Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “ISOTOPE RATIO”

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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Radionuclide measurements by accelerator mass spectrometry at Arizona

Over the past years, Tandem Accelerator Mass Spectrometry (TAMS) has become established as an important method for radionuclide analysis. In the Arizona system the accelerator is operated at a thermal voltage of 1.8MV for C-14 analysis, and 1.6 to 2MV for Be-10. Samples are inserted into a cesium sputter ion source in solid form. Negative ions sputtered from the target are accelerated to about 25kV, and the injection magnet selects ions of a particular mass. Ions of the 3+ charge state, having an energy of about 9MeV are selected by an electrostatic deflector, surviving ions pass through two magnets, where only ions of the desired mass-energy product are selected. The final detector is a combination ionization chamber to measure energy loss (and hence, Z), and a silicon surface-barrier detector which measures residual energy. After counting the trace iosotope for a fixed time, the injected ions are switched to the major isotope used for normalization. These ions are deflected into a Faraday cup after the first high-energy magnet. Repeated measurements of the isotope ratio of both sample and standards results in a measurement of the concentration of the radionuclide. Recent improvements in sample preparation for C-14 make preparation of high-beam current graphite targets directly from CO2 feasible. Except for some measurements of standards and backgrounds for Be-10 measurements to date have been on C-14. Although most results have been in archaeology and quaternary geology, studies have been expanded to include cosmogenic C-14 in meteorites. The data obtained so far tend to confirm the antiquity of Antarctic meteorites from the Allan Hills site. Data on three samples of Yamato meteorites gave terrestrial ages of between about 3 and 22 thousand years.

Jull, A. J. T.↗
NASA NTRS1986

Production rates of neon xenon isotopes by energetic neutrons

As a first step in an experimental program to study the behavior of noble gases produced in situ in minerals, a suite of minerals and pure chemicals were irradiated with 14.5 MeV neutrons at LLNL's Rotating Target Neutron Source (RTNS-II) and production rates for noble gases were determined. While neutron effects in meteorites and lunar samples are dominated by low-energy neutron capture, more energetic cosmic-ray secondary neutrons can provide significant depth-dependent contributions to production of cosmogenic nuclides through endothermic reactions such as (n,2n), (n,np), (n,d) and (n,alpha). Production rates for nuclides produced by cosmic-ray secondary neutrons are therefore useful in interpreting shielding histories from the relative abundances of cosmogenic nuclides. Absolute production cross sections were calculated from isotope dilution analyses of NaCl, Mg, CsCl, and Ba(NO3)2 samples, assuming purity, stoichiometry, and quantitative noble gas retention and extraction. Relative production cross sections determined from neon isotopic ratios in the mineral samples were also considered in evaluating the neon production cross sections. Results are presented.

Leich, D. A.↗
NASA NTRS1985

Laboratory and astronomical detection of the deuterated ethynyl radical CCD

Two rotational transitions of CCD, N = 1-2 at 144 GHz and 2-3 at 216 GHz, were detected in a laboratory glow discharge through deuterated acetylene and helium, after which one, N = 2-1, was detected toward the rich molecular cloud behind the Orion Nebula. The 144 GHz transition is a well-resolved spin doublet split by 55 MHz, the components of which contain hyperfine structure of the order of 1 MHz, so far only partially resolved. From observations toward two positions in Orion, at and near the Kleinmann-Low nebula, the column density of CCD is determined to be 1.8 x 10 to the 13th/sq cm and the isotopic ratio CCD/CCH = 0.05. CCD was not detected at two positions in TMC-1.

Vrtilek, J. M.↗
NASA NTRS1985

Project Cassini - A potential collaborative ESA/NASA Saturn Orbiter and Titan Probe mission

Project Cassini is concerned with a potential collaborative mission to Saturn and Titan. The project has been the subject of a joint ESA/NASA Technical Assessment Study from April 1984 to July 1985. The spacecraft consists of a Saturn Orbiter and a Titan entry Probe. Science objectives with respect to Titan include a determination of the abundances of atmospheric constituents and isotope ratios, a study of the distribution of trace gases, cloud physics, and a determination of the state of the surface. Other science objectives are related to Saturn, the rings, icy satellites, the magnetosphere of Saturn, and asteroids. Attention is given to mission design, the spacecraft, and future prospects.

