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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Gain measurements of the Ca-Xe charge exchange system

Charge-exchange-pumped Ca(+) was studied for possible positive laser gain at 370.6 and 315.9 nm using an Xe MPD arc as the Xe(+) source. The present paper describes the MPD arc, the calcium injection system, the diagnostics for gain, and spontaneous emission measurements and results. No positive gain measurements were observed. A small Xe-Ca charge exchange cross section compared to He-metal laser systems charge exchange cross sections is the most probable reason why the result was negative.

Michels, C. J.↗
NASA NTRS1980

Proportional retention of I and radiogenic Xe-129 in preheated Allende

In order to study the effects of heat loss and redistribution of iodine and xenon on the I-Xe age, the iodine-xenon dating technique was applied to a bulk sample of the Allende chondrite and a similar sample preheated to 900 C prior to the irradiation. Little iodine or Xe-129 were lost from the high-temperature fractions of the preheated sample, and the high-temperature isochron was fully preserved. The amounts of I and radiogenic Xe-129 retained by the preheated sample were nearly proportional in those temperature fractions (900-1150 C) where most I and radiogenic X-129 were lost. However, the intermediate-temperature isochron was modified, resulting in an apparent isochron which would predate the high-temperature isochron by 6.7 + or - 1.1 m.y.

Rison, W.↗
NASA NTRS1982

Shock disturbance of the I-Xe system

Three separate samples of the meteorite Bjurbole were artificially shocked at pressures of 70 kb, 200 kb, and 400 kb. Analysis of xenon released in stepwise heating shows that the I-Xe system of the 400 kb sample is substantially altered by the shock loading, and it is no longer possible to infer an age or trapped xenon composition for that sample. The 200-kb and 70-kb samples display isotopic structures progressively less altered demonstrating the gradations in shock disturbance likely to be found in natural systems. Interpretations of the I-Xe and Ar-40-Ar-39 systems for several naturally shocked meteorites are also presented. New data for Arapahoe do not confirm the previously reported age and trapped xenon composition, demonstrating instead that its I-Xe structure has been strongly disturbed by shock.

Caffee, M. W.↗
NASA NTRS1986

I-Pu-Xe dating and the relative ages of the earth and moon

The ages of the earth and moon as determined by various chronometric systems are discussed with primary emphasis placed on the development of an I-Pu-Xe chronometer. Data on excess fission xenon are reviewed with attention given to the strengths and weaknesses of the assumptions required for lunar I-Pu-Xe chronometry. Using I-Pu-Xe dating, it is estimated that the retention of excess fission xenon in lunar samples began no more than 63 + or - 42 m.y. after the time of primitive meteorite formation.

Swindle, T. D.↗
NASA NTRS1993

I-Xe structure of ILAFEGH 009 and shallowater: Evidence for early formation and rapid cooling of impact-derived enstatite meteorites

Enstatite meteorites have proven to be ideal samples for past studies of the I-Xe system. This work focuses on two enstatite meteorites that were formed by impact processes. Ilafegh 009 is a clast-free impact melt rock from the EL chondrite parent body. The Shallowater aubrite likely formed when a fully molten planetesimal collided with a solid planetesimal, mixing fragments of the solid planetesimal into the enstatite mantle of the molten planetesimal. A complex three-stage cooling history resulted from this mixing and later break-up and reassembly of the parent body. The present study indicates that the I-Xe structure of these two meteorites resulted from in situ decay of live I-129 and that both experienced xenon closure of the iodine host phase at approximately the same time. I-Xe cooling rates are consistent with the cooling rates derived from mineralogic and petrologic studies of these objects. The similarities in ages suggest that the region of the nebula in which enstatite parent bodies formed must have experienced an intense early bombardment.

