Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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31745 Publications
EXAFS structural analysis of aqueous uranium(VI) complexes with lignin degradation products
Reich, T.; Roßberg, A.; Baraniak, L.; Hennig, C.; Bernhard, G.; Nitsche, H.;
Uranium LIII-edge extended X-ray absorption fine structure (EXAFS) analysis was carried out on aqueous uranium(VI) complexes with monomeric intermediates of the natural wood-degradation process such as protocatechuic acid (3,4-dihydroxybenzoic acid), catechol (2-hydroxyphenol), pyrogallol (1,2,3-trihydroxybenzol), and vanillic acid (4-hydroxy-3-methoxybenzoic acid). The structural parameters of the 1:1 complexes with protocatechuic acid and vanillic acid in weakly acidic solution indicate that the carboxylic group coordinates the uranyl cation in a bidentate mode in the equatorial plane. Based on the identical structural parameters observed for protocatechuic acid, catechol and pyrogallol in alkaline media, we conclude that the coordination with uranium occurs via the neighboring OH groups of the phenols under formation of a five-membered ring.
  • Radiochimica Acta 88, 593-597 (2000)

Publ.-Id: 6441 - Permalink


Soluble humic substances from in situ precipitated microcrystalline calcium carbonate, internal sediment, and spar cement in a Cretaceous carbonate mud-mound
Neuweiler, F.; Rutsch, M.; Geipel, G.; Reimer, A.; Heise, K. H.;
Albian carbonate mud-mound limestones exposed near Iraneta, northern Spain, show a fabric- and particle-specific fluorescence. Intense fluorescence is restricted to in situ precipitated microcrystalline (automicritic) fabrics, calcified demosponges, and coralline sponges. Intermediate intensity derives from micritized bioclasts, pellets, and a rim of marine bladed cement. Most invertebrate skeletons, late-diagenetic equant cement, and crosscutting zones of dolomitization are weakly to nonfluorescent. Internal microcrystalline sediment (allomicrite) and red algae debris have variable fluorescence. Correlation between rock fluorescence and soluble humic substances was evaluated from 3 g of automicrite, allomicrite, and cement. Time-resolved laser-induced fluorescence spectroscopy (TRLFS) with ultra-short pulses on two extracrystalline fractions (NaOH-soluble) and two intracrystalline fractions (HCl-soluble and NaOH-soluble) showed that most of the soluble humic substances of automicrite are within the crystals; but conversely, are significantly enriched on outer surfaces of allomicrite. Spar cement is close to detection limits. Fluorescence lifetimes are in the range of 0.5–2 ns and 3.5–6 ns. We conclude that precipitation of automicrite took place during oxidative organic matter diagenesis, i.e., during condensation reactions of degradation products of marine biopolymers. By contrast, allomicrite formed by skeletal breakdown followed by ingestion, organic coating, and reingestion during deposit feeding. A humic-substance–based model of marine polymer gels represents a new approach for the understanding of ancient polygenetic carbonate muds, so typical of Phanerozoic mud-mounds in deeper water settings.
  • Geology 28, 851-854 (2000)

Publ.-Id: 6440 - Permalink


Solution coordination chemistry of uranium in the binary UO22+}-SO42- and the ternary UO22+-SO42--OH- system
Moll, H.; Reich, T.; Hennig, C.; Roßberg, A.; Szabó, Z.; Grenthe, I.;
The structure and reaction dynamics in the systems UO22+-SO42- and UO22+-SO42--OH- were investigated using EXAFS and 17O-NMR spectroscopy. Uranium LIII edge EXAFS indicated a bidentate coordination mode of sulfate to uranyl. In solution, this is characterized by an U-S distance of 3.11 Å. Approximately 5 oxygen atoms were observed in the equatorial plane at 2.39-2.43 Å. The kinetics in the binary uranyl sulfate system can be described by four dominant exchange reactions: (1) UO22+ + SO42- ⇔ UO2SO4 (k1), (2) U*O22+ + UO2SO4 ⇔ U*O2SO4 + UO22+ (k2), (3) UO22+ + UO2(SO4)22- ⇔ 2 UO2SO4 (k3), and (4) UO2SO4 + SO42- ⇔ UO2(SO4)22- (k4). These reactions have rate constants indicating that the exchange is not of the simple Eigen-Wilkins type. Ternary uranyl sulfate hydroxide species were characterized by their 17O chemical shift and by potentiometry. There are no separate signals for the possible isomers of the ternary species indicating that they are in fast exchange with each other.
  • Radiochimica Acta 88, 559-566 (2000)

Publ.-Id: 6439 - Permalink


Spectroscopic properties of uranium(VI) minerals studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS)
Geipel, G.; Bernhard, G.; Rutsch, M.; Brendler, V.; Nitsche, H.;
We studied the fluorescence properties of 120 uranium minerals in order to provide a data base of potential secondary solids that may form in the flooding process of defunct uranium mines. This data base may provide a link of the fluorescence spectra of the minerals to solution spectra containing so far unknown tertiary and quaternary solution complexes in environmental uranium(VI)-containing waters. In our initial effort to establish the data base, we are focusing on phosphate- and arsenate-containing uranium(VI) minerals. Except for chernikovite [(H3O)2(UO2)2(PO4)2 · 6H2O], these minerals show intensive fluorescence emissions bands with a bathochromic shift of up to 18 nm compared to the fluorescence of the uranyl ion in solution. We found hypsochromic shifts of the fluorescence emission for the uranyl phosphates and bathochromic shifts for the uranyl arsenates. The band spacing of the fluorescence emissions from the 20502 cm-1 level are comparable to the vibrational frequency available from IR-data. Using these data we estimated the axial U-O bond lengths. The decrease of the vibration frequency from 813 cm-1 (saleiite, [Mg(UO2)2(PO4)2 · 10(H2O)]) to 794 cm-1 (uranocircite, [Ba(UO2)2(PO4)2 · 12(H2O)]) is related to a slight increase of the U-O bond length by 3 pm. In both series, the fluorescence lifetime decreases with decreasing crystal water. This is different compared to solution spectra where the lifetime increases as water is displaced from the inner coordination shell. Comparing the fluorescence data of the mineral troegerite [H2(UO2)2(AsO4)2 · 8(H2O)] with the fluorescence spectra of uranyl arsenate solutions, we identified one of the three uranyl arsenate complexes in solution as HUO2(AsO4)(aq.).batch experiment at anoxic conditions proved that pyrite can immobilise U(VI).
  • Radiochimica Acta 88, 757-762 (2000)

Publ.-Id: 6438 - Permalink


The MHD α²-dynamo, Z2-graded pseudo-Hermiticity, level crossings, and exceptional points of branching type
Günther, U.; Stefani, F.; Gerbeth, G.;
The spectral branching behavior of the 2×2 operator matrix of the magneto-hydrodynamic α²-dynamo is analyzed numerically. Some qualitative aspects of level crossings are briefly discussed with the help of a simple toy model which is based on a Z2-graded-pseudo-Hermitian 2×2 matrix. The considered issues comprise: the underlying SU(1,1) symmetry and the Krein space structure of the system, exceptional points of branching type and diabolic points, as well as the algebraic and geometric multiplicity of corresponding degenerate eigenvalues.
  • Czechoslovak Journal of Physics 54(2004), 1075-1089

Publ.-Id: 6437 - Permalink


Chemical characterization of bohrium (element 107)
Eichler, R.; Brüchle, W.; Dressler, R.; Düllmann, C. E.; Eichler, B.; Gäggeler, H. W.; Gregorich, K. E.; Hoffmann, D. C.; Hübener, S.; Jost, D. T.; Kirbach, U. W.; Laue, C. A.; Lavanchy, V. M.; Nitsche, H.; Patin, J. B.; Piguet, D.; Schädel, M.; Shaughnessy, D. A.; Strellis, D. A.; Taut, S.; Tobler, L.; Tsyganov, Y. S.; Türler, A.; Vahle, A.; Wilk, P. A.; Yakushev, A. B.;
The arrangement of the chemical elements in the periodic table highlights resemblances in chemical properties, which reflect the elements' electronic structure. For the heaviest elements, however, deviations in the periodicity of chemical properties are expected1-3: electrons in orbitals with a high probability density near the nucleus are accelerated by the large nuclear charges to relativistic velocities, which increase their binding energies and cause orbital contraction. This leads to more efficient screening of the nuclear charge and corresponding destabilization of the outer d and f orbitals: it is these changes that can give rise to unexpected chemical properties. The synthesis of increasingly heavy elements4-6, now including that of elements 114, 116 and 118, allows the investigation of this effect, provided sufficiently long-lived isotopes for chemical characterization are available7. In the case of elements 104 and 105, for example, relativistic effects interrupt characteristic trends in the chemical properties of the elements constituting the corresponding columns of the periodic table8, whereas element 106 behaves in accordance with the expected periodicity9-12. Here we report the chemical separation and characterization of six atoms of element 107 (bohrium, Bh), in the form of its oxychloride. We find that this compound is less volatile than the oxychlorides of the lighter elements of group VII, thus confirming relativistic calculations13 that predict the behaviour of bohrium, like that of element 106, to coincide with that expected on the basis of its position in the periodic table.
  • Nature 407, 63-65 (2000)

Publ.-Id: 6435 - Permalink


Application of LIBD to the determination of the solubility product of thorium(IV)-colloids
Bundschuh, T.; Knopp, R.; Müller, R.; Kim, J. I.; Neck, V.; Fanghänel, T.;
A new experimental method is presented for the determination of solubility data, which is based on the laser-induced breakdown detection (LIBD). The method is capable of monitoring the initial colloid generation when the metal ion concentration reaches or just exceeds the solubility at given pH. The application is made to determine the solubility of Th(IV) in acidic solutions at I = 0.5 M (NaCl) and 25 °C. The initial colloid formation is determined as a function the H+ concentration in a series of 2.8 × 10-2 - 8.9 × 10-5 M thorium solutions. The conditional solubility product (log K'sp = -49.54 ± 0.22) obtained in this study corresponds to an equilibrium between solution and colloidal thorium dioxide particles. The solubility product at I = 0 (log K°sp = -52.8 ± 0.3) is calculated with the SIT coefficients of the NEA-TDB. It corresponds to the known value for crystalline ThO2(cr), in particular if the small particle size of about 20 nm is taken into account. The present results indicate that the high thorium solubilities measured in the previous studies for amorphous Th(IV) hydroxide or hydrous oxide are primarily caused by the inclusion of polynuclear species or Th(IV) colloids of very small size.
  • Radiochimica Acta 88, 625-629 (2000)

