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NASA NTRS1983

Cosmic-ray isotopic composition

The 'age' of the cosmic rays and the origin of cosmic ray source matter are discussed. General approaches to the interpretation of the abundances of secondary radioactive nuclides are reviewed using Be-10 as an illustration. The present state of Be-10 based cosmic ray age determinations are summarized, briefly mentioning some recent results based on the isotope Al-26. The effects of nonhomogeneous propagation models on the interpretation of the radioactive isotope observations are mentioned. The cosmic ray source isotopic composition is discussed, emphasizing the neon composition. Mass histogram findings on the abundance of various elements in cosmic rays are described and possible reasons for some abundance enhancements are considered.

Wiedenbeck, M. E.↗

NASA NTRS1983

The effect of cross-section uncertainties on the derivation of source abundances from cosmic-ray composition observations

It is pointed out that the derivation of source abundances from the composition observed near the earth requires that the secondary contribution to the observed nuclidic abundances be calculated from a model of cosmic-ray propagation. A crucial element in such a calculation is the choice of nuclear fragmentation cross sections. Uncertainties in these cross sections give rise to uncertainties in the derived source abundances. It is shown here that the uncertainties in fragmentation cross sections can, in certain important cases, contribute significantly to the uncertainties in cosmic ray source abundances deduced from the observed composition. For this reason, it is essential that reliable estimates of the effect of cross section uncertainties be made when interpreting the source composition deduced from cosmic ray observations. In addition, formulas are presented which can be used to obtain such estimates with a minimum of computational effort.

Wiedenbeck, M. E.↗

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 NTRS1985

Instrumental background in balloon-borne gamma-ray spectrometers and techniques for its reduction

Instrumental background in balloon-borne gamma-ray spectrometers is presented. The calculations are based on newly available interaction cross sections and new analytic techniques, and are the most detailed and accurate published to date. Results compare well with measurements made in the 20 keV to 10 MeV energy range by the Goddard Low Energy Gamma-ray Spectrometer (LEGS). The principal components of the continuum background in spectrometers with GE detectors and thick active shields are: (1) elastic neutron scattering of atmospheric neutrons on the Ge nuclei; (2) aperture flux of atmospheric and cosmic gamma rays; (3) beta decays of unstable nuclides produced by nuclear interactions of atmospheric protons and neutrons with Ge nuclei; and (4) shield leakage of atmospheric gamma rays. The improved understanding of these components leads to several recommended techniques for reducing the background.

Gehrels, N.↗

NASA NTRS1985

Variations in the propagation of UH-nuclei

Calculations of the propagation of UH-nuclei were improved by extending the number of individual nuclides considered, and by using more recent evaluations of the rigidity dependence of the escape length, the possible source composition, and altered cross sections. The effects of using different expressions for the dependence of abundances on first ionization potentials (FIP) are outlined. The sensitivity of the calculated elemental abundances to the various changes made in the propagation assumptions are discussed.

Waddington, C. J.↗

NASA NTRS1985

Nineteenth International Cosmic Ray Conference. SH Sessions, Volume 5

Papers submitted for presentation at the 19th International Cosmic Ray Conference are compiled. This volume contains papers addressing cosmic ray gradients in the heliosphere; siderial, diurnal, and long term modulations; geomagnetic and atmospheric effects; cosmogenic nuclides; solar neutrinos; and detection techniques.

Jones, F. C.↗

NASA NTRS1984

The isotopic composition of cosmic rays

Observations of the isotopic composition of the elements carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, calcium, iron and nickel are reviewed and 'best' values of selected isotope ratios are derived from the collected data. A simple model of cosmic ray propagation is employed to deduce cosmic ray source abundance ratios both from these best values and from the observations made in a recent high resolution satellite experiment. General features of the cosmic ray source composition are discussed, with emphasis on the enhanced abundances of various neutron rich nuclides and on the distinctly nonsolar character of the nitrogen isotopic abundances.

Wiedenbeck, M. E.↗

NASA NTRS1985

History and current understanding of the Suess abundance curve

By means of a gamma ray sum spectrometer currently under development, many neutron capture cross sections will be determined with high accuracy, thereby facilitating the inferrence of the abundance ratios of neighboring elements with the same accuracy. Attention is presently given to this and other prospects emerging in the study of nuclide abundances and the data base for theories of element synthesis. Poorly understood structures in the abundance curve are discussed in view of recent models on nuclear structure and stability.

