Large internal isotopic variations are observed, deriving from parent body and possibly nebular processes. Chondrule separates from Allende demonstrate isotopic compositions which vary as a function of diameter. High-temperature gas-solid exchange and a two-component mixing model may account for the observations. High-resolution isotopic data and structural information are reported for organic sulfur compounds separated by chemical extractions. The insoluble organics appear to be of either aliphatic or alicyclic structure and are dominant phases.

Gao, Xia↗
NASA NTRS1993

MAST - A mass spectrometer telescope for studies of the isotopic composition of solar, anomalous, and galactic cosmic ray nuclei

The Mass Spectrometer Telescope (MAST) on SAMPEX is designed to provide high resolution measurements of the isotopic composition of energetic nuclei from He to Ni (Z = 2 to 28) over the energy range from about 10 to several hundred MeV/nuc. During large solar flares MAST will measure the isotopic abundances of solar energetic particles to determine directly the composition of the solar corona, while during solar quiet times MAST will study the isotopic composition of galactic cosmic rays. In addition, MAST will measure the isotopic composition of both interplanetary and trapped fluxes of anomalous cosmic rays, believed to be a sample of the nearby interstellar medium.

Cook, Walter R.↗
NASA NTRS1993

Oxygen isotopes as tracers of tektite source rocks: An example from the Ivory Coast tektites and Lake Bosumtwi Crater

Oxygen isotope studies of tektites and impact glasses provide an important tool to help in identifying the target lithologies for terrestrial impacts, including the K-T boundary impact. However, such studies may be complicated by modification of the original oxygen isotope values of some source rocks during the tektite formation process either by vapor fractionation or incorporation of meteoric water. To further investigate the relationship between the oxygen isotopic composition of tektites and their source rocks, Ivory Coast tektites and samples of impact glasses and bedrock lithologies from the Bosumtwi Crater in Ghana--which is widely believed to be the source crater for the Ivory Coast tektites--were studied. Our preliminary results suggest that the phyllites and metagraywackes from the Bosumtwi Crater were the predominant source materials for the impact glasses and tektites and that no significant oxygen isotope modification (less than 1 percent delta(O-18)) took place during impact melting. This contrasts with previous studies of moldavites and Australasian tektites and their sedimentary source materials which suggests a 4 to 5 percent lowering of delta(O-18) due to meteoric water incorporation during impact melting.

Blum, Joel D.↗
NASA NTRS1993

Martian carbon dioxide: Clues from isotopes in SNC meteorites

Attempts to unravel the origin and evolution of the atmosphere and hydrosphere on Mars from isotopic data have been hampered by the impreciseness of the measurements made by the Viking Lander and by Earth-based telescopes. The SNC meteorites which are possibly pieces of the Martian surface offer a unique opportunity to obtain more precise estimates of the planet's volatile inventory and isotopic composition. Recently, we reported results on oxygen isotopes of water extracted by pyrolysis from samples of Shergotty, Zagami, Nakhla, Chassigny, Lafayette, and EETA-79001. Now we describe complementary results on the stable isotopic composition of carbon dioxide extracted simultaneously from those same samples. We will also report on C-14 abundances obtained by accelerator mass spectrometry (AMS) for some of these CO2 samples.

Karlsson, H. R.↗