Publ.-Id: 6427 - Permalink


Spectroscopic characterization of alkaline earth uranyl carbonates
Amayri, S.; Reich, T.; Arnold, T.; Geipel, G.; Bernhard, G.;
A series of alkaline uranyl carbonates, M[UO2(CO3)3]•nH2O (M = Mg2, Ca2, Sr2, Ba2, Na2Ca, and CaMg) was synthesized and characterized by inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS) after nitric acid digestion, X-ray powder diffraction (XRD), and thermal analysis (TGA/DTA). For the first time, the molecular structure of these compounds was characterized by extended X-ray absorption fine-structure (EXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). Crystalline Ba2[UO2(CO3)3]•6H2O was obtained for the first time. The EXAFS analysis showed that this compound consists of (UO2)(CO3)3 clusters similar to the other alkaline earth uranyl carbonates. The average U-Ba distance is 3.90±0.02 Å. Fluorescence wavelengths and life times were measured using time-resolved laser-induced fluorescence spectroscopy (TRLFS). The U-O bond distances determined by EXAFS, TRLFS, and XPS agree within the experimental uncertainties. The spectroscopic signatures observed could be useful for identifying uranyl carbonate species adsorbed on mineral surfaces.
Keywords: uranium, alkaline earth, carbonate, synthesis, structure, spectroscopy, EXAFS, TRLFS, XPS

Publ.-Id: 6426 - Permalink


Institute of Radiochemistry - Profile, goals and scientific results -
Bernhard, G.;
  • Lecture (others)
    Invited Talk, Group Meeting, 05.04.2004, UC at Berkeley, LBNL, United States

Publ.-Id: 6424 - Permalink


Radioecological research at the Institute of Radiochemistry, FZR
Bernhard, G.;
Radioecological research at the Institute of Radiochemistry, FZR
  • Lecture (others)
    Shanghai Institute of Applied Physics, 26.04.2004, Shanghai, China

Publ.-Id: 6423 - Permalink


Transfer of Uranium to Plants - Aspects of Chemical Speciation
Bernhard, G.;
Transfer of Uranium to Plants - Aspects of Chemical Speciation
  • Lecture (others)
    Invited Talk - American Chemical Society, Symposium 2004, Anaheim, United States

Publ.-Id: 6422 - Permalink


Wechselwirkung von Uran mit Biosystemen - Aspekte der chemischen Speziation
Bernhard, G.;
Wechselwirkung von Uran mit Biosystemen
  • Lecture (others)
    Freie Universität Berlin 21.06.2004, Berlin, Germany

Publ.-Id: 6421 - Permalink


Blind prediction of Cu(II) sorption onto goethite: Current capabilities of surface complexation modeling
Richter, A.; Brendler, V.; Nebelung, C.;
The paper presents examples illustrating the current blind predictive capabilities of surface complexation models (SCM) in combination with a respective database RES3T (Rossendorf Expert System for Surface and Sorption Thermodynamics).
The general strategy for the selection of numerical data is discussed. Based on the information about the minerals collected in the sorption database RES3T, first a set of relevant surface species must be formed. Then respective surface complexation parameters are taken from RES3T: the binding site density for the minerals, the surface protolysis constants, and the stability constants for all relevant surface complexes. To be able to compare and average thermodynamic constants originating from different sources a normalization concept is applied.
In a second part the current capabilities of SCM is demonstrated based on a blind prediction exercise. The system covered is the Cu(II) sorption onto goethite, with the predictions compared to raw data from three independent experimental investigations. To keep the number of parameters at a minimum, the Diffuse Double Layer model was selected to account for electrostatics. The calculations were performed with the FITEQL code, version 3.2.
In most cases the model prediction represented the experimental values for the sorbed amount of Cu(II), expressed as conventional distribution coefficients KD, within one order of magnitude or better.
We conclude, that the application of SCM can indeed be very valuable for estimating distribution coefficients for contaminants in well defined mineral systems. The SCM database so far assembled within the RES3T project is able to provide the respective parameter sets following the outlined stepwise strategy of species selection, data collection, normalization and averaging.


Keywords: surface complexation, database, blind prediction, adsorption, copper, goethite, distribution coefficient
  • Geochimica et Cosmochimica Acta 69(2005), 2725-2734

Publ.-Id: 6419 - Permalink


Comparative investigations to corrosion fatigue of Al-Cu and Al-Mg-Si alloys
Thieme, M.; Bergner, F.; Haase, I.; Worch, H.;
One of the serious problems encountered in the use of various materials in technology is the occurrence of fatigue phenomena as an undesirable material damage under cyclic mechanical load. For aluminium alloys this issue is of extremely high importance in case of their utilisation for aircraft purposes, e.g., where a very wide spectrum of frequencies occur. Moreover, the cyclic loading may be joined by the presence of specific electrolyte media. Therefore, the material candidates must be thoroughly examined in view of their sensitivity to fatigue as well as to corrosion fatigue. Usually, the Cu-containing alloy EN-AW 2024 T3 is applied besides 7075 T6 in Airbus aircrafts, but the weldable alloy 6013 T6 is considered to be a potential alternative. Referring to former investigations on the environmental sensitivity in the fatigue behaviour this paper brings up experimental findings as well as expanded considerations about damaging mechanisms and modelling. The situation with the alloy 6013 T6 is emphasized. The propagation of cracks under cyclic load in different environments, such as vacuum, air or aqueous media, is described by means of fracture mechanics. This enables discrimination in view of the influence of environmental factors and, hence, the participation of corrosion processes.
Keywords: Fatigue Crack Growth, Corrosion Fatigue, Aluminium Alloy
  • Lecture (Conference)
    4th Kurt Schwabe Corrosion Symposium 'Mechanisms of Corrosion and Corrosion Prevention', 13.-17.06.2004, Espoo, Finland
  • Contribution to proceedings
    Proceedings 4th Kurt Schwabe Corrosion Symposium 'Mechanisms of Corrosion and Corrosion Prevention', 13.-17.06.2004, Espoo, Finland, pp. 103-111

Publ.-Id: 6418 - Permalink


Silicon-based electrically driven microcavity LED
Potfajova, J.; Sun, J. M.; Winnerl, S.; Dekorsy, T.; Skorupa, W.; Schmidt, B.; Helm, M.; Mantl, S.; Breuer, U.;
A silicon pn -diode was embedded into a microcavity composed of a buried metal silicide as bottom reflector and a Si/SiO 2 Bragg mirror as top reflector. Spectral narrowing and an increased intensity of the Si bandgap electroluminescence was observed.
Keywords: silicon based light emission, mirocavity, optical interconnects

Publ.-Id: 6417 - Permalink


Measurements of the bulk velocity during solidification of metallic alloys
Eckert, S.; Willers, B.; Gerbeth, G.;
The melt convection during solidification has a considerable influence on the nucleation process and the grain structure. The ultrasound Doppler velocimetry (UDV) was applied to measure the bulk flow in a Sn-15wt%Pb alloy solidified directionally from a water cooled copper chill. The flow is driven by a rotating magnetic field. We present velocity profiles measured during the solidification process. Moreover, from the UDV measurements the current position of the solidification front can be estimated.
Keywords: solidification, forced convection, velocity measurements, UDV
  • Metallurgical and Materials Transactions A 36(2005), 267-270

Publ.-Id: 6416 - Permalink


Excitons in Si nanocrystals: Confinement and migration effects
Heitmann, J.; Muller, F.; Yi, L.; Zacharias, M.; Kovalev, D.; Eichhorn, F.;
A detailed analysis of the strong room-temperature photoluminescence (PL) signal of size controlled nc-Si is reported. The size control of nc-Si is realized by evaporation of SiO/SiO2 superlattices and subsequent thermally induced phase separation. By this method the synthesis of completely SiO2 passivated Si nanocrystals with a controlled size is demonstrated. A strong blueshift of the photoluminescence signal from 1.3 to 1.65 eV with decreasing crystal size is observed. Resonant photoluminescence measurements prove the breakdown of the k-conservation rule for nc-Si by showing an increase in the no-phonon transition probability with decreasing crystal size. A no-phonon to phonon assisted transition probability ratio above 1 is detected at 4.5 K. These results confirm quantum confinement as the origin of the investigated luminescence signal. The size dependence of the different luminescence properties and the very high no-phonon transition probability indicate a lower confin!
ement barrier compared to other systems containing nc-Si and additional migration effects of the excitons between the nanocrystals. A separation of quantum confinement and migration effects on the PL signal is possible due to the very narrow size distribution of the nc-Si and detailed time and temperature dependent investigations of the photoluminescence.
  • Physical Review B 6919(2004)19, 5309-5309

Publ.-Id: 6415 - Permalink


Virtues and pitfalls in structural analysis of compound semiconductors by the complementary use of RBS/channeling and high resolution X-ray diffraction
Turos, A.; Gaca, J.; Wojcik, M.; Nowicki, L.; Ratajczak, R.; Groetzschel, R.; Eichhorn, F.; Schell, N.;
InP single crystals implanted with 1.2 MeV As-ions and InP/InGaAsP superlattices were studied with RBS/c and high resolution X-ray diffraction (HRXRD). The principal advantage of HRXRD is the determination of interplanar spacing with an accuracy of 10(-5). One should be aware of a variety of pitfalls inherently involved with the nature of X-ray diffraction. These are primarily due to the ambiguity of X-ray rocking curves evaluation. Additional peaks appearing in X-ray rocking curves after ion implantation are usually attributed to regions with distinct lattice parameter. Calculations using damage profiles measured by RBS/c indicated that they could result from X-ray interference effects. For quaternary compounds where HRXRD can hardly detect local change of composition RBS was applied for this purpose. The great accuracy of HRXRD is also advantageous for the RBS analysis: determination of lattice spacing allows the layer density determination hence, the depth scale can be expressed in linear units and stopping power data can be adjusted accurately.
Keywords: III-V semiconductors; Heterostructures; RBS/channeling; HRXRD
  • Nuclear Instruments and Methods in Physics Research B 219-20(2004), 618-625

Publ.-Id: 6414 - Permalink


On the Hydrodynamics of Bubble Columns: Comparison of Experimental Measurements with Computational Fluid Dynamics Calculations
Vanga, B. N. R.; Krepper, E.; Zaruba, A.; Prasser, H.-M.; de Bertodano, L.;
The hydrodynamics of a rectangular bubble column operating in the dispersed bubbly regime has been experimentally and numerically investigated. The bubble column was locally aerated using a sparger located in the center of the bottom plate. The experimental studies involve wire-mesh tomography measurements for void fraction and bubble size distributions and digital image rocessing of high speed camera images for estimation of bubble velocities, size distributions and flow patterns. Experiments were performed for various aspect ratios (height of water column to width ratio) and superficial gas velocities. Computational fluid dynamic calculations of transient flow for the above bubble column were performed using an Eulerian-Eulerian two fluid model. Turbulence in the liquid phase was accounted for, using the k-epsilon turbulence model. The effect of the gas phase on turbulence in the continuous phase was modeled using Sato’s eddy viscosity model for bubble-induced turbulence. A systematic analysis of the interfacial momentum transfer terms has been conducted. It was found that the non-drag bubble forces play a very prominent role in the predicting the correct flow pattern and void fraction distributions. In the calculations, the lift force and the wall force were considered. A ‘wall peak’ in the time averaged void fraction distribution has been experimentally observed and this cannot be predicted without including these non-drag forces in the numerical calculations. In this paper, experimental data are compared with the results of the numerical simulations.
Keywords: bubble columns, two-phase flow, modelling, turbulence, interfacial momentum transfer
  • Lecture (Conference)
    5th International Conference on Multiphase Flow, ICMF 04, 30.05.-04.06.2004, Yokohama, Japan
  • Contribution to proceedings
    5th International Conference on Multiphase Flow, ICMF 04, 30.05.-04.06.2004, Yokohama, Japan, paper 264