Marti, K.↗

NASA NTRS1985

Solar abundances and the role of nucleogenesis in low-to-medium mass stars in the galaxy

The pattern of solar elemental abundances agrees well with that shown by Cl chondrites for nonvolatile elements. For metals of the iron peak, the chief source of uncertainty seems to be the structure of the solar atmosphere. Lines of rare elements are frequently masked by atomic and molecular lines of abundant species. The vast majority of stars (including the sun) will do little to change the bulk composition of the interstellar medium from which new stars are formed. He, C, and N in small quantities are supplied by stars from 1 to 8 solar masses as they evolve and produce nebular envelopes that dissipate into the interstellar medium, but as has long been recognized, oxygen, heavier elements, and all r-process and proton-rich nuclides are made in massive stars.

Aller, L. H.↗

NASA NTRS1985

The propagation of ultraheavy cosmic-ray nuclei revisited

The propagation of ultraheavy (Z = 30 or more) cosmic-ray nuclei is investigated theoretically in the leaky-box model, expanding the analysis of Brewster et al. (1983) to include more individual nuclides and account for recent findings with respect to source compositions, cross sections, and the dependence of escape length and spectral characteristics on rigidity. The results of numerical computations are presented in tables and graphs and compared with observational data (from HEAO-3) and other theoretical predictions. Good agreement is obtained for Z = 32-42 and 50-58 using a solar-system source with a first-ionization-potential correlation factor and the solar-system abundances given by Anders and Ebihara (1982).

Brewster, N. R.↗

NASA NTRS1986

Cosmic ray records in Antarctic meteorites

The cosmogenic radionuclides Be(10), Al(26), and Mn(53) and noble gases were determined in more than 28 meteorites from Antarctica by nuclear analytical techniques and static mass spectrometry, respectively. The summarized results are listed. The concentrations of Al(26) and Mn(53) are normalized to the repective main target elements and given in dpm/kg Si sub eq and dpm/kg Fe. The errors stated include statistical as well as systematical errors. For noble gas concentrations estimated errors are 5% and for isotopic ratios 1.5%. Cosmic ray exposure ages T sub 21 were calculated by the noble gas concentrations and the terrestrial residence time (T) on the basis of the spallogenic nuclide Al(26). The suggested pairing of the LL6 chondrite RKPA 80238 and RKPA 80248 and the eucrites ALHA 76005 and ALHA 79017 is confirmed not only by the noble gas data but also by the concentrations of the spallation produced radionuclides. Futhermore, ALHA 80122, clasified as an H6 chondrite, has a noble gas pattern which suggest that this meteorite belongs to the ALHA 80111 shower.

Vogt, S.↗

NASA NTRS1986

Cosmogenic nuclei

Cosmogenic nuclei, nuclides formed by nuclear interactions of galactic and solar cosmic rays with extraterrestrial or terrestrial matter are discussed. Long lived radioactive cosmogenic isotopes are focused upon. Their uses in dating, as tracers of the interactions of cosmic rays with matter, and in obtaining information on the variation of primary cosmic ray flux in the past are discussed.

Raisbeck, G. M.↗

NASA NTRS1986

Production of radionuclides in artificial meteorites irradiated isotropically with 600 MeV protons

The understanding of the production of cosmogenic nuclides in small meteorites (R is less than 40 cm) still is not satisfactory. The existing models for the calculation of depth dependent production rates do not distinguish between the different types of nucleons reacting in a meteorite. They rather use general depth dependent particle fluxes to which cross sections have to be adjusted to fit the measured radionuclide concentrations. Some of these models can not even be extended to zero meteorite sizes without logical contradictions. Therefore, a series of three thick target irradiations was started at the 600 MeV proton beam of the CERN isochronuous cyclotron in order to study the interactions of small stony meteorites with galactic protons. The homogeneous 4 pi irradiation technique used provides a realistic meteorite model which allows a direct comparison of the measured depth profiles with those in real meteorites. Moreover, by the simultaneous measurement of thin target production cross sections one can differentiate between the contributions of primary and secondary nucleons over the entire volume of the artificial meteorite.

