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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Noble gas trapping by laboratory carbon condensates

Trapping of noble gases by carbon-rich matter was investigated by synthesizing carbon condensates in a noble gas atmosphere. Laser evaporation of a solid carbon target yielded submicron grains which proved to be efficient noble gas trappers (Xe distribution coefficients up to 13 cu cm STP/g-atm). The carbon condensates are better noble gas trappers than previously reported synthetic samples, except one, but coefficients inferred for meteoritic acid-residues are still orders of magnitude higher. The trapped noble gases are loosely bound and elementally strongly fractionated, but isotopic fractionations were not detected. Although this experiment does not simulate nebular conditions, the results support the evidence that carbon-rich phases in meteorites may be carriers of noble gases from early solar system reservoirs. The trapped elemental noble gas fractionations are remarkably similar to both those inferred for meteorites and those of planetary atmospheres for earth, Mars and Venus.

Niemeyer, S.↗
NASA NTRS1983

Experiments with phase transitions at very high pressure

Diamond cells were constructed for use to 1 Mbar. A refrigerator for cooling diamond cells was adapted for studies between 15 and 300 K. A cryostat for superconductivity studies between 1.5 to 300 K was constructed. Optical equipment was constructed for fluorescence, transmission, and reflectance studies. X-ray equipment was adapted for use with diamond cells. Experimental techniques were developed for X-ray diffraction studies using synchrotron radiation. AC susceptibility techniques were developed for detecting superconducting transitions. The following materials were studied: compressed solidified gases (Xe, Ar), semiconductors (Ge, Si, GaAs), superconductors (Nb3Ge, Nb3Si, Nb3As, CuCl), molecular crystals (I).

Spain, I. L.↗
NASA NTRS1982

Lunar and Planetary Science Conference, 13th, Houston, TX, March 15-19, 1982, Proceedings. Part 1

The present conference on planetary and lunar science considers theoretical models for the composition of the Venus crust, the lunar crust, the prediction of phase relationships in planetary mantles, the volumetric analysis of complex lunar craters, grazing impacts on Mars, the determination of lunar structure by means of electrical conductivity and seismic experiments, results of studies on the Apollo 16 site rocks, as well as Apollo 14, 15 and 17 lunar glasses and regoliths, and carbon components and isotopic compositions of chondritic meteorites. Also discussed are iron meteorites, interplanetary dust and tektites, and such theoretical and experimental issues as refractory condensates and chondrules from solar furnace experiments, molecular synthesis through the irradiation of silicates, and the adsorption of excess fission Xe.

Boynton, W. V.↗
NASA NTRS1982

The Apennine Front core 15007/8 - Irradiational and depositional history

The Apennine Front core 15007/8 was subjected to ferromagnetic resonance (FMR), magnetic and noble gas measurements. Ferromagnetic resonance intensity I(s)/FeO surface exposure (maturity) index values and FeO concentrations are reported for 0.5 cm depth intervals, and noble gas and FMR data are given for 13 soils and two clasts selected from various depths in the 57 cm-long core. These data indicate four core units whose contacts are at about 18, 49 and 55 cm. The 0-18 cm unit has cosmogenic Xe-131/Xe-126 values that are higher than underlying soil and indicate irradiation at depths greater than 50 cm, showing it to probably be the ejecta from the crater on whose rim the core was collected. The 18-49 cm unit has relatively constant I(s)/FeO values and cosmogenic gas abundances, and may represent a mixed soil zone. I(s)/FeO drops by 30 units at the 49-cm contact. Comparison of the 15007/8 core to other ones suggests that soils on slopes tend to be mixed and to not contain units with long in situ radiation profiles.

Bogard, D. D.↗
NASA NTRS1982

Advances in NASA research on nuclear-pumped lasers

NASA has been primarily interested in nuclear-pumped lasers using the He-3 or U-235F6 reaction for lasant excitation. With He-3 excitation, a large volume, multiple-path He-3-Ar nuclear laser has produced an output of 1 kilowatt. Power deposition was shown to be homogeneous over this volume. The CO laser has been pumped for the first time using the He-3 reaction, producing approximately 200 Watts. Using a boron-10 coating to excite N2, nuclear lasing has been achieved in CO2 in a transfer laser configuration. Nuclear lasing of Ar-Xe has been demonstrated using fission fragment excitation from U-235F6. Research on the gas core reactor has resulted in a steady state operational power of 30 kilowatts with flowing U-235F6 in an argon vortex.