Beckman, J.↗
NASA NTRS1985

Lunar and Planetary Science Conference, 16th, Houston, TX, March 11-15, 1985, Proceedings. Part 1

Various papers on lunar studies, meteorites and cosmic dust, Mars, and tektites are presented. The topics discussed include: very high potassium basalt; complications in mare basalt petrogenesis; mare basalt genesis; modeling trace elements and isotopic ratios; Zr-Hf-Ta fractionation during lunar evolution; petrology of the Apollo 12 highland component; petrologic province maps of the lunar highlands derived from orbital geochemical data; petrology and chemistry of Apollo 12 regolith breccias; chemical variability and origin of agglutinitic glass; multistage exposure history of the 74261 soil constituents; and an experimental investigation of agglutinate melting mechanisms; shocked mixtures of sodium and potassium feldspars. Also addressed are: cooling history of some Antarctic ureilites; the Leoville accretionary breccia; ubiquitous brecciation after metamorphism in equilibrated ordinary chondrites; layer silicates in a chondritic porous interplanetary dust particle; estimates of rheological properties for flows on the Martian volcano Ascraeus Mons; the Martian dust storm of Sol 1742; and late Eocene North American microtektites and clinopyroxene-bearing spherules.

Ryder, G.↗
NASA NTRS1986

Apollo 16 regolith breccias - Characterization and evidence for early formation in mega-regolith

The Apollo 16 regolith breccias were characterized in terms of petrography, grain-size distribution, porosity, major and trace element composition, noble gas contents, and ferromagnetic resonance properties. Significant variation was found with respect to density and porosity; the more dense breccias displayed substantial shock damage. The breccias resembled the soils in grain-size distribution and in petrological components, though many were found to be compositionally different from the Apollo 16 soils in that a mafic component was lacking. Nearly all breccias showed evidence of irradiation at the lunar surface, and analyses of disaggregated breccias indicated that this irradiation occurred before compaction. The concentration of surface irradiation parameters were far less than those of lunar soils or breccias of other Apollo missions. Observations with respect to the argon isotope ratio and Xe presence have led to the possibility that breccia-surface irradiation occurred as early as four billion years ago, and that most Apollo 16 regolith breccias were not formed from any known Apollo 16 soil.

Mckay, D. S.↗
NASA NTRS1986

Fractionation of noble gases by thermal escape from accreting planetesimals

Assuming solar initial elemental and isotopic ratios and a determination of the degree of fractionation occurring by competition between gravitational binding and escape, a model is developed for selective noble gas loss through escape during the growth of planetesimals to form the terrestrial planets. Of the two classes of planetesimals that can form on a time scale that is consistent with modern accretion models, one is depleted in neon while the other is neon-rich. The mechanism is noted to be capable of accounting for all known properties of the noble gas volatiles on the terrestrial planets, with only one exception, namely the Ar-36/Ar-38 ratios for Mars and the earth, which are much lower than observed.

Donahue, T. M.↗
NASA NTRS1986

Nitrogen and light noble gases in Shergotty

Two samples of Shergotty and a sample of EETA 79001's lithology A have been analyzed for N, He, Ne, and Ar abundances and isotopic composition. After correcting for spallogenic nitrogen, the nitrogen isotopic ratios were found to be close to that of the terrestrial atmosphere. The spallogenic noble gas data are consistent with cosmic ray irradiation of both Shergotty and EETA 79001 at shallow shielding depths. Cosmic ray exposure ages were estimated to be in the range of 0.5-0.8 Myr for EETA 79001, and of 2.0-5.2 Myr for Shergotty, depending on the choice of object size and shielding. Among the two Shergotty samples, the contents of Ar-40 differed by a factor of 3. This difference can be attributed to either a small-scale mineralogical inhomogeneity or a significant variation in the degree of degassing of minerals during shock, although the presence of trapped argon with a high Ar-40/Ar-36 ratio and its heterogeneous distribution cannot be ruled out.