Publ.-Id: 6413 - Permalink


Angular distributions for H-3,4(Lambda) bound states in the He-3,He-4(e,e ' K+) reaction
Dohrmann, F.; Ahmidouch, A.; Armstrong, C. S.; Arrington, J.; Asaturyan, R.; Avery, S.; Bailey, K.; Baker, O. K.; Bitao, H.; Breuer, H.; Brown, D. S.; Carlini, R.; Cha, J.; Chant, N.; Christy, E.; Cochran, A.; Cole, L.; Crowder, J.; Danagoulian, S.; Elaasar, M.; Ent, R.; Fenker, H.; Fujii, Y.; Gan, L.; Garrow, K.; Geesaman, D. F.; Gueye, P.; Hafidi, K.; Hinton, W.; Juengst, H.; Keppel, C.; Liang, Y.; Liu, J. H.; Lung, A.; Mack, D.; Markowitz, P.; Mitchell, J.; Miyoshi, T.; Mkrtchyan, H.; Mtingwa, S. K.; Mueller, B.; Niculescu, G.; Niculescu, I.; Potterveld, D. H.; Raue, B. A.; Reimer, P. E.; Reinhold, J.; Roche, J.; Sarsour, M.; Sato, Y.; Segel, R. E.; Semenov, A. Y.; Stepanyan, S.; Tadevosian, V.; Tajima, S.; Tang, L.; Uzzle, A.; Wood, S.; Yamaguchi, H.; Yan, C.; Yuan, L.; Zeier, M.; Zeidman, B.; Zihlmann, B.;
The H-3,4(Lambda) and H-4(Lambda) hypernuclear bound states have been observed for the first time in kaon electroproduction on He-3,He-4 targets. The production cross sections have been determined at Q(2)=0.35 GeV2 and W=1.91 GeV. For either hypernucleus the nuclear form factor is determined by comparing the angular distribution of the He-3,He-4(e,e(')K(+))(Lambda)H-3,H-4 processes to the elementary cross section H-1(e,eK(+))Lambda on the free proton, measured during the same experiment.
  • Physical Review Letters 93(2004), 242501

Publ.-Id: 6412 - Permalink


Tailoring Magnetism by Light-Ion Irradiation
Fassbender, J.ORC; Ravelosona, D.; Samson, Y.
Due to the reduced dimension, the magnetic properties of ultrathin magnetic films and multilayers, e.g. magnetic anisotropies and exchange coupling, often depend strongly on the surface and interface structure. In addition chemical composition, crystallinity, grain sizes and their distribution govern the magnetic behaviour. All these structural properties can be modified by light-ion irradiation in an energy range from 5 - 150 keV due to the energy loss of the ions in the solid along their trajectory. Consequently the magnetic properties can be tailored by ion irradiation. Similar effects can also be observed using Ga+ ion irradiation, which is the common ion source in focused ion beam lithography.

Examples for ion induced modifications of magnetic anisotropies and exchange coupling are presented. This review is limited to radiation induced structural changes giving rise to a modification of magnetic parameters. Ion implantation is discussed only in special cases.

Due to the local nature of the interaction, magnetic patterning without affecting the surface topography becomes feasible, which may have an applied interest. The main patterning technique is homogeneous ion irradiation through masks. Focused ion beam and ion projection lithography are commonly only relevant for larger ion masses. The creation of magnetic feature sizes below 50 nm is shown. In contrast to topographic nanostructures the surrounding area of these nanostructures can be left ferromagnetic leading to new phenomena at their mutual interface.

Most of the material systems discussed here are important for technological applications. The main areas are magnetic data storage applications, such as, hard magnetic media with a large perpendicular magnetic anisotropy or patterned media with an improved signal to noise ratio, and magnetic sensor elements. It will be shown that light-ion irradiation has many advantages for design of new material properties and the fabrication technology of actual devices.

Keywords: magnetism, light ion irradiation, magnetic patterning, exchange bias, magnetic anisotropy, exchange coupling

Publ.-Id: 6411 - Permalink


Ex-vessel Core Melt Stabilization Research (ECOSTAR)
Alsmeyer, H.; Albrecht, G.; Meyer, L.; Häfner, W.; Journeau, C.; Fischer, M.; Hellmann, S.; Eddi, M.; Allelein, H.-J.; Bürger, M.; Sehgal, B. R.; Koch, M. K.; Alkan, Z.; Petrov, J.; Gaune-Escard, M.; Altstadt, E.; Bandini, G.;
The Project on Ex-Vessel Core Melt Stabilization Research (ECOSTAR) started in January 2000 and to be concluded by end of 2003. The project is performed by 14 partner institutions from 5 European countries and involves a large number of experiments with low and high temperature simulant melts and real corium at different scales. Model development and scaling analysis allows application of the research results to existing and to future LWRs in the area of reactor design and accident mitigation.
The project is oriented toward the analysis and mitigation of severe accident sequences that could occur in the ex-vessel phase of a postulated core melt accident. The issues are: (1) the release of melt form the pressure vessel, (2) the transfer and spreading of the melt on the basement, (3) the analysis of the physical-chemical processes that are important for corium behavior especially during concrete erosion with onset of solidification, and (4) stabilization of the melt by cooling through direct water contact.
The results achieved so far, resolve a number of important issues: The amount of melt that could be transferred at RPV failure from the RPV into the containment can be substantially reduced by lowering the residual pressure in the primary circuit. It is found that melt dispersion is also strongly depending on the location of the RPV failure, and that lateral failure results in substantially less melt dispersion. During melt release, the impinging melt jet could erode parts of the upper basement surface. Jet experiments and a derived heat transfer relation allow estimation of its contribution to concrete erosion. Spreading of the corium melt on the available basement surface is an important process, which defines the initial conditions for concrete attack or for the efficiency of cooling in case of water contact, respectively. Validation of the spreading codes based on a large-scale benchmark experiment, is underway and will allow determination of the initial conditions, for which a corium melt can be assumed to spread homogeneously over the available surface. Experiments with UO2-based corium melts highlight the role of phase segregation during onset of melt solidification and during concrete erosion. To cool the spread corium melt, the efficacy of top flooding and bottom flooding is investigated in small scale and in large-scale experiments, supported by model developments. Project assessment is continuing to apply the results to present and future reactors.
Keywords: Ex-vessel corium melt stabilization Molten-corium-concrete interaction Corium properties Melt spreading Melt cooling
  • Nuclear Engineering and Design, 235(2005)2-4, 271-284

Publ.-Id: 6410 - Permalink


Untersuchung strahleninduzierter Chromosomenaberrationen
Wolfring, E.;
Bestrahlung der Brustdrüsenepithelzellinie MCF-12A mit Röntgenstrahlung aus konventionellen Röntgenröhren und Bestimmung der Dosis-Effekt-Kurve
  • Open Access LogoWissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-404 2004

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Publ.-Id: 6409 - Permalink


Boron-dilution Transients in PWRs
Weiß, F.-P.;
Boron 10 is a strong thermal neutron absorber. As boric acid solved in the coolant of PWRs, it is used to compensate the excess reactivity of the reactor especially at the beginning of the fuel cycle. Therefore, an inadvertent or even unavoidable reduction of the boron concentration means an increase of reactivity that might result in a power excursion, a so called boron dilution transient.
The reactivity increase depends on the of mixing of the underborated water with the coolant inventory in the downcomer (DC) and lower plenum (LP) where the boron concentration is still on the normal high level.
After introducing the different boron dilution scenarios, results of the latest PKL (Primärkreislauf test facility at FANP) experiments will be presented aiming at the assessment of the maximum slug volume for the different safety injection cases. Moreover, the ROCOM (Rossendorf Coolant Mixing) tests will be discussed that were performed to study mixing phenomena inside the reactor pressure vessel (RPV) and which provided typical boron concentration profiles at the core entrance for the different flow regimes. These experiments are used to develop and validate calculation models for boron mixing. Such models are needed to get information about realistic time dependent boron concentration profiles over the cross section of the core entrance. The obtained concentration profiles are used as boundary conditions for coupled neutronic / thermal-hydraulic core calculations to show whether recriticality of the shut down reactor is reached and how high the maximum power peak can be. The results of such calculations will be presented proving that these deborated slugs can cause prompt criticality. Nevertheless, due to the fast Doppler feedback no core damage occurs even with the maximum slug volume of 36m³ .
  • Lecture (others)
    29. Fachsitzung der Internationalen Länderkommision Kerntechnkik, 24.05.2004, Stuttgart, Germany

Publ.-Id: 6408 - Permalink


Aktuelle Entwicklungen auf dem Gebiet der nuklearen Sicherheitsforschung
Weiß, F.-P.;
The presentation starts from sustainability considerations of different energy carries like oil, hoard cool, or nuclear energy.
It is highlighted, that in terms of sustainability nuclear energy suffers from the danger of proliferation of weapons grade material, the maximum damage of a conceivable severe accident, and the extremely long isolation period of the nuclear waste in the final disposal. Discussing the safety analysis of so called boron dilution transients in PWRs, the behaviour of the reactor pressure vessel during a severe core melt accident, and the possibilities of the transmutation of long lived radio-nuclides it is shown how modern nuclear safety research can contribute to improved sustainability performance of nuclear energy.
  • Invited lecture (Conferences)
    Kolloquium: Kompetenz zur Kerntechnik in Lehre und Forschung, 25.-26.06.2004, Zittau, Germany

Publ.-Id: 6407 - Permalink


Recents Developments in Nuclear Safety Research
Weiß, F.-P.;
The presentation starts from sustainability considerations of different energy carries like oil, hoard cool, or nuclear energy.
It is highlighted, that in terms of sustainability nuclear energy suffers from the danger of proliferation of weapons grade material, the maximum damage of a conceivable severe accident, and the extremely long isolation period of the nuclear waste in the final disposal. Discussing the safety analysis of so called boron dilution transients in PWRs, the behaviour of the reactor pressure vessel during a severe core melt accident, and the possibilities of the transmutation of long lived radio-nuclides it is shown how modern nuclear safety research can contribute to improved sustainability performance of nuclear energy.
  • Invited lecture (Conferences)
    Conference on Multi-Phase Flow: Simulation, Experiment and Application, 28.-30.06.2004, Dresden, Germany

Publ.-Id: 6406 - Permalink


Migration of di- and tri-interstitials in silicon
Posselt, M.; Gao, F.; Zwicker, D.;
Small self-interstitial clusters play an important role in the evolution of radiation damage in Si during post-implantation annealing. These small clusters can be formed by two different processes: (i) fast relaxation of highly disordered cascade regions immediately after ion impact and (ii) clustering of diffusing self-interstitials in the first stage of the annealing process.
The structure and energetics of the small self-interstitial clusters have been investigated by different theoretical methods. However, there are only few data on their mobility and migration mechanisms. They have been mainly obtained by static potential energy calculations. This work presents results of a systematic molecular dynamics study of the migration of di- and tri-interstitials. A classical potential approach is employed since it allows the investigation of defect migration under relatively realistic conditions, by considering large computational cells, long periods and different initial conditions. The di- and tri-interstitial configurations with the lowest formation energies are determined, and the results are compared with literature data. Then, the migration of di- and tri-interstitials is followed for 10 – 30 ns, at temperatures between 800 and 1600 K. Defect diffusivity and self-diffusion coefficient per defect are obtained by the Wigner-Seitz cell analysis and the Guinan method as well as from the mean square displacements of all atoms. The effective migration energies are determined and compared with the data from literature. The elementary migration mechanisms are discussed in detail by the analysis of movies and snapshots of defect trajectories and atomic rearrangements.