De Young, R. J.↗
NASA NTRS1983

Temperature-dependent ion mixing and diffusion during sputtering of thin films of CrSi2 on silicon

Measurements of sputtering yields and composition profiles have been carried out using backscattering spectrometry for samples of CrSi2 on Si irradiated wth 200-keV Xe ions. When the CrSi2 layer is thinner than the ion range, the sputtering yield ratio of Si to Cr increases from 3.5 for room-temperature irradiation to 65 at 290 C. For a thick sample, the corresponding increase is from 2.4 to 4.0 only. These changes are explained in terms of a rise in the Si surface concentration ot 290 C. The driving force for this process seems to be the establishment of stoichiometric CrSi2 compound. Transport of Si to the surface is by ion mixing in the thin sample and thermal diffusion through the thick layer.

Shreter, U.↗
NASA NTRS1984

Test bed ion engine development

A test bed ion (TBI) engine was developed to serve as a tool in exploring the limits of electrostatic ion thruster performance. A description of three key ion engine components, the decoupled extraction and amplified current (DE-AC) accelerator system, field enhanced refractory metal (FERM) hollow cathode and divergent line cusp (DLC) discharge chamber, whose designs and operating philosophies differ markedly from conventional thruster technology is given. Significant program achievements were: (1) high current density DE-AC accelerator system operation at low electric field stress with indicated feasibility of a 60 mA/sq cm argon ion beam; (2) reliable FERM cathode start up times of 1 to 2 secs. and demonstrated 35 ampere emission levels; (3) DLC discharge chamber plasma potentials negative of anode potential; and (4) identification of an efficient high plasma density engine operating mode. Using the performance projections of this program and reasonable estimates of other parameter values, a 1.0 Newton thrust ion engine is identified as a realizable technology goal. Calculations show that such an engine, comparable in beam area to a J series 30 cm thruster, could, operating on Xe or Hg, have thruster efficiencies as high as 0.76 and 0.78 respectively, with a 100 eV/ion discharge loss.

Aston, G.↗
NASA NTRS1983

Measurements of I-129 in meteorites and lunar rock by tandem accelerator mass spectrometry

Precise measurements of the half-life of I-129 in three different meteorites and one lunar surface rock are reported. The meteorite source of I-129 was produced by cosmic ray secondary neutron reactions on Te, while the source in lunar materials in spallation on barium and rare earth elements. The Abee, Allende, and Dhajala meteorites were examined, together with the lunar rock 14310. Details of the process used to extract the iodine are provided. The Abee and Allende samples exhibited a production of 0.5 atom/min per gm of Te from the (n,2n) reaction and 0.05 atom/min/gm for the (n,gamma) reaction. The I-129 is concluded to be a viable tool for long-lived cosmogenic nuclide studies. Further work to extend the data to include the constancy of the cosmic ray flux, the meteorite bombardment history, and the cosmic exposure age dating by means of the I-129 and Xe-129 method is indicated.

Nizhiizumi, K.↗
NASA NTRS1984

Nitrogen isotopes in lunar highlands breccias

Lunar highlands breccias from Apollo 14 and 16 which may have trapped solar wind gases at a very early epoch in the history of the moon, as implied by their high content of parentless fissiogenic xenon and sometimes of parentless radiogenic Xe-129, are analyzed for nitrogen content and isotopic composition using stepwise heating techniques. The results show that the nitrogen is not particularly light and was not acquired in very ancient times. The conflicting presence of both parentless xenon and nitrogen of relatively recent isotopic signature can be explained if the hypothetical light nitrogen is diluted by more abundant, heavier nitrogen. Accordingly, the very ancient soil components implied in these breccias by the presence of excess fission xenon were reexposed at a much later epoch, or mixed with younger soil components, before the compaction event.

