Biography:

In the past K. Thiel has collaborated on articles with D.E. Arafah and H. Kochan. One of their most recent publications is Uranium distribution in basalt fragments of five lunar samples. Which was published in journal Earth and Planetary Science Letters.

More information about K. Thiel research including statistics on their citations can be found on their Copernicus Academic profile page.

K. Thiel's Articles: (14)

Uranium distribution in basalt fragments of five lunar samples

AbstractThe U-distribution in selected basaltic fragments from lunar fines 10084, rocks 12021 and 12053 and breccias 14305 and 14321 was studied by means of fission tracks in mica detectors. U-enriched mineral phases were localized with an accuracy of better than 1 μm by applying a photographic mapping technique. Identification and elemental analysis were carried out using an electron microprobe. Phases of high U-content were identified as apatite (30–280 ppm), SiO2-rich glasses (3–150 ppm) or fine-grained Zr-containing minerals of grain sizes < 1 μm (50–450 ppm). These U-enriched phases preferentially occur in the mesostasis. An inverse relationship was found between the U-concentration and the volume of the mesostasis regions. The ratio of the U-contents of the residual liquid and the major phases is comparable with that found in terrestrial basalts.

235U fission tracks and238U-series disequilibria as a means to study recent mobilization of uranium in Archaean pyritic conglomerates

AbstractUranium transport in Archaean sediments was investigated by means of two carefully selected suites of variably weathered sediment samples from different localities of the Pongola Supergroup, Kaapvaal Craton, South Africa. U remobilization on a micron- to meter-scale was analyzed using U fission-track micromapping and α-spectrometry bulk measurements. These studies were supported by ore microscopy, neutron activation analysis and γ-spectrometry. By combining particle track technique and α-spectrometry detailed information is obtained on the U transport history of weathered surface samples. A simple model of U redistribution processes in geologic systems based on238U-series disequilibria is developed and applied to the samples investigated. Some implications for γ-radiometric field methods are discussed.

A sensitive mapping technique for bismuth using α-particle tracks☆

AbstractBi-containing metals, minerals, and glasses were irradiated with thermal neutrons to produce the isotope 210Po from the reaction 209Bi (n, γ) 210Biβ−→210Po. After n-irradiation the Bi microdistributions in the samples were recorded in cellulose nitrate and cellulose triacetate by α-track radiography using the induced 210Po α-activity. The possible interference of U and Th and their decay series is discussed and taken into account. The ultimate Bi-detection limit was found to be ca. 10 ppb, and the spatial resolution achievable at Bi concentrations in the ppm range is ≈ 10 μm.

Phenomenology and dynamic behavior of the dust component in the KOSI experiments

AbstractThe KOSI (Kometensimulation) project (1987–1993) was intended as a series of multi-discipline experiments to investigate porous ice-dust mixtures under space conditions in view of a better understanding of comets. The present paper gives a synoptic summary of results obtained in the simulation experiments that are related particularly to the phenomenology and dynamic behavior of the dust component. Sample preparation was achieved by spraying aqueous suspensions of mineral powders (olivine, montmorillonite) into liquid nitrogen, which implies contact to liquid water. After sublimation of the ice both montmorillonite and olivine containing residues show a size dependence in porosity and mass density that is typical for fractal-like particles. The montmorillonite containing dust residues after artificial insolation were found to form coherent “tactoids” of high electrical conductivity. The decrease of the dust emission activity of fresh ice-dust mixtures with increasing time of insolation is explained by the formation of a volatile-depleted dust mantle that quenches further activity. The surface temperature was found to be directly related to the thickness of the ice-free dust cover and to the elevation angle of the light source above the local horizon. The surface topography of the sample after irradiation indicates the occurrence of local mantle displacements (“dust avalanches”) on inclined surfaces due to gas drag induced lifting and slipping down of parts of the dust cover. The local dust removal and deposition leads to the formation of valleys and ridges parallel to the gradient of inclimation. Similar features are expected to occur on cometary nuclei. Test particles of defined size and density were used to simulate meteoroid impact events on a developed dust mantle during insolation. The mean local surface temperature was found to drop immediately after impact by 1–7 K, depending on the total cross-section of the particles. A simultaneous enhancement of the gas emission was observed, the increase of the local gas flux density being anticorrelated to the surface temperature. Particle acceleration due to the enhanced gas drag was found to vary from <10 to 17 m s−2 depending on the particle size. Implications for impact induced phenomena on comets are discussed.