Kehm, K.↗
NASA NTRS2005

Ar-Ar and I-Xe Ages and the Thermal History of IAB Meteorites

Studies of several samples of the large Caddo County IAB iron meteorite reveal andesitic material, enriched in Si, Na, Al and Ca, which is essentially unique among meteorites. This material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening. Such an origin implies extended metamorphism of the IAB parent body. New Ar-39- Ar-40 ages for silicate from three different Caddo samples are consistent with a common age of 4.50-4.51 Gyr ago. Less well defined Ar-Ar degassing ages for inclusions from two other IABs, EET8333 and Udei Station, are approx.4.32 Gyr, whereas the age for Campo del Cielo varies considerably over approx.3.23-4.56 Gyr. New I-129-Xe-129 ages for Caddo County and EET8333 are 4557.9+/-0.1 Myr and 4557-4560 Myr, respectively, relative to an age of 4562.3 Myr for Shallowater. Considering all reported Ar-Ar degassing ages for IABs and related winonaites, the range is approx.4.32-4.53 Gyr, but several IABs give similar Ar ages of 4.50-4.52 Gyr. We interpret these older Ar ages to represent cooling after the time of last significant metamorphism on the parent body, and the younger ages to represent later 40Ar diffusion loss. The older Ar-Ar ages for IABs are similar to Sm-Nd and Rb-Sr isochron ages reported in the literature for Caddo County. Considering the possibility that IAB parent body formation was followed by impact disruption, reassembly, and metamorphism (e.g., Benedix et al. 2000), the Ar-Ar ages and IAB cooling rates deduced from Ni concentration profiles in IAB metal (Herpfer et al., 1994) are consistent if the time of the post-assembly metamorphism was as late as approx.4.53 Gyr ago. However, I-Xe ages reported for some IABs define much older ages of approx.4558-4566 Myr, which cannot easily be reconciled with the much younger Ar-Ar and Sm-Nd ages. An explanation for the difference in radiometric ages of IABs may reside in combinations of the following: a) I-Xe ages have very high closure temperatures and were not reset during metamorphism approx.4.53 Gyr ago; b) a bias exists in the 40K decay constants which makes these Ar-Ar ages approx.30 Myr too young; c) the reported Sm-Nd and Rb-Sr ages for Caddo are in error by amounts equal to or exceeding their reported 2-sigma uncertainties; and d) about 30 Myr after the initial heating that produced differentiation of Caddo silicate and mixing of silicate and metal, a mild metamorphism of the IAB parent body reset the Ar-Ar ages.

Bogard, Donald D.↗
NASA NTRS2021

Long-Duration Wear Testing of the ASTRAEUS Thruster, Phase I: 50 kg Xe Throughput

The thruster component of the Ascendant Sub-kW Transcelestial Electric Propulsion System successfully completed a propellant throughput demonstration of 50 kg Xe. The thruster was operated for a total of 3,287 h, of which 97.9% was spent at 300 V – 1000 W, and generated 790 kN-s of total impulse. Thrust, telemetry, and temperature measurements periodically recorded at four operating conditions spanning 200 – 500 V and 200 – 1000 W revealed constant performance across the full test duration. High-fidelity plasma simulations of the erosion of the thruster’s downstream faces suggest a theoretical lifetime capability of >30 kh, corresponding to propellant throughput of >450 kg Xe and a total impulse of >7 MNs, at the 300 V – 1000 W operating condition. The test was voluntarily paused after 50 kg Xe throughput to take these erosion measurements before the test was resumed at a different operating condition.

Goebel, Dan↗
NASA NTRS1971

Xe and Kr analyses of silicate inclusions from iron meteorites.

Measurements have been conducted of the amounts and isotopic composition of Xe and Kr in silicate inclusions of several iron meteorites. It is shown that the Xe and Kr contents are comparable to chondritic values. The isotopic compositions show trapped gas of both chondritic and atmospheric composition. Large spallation effects occur in some of the meteorites; the spallation spectra in some instances differ from those reported for stone meteorites. In several meteorites, very large neutron capture effects on Br and I occur. All samples have pronounced Xe129 excesses which apparently indicate differences in the formation times from chondrites of less than about 100 million years; however, the presence of trapped Xe132 in silicates which were enclosed in molten Fe-Ni and cooled slowly proves that they were not entirely outgassed, so that some of the Xe129 excess may also be trapped.

Bogard, D. D.↗
NASA NTRS1978

Limitation of 3547 to 1182 A conversion efficiency in Xe

It is found that the 3547 to 1182 A conversion efficiency in Xe-Ar mixtures is limited to about 0.9% by Kerr-induced dispersion. The primary effect is felt to be produced by a mixed-frequency Kerr nonlinearity which changes the index of refraction at 1182 A in the presence of large power densities at 3547 A, affecting phase matching. It is suggested that a Xe-Mg-Ar mixture compensates for the Kerr-induced dispersion and permits increased efficiencies.

Zych, L. J.↗
NASA NTRS1980

Search for Xe-129 in mineral grains from Allende inclusions - An exercise in miniaturized rare gas analysis

The paper considers the use of a static mass spectrometer modified by installation of a Baur-Signer ion source and an ion-counting system to detect xenon from single grains from inclusions in the C3 meteorite Allende. The inferred iodine concentrations increased from pentlandite to melilite, but vary widely from one grain to another, indicating that the iodine resides in a minor phase which is included 'spottily' in the bulk phases. It is likely that the detectability for excess Xe achieved here could be improved; about 23,000 of fissiogenic Xe-132 was detected in the largest melelite sample analyzed, but it cannot be determined from this study whether the PU-244 is 'spotty' like I-129 or is uniformly distributed in the melilite.