Keywords: defects, diffusion, migration, silicon, computer simulation
  • Lecture (Conference)
    7th International Conference on Computer Simulation of Radiation Effects in Solids (COSIRES 2004), June 28 - July 2, 2004, Helsinki, Finland

Publ.-Id: 6405 - Permalink


Bacterial mediated formation of Pd-nanoclusters for the development of biocatalysts
Pollmann, K.; Raff, J.; Merroun, M. L.; Fahmy, K.; Mikeehenko, I.; Creamer, N.; Macaskie, L.; Selenska-Pobell, S.;
The bacterial strain Bacillus sphaericus JG-A12 was isolated from a uranium mining waste pile near the town of Johanngeorgenstadt, Germany. The cells of this strain are capable of selective and reversible accumulation of U, Cu, Pb, Al, and Cd from the U waste waters [1]. Further, they are enveloped by a surface layer (S-layer) which differs significantly in its primary structure from the other B. sphaericus S-layers studied up to date [2]. The highly ordered crystalline S-layers are one of the most common cell envelope structures of bacteria and archaea [3]. They are composed of protein monomers which possess the ability to self-assemble into two-dimensional paracrystalline lattices. The highly regular structure of the S-layers with many pores of identical size offers good binding sites for different metals and nucleation sites for the formation of metal nanoclusters or minerals.
EXAFS and IR analyses of the interactions of purified and recrystallised S-layer with Pd(II) demonstrated, that Pd(II) is bound to the carboxyl groups of the protein. In accordance to these results the C-terminal domain of the protein was found to contain aspartate and glutamate rich stretches which we assume to be responsible for the deposition of Pd in the pores of the protein lattices. Interestingly, the Laser Induced Infrared spectroscopic analysis showed that the deposition of the Pd complexes stabilizes the structure of the protein. The bound Pd(II) is reduced to metallic Pd nanoclusters in the presence of reducing agents which is in agreement with previous studies on another S-layer [4]. In addition, Pd nanoclusters are formed on the intact cells of B. sphaericus JG-A12 via a two step process consisting of biosorption (I) and of consequent metal reduction (II) in the presence of molecular H2 as an electron donor. The metallic nature of the clusters was confirmed by EXAFS and their size was estimated to be about 9-10 Ǻ. TEM analysis showed that the clusters are localised at specific sites of the bacterial cell wall, indicating the primary role of the S-layer in their formation.
The Pd nanoclusters formed on S-layer and bacterial cell templates are highly interesting for technical applications, e. g for the development of novel catalysts. Genetic engineering of the S-layer is applied for optimising its capability to form Pd nanoclusters with increased catalytic activity and also in order to extend its binding capacity to other metals of industrial interest.

1. Selenska-Pobell, S. et al. (1999) FEMS Microbiol. Ecol. 29, 59-67.
2. Raff, J. (2002) “Wechselwirkungen der Hüllproteine von Bakterien aus Uranabfallhalden mit Schwermetallen“ PhD Thesis, FZR-Report No. 358.
3. Sleytr, U. et al. (1997) FEMS Microbiol. Rev. 20, 47-97.
4. Wahl et al. (2001) Advanced Materials 13, 736-740.
  • Lecture (Conference)
    Biometals 2004, 03.-05.09.2004, Garmisch-Partenkirchen, Germany
  • Contribution to proceedings
    Biometals 2004, 03.-05.09.2004, Garmisch-Partenkirchen, Germany
    Proceedings

Publ.-Id: 6404 - Permalink


Interaction of Uranium with bacterial strains isolated from Uranium contaminated environments: Microscopic and spectroscopic studies
Merroun, M. L.;
Uranium is a long-living radionuclide that represents ecological and human health hazards. The mining and processing of uranium during the last decades for nuclear fuel and nuclear weapon production resulted in generation of significant amounts of radioactive waste. It is critical that the uranium in these wastes has to be effectively immobilized and removed away in order to prevent ground water contamination. Microbial biosorption of U(VI) was proposed as one of the methods for uranium immobilization. In this paper, we describe the isolation of bacterial strains from water and soil samples collected from different uranium contaminated environments. Phylogenetic analysis of these strains revealed that they are related to Stenotrophomonas maltophilia, Bacillus sphaericus, Microbacterium oxydans, Pseudomonas rhodesiae etc. As estimated by using ICP-MS these natural isolates possess a high ability to accumulate uranium and other heavy metals such as Cd, Pb, Ni and Ag. The bacterial uranium tolerance was studied using flow cytometry techniques. X-ray absorption near-edge structure analysis showed that the cells of these strains precipitate U(VI) as autunite-like phase (inorganic uranyl phosphate phase) at pH 4.5, probably due to the release of the inorganic phosphate from the cells. However, at pH 2 the uranium bonding was consistent with the formation of complexes with organic bound phosphate of the cell surface. These results are in agreement with those found by infrared measurements.Transmission electron microscope and energy dispersive X-ray analysis showed strain-specific extracellular and/or intracellular uranium accumulation to varying degrees.
  • Lecture (others)
    Invited Talk at the Institut for Nuclear Chemistry, University Mainz, 06.06.2004, Mainz, Germany

Publ.-Id: 6403 - Permalink


Isolation, characterization and U(VI)-Immobilizing potential of bacterial Strains from Uranium contamined environments
Merroun, M. L.; Rossberg, A.; Hennig, C.; Selenska-Pobell, S.;
Uranium is a long-living radionuclide that represents ecological and human health hazards. The mining and processing of uranium during the last decades for nuclear fuel and nuclear weapon production resulted in generation of significant amounts of radioactive waste. It is critical that the uranium in these wastes has to be effectively immobilized and removed away in order to prevent ground water contamination. Microbial biosorption of U(VI) was proposed as one of the methods for uranium immobilization. In this paper, we describe the isolation of bacterial strains from water and soil samples collected from different uranium contaminated environments. Phylogenetic analysis of these strains revealed that they are related to Stenotrophomonas maltophilia, Bacillus sphaericus, Microbacterium oxydans, Pseudomonas rhodesiae etc. As estimated by using ICP-MS these natural isolates possess a high ability to accumulate uranium and other heavy metals such as Cd, Pb, Ni and Ag. The bacterial uranium tolerance was studied using flow cytometry techniques. X-ray absorption near-edge structure analysis showed that the cells of these strains precipitate U(VI) as autunite-like phase (inorganic uranyl phosphate phase) at pH 4.5, probably due to the release of the inorganic phosphate from the cells. However, at pH 2 the uranium bonding was consistent with the formation of complexes with organic bound phosphate of the cell surface. These results are in agreement with those found by infrared measurements.Transmission electron microscope and energy dispersive X-ray analysis showed strain-specific extracellular and/or intracellular uranium accumulation to varying degrees.
  • Poster
    Biometals 2004 (03.-05.09.2004), Garmisch-Partenkirchen
  • Contribution to proceedings
    Biometals 2004, 03.-05.09.2004, Garmisch-Partenkirchen, Germany

Publ.-Id: 6402 - Permalink


Atomistic study of intrinsic defect migration in 3C-SiC
Gao, F.; Weber, W. J.; Posselt, M.; Belko, V.;
Atomic-scale computer simulations, both molecular dynamics (MD) and the nudged-elastic band methods, have been applied to investigate long-range migration of point defects in cubic SiC (3C-SiC) over the temperature range from 0.36Tm to 0.95Tm (melting temperature). The point defect diffusivities, activation energies, and defect correlation factors have been obtained. Stable C split interstitials can migrate via the first- or second-nearest-neighbor sites, but the relative probability for the latter mechanism is very low. Si interstitials migrate directly from one tetrahedral position to another neighboring equivalent position by a kick-in/kick-out process via a split-interstitial configuration. Both C and Si vacancies jump to one of their equivalent sites through a direct migration mechanism. The migration barriers obtained for C and Si interstitials are consistent with the activation energies observed experimentally for two distinct recovery stages in irradiated SiC. Also, energy barriers for C interstitial and vacancy diffusion are in reasonable agreement with ab initio data.
Keywords: Defects, diffusion, SiC, computer simulation

Publ.-Id: 6401 - Permalink


He Ion Irradiation Induced Phase Transformation of Icosahedral FePt Nanoparticles
Dmitrieva, O.; Acet, M.; Liedke, M. O.; Rellinghaus, B.; Fassbender, J.ORC
One potential material system for high density magnetic recording consists of monodisperse, highly anisotropic FePt nanoparticles. In order to exhibit high magnetic anisotropy the fct-L10 phase is required.
FePt nanoparticles (dP = 6 nm) with concentration close to the stoichiometric composition have been prepared by DC magnetron sputtering in Ar and subsequent thermal annealing in the gas phase [1]. Recently, we have shown that under appropriate preparation conditions (pAr = 0.5 mbar, rAr = 100 sccm, Tanneal = 600 – 1000 °C), the obtained particles are monodisperse but unfortunately of icosahedral structure [1,2]. Strikingly, these icosahedra are very stable and even post-deposition annealing of these FePt icosahedra for 30 min at T = 1000 °C did not result in a structural phase transition of the particles, although L10 phase is known to be the thermodynamic equilibrium structure of (bulk) FePt.
In order to investigate both the stability of the icosahedral structure and the potential of He ion irradiation for inducing a structural phase transformation, likewise prepared FePt nanoparticles were deposited onto amorphous carbon films (TEM grids) and irradiated with He ions (5 keV) at room temperature. The ion fluence was varied in the range between 1•1016 and 3•1017 ions/cm2. HRTEM investigations on the irradiated particles show that with increasing fluence, the icosahedral particles become increasingly destabilized at the expense of fcc polycrystalline and single crystal particles. Adjacent particles are observed to coalesce (sinter) under the impact of the ion beam thereby indicating an enhanced diffusivity due to (temporarily) increased defect concentrations. The influence of the modified particle structure (with lower symmetry) on the result of a subsequent post-deposition annealing of the irradiated FePt nanoparticles is discussed.

Part of this work is supported by the Deutsche Forschungsgemeinschaft within SFB 445.