Ion range studies of cerium implanted CaF2

AbstractRange measurements for cerium ions implanted in CaF2 have been made for the energy range 50 to 400 keV. The projected ranges and range distributions determined by RBS have been compared with theoretical estimates. Calculations were made for a selection of nuclear interaction potentials and various electronic stopping factors. Good agreement between theory and experiment has been achieved by means of a Molière potential.

Track radiography of heavy-ion induced sputtering☆

AbstractTo study heavy-ion induced sputter phenomena, several glasses of different chemical composition were irradiated with 56Fe+-ions in the energy of 70.5–100 keV at the GSI-UNILAC, Darmstadt. A sensitive particle track technique was employed to measure differential sputter yields and the angular distribution of sputtered atoms in 2π-geometry.This techniques utilizes U, Th, Li and B contained in the target matrix as tracer elements. The sputtered target atoms are collected on a cone-shaped catcher (Makrofol polycarbonate or Teflon foil) and the target atoms are recorded via (n, f) and (n, ƒ) reactions as particle-track radiographs of the catcher on mica or CA 80-15 cellulose nitrate detectors.The cone shape of the catcher foils allows one to scan the sputtering process in 2π-geometry in a single irradiation experiment. The detection sensitivity for sputtered atoms is estimated to be <5×1011 target atoms cm−2 in the case phosphate glass containing 0.06% 235U and <1013 target atoms cm-2 in the case Li2B4O7 glass.Angular distributions for U-phosphate and Li2B4O7 glass sputtered by 100 keV 56Fe+-ions are presented.

Comet simulation experiments in the DFVLR space simulators

AbstractThe experiments are performed in two space simulation facilities of different dimensions (the simulated characteristics are vacuum, background temperature and solar irradiation). 1) The big Space Simulator is a horizontal cylinder of 4.8 m length and 3.5 m diameter. The test volume is surrounded by a liquid nitrogen-cooled shroud. The comet sample is cylindrical, 30 cm in diameter and 15 cm in height. 2) Height and diameter of the small chamber are around 1 m each. The model comet has dimensions of 10 × 7 × 6 cm3. The samples are prepared by injecting well-defined suspensions of water and minerals into liquid nitrogen. Results, obtained in the initial simulation tests are: The activity (gas and dust emission from the sample upon irradiation with light) of a model comet of given composition depends on the chamber pressure and the surface temperature. The angular distribution of the dust emission has a maximum towards the surface normal with a slight shift in the sunward direction. Particle velocities lie around a few m/s for 0.1 to 1 mm dust particles. The residual dust grains resemble Brownlee particles in porosity and mass density.

Mechanisms of dust emission from the surface of a cometary nucleus

AbstractCometary nuclei consist of mixtures of ices and non-volatile particulates in mass proportions of similar magnitudes. Dust is carried by the gas drag of the sublimating ices. Gas drag from free expansion of the gases from the nucleus dominates the acceleration of dust particles after their release from the surface. Significant gas pressure can build up in the ice matrix by sublimation of ices of different volatility and by non-uniform heat absorption in the ice. A dust mantle of several mm thickness which builds up over the sublimating ice surface quenches the gas outflow and causes a pressure build up across the mantle. The pressure gradient can become unstable and microeruptions of gas and dust occur. These mechanisms are in agreement with findings from recent simulation experiments in which emission speeds of 1 to 2 m/s have been observed for particles of about 10−4 m in radius

Uranium redistribution in weathered conglomerates of the early precambrian pongola supergroup, South Africa - inferences from a study by alpha spectrometry and fission track micromapping

AbstractSamples of potentially uranium-bearing conglomerates from deeply weathered outcrops of the 2900 to 3200 m.y. old Pongola Supergroup were investigated from three different localities in the southeastern Transvaal and northern Zululand. Uranium isotope and uranium fission track analyses were carried out to study and to unravel the complex uranium redistribution processes which took place and which are still in progress in the weathering zone of the conglomerates. It is proposed that the combination of the two radiochemical methods can provide valuable information assisting the exploration of uranium mineralization in early Precambrian quartz-pebble conglomerates.