Becker, R. H.↗
NASA NTRS1986

Xenon and other noble gases in shergottites

The isotopic composition of the xenon component trapped in EETA 79001's lithologies B and C has been determined, and other noble gases were measured in some samples. The Xe component was found to have light isotope ratios indistinguishable from those of the terrestrial atmosphere. The trapped component has a Xe-129/Xe-132 ratio of about 2.4, and is enhanced in Xe-134 and Xe-136 relative to the terrestrial atmosphere or the average carbonaceous chondrite. Similarities between values for Ar-40/Ar-36, Xe-129/Xe-132, and N-15/N-14 and the corresponding Martian atmospheric values suggest Martian origin of the trapped gases.

Swindle, T. D.↗
NASA NTRS1986

Spectroscopic observations of the planets

The first phase of a program to determine D/H in the atmospheres of all of the major planets and Titan was completed. D/H on Titan and Uranus is significantly higher than on Jupiter and Saturn. Titan has the highest value, consistent with the possibility for hydrogen escape. These measurements were all made in terms of CH3D/CH4. They suggest that at least two reservoirs for deuterium exist in the outer solar system - a large one in hydrogen and a much smaller one in hydrogen-containing compounds that have not isotopically equilibrated with the hydrogen. This result also demonstrates that a large fraction of the carbon in the outer solar nebula was in the form of methane and it supports the nucleation model for the formation of the outer planets. Other studies included an investigation of the phosphine abundance over the Great Red Spot (GRS) of Jupiter, pre-Voyager determination of the amount of acetylene on Uranus, and the carbon isotope ratio in Halley's Comet. UV spectra of the GRS obtained with the IUE show no evidence of the enhancement of phosphine that would be expected if the red color of this object is caused by P4.

Owen, T.↗
NASA NTRS1986

Isotopic constraints on planetary evolution

Direct observations are considered that have been made on terrestrial and lunar materials in the search for answers to questions such as: when did the major structural elements (core, mantle, and crust) come into being and evolved. Measurements of isotope ratios of Pb, Sr, Nd, Hf, Ar, and other elements provide information about the internal evolution of the earth and moon since their formation 4.5 Gyr ago. The existing evidence indicates the core formed more than 4.4 Gyr ago, possibly while the earth was still accreting. Though the moon crust is similarly very old, the moon ceased to differentiate after about 1 Gyr whereas earth differentiation has continued to the present. Cataclysmic episodes have occurred, the largest 2.8 Gyr ago, when about half of the present continental mass formed.

Depaolo, D. J.↗
NASA NTRS1986

Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the earth's core

The hypothesis that the mantle Pb isotope ratios reflect continued extraction of Pb into the earth's core over geologic time is evaluated by studying the depeletion of chalcophile and siderophile elements in the mantle. Oceanic basalt samples are analyzed in order to determine the Pb, Sr, and Nd isotropic compositions and the abundances of siderophile and chalcophile elements and incompatible lithophile elements. The data reveal that there is no systematic variation of siderophile or chalcophile element abundances relative to abundances of lithophile elements and the Pb/Ce ratio of the mantle is constant. It is suggested that the crust formation involves nonmagmatic and magmatic processes.

Newsom, H. E.↗
NASA NTRS1988

Pb, Sr, and Nd isotopic compositions of a suite of Large Archean, igneous rocks, eastern Beartooth Mountains - Implications for crust-mantle evolution

Compositionally diverse Late Archean rocks (2.74-2.79 Ga old) from the eastern Beartooth Mountains (Montana and Wyoming) were studied and shown to have the same initial Pb, Sr, and Nd isotopic ratios. Lead and Sr initial ratios are higher and Nd initial values lower than predicted for rocks derived from model mantle sources and strongly indicate the involvement of an older crustal reservoir in the genesis of these rocks. A model involving subduction of continental detritus and contamination of the overlying mantle is suggested.