Michel, R.↗

NASA NTRS1986

Compilation of cosmogenic radionuclides in meteorites

All available data for the concentration of cosmogenic nuclides Mn-53(t(1/2) = 3.7 x 10(6) years), Al-26 (7.05 x 10(5) years), Be-10 (1.6 x 10(6) years), Cl-36 (3.0 x 10(5) years) and Ne-21, and Ne-22/Ne-21 ratios in stony, iron, and stony-iron meteorites were compiled. For iron meteorites, the He-4/Ne-21 ratio was adopted instead of Ne-22/Ne-21 ratio, because the He-4/Ne-21 ratio in iron meteorites indicates the shielding condition of the sample. The compilation contains over 2000 different analyses for four cosmogenic radionuclides.

Nishiizumi, K.↗

NASA NTRS1986

Simulation of cosmic irradiation conditions in thick target arrangements

One approach to simulate 2-pi irradiation conditions of planetary surfaces which has been widely applied in the past are bombardments of so called thick targets. A very large thick target was exposed recently to 2.1 GeV protons at the Bevatron-Bevalac in Berkeley. In a 100x100x180 cm steel-surrounded granodiorite target radioactive medium and high energy spallation products of the incident primary and of secondary particles were analyzed along the beam axis down to depths of 140 g/cm(2) in targets such as Cu, Ni, Co, Fe, T, Si, SiO2 and Al. Activities of these nuclides were exclusively determined via instrumental gamma-ray spectroscopy. Relative yields of neutron capture and spallation products induced in Co and Cu targets during the thick target bombardment are shown as a function of depth. The majority of the medium energy products such as Co-58 from Co targets exhibit a maximum at shallow depths of 40-60 g/cm(2) and then decrease exponentially. In a comparable 600 MeV proton bombarded thick target such a slight maximum for medium energy products was not observed. Rather, Co-58 activities in Co decreased steadily with the highest activity at the surface. The activities of the n-capture product Co-60 increase steadily starting at the surface. This indicates the rapidly growing flux of low energy neutrons within the target.

Theis, S.↗

NASA NTRS1986

Formation of the sun and its planets

Aspects of star formation are considered, taking into account those observations and models which have a relation to the formation of the solar system. Attention is given to the galactic structure, the stellar properties, planetary indications, interstellar clouds, observations of forming stars, the conditions for cloud collapse, models of star formation, the modeling of secondary features, and aspects of dynamical evolution. Aspects of cosmochemistry are also explored. Questions of chronology are discussed along with nuclide variations, chemical variations among chondritic meteorites, and chemical variations among differentiated objects. A description of planet formation is provided, taking into consideration, gas-dust interaction, sun and accretion disk dynamics, planetesimal swarms, planet-disk interactions, planetary systems, terrestrial planet formation, and gaseous protoplanet contraction.

Kaula, W. M.↗

NASA NTRS1986

Lunar and Planetary Science Conference, 16th, Houston, TX, March 11-15, 1985, Proceedings. Part 2

The present conference presents papers on the criteria, data and implications of pristine lunar glasses, lunar granulities and their precursor anorthositic norites of the early lunar crust, characterization and evidence for early formation in the megaregolith of Apollo 16 regolith breccias, and anorthosite assimilation and the origin of the Mg/Fe-related bimodality of pristine moon rocks in support of the magmasphere hypothesis. Other topics include the mineralogy of Yamato 791073 with reference to crystal fractionation of the howardite parent body, the geology and geomorphology of the Venus surface as revealed by the radar images obtained by Veneras 15 and 16, tidal dissipation in a viscoelastic planet, and cosmogenic neutron-capture-produced nuclides in stony meteorites. Also considered are the first results of hydrous alteration of amorphous silicate smokes, elemental analysis of a comet nucleus by passive gamma ray spectrometry from a penetrator, and uranium series dating of Allan Hills ice.

Ryder, G.↗

NASA NTRS1986

Uranium series dating of Allan Hills ice

Uranium-238 decay series nuclides dissolved in Antarctic ice samples were measured in areas of both high and low concentrations of volcanic glass shards. Ice from the Allan Hills site (high shard content) had high Ra-226, Th-230 and U-234 activities but similarly low U-238 activities in comparison with Antarctic ice samples without shards. The Ra-226, Th-230 and U-234 excesses were found to be proportional to the shard content, while the U-238 decay series results were consistent with the assumption that alpha decay products recoiled into the ice from the shards. Through this method of uranium series dating, it was learned that the Allen Hills Cul de Sac ice is approximately 325,000 years old.

Fireman, E. L.↗