Fourcade, S.↗
NASA NTRS1984

Ion thruster performance model

A model of ion thruster performance is developed for high flux density, cusped magnetic field thruster designs. This model is formulated in terms of the average energy required to produce an ion in the discharge chamber plasma and the fraction of these ions that are extracted to form the beam. The direct loss of high energy (primary) electrons from the plasma to the anode is shown to have a major effect on thruster performance. The model provides simple algebraic equations enabling one to calculate the beam ion energy cost, the average discharge chamber plasma ion energy cost, the primary electron density, the primary-to-Maxwellian electron density ratio and the Maxwellian electron temperature. Experiments indicate that the model correctly predicts the variation in plasma ion energy cost for changes in propellant gas (Ar, Kr and Xe), grid transparency to neutral atoms, beam extraction area, discharge voltage, and discharge chamber wall temperature. The model and experiments indicate that thruster performance may be described in terms of only four thruster configuration dependent parameters and two operating parameters. The model also suggests that improved performance should be exhibited by thruster designs which extract a large fraction of the ions produced in the discharge chamber, which have good primary electron and neutral atom containment and which operate at high propellant flow rates.

Brophy, J. R.↗
NASA NTRS1985

The degassing history of the Earth: Noble gas studies of Archaean cherts and zero age glassy submarine basalts

Recent noble gas studies suggests the Earth's atmosphere outgassed from the Earth's upper mantle synchronous with sea floor spreading, ocean ridge hydrothermal activity and the formation of continents by partial melting in subduction zones. The evidence for formation of the atmosphere by outgassing of the mantle is the presence of radionuclides H3.-4, Ar-040 and 136 Xe-136 in the atmosphere that were produced from K-40, U and Th in the mantle. How these radionuclides were formed is reviewed.

Hart, R.↗
NASA NTRS1985

Noble gases in meteorites and terrestrial planets

Terrestrial planets and chondrites have noble gas platforms that are sufficiently alike, especially Ne/Ar, that they may have acquired their noble gases by similar processes. Meteorites presumably obtained their noble gases during formation in the solar nebula. Adsorption onto C - the major gas carrier in chondrites - is the likely mechanism for trapping noble gases; recent laboratory simulations support this hypothesis. The story is more complex for planets. An attractive possibility is that the planets acquired their noble gases in a late accreting veneer of chondritic material. In chondrites, noble gases correlate with C, N, H, and volatile metals; by Occam's Razor, we would expect a similar coupling in planets. Indeed, the Earth's crust and mantle contain chondritic like trace volatiles and PL group metals, respectively and the Earth's oceans resemble C chondrites in their enrichment of D (8X vs 8-10X of the galactic D/H ratio). Models have been proposed to explain some of the specific noble gas patterns in planets. These include: (1) noble gases may have been directly trapped by preplanetary material instead of arriving in a veneer; (2) for Venus, irradiation of preplanetary material, followed by diffusive loss of Ne, could explain the high concentration of AR-36; (3) the Earth and Venus may have initially had similar abundances of noble gases, but the Earth lost its share during the Moon forming event; (4) noble gases could have been captured by planetestimals, possibly leading to gravitational fractionation, particularly of Xe isotopes and (5) noble gases may have been dissolved in the hot outer portion of the Earth during contact with a primordial atmosphere.

Wacker, J. F.↗
NASA NTRS1985

Noble gas patterns in the atmospheres of Mars and Earth: A comparison via the SNC meteorites

Noble gas and nitrogen compositions in the glassy phase of the EETA 79001 shergottite correspond closely with Viking measurements. This direct evidence for the origin of the SNC meteorites on Mars, and for trapping of an unfractionated sample of Martian atmospheric gases in the 79001 glass, provides a reasonable basis for comparing the Martian and terrestrial atmospheres with more precision than that afforded by the Viking data set. Results are that, with one exception, elemental and isotopic compositions of nonradiogenic Martian noble gases are similar to those in the Earth's atmosphere; relatively small isotopic discrepancies in Kr and perhaps Xe may be attributable to different degrees of mass fractionation of a common parent reservoir. The anomaly is in Ar composition, where Martian Ar-36/AR-38 approx. 4 is strikingly lower than the values near 5.3 that characterize both the Earth and major meteoritic gas carriers. Although a primordial Martian ratio of 5.3 could in principle be altered by some planet specific process (e.g., cosmic ray spallation of surface materials) operating over geologic time, one has not been found that works.