Wooden, J. L.↗
NASA NTRS1988

Cosmic-ray lifetime in the Galaxy - Experimental results and models

The separation of the cosmic-ray Be isotopes achieved by two satellite experiments is discussed. From the measured isotopic ratio between the radioactive Be-10 (half-life = 1,500,000 years) and the stable Be-9, it is deduced that the cosmic rays propagate through matter with an average density of 0.24 = or - 0.07 atoms/ cu cm. The implications of this result for the cosmic-ray age is reviewed in the context of two models of confinement and propagation: the homogeneous model, normally identified with confinement ot the galactic gaseous disk, and a diffusion model in which the cosmic rays extend into a galactic halo. A newly deduced cosmic-ray pathlength distribution, a self-consistent model of solar modulation, and a set of fragmentation cross sections are calculated. It is concluded that in a homogeneous model, the cosmic-ray age is 15 (+7, -4) million years, about a factor longer than early estimates based on the abundances of the light nuclei Li, Be, and B and a nominal interstellar density of 1 atom/ cu cm.

Simpson, J. A.↗
NASA NTRS1987

Diatomic molecules in the solar infrared spectrum from ATMOS-SL 3 high resolution observations

The ATMOS Fourier Transform Spectrometer has observed, for the first time, the solar infrared spectrum, from 2 to 16 microns, free of any telluric absorption. A very large number of molecular lines are present on these high quality spectra: vibration-rotation lines of CO (Delta-v = 1 and 2; including the isotopic species C-13, O-18 and O-17), CH, NH, OH and pure rotation lines of OH. The analysis of these lines will allow to derive accurate values for the solar abundances of C, N and O and the isotopic ratios C-13/C-12, O-18/O-16, and O-17/O-16, to test the photospheric model from high to deep layers, to test the electric dipole moment functions of the different molecules and to derive much better molecular constants for CH.

Grevesse, Nicolas↗
NASA NTRS1988

Overview of Mars: SNC meteorite results

The SNC meteorites according to their oxygen isotope ratios and various trace element ratios form a distinct group of 8 achondrites. Their young crystallization ages and fractionated REE pattern which exclude an asteroidal origin, were the first observations to point towards Mars as their parent body. In spite of the many arguments for Mars as the parent body of the SNC meteorites there does not exist a generally accepted model for the ejecting process and other dynamical problems involved. In this discussion it is, however, assumed that Mars is the SNC parent body. The chemical composition of Mars is examined.

Waenke, H.↗
NASA NTRS1988

Moderately volatile elements

That the fractionation of moderately volatile and highly volatile elements was a major process in the early solar system is reflected in the variable concentrations of Rb and associated variations in initial Sr-isotopic ratios in chondritic meteorites. Greater knowledge of processes leading to volatile depletion would place stronger constraints on the formation conditions of solid material in the solar system. It will especially become possible to ascertain whether evaporation or incomplete condensation was the major process in establishing the elemental abundance patterns observed in primitive meteorites and planets.

Palme, H.↗
NASA NTRS1989

High-resolution infrared spectroscopy of Io and possible surface materials

A comparison of new spectra of Io with laboratory-simulated frosts confirms that the dominant materials on Io are SO2 frost in conjunction with a spectrally neutral material presumed to be sulfur. While the 4-micron region spectra are largely explainable in these terms, attention is drawn to a shoulder in the spectrum at 4.04 microns that is suggestive of adsorbed SO2 gas; two shallow, unidentified bands are also noted at 3.85 and 3.91 microns. The isotopic ratios of oxygen and sulfur appear to be normal. The absence of distinct bands in the new spectra in the 5-micron region limits the abundance of sulfate and sulfite compounds.

Howell, Robert R.↗