Reynolds, J. H.↗
NASA NTRS1980

Na + Xe collisions in the presence of two nonresonant lasers

Na+Xe collisions in the presence of two distinct laser fields (rhodamine 110 and Nd:glass) are investigated with reference to the response to nonresonant radiation of alkali metals collisionally perturbed by a buffer gas. It is found that the excited Na-asterisk (4s)+Xe state is produced with a measurable cross section due to two-photon absorption with field intensities as low as 10 MW/sq cm.

De Vries, P. L.↗
NASA NTRS1980

Computational study of alkali-metal-noble gas collisions in the presence of nonresonant lasers - Na + Xe + h/2/pi/omega sub 1 + h/2/pi/omega sub 2 system

The collision of Na with Xe in the presence of both the rhodamine-110 dye laser and the Nd-glass laser is investigated within a quantum-mechanical close-coupled formalism, utilizing ab initio potential curves and transition dipole matrix elements. Both one- and two-photon processes are investigated; the Na + Xe system is not asymptotically resonant with the radiation fields, so that these processes can only occur in the molecular collision region. The one-photon processes are found to have measurable cross sections at relatively low intensities; even the two-photon process has a significant section for field intensities as low as 10 MW/sq cm.

Devries, P. L.↗
NASA NTRS1980

I-Xe dating of Allende inclusions - Antiquity and fine structure

Two inclusions from the Allende meteorite exhibit multiple I-Xe isochrons corresponding to an age difference of 3.7 + or - 1.5 m.y. for their constituent minerals. Consequently, I-Xe dating can establish, independently of thermodynamic models, a partial sequence of formation for minerals that incorporated radioactive I-1290. Additionally, the results give an estimate of the cooling rate for the solar nebula when validity of thermodynamic models is assumed.

Zaikowski, A.↗
NASA NTRS1981

I-Xe dating - Intercomparisons of neutron irradiations and reproducibility of the Bjurbole standard

Previously unpublished data from the stepwise heating analysis of the Valecitos 1 (V1) Bjurbole standard are presented together with the results of an isotopic dilution measurement of a KI monitor included in the V1 irradiation package. The results of these analyses are compared with the values observed in other irradiations. Apparent I-Xe 'formation' ages are reproducible for three different samples of Bjurbole, suggesting isotopic homogeneity for initial iodine in the bulk material. The systematics of neutron capture in Xe-135 (produced from U-235 neutron fission) are examined and verified in irradiated BCR-1.

Hohenberg, C. M.↗
NASA NTRS1981

Multiple-path fission-foil nuclear lasing of Ar-Xe

Nuclear lasing of Ar 10% Xe at 760 torr with a thermal neutron flux of 3.5 x 10 to the 16th n/sq cm s has produced an output power of 50 W. Lasing occurred at 2.6 microns in Xe-I. The argon buffer gas is shown to efficiently stop fission fragments at reasonable pressures emitted from (U-235)3O8 coatings. A unique folded optical path was used to increase the excited gas volume which in turn increases laser output.

De Young, R. J.↗
NASA NTRS1984

The I-Pu-Xe age of the Moon

The Rb-Sr analyses of some lunar samples which indicate that the Moon is close to the age of primitive meteorites are only reliable to within about 100 m.y. A potentially more precise chronometer is the I-Pu-Xe system. I129 has a 17 m.y. halflife and decays to Xe129; Pu244, with an 82 m.y. halflife, produces Xe131 to Xe136 in fission. The I129/Pu244 ratio has a halflife of 21 m.y. Xenon retention for the Earth could have begun as late as the event that gave birth to the Moon. For the Moon, it is hard to imagine that xenon retention could have begun before re-accretion of the fissioned (and initially dispersed?) material, particularly if that material got hot enough to account for the depletion of the volatile elements. Thus, if fission model are correct, xenon retention in the Earth certainly began no later than in the Moon, and possibly began earlier. Therefore, the I-Pu-Xe system is only marginally consistent with a fission origin. If further study confirms that the I/U ratio of the Moon is .01 or less, or if gas-rich lunar highland breccias with higher ratios of I129 to Pu244 are found, it would be difficult to explain the results in an earth-fission model of lunar origin.

Swindle, T. D.↗