Pepin, R. O.↗
NASA NTRS1984

Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites

Three meteorites belonging to the rare group of SNC achondrites, which may have originated in the planet Mars, have been subjected to noble gas isotopic concentration measurements. The elemental and isotopic ratios obtained are unlike those for any other noble gas components except those obtained in analyses of the Martian atmosphere by Viking spacecraft. It is hypothesized that the Kr and Xe gases represent a portion of the Martian atmosphere which was shock-implanted in the case of Elephant Moraine A79001, and that they constitute direct evidence of a Martian origin for the shergottite meteorites. If the SNC meteorites were ejected from Mars at the shergottite shock age of about 180 My ago, they must have been objects more than 6 m in diameter which experienced at least three space collisions to initiate cosmic ray exposure.

Bogard, D. D.↗
NASA NTRS1983

Cosmic-ray abundances of the even charge elements from Sn-50 to Ce-58 measured on HEAO-3

Elements with even atomic number (Z) in the interval Z = 50-58 have been resolved in the cosmic radiation using the Heavy Nuclei Experiment on the HEAO-3 satellite. The observation that Sn-50 and Ba-56 are more abundant than Te-52 and Xe-54 indicates a substantial s-process contribution to the cosmic ray source. A significant abundance of Ce-58 provides further support for this finding.

Stone, E. C.↗
NASA NTRS1985

Ion-beam-assisted etching of diamond

The high thermal conductivity, low RF loss, and inertness of diamond make it useful in traveling wave tubes operating in excess of 500 GHz. Such use requires the controlled etching of type IIA diamond to produce grating like structures tens of micrometers deep. Previous work on reactive ion etching with O2 gave etching rates on the order of 20 nm/min and poor etch selectivity between the masking material (Ni or Cr) and the diamond. An alternative approach which uses a Xe(+) beam and a reactive gas flux of NO2 in an ion-beam-assisted etching system is reported. An etching rate of 200 nm/min was obtained with an etching rate ratio of 20 between the diamond and an aluminum mask.

Efremow, N. N.↗
NASA NTRS1985

Operation of a microchannel plate counting system in a mass spectrometer

A multiplier detector system has been developed as part of a mass spectrometer in an ion counting mode. During its operation ions striking a microchannel plate release pulses of electrons which are accelerated to a phosphor layer. The resulting flash of light is carried by a fiber-optic bundle to a photodiode array. The detector simultaneously counts ions throughout a mass spectrum covering more than 30 amu. It extends the mass spectrometer's operation toward low count rates for trace gas analysis. Each mass peak can be independently measured at count rates between 0.1 and 1000 counts/s. Higher rates on a few peaks do not blind the entire detector. The detector's capabilities have been demonstrated by measuring Kr and Xe isotopes in air at natural abundances. The mass spectrometer, detector, and readout electronics are sufficiently compact to permit use in a balloon-borne experiment.

Murphy, D. M.↗
NASA NTRS1984

Substrate heating rates for planar and cylindrical-post magnetron sputtering sources

Results are presented for the substrate heating energy/atom required in the planar magnetron sputtering of Al, Cr, Ni, Cu, Mo, In, Ta, W, and Pt in Ar, as well as Al and Cr in O2. Data are also obtained for cylindrical magnetron sputtering of Nb, Ag, Ta, W, and Pb-Sn in Ar, and Mo sputtered in Ne, Ar, Kr, and Xe. Planar and cylindrical magnetron heating rates were comparable. Special experiments were conducted to examine the contributions to substrate heating of plasma species and ion neutralization and reflection at the cathode; the results obtained indicate that charged plasma species do not significantly contribute to the heating, but that neutralized and reflected ions play a significant role in the planar as well as cylindrical cases despite the differences in cathode geometry.

Thornton, J. A.↗