Some applications of uranium tracing in geological samples using thermal neutron induced fission

AbstractThe mode of distribution of uranium in geological samples is closely related to the type of geochemical environment during the time of formation or alteration of a specific specimen. By high resolution micromapping of U in sediment samples from the Kaapvaal Craton, South Africa, using n-induced U-235 fission tracks in mica detectors, information is obtained about the redox potential of the hydrosphere- atmosphere system at the time of U-deposition or remobilization. Mineralogical and textural considerations as well as radioactive disequilibria in the natural U-decay series, measured by α-spectrometry, were used to identify the type of U-mobilization. The U-distribution patterns of carefully selected conglomerate samples from the Witwatersrand basin are in agreement with the assumption of a predominantly reducing Earth atmosphere at the time of basin formation some 2.6 × 109 y ago.

Heterogeneous catalysis with supported platinum colloids: A systematic study of the interplay between support and functional ligands

AbstractWhereas colloidal metal nanoparticles have attracted considerable interest in homogeneous catalysis, the effect of organic ligands has been less systematically investigated in heterogeneous gas-phase catalysis. This paper aims at elucidating this aspect for nanoparticles capped with dodecylamine (DDA), which have been deposited on three different support materials with varying acid/base properties, namely γ-Al2O3, SiO2 and MgO. For this purpose, a synthetic approach was applied that is based on the preparation of ligand-free Pt nanoparticles in ethylene glycol. By functionalizing these particles subsequently with ligands, it is possible to obtain ligand-free and ligand-capped particles with the same metal core (e.g. identical size and shape), thus allowing to investigate the influence of the ligands without changing any other parameter. After deposition on the different supports, the Pt nanoparticles were characterized by STEM, AAS and DRIFTS. The catalytic properties of these catalysts were investigated under two different reaction regimes: first, octadiene hydrogenation served as a test reaction to probe the influence of the ligands on larger molecules under reducing conditions at low temperatures (T < 100 °C) where the ligand shell is intact. The results show that ligands can strongly modify metal–support interactions and exert a protecting effect with respect to support induced oxidation of Pt surface atoms that occurs during particle deposition. In particular, in the case of the Brønsted acidic SiO2 support, where surface oxidation of Pt is most pronounced, the ligand-capped sample is significantly more active for octadiene hydrogenation than the ligand-free counterpart. Second, the samples were tested with respect to CO oxidation at high temperatures (T ∼ 200 °C) where processes like decomposition/desorption and spillover of ligands on the support become important. Depending on the acid/base and adsorption properties of the different supports, the spillover of DDA turns out to be the main reason for diminishing the ligand coverage of the nanoparticles under these conditions. Whereas spillover is most pronounced on Lewis acidic γ-Al2O3, a specific interaction between the basic MgO and DDA, namely its catalytic transformation into a nitrile, leads to enhanced spillover when compared to the Brønsted acidic SiO2. These observed ligand effects are not limited to catalysts synthesized with the ethylene glycol method but are also observed in the case of the particles prepared by a classical colloidal approach.

Electrochemical silver deposition on adsorbate modified Au(1 1 1) electrode

AbstractThe silver underpotential deposition (UPD) on thymine modified and unmodified electrodes was investigated using classical electrochemical methods combined with photoemission experiments. On unmodified electrodes positive of the Nernst potential two silver monolayers were formed. The first formed monolayer differs in its work function as well as in its 3d-band binding energy from the second monolayer/bulk material. On thymine modified electrodes only one silver monolayer is formed whereas the formation of the second monolayer is completely blocked in the UPD region. On both, the deposited silver monolayer and the bulk silver, thymine is adsorbed topmost, interacting via its N(3) atom with the surface. XPS spectra reveal an increased electron density at the N(3) in the adsorbed state compared to the signal arising from vacuum deposited thymine multilayers. The metal deposition in electrochemical environment includes two steps, firstly on the electrode adsorbed thymine has to be removed by silver ions; secondly thymine has to be readsorbed on the topmost metal layer. Therefore the influence of adsorbed thymine on metal deposition processes is strongly correlated to the energy balance between the adsorption energies of metal adatoms, thymine on the electrode and thymine on the final topmost layer. Adjusting these energies leads to the different influence of chemisorbed thymine on the formation of the first and second silver monolayer.

Advertisement
Join Copernicus Academic and get access to over 12 million papers authored by 7+ million academics.
Join for free!

Contact us