[1] S. Stappert, B. Rellinghaus, M. Acet, and E.F. Wassermann, J. Cryst. Growth 252 (2003) 440-450.
[2] S. Stappert, B. Rellinghaus, M. Acet, and E.F. Wassermann, Proc. Mat. Res. Soc. Vol. 704, pp. 73-78 (2002).

Keywords: magnetism, nanoparticles, FePt, hard magnetic materials, He ion irradiation, structural phase transformation
  • Poster
    Conference on Magnetism and Magnetic Materials, Jacksonville, USA, 07.-11.11.2004, Abstract DR-5

Publ.-Id: 6400 - Permalink


Anisotropy patterning of amorphous CoFeSiB films by He -ion radiation
McCord, J.; Fassbender, J.ORC; Liedke, M. O.; Fromberger, M.; Quandt, E.; Schultz, L.
The alteration of magnetic properties in magnetic thin films by ion radiation has gained increasing attention in recent years. Most of the experiments focus on magnetic multilayers, e.g. Pt/Co [1] or exchange bias [2] systems. Only a few results are obtained from soft magnetic polycrystalline single-layer films [3] using Ga - ions. Here, we present data on the local alteration of the magnetic anisotropy axis in amorphous soft magnetic (Fe9090Fe10)78B10Si12 films by He -ion radiation in an applied magnetic in-plane field. Sputtered CoFeSiB (thickness 30 nm) were irradiated with 5 keV He -ions with a fluence ranging from 5 10^14 ions/cm2 to 5 10^16 ions/cm2. A magnetic field of 600 Oe was applied during irradiation aligned under an angle to the initial easy axis of anisotropy. Above a fluence of 1016 ions/cm2 a rotation of uniaxial anisotropy in the applied field direction in the irradiated areas is observed by MOKE magnetometry and complementary domain observation by Kerr microscopy in the longitudinal mode. No degradation in magnetic properties relative to the as-deposited state is found from the magnetization loops on non-patterned samples. The easy axis coercivity is typically around Hc,ea = 2 Oe and the anisotropy field around Hk = 18 Oe. Using irradiation together with photolithography the films were irradiated locally, thus resulting in "anisotropypatterned" structures. Domain patterns in differently anisotropy-patterned elements with varying angles of anisotropy and edge orientation, separating regions of different anisotropy alignment, are shown. The influence of the patterning on the (still) full film reversal and the resulting border domain structures are discussed in detail.
References:
[1] C. Chappert et al., Science 280, 1919 (1998)
[2] A. Mougin et al., Phys. Rev. B 63, 060409 (2001)
[3] Woods et al., Appl. Phys. Lett. 81(7), 1267-1269 (2002)
Keywords: magnetism, anisotropy patterning, He ion irradiation, soft magnetic materials
  • Lecture (Conference)
    Conference on Magnetism and Magnetic Materials, Jacksonville, USA, 07.-11.11.2004, Abstract HC-1, Jacksonville, USA

Publ.-Id: 6399 - Permalink


Modified Gilbert Damping due to Exchange Bias in NiFe/FeMn Bilayers: Abstract DF-12
Weber, M.; Nembach, H.; Hillebrands, B.; Fassbender, J.ORC
An all-optical pump probe technique is used to investigate the magnetization dynamics and the magnetic damping behaviour in the polycrystalline Ni80Fe20/Fe50Mn50 exchange bias system. It is demonstrated that the magnetic damping increases linearly with the exchange bias field magnitude. The results can be understood taking the exchange bias effect as an additional dissipation mechanism into account.
In order to investigate the magnetization dynamics an 8.5 ps laser excitation is used. Within the laser pulse duration the temperature is raised, resulting in a collapse of the exchange bias field. Due to the fast recovery time on the order of 160 ps a magnetization precession is induced and the Gilbert magnetic damping parameter is determined by comparison of the experimental data to macrospin simulations on the basis of the Landau-Lifshitz-Gilbert equation. The exchange bias field dependence is studied employing the antiferromagnetic thickness dependence by preparing a wedge shaped FeMn layer (0 – 10 nm) on top of a 5 nm NiFe layer.

Keywords: magnetism, magnetization dynamics, magnetic damping, exchange bias
  • Lecture (Conference)
    Conference on Magnetism and Magnetic Materials, 07.-11.11.2004, Jacksonville, USA

Publ.-Id: 6398 - Permalink


All-Optical Probe of Magnetization Dynamics in exchange coupled NiFe/FeMn Bilayers on the Picosecond Timescale
Weber, M.; Nembach, H.; Hillebrands, B.; Fassbender, J.ORC
A 8.5 ps short laser excitation (pulse energy: 10 nJ, spot size: 30 µm) is used to thermally control the magnetization of a polycrystalline, mesoscopic NiFe/FeMn exchange biased bilayer system on a picosecond time scale. Due to the creation of a hot spin and phonon gas the exchange coupling across the interface between the ferromagnetic and antiferromagnetic layers is reduced upon arrival of the excitation pulse.

Within 10 picoseconds a fast optical unpinning of the exchange bias coupling is observed for the easy axis geometry as well as for the hard axis case by longitudinal Kerr effect of a time-delayed weak probe pulse. This leads to a reduction of the exchange bias field Heb of about 50 percent (shown in Fig. 1) and to a dramatic increase of the zero-field susceptibility, respectively [1]. A similar time dependence is found for the coercive field Hc of the 50µm x 50µm sample structure under investigation. These effects indicate that the lattice temperature at the FM/AFM interface is elevated close to the blocking temperature Tb (155°C for FeMn) of the exchange coupled system. The fast optical unpinning is followed by a slower recovery of the exchange bias field and the zero-field susceptibility governed by a phenomenological spin-lattice time constant of 160 ps. Pump-probe measurements on a sample with different thicknesses of FeMn, thus different exchange bias field values, show that this relaxation time is independent of the bias field strength.

Keywords: magnetism, exchange-bias, pump-probe, time-resolved, MOKE
  • Lecture (Conference)
    International Workshop on Exchange Bias in Magnetic Nanostructures, 16.-18.09.2004, Biarritz, France
  • Contribution to proceedings
    International Workshop on Exchange Bias in Magnetic Nanostructures, Biarritz, France, 16.-18.09.2004, S. 35-36

Publ.-Id: 6397 - Permalink


Investigation of the Fe19Ni81/Fe50Mn50 exchange bias system with varying Cu spacer layer for partial decoupling
Liedke, M. O.; Nembach, H.; Hillebrands, B.; Fassbender, J.ORC
The exchange coupling across an interface between a ferromagnetic and an antiferromagnetic layer may result in the so-called exchange bias effect evidenced by a shift of the hysteresis loop along the magnetic field axis. To study the role of the exchange interaction at and near the interface, Ni81Fe19/Fe50Mn50 bilayers have been studied, which have an intervening Cu layer of varying thickness and position in the antiferromagnetic Fe50Mn50 layer. The role of the intervening Cu layer is to generate partial exchange decoupling. The main motivation is to investigate in which way each antiferromagnetic monolayer contributes to the total exchange bias and what governs the AF thickness dependence? In general introducing an intervening layer in the antiferromagnet can be understood as a well defined layer of defects. The interpretation of the results might be easier than for dilution experiments [1] and contribute to a deeper understanding of the correlation between exchange bias field and coercivity.
We have prepared and investigated a polycrystalline Ni81Fe19/Fe50Mn50 bilayer with an intervening wedge-shaped (0 – 7 Å) Cu layer. As a growth template a Si(100) wafer with a 150 Å thick Cu buffer layer was used. The thickness of the ferromagnetic Ni81Fe19 layer is 50 Å. On top a wedge (0 – 100 Å) of the antiferromagnetic Fe50Mn50 was grown. Next, a Cu spacer wedge was grown perpendicular to the wedge direction of the Fe50Mn50 film. Finally, an additional wedge of Fe50Mn50 was grown on top with the opposite direction in order to keep the total Fe50Mn50 thickness at 100 Å throughout the whole sample (see the sketches in Fig. 1). For protection a 20 Å thick Cr layer was deposited. As a result, a sample was obtained, where in one direction the position of the intervening Cu layer varies from the interface to the top surface of the Fe50Mn50 layer at constant Cu thickness, and in the other direction the Cu thickness varies. To initialize the exchange bias effect the sample was annealed above the Néel temperature and cooled in an applied field back to room temperature.
Hysteresis loops were measured as a function of position on the sample using longitudinal magneto-optic Kerr effect magnetometry. From the loops the exchange bias field Heb and the coercive field Hc were extracted as a function of the Cu spacer thickness, tCu, and of the thickness tFeMn of the lower Fe50Mn50 layer, which is directly exchange coupled to the Ni81Fe19 layer. The results are shown in Fig.1.
The obtained two-dimensional maps provide us easily with global information about the influence of the partial decoupling of the antiferromagnet as a function of tCu and tFeMn.
To obtain more quantitative information several line scans along the horizontal and vertical axes in Fig. 1 were evaluated (not shown here).
For the exchange bias field two different behaviors appear with increasing tFeMn. On one hand Heb decreases for a small Cu thickness close to the ferromagnetic/antiferromagnetic interface, on the other hand Heb increases for a small Cu thickness distant to the interface. For larger tFeMn the typical antiferromagnetic thickness dependence can be found. With increasing tCu a shift of the Heb maximum to higher tFeMn is observed. For a line scan along the Cu wedge for tFeMn = 0 a good agreement with previous results is found [2].
The evolution of the coercivity is different. For small Cu thickness tCu no variation of Hc is observed for scans along tFeMn. It can be concluded that there is no direct connection between exchange bias field Heb and coercive field Hc. The position of the Heb maximum depends on Cu thickness and position within the antiferromagnetic layer but the maximum in Hc always stays the same. In addition it is found that the exchange bias field also depends slightly on cap layer thickness.

Keywords: magnetism, exchange bias, exchange coupling
  • Poster
    International Workshop on Exchange Bias in Magnetic Nanostructures, Biarritz, France, 16.-18.09.2004

Publ.-Id: 6396 - Permalink


Damping in thin NiFe films
Nembach, H.; Weber, M.; Fassbender, J.ORC; Hillebrands, B.
We investigated the damping and spin wave creation in thin NiFe films with time-resolved magneto-optic Kerr effect magnetometry for small and large angle excitations. To gain further insight in the underlying damping process we employ a special measurement procedure, which allows us to determine all three magnetization vector components and therefore the length of the magnetization vector. A reduction of the magnitude of the magnetization vector indicates the creation of spin waves. These spin waves can be created directly by the stripline or by multi-magnon processes.
The experimentally determined magnetization trajectory is compared with numerical simulations based on the Landau-Lifshitz equation, and the damping constant is extracted from these simulations
Keywords: magnetism, permalloy, magnetization dynamics, magnetic damping
  • Poster
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 08.-12.03.2004

Publ.-Id: 6395 - Permalink


Strukturelle Untersuchung ionenbestrahlter Austausch-Verschiebungsschichtsysteme
Blomeier, S.; Fassbender, J.ORC; Hillebrands, B.; Mcgrouther, D.; Mcvitie, S.; Chapman, J. N.
Vor Kurzem konnte gezeigt werden, dass die magnetischen Eigenschaften von Austausch-Verschiebungssystemen durch Ionenbestrahlung gezielt beeinflusst werden können. Ein Modell wurde vorgeschlagen, welches die Änderung der magnetischen Eigenschaften der bestrahlten Systeme einer ioneninduzierten Änderung ihrer Struktur zuschreibt. Insbesondere wird die Erzeugung von Punktdefekten für die beobachteten Veränderungen verantwortlich gemacht. Um dieses Modell zu testen, wurde das ionenbestrahlte System NiFe/FeMn mit Hilfe von Transmissionselektronenmikroskopie untersucht. Es wurden Hellfeld-, Dunkelfeld- und Beugungsbilder von dem System aufgenommen und ausgewertet. Daraus wurden Morphologie, Partikelgrößen und kristallographische Struktur des Systems bestimmt und deren Abhängigkeit von der Bestrahlung durch 5 keV He+-Ionen und 30 keV Ga+-Ionen untersucht. Es konnte gezeigt werden, dass sich weder Partikelgrößen noch kristallographische Struktur mit zunehmender Ionendosis signifikant verändern. Diese Ergebnisse unterstützen die Theorie der defektinduzierten Modifikation der Austausch-Verschiebung.
Keywords: magnetism, exchange-bias, TEM, structural characterization
  • Poster
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 08.-12.03.2004

Publ.-Id: 6394 - Permalink


Investigation of the Fe19Ni81/Fe50Mn50 exchange bias system with varying Cu spacer layer for partial decoupling
Liedke, M. O.; Nembach, H.; Fassbender, J.ORC; Hillebrands, B.
In order to modify the exchange interaction in a Fe19Ni81/Fe50Mn50 exchange bias system a Cu interlayer of varying thickness and position with respect to the interface has been introduced. In one direction the position of the intervening Cu layer varies from the interface to the top surface of the Fe50Mn50 layer at a constant Cu thickness, and in the other direction the Cu thickness itself varies. The introduction of the Cu layer leads to a partial exchange decoupling. The samples were grown in a UHV MBE system and investigated by magneto-optic Kerr effect magnetometry. Two-dimensional maps are obtained, which provide global information about the influence of the partial decoupling of the antiferromagnet as a function of Cu thickness tCu and FeMn thickness tFeMn. In order to obtain more quantitative information several line scans along the horizontal and vertical axes were evaluated. The influence of the partial decoupling on the origins of exchange bias will be discussed.
Keywords: magnetism, exchange-bias, exchange coupling
  • Lecture (Conference)
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 08.-12.03.2004, Regensburg, Deutschland

Publ.-Id: 6392 - Permalink


Modifikation der magnetischen Eigenschaften austausch-gekoppelter NiFe/FeMn Filme durch Beschuss mit Ga+-Ionen
Blomeier, S.; Fassbender, J.ORC; Hillebrands, B.; Mcgrouther, D.; O'Neill, R.; Mcvitie, S.; Chapman, J. N.
Der Einfluss der Bestrahlung von 30 keV Ga+-Ionen auf die magnetischen Eigenschaften des polykristallinen NiFe/FeMn Austausch-Bias-Systems wurde untersucht. In diesem Zusammenhang wurde ein Modell getestet, welches die Änderung der magnetischen Eigenschaften der bestrahlten Systeme einer ioneninduzierten Änderung ihrer Struktur zuschreibt. Dieses Modell war ursprünglich für die Bestrahlung mit He+-Ionen entwickelt worden und macht die Erzeugung von Punktdefekten für die beobachteten Veränderungen verantwortlich. Diesbezüglich wurde auch eine Simulation durchgeführt, die den Beschuss durch He+- und Ga+-Ionen vergleicht. Es konnte gezeigt werden, dass sich das oben genannte Modell auf die Bestrahlung mit Ga+-Ionen übertragen lässt und durch die Ergebnisse der Simulation unterstützt wird. Überdies wurde getestet, ob sich das untersuchte System durch die Bestrahlung mit Ga+-Ionen magnetisch strukturieren lässt. Es konnte nachgewiesen werden, dass auf einer Skala von 10^2 - 10^3 nm eine magnetische Strukturierung mit bestimmten Einschränkungen möglich ist
Keywords: magnetism, exchange-bias, focused ion beam, Ga irradiation, magnetic patterning
  • Lecture (Conference)
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Regensburg, 08.-12.03.2004

Publ.-Id: 6391 - Permalink


Optische Kontrolle der Magnetisierung in austauschgekoppelten NiFe/FeMn Filmen auf der Pikosekundenzeitskala
Weber, M.; Nembach, H.; Fassbender, J.ORC
Eine 9 Pikosekunden kurze Laseranregung (Pulsenergie: 10 nJ, Spotgröße: 25 mm) wird zur thermischen Kontrolle der Magnetisierung eines NiFe/FeMn Bilagensystems auf der Pikosekundenzeitskala eingesetzt. Aufgrund der Erzeugung eines heissen Phononen- /Spingases durch den anregenden Laserpuls wird die Austauschkopplung an der Grenzfläche zwischen Ferro- und Antiferromagnet teilweise unterdrückt. Innerhalb von 20 Pikosekunden wird eine schnelle Reduktion der Austauschkopplung mittels longitudinalem Kerr-Effekt des zeitverzögerten Probepulses sowohl für die magnetisch leichte als auch harte Richtung nachgewiesen. Dies führt zu einer Reduktion des Austausch-Verschiebungsfeldes von ca. 50 Prozent, bzw. zu einem dramatischen Anstieg der Nullfeld-Suszeptibilität. Diese Effekte können durch ein nahe an die sogenannte Blocking-Temperatur Tb (155o C für NiFe/FeMn) erwärmtes Gitter an der FM/AFM-Grenzfläche verstanden werden. Dem schnellen thermischen Unterdrücken der Austauschkopplung folgt eine langsamere Erholung des Austausch-Verschiebungsfeldes bzw. der Nullfeld-Suszeptibilität mit einer Zeitkonstante von ca. 170 ps.
Keywords: magnetism, exchange-bias, pump-probe, time-resolved
  • Lecture (Conference)
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 08.-12.03.2004, Regensburg, Deutschland

Publ.-Id: 6390 - Permalink


Structure and interface characterisation of Exchange Bias FeMn-FeNi films
Solina, D.; Liedke, M. O.; Tietze, U.; Fassbender, J.ORC; Schreyer, A.
X-ray and neutron scattering studies have been carried out on thermally evaporated FeMn-FeNi exchange bias systems before and after ion irradiation by helium ions. Helium ion dosages of 10^14 - 1017 ions /cm-2 have been used. The ion irradiation has shown to alter the exchange bias characteristics of the samples with dose and rate. X-ray diffraction measurements on the samples were made to assess the epitaxy of the layers to the substrate. In order to assess the effect of ion irradiation on these films, both neutron and x-ray reflectivity measurements were performed and the data modelled simultaneously. A final model was chosen that approximated both sets of data well. The reflectivity data suggest that the samples are prone to etching of the surface layer by the ions and that ion irradiation encourages interlayer mixing resulting in a smearing of the oscillations. Increased ion dosages result in a complete loss of exchange bias properties which is supported with loss in inter-layer definition by ion irradiation.
Keywords: magnetism, exchange-bias, x-ray scattering, neutron scattering, thin film magnetism
  • Poster
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Regensburg, 08.-12.03.2004

Publ.-Id: 6389 - Permalink


Developing Air-Water Flow in a Vertical 200 mm Tube (TOPFLOW)
Prasser, H.-M.;
In the lecture, an overview of the experimental work perfromed at the Rossendorf multi-purpose thermal hydaulic test facility TOPFLOW concerning the flow structure in vertical pipes as well as of the function of wire-mesh sensors was given. Experimental results on the evolution of the radial gas fraction profiles, gas velocity profiles and bubble size distributions in a gas-liquid two-phase flow along a large vertical pipe of 194 mm inner diameter are presented. The tests were performed with two wire-mesh sensors. They were used to record sequences of two-dimensional distributions of local instantaneous gas fraction within the complete pipe cross-section with a lateral resolution of 3 mm and a sampling frequency of 2500 Hz. This data is the basis for a fast flow visualization and for the calculation of the mentioned profiles. The gas fraction profiles were obtained by averaging the sequences over time, velocities were measured by cross-correlation of the signals of the two sensors, which were located on a short (63 mm) distance behind each other. The high resolution of the mesh sensors allows to identify regions of connected measuring points in the data array, which are filled with the gas phase. This method was used to obtain the bubble size distributions.
In the experiments, the superficial velocities ranged from 0.04 to 8 m/s for the gas phase and from 0.04 to 1.6 m/s for the liquid. In this way, the experiments cover the range from bubbly to churn turbulent flow regimes. The evolution of the flow structure was studied by varying the distance between the gas injection and the sensor position. This distance was changed by the help of a so-called variable gas injection set-up. It consists of 6 gas injection units, each of them equipped with three rings of orifices in the pipe wall for the gas injection. These rings are fed with the gas phase from ring chambers, which can be individually controlled by valves. The middle ring has orifices of 4 mm diameter, while the upper and the lower rings have nozzles of 1 mm diameter. In this way, 18 different inlet lengths and two different gas injection geometries can be chosen. The latter allows to vary the initial bubble diameter at identical superficial velocities. The test setup is designed for steam-water operation at up to 7 MPa saturation pressure as well, which will be performed later. The paper presents the results of air-water tests.
A special data evaluation technique allows to study the evolution of radial gas fraction profiles that are decomposed according to bubble size classes. In this way, the behavior of bubbles of different
diameter can be observed, which experience different non-drag forces depending on their diameter. A parallel test section of 52.3 mm inner diameter allows the study of scaling effects.
Keywords: air-water flow, two-phase flow, flow structure, radial gas fraction profiles, gas velocity profiles, bubble size distributions, wire-mesh sensors
  • Lecture (Conference)
    Multi-phase Flow: Simulatio, Experiment and Applications, ANSYS-CFX / FZR workshop, 28.-30.06.2004, Dresden, Germany

Publ.-Id: 6388 - Permalink


Measurement Techniques for Multiphase Flows
Prasser, H.-M.;
The lecture presented an overview of the state of art of instrumentation for transient and staedy state multi-phase flows with a special attention to gas-liquid flows. The methods discussed are: impedance probes, wire-mesh sensors, optical fibre probes, gamma tomography, X-ray tomography, 3D pulsed holography, neutron radiography. Short descriptions of the measuring methods developed in Rossendorf were included. This concerns electrical impedance probes, wire-mesh sensors, gamma-tomography for periodically changing transient gas fraction fiels (e.g. in axial pumps and turbo couplings) and highthe status of the development of a -speed X-ray tomography system on the basis of a scanned electron beam.
Keywords: multi-phase flow, impedance probes, wire-mesh sensors, optical fibre probes, gamma tomography, X-ray tomography, 3D pulsed holography, neutron radiography
  • Lecture (Conference)
    Multiphase Flows in Industrial Applications CFX-Germany, ANSYS-CFX / FZR workshop, Lecture on Short Course on Multiphase Flows, 28.-30.06.2004, Dresden, Germany
  • Lecture (Conference)
    Multi-phase Flow: Simulatio, Experiment and Applications, ANSYS-CFX / FZR workshop, Lecture on Short Course on Multiphase Flows, 28.-30.06.2004, Dresden, Germany

Publ.-Id: 6387 - Permalink


Wire-Mesh Sensors - a Useful Tool for Two-phase Flow Model Development
Prasser, H.-M.;
Wire-mesh sensors have achieved a high spatial and time resolution and can therefore contribute considerably to the development of models for CFD codes
Keywords: gas-liquid flow, two-phase flow, wire-mesh sensors
  • Lecture (Conference)
    Multiphase Flows in Industrial Applications CFX-Germany, ANSYS-CFX / FZR workshop, FZ-Rossendorf 28.-30.04.2003.

Publ.-Id: 6386 - Permalink


Air-Water Flow in a Vertical Pipe with Sudden Changes of the Superficial Water Velocity
Prasser, H.-M.; Krepper, E.;
A constant air flow rate was injected into an upwards water flow, while the superficial water velocity was suddenly changed by activating a butterfly valve, the flap of which was perforated. In this way, both a jump-like decrease and increase of the liquid flow rate were studied. Measurements were taken by two wire-mesh sensors mounted at a short distance behind each other. They were placed 3030 mm downstream of the gas injection. Time sequences of the average gas fraction, radial gas fraction and gas velocity profiles as well as bubble size distributions were derived from the wire-mesh sensor data. The velocities were obtained by cross-correlating the signals of the two successive sensors.
Closely after a sudden increase of the liquid superficial velocity, the content of the pipe segment between gas injection and sensor contains still bubbles that were generated before the perturbation. In the first approximation, the cross-section averaged gas fraction should therefore remain the same as before for the duration of the passage of this portion of two-phase mixture. However, the wall peak in the radial gas fraction profile became more pronounced and shifted towards the wall. This was predicted with a CDF code (CFX-4) as well. However, the experimental observations are indicating a significant decrease of the cross-section averaged gas fraction, which can only be explained by an increase of the drift velocity. The latter was supported by gas velocity profiles obtained by cross-correlation of the signals of the two available sensors. We conclude that there is an effect of growing bubble rise velocity with increasing liquid superficial velocity. The nature of this effect is still unclear. In addition, the described effect was observed with inversed tendencies in case of a sudden reduction of the liquid flow rate.

Keywords: pulsed flow, gas-liquid flow, transient two-phase flow, wore-mesh sensor, CFD, drift flux model, vertical pipe flow
  • Lecture (Conference)
    3rd International Symposium on Two-Phase Flow Modelling and Experimentation 22.-24.09.2004, Pisa, Italy
  • Contribution to proceedings
    3rd International Symposium on Two-Phase Flow Modelling and Experimentation 22.-24.09.2004, Pisa, Italy, proceedings on CD-ROM, paper ms14.

Publ.-Id: 6384 - Permalink


Measurement of Bubble Velocity Profiles and Turbulent Diffusion Coefficients of the Gaseous Phase in Rectangular Bubble Column using Image Processing
Zaruba, A.; Krepper, E.; Prasser, H.-M.; Schleicher, E.;
The flow in a rectangular bubble column with a cross-section of 100x20 mm and a height of 1500 mm was studied using a high-speed video system. Series of the images were taken at a frequency of 500 Hz at different elevations. The images were treated by means of a bubble recognition algorithm. It allows the tracking of most of the individual bubbles in a bubble swarm. Time-averaged velocity profiles and turbulent diffusion coefficients were derived as a function of the superficial gas velocity.
To measure the turbulent diffusion coefficient of the gaseous phase the lateral displacement of bubbles traveling over a certain vertical distance was transformed into a probability density distribution. It was found that the shape of distributions fits well to a Gaussean standard distribution. Dispersion coefficients were found to grow nearly proportionally to the square root of the vertical distance. Diffusion coefficients were calculated from the proportionality factor and were compared to correlations from the literature. Experiments were performed at superficial gas velocities ranging from 0.5 to 4 mm/s. The bubbles were generated either by a porous sparger or a set of capillaries placed at the bottom of the column. Measurements were taken at heights between 1 and 1.5 m, where the bubble cloud was occupying the entire cross section.
Keywords: bubble column, turbulence, high-speed imaging, image processing, turbulent dispersion, bubble size, bubble velocity
  • Lecture (Conference)
    3rd International Symposium on Two-Phase Flow Modelling and Experimentation, 22.-24.09.2004, Pisa, Italy
  • Contribution to proceedings
    3rd International Symposium on Two-Phase Flow Modelling and Experimentation, 22.-24.09.2004, Pisa, Italy, Proceedings on CD-ROM, paper ms19.

Publ.-Id: 6383 - Permalink


A study of non-symmetric air water flow using wire-mesh sensor
Wangjiraniran, W.; Aritomi, M.; Kikura, H.; Motegi, Y.; Prasser, H.-M.;
This study investigates the characteristics of upward air-water flow for non-symmetric initial condition in the 50 mm vertical tube. Wire Mesh Sensor is utilized to obtain the local properties of the gas phase i.e. void fraction, gas velocity and bubble size. To generate the non-symmetric initial flow condition, air is supplied from a single injecting hole and mixed into the liquid flow inside the pipe through a narrow-ring shaped exit. In this case, large bubble can be generated at the inlet. In this experiment, the characteristics of bubble formation at the inlet and flow evolution along the pipe are emphasized. The results show the capability of the measurement technique to analyze the non-symmetric distribution of the gas phase. The effects of superficial gas and liquid velocity on the bubble distributions and on the local void fraction spreading at the inlet are clarified. The flow evolution dominated by bubble coalescence and break up is discussed in term of the development of Sauter mean diameter over the cross- section of the pipe. The results also show an inconsistency with uniform flow conditions at low gas flux. Hence, the effect of phase distribution spreading is proposed. In addition, the development of rising gas velocity in term of mean bubble size during coalescence and break up is revealed.

Keywords: gas-liquid flows, non-symmetric flows, wire mesh sensor, flow evolution, bubble rising velocity
  • Experimental Thermal and Fluid Science 29(2005)3, 315-322

Publ.-Id: 6382 - Permalink


Influence of the Gas Injection on the Void Fraction Profiles and Bubble Size Distributions of a Air-Water Flow in Vertical Pipes
Prasser, H.-M.;
The paper reports about air-water tests in two different vertical pipes, one of 52.3 mm inner diameter, the other of 194.1 mm. In both cases, wire-mesh sensors with a measuring frequency of 2500 Hz were applied. They were used to measure radial gas fraction profiles, velocity profiles and bubble size distributions. Scaling effects were found by comparing the results for the two different pipe diameters. A variation of the air injection conditions allowed to study the influence of the primary bubble size. It was found that the kind of gas injection has a deciding influence at the appearance of a wall peak in the gas fraction profile as well as at the establishing gas fraction profiles and bubble size distributions in general.
Keywords: Gas-liquid flow, two-phase flow, wire-mesh sensor, gas fraction profiles, gas velocity profiles, bubble size
  • Lecture (Conference)
    5th International Conference on Multiphase Flow, ICMF’04, 30.05.-04.06.2004, Yokohama, Japan
  • Contribution to proceedings
    5th International Conference on Multiphase Flow, ICMF’04, 30.05.-04.06.2004, Yokohama, Japan, paper 187

Publ.-Id: 6381 - Permalink


Leserbriefe
Prasser, H.-M.;
Die friedliche Nutzung der Kernenergie hat heute einen besonderen Bedarf an Öffentlichkeitsarbeit. In keiner anderen Technologiesparte ist die Diskrepanz zwischen dem Nutzen und dessen Bewertung durch die Öffentlichkeit, zwischen den tatsächlichen Risiken und den Ängsten in der Bevölkerung, zwischen dem Stand der Technik und veröffentlichter Meinung so gravierend, wie hier. Die Ursache liegt zum großen Teil auch in Versäumnissen in der Vergangenheit. Die Kernenergiebefürworter haben durch ihre zurückhaltende Öffentlichkeitsarbeit den mehrheitlich gegen die Kernenergie eingestellten Umweltverbänden und Bürgerbewegungen in den Medien praktisch vollständig das Feld überlassen. Dies hat über eine Reihe von Rückkopplungen zu Verstärkungseffekten geführt, die es nun schwer machen, die Öffentlichkeit mit sachlichen Argumenten zu erreichen. Nur mühsam gelingt es, den verlorenen Boden zurückzugewinnen. Die Kerntechnische Gesellschaft muß den Part der Argumentation für die Kernenergie als Zukunftsoption übernehmen und stärker die in der wissenschaftlich-technischen Weiterentwicklung liegenden Potenzen für Umwelterhalt, Ressourcensicherung und Sicherheit herausstellen.
Die Kerntechnische Gesellschaft stützt sich als gemeinnütziger Verein naturgemäß in hohem Maß auf das Engagement seiner Mitglieder. Das Verfassen von Leserbriefen kann in diesem Zusammenhang gewissermaßen als dezentrale Form der Öffentlichkeitsarbeit angesehen werden. Leserbriefe können und sollen jedoch die persönliche Meinung des Verfassers zum Ausdruck bringen. Leserbriefaktionen, bei denen zentral vorgefasste Parolen an die Öffentlichkeit gebracht werden sollen, entsprechen meiner Meinung nach nicht dem Niveau unserer Kerntechnischen Gesellschaft und seiner Mitglieder. Wohl aber ist es nötig und zulässig, zu einem stärkeren Engagement auf diesem Gebiet aufzurufen. Der vorliegende Beitrag soll deshalb nicht als starrer Leitfaden für eine inhaltliche Gestaltung von Leserbriefen verstanden werden. Es soll vielmehr die persönlichen Ansichten des Autors zur Diskussion stellen und seine Erfahrungen vermitteln, um damit alle diejenigen, die es noch nicht versucht haben, zum Einbringen ihrer persönlichen Meinung auf dem Weg des Leserbriefes zu ermutigen.
Keywords: Kernenergie, Öffentlichkeitsarbeit, Leserbriefe
  • Lecture (Conference)
    Jahrestagung Kerntechnik 2000, Bonn, 23.-25. Mai, Fachsitzung Kommunikation mit der Öffentlichkeit, Herausgeber: Deutsches Atomforum e.V., Bonn, Juni 2000, S. 39-48.
  • Contribution to proceedings
    Jahrestagung Kerntechnik 2000, Bonn, 23.-25. Mai, Fachsitzung Kommunikation mit der Öffentlichkeit, Herausgeber: Deutsches Atomforum e.V., Bonn, Juni 2000, S. 39-48.

Publ.-Id: 6380 - Permalink


Evolution of the structure of a gas-liquid two-phase flow in a large vertical pipe
Prasser, H.-M.; Beyer, M.; Carl, H.; Gregor, S.; Lucas, D.; Pietruske, H.; Schütz, P.; Weiss, F.-P.;
Experimental results on the evolution of the radial gas fraction profiles, gas velocity profiles and bubble size distributions in a gas-liquid two-phase flow along a large vertical pipe of 194 mm inner diameter are presented. The tests were performed at the TOPFLOW facility in Rossendorf, where two wire-mesh sensors were used to measure sequences of two-dimensional distributions of local instantaneous gas fraction within the complete pipe cross-section with a lateral resolution of 3 mm and a sampling frequency of 2500 Hz. This data is the basis for a fast flow visualization and for the calculation of the mentioned profiles. The gas fraction profiles were obtained by averaging the sequences over time, velocities were measured by cross-correlation of the signals of the two sensors, which were located on a short (63 mm) distance behind each other. The high resolution of the mesh sensors allows to identify regions of connected measuring points in the data array, which are filled with the gas phase. This method was used to obtain the bubble size distributions.
In the experiments, the superficial velocities ranged from 0.04 to 8 m/s for the gas phase and from 0.04 to 1.6 m/s for the liquid. In this way, the experiments cover the range from bubbly to churn turbulent flow regimes. The evolution of the flow structure was studied by varying the distance between the gas injection and the sensor position. This distance was changed by the help of a so-called variable gas injection set-up. It consists of 6 gas injection units, each of them equipped with three rings of orifices in the pipe wall for the gas injection. These rings are fed with the gas phase from ring chambers, which can be individually controlled by valves. The middle ring has orifices of 4 mm diameter, while the upper and the lower rings have nozzles of 1 mm diameter. In this way, 18 different inlet lengths and two different gas injection geometries can be chosen. The latter allows to vary the initial bubble diameter at identical superficial velocities. The test setup is designed for steam-water operation at up to 7 MPa saturation pressure as well, which will be performed later. The paper presents the results of air-water tests.
A special data evaluation technique allows to study the evolution of radial gas fraction profiles that are decomposed according to bubble size classes. In this way, the behavior of bubbles of different diameter can be observed, which experience different non-drag forces depending on their diameter. A parallel test section of 52.3 mm inner diameter allows the study of scaling effects.
Keywords: Gas-liquid flow, two-phase flow, wire-mesh sensor, flow structure, bubble size, gas fraction, vertical pipe flow
  • Lecture (Conference)
    Eleventh International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 11), 02.-06.10.2005, Avignon, France
  • Contribution to proceedings
    Eleventh International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 11), 02.-06.10.2005, Avignon, France, paper 399.
  • Nuclear Engineering and Design 237(2007), 1848-1861

Publ.-Id: 6379 - Permalink


Evolution of Interfacial Area Concentration in a Vertical Air-Water Flow Measured by Wire-Mesh Sensors
Prasser, H.-M.; Gregor, S.;
Conservation equations for one or more classes of the interfacial area concentration is one of the concepts to take into account the evolution of the flow structure in a gas-liquid two-phase flow in thermal hydraulic or, respectively, CFD modeling. The measurement of the interfacial area concentration is therefore of large importance for the development and validation of those models. The paper presents the results of an application of wire-mesh sensors to obtain the interfacial area concentration in vertical pipes of 52.3 and 194.1 mm inner diameter as an alternative to the widely used multiple-tip optical fiber probes. The measuring data of a mesh sensor is represented by a threedimensional matrix of local instantaneous gas fractions recorded as a time sequence (sampling frequency 2.5 - 10 kHz) at the crossing points of the wires (resolution in x-y direction: 3 mm).
Velocity information necessary to convert the time axis in an Eulerian z-axis was obtained by means of cross-correlation calculated between the signals of two identical mesh sensors located at a small distance behind each other. Three methods are used to deduce the interfacial area concentration from the wire-mesh sensor data:
• Calculation of the sum of the interfacial area represented by bubbles, which are identified as regions of connected gas filled elements in the data array, where the superficial area of an individual bubble is calculated under the assumption of a ellipsoid bubble shape. This method is restricted to low gas fractions, where bubble shapes are still close to the assumed geometry.
• A second method consists in the calculation of the sum of the contribution of the gas fraction in each local control volume of the sensor formed by two crossing wires. This control volume is approximated by a sphere with a volume-equivalent diameter. Furthermore, it is assumed, that the gas-liquid interface is a plane that cuts this sphere in a part filled with gas and another part that is filled with liquid. In this way, a correlation between the local interfacial area and the local instantaneous gas fraction can be found, in which the local interfacial area concentration is at its maximum at 50 % volumetric gas fraction and decreases to zero at both 0 % and 100 % local instantaneous gas fraction. This method can be applied to large bubbles with an arbitrary shape.
• In order to decrease the measuring error arising from the fact that gas fractions in measuring points inside large bubble as well as inside completely liquid-filled regions are not exactly equal to 100 % or, respectively, 0 %, and which therefore may significantly contribute to an overestimation of the interfacial area, a third method was applied, which is based on the identification of points that belong to the surface of the bubbles.
All three methods were applied to a vertical air-water flow. Since the distance between the sensors and the gas injection was varied, it was possible to characterize the change of the interfacial area concentration along the pipe. By combining the method with bubble size measurements, a decomposition of the interfacial area concentration into more then one bubble size classes was performed as well. The obtained results were compared to the findings reported in literature. The use of mesh sensors proved to be very efficient, since a bubble-size decomposed interfacial area concentration measurement in an entire pipe cross-section can be performed in a few seconds, which is very efficient compared to the use of local probes.
Keywords: gas-liquid flow, wire-mesh sensor, interfacial area concentration, vertical flow, two-phase flow
  • Lecture (Conference)
    Eleventh International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 11), 02.-06.10.2005, Avignon, France
  • Contribution to proceedings
    Eleventh International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 11), 02.-06.10.2005, Avignon, France. paper 398.

Publ.-Id: 6378 - Permalink


Experiments on Turbulent Diffusion of the Gaseous Phase in Rectangular Bubble Column Using Image Processing
Zaruba, A.; Krepper, E.; Prasser, H.-M.;
The paper describes the application of high-speed video observation combined with digital image processing techniques to measure the turbulent diffusion coefficient of the gaseous phase in a bubble column. The test channel is a rectangular bubble column with a cross-section of 100x20 mm and a height of 1500 mm. Sequences of the images were taken at different heights of the column with a high-speed video system. A diffuse illumination from the backside of the column was found to produce bubble images with a characteristic bright spot in the center. This bright spot was used for a bubble tracking that tolerates some overlapping of the bubbles. This allows to enhance the applicability of the bubble tracking to higher gas fractions compared to the tracking of the entire bubble shadows. After the bubble trajectories were extracted from the image sequence, the lateral displacement of bubbles between two virtual horizontal measuring planes was calculated for each bubble that crosses both planes. This lateral displacement is caused by the turbulent fluctuations of the bubble velocity. It is analyzed statistically by constructing probability density functions of the lateral displacement, which can be well described by Gaussean standard distributions. The dispersion coefficient of the standard distributions that were fitted to the experimental data shows a linear dependency from the square-root of the vertical distance between the two control planes, which supports the assumption of a diffusion model for the lateral displacement. Deviations from the linear dependency were identified to be the result of deterministic oscillatory motions of the rising bubbles. This effect is typical for low-viscosity liquids, while the bubble oscillations are damped at higher viscosities. Results are presented for different gas flow rates (superficial gas velocity ranged from 0.5 to 4 mm/s), different primary bubble sizes produced by a variation of the sparger and for liquids with different viscosity (water, ethylene glycol, mixtures of both). All measurements were taken at heights between 1 and 1.5 m, where the bubble cloud was occupying the entire cross section. Beside the turbulent diffusion study, the optical observation was used to obtain bubble sizes, gas fractions and rise velocities. Measured diffusion coefficients were compared to correlations from the literature and CFD simulations of the column using the code CFX.
Keywords: Gas-liquid flow, bubble columns, image processing, turbulence, turbulent diffusion
  • Lecture (Conference)
    Eleventh International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 11), 02.-06.10.2005, Avignon, France
  • Contribution to proceedings
    Eleventh International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 11, 02.-06.10.2005, Avignon, France, Proceedings CD-ROM, paper 388

Publ.-Id: 6377 - Permalink


Polymerization Processes of Epoxy Matric Composites under Simulated Free Space Conditions
Kondyurin, A. V.; Lauke, B.; Richter, E.;

Publ.-Id: 6376 - Permalink


Analysis of the Ni81Fe19/Fe50Mn50 exchange bias system with a varying Cu spacer thickness and position for partial decoupling
Liedke, M. O.; Nembach, H.; Hillebrands, B.; Fassbender, J.ORC
In order to study the role of the exchange interaction at and near the interface, Ni81Fe19/Fe50Mn50 bilayers have been studied, which have an intervening layer of varying thickness and position in the anti ferromagnetic Fe50Mn50 layer. The role of the intervening layer is to generate partial exchange decoupling. As a result, samples were obtained, where in one in-plane direction the position of the intervening layer varies from the interface to the top surface of the Fe50Mn50 layer at a constant intervening layer thickness, and in the other in-plane direction the intervening layer thickness varies. Two-dimensional maps of the resulting exchange bias field and the coercive field were obtained from magneto-optic Kerr effect magnetometry measurements. The role of the position and strength of the partial decoupling within the anti ferromagnetic layer on the exchange bias effect and the coercive field is discussed.
Keywords: magnetism, exchange bias, thin films, exchange coupling
  • Poster
    JEMS'04, Dresden, 06.-10.09.2004
  • Contribution to external collection
    JEMS'04, Dresden, 06.-10.09.2004

Publ.-Id: 6375 - Permalink


Modification of the magnetic properties of exchange coupled NiFe/FeMn films by Ga+ ion irradiation
Blomeier, S.; Mcgrouther, D.; Neill, R. O.; Mcvitie, S.; Chapman, J. N.; Weber, M. C.; Hillebrands, B.; Fassbender, J.ORC
The influence of 30 keV Ga+ ion irradiation on the magnetic properties of polycrystalline NiFe/FeMn exchange bias layers were investigated. In this context, it was of particular interest to determine whether a previously observed enhancement of the bias field value for the irradiation with 5 keV He+ ions is a material-specific or an ion-specific effect. Moreover, the capability of magnetic micropatterning using Ga+ ions from a Focused Ion Beam source was tested. It could be demonstrated that magnetic patterning with a lower limit in the range of 100 – 1000 nm is possible.
Keywords: magnetism, exchange-bias, focussed ion beam, magnetic patterning
  • Poster
    JEMS'04, Dresden, 06.-10.09.2004
  • Journal of Magnetism and Magnetic Materials 290(2005)1, 731-734

Publ.-Id: 6374 - Permalink


Reflectivity characterization of ion irradiated exchange bias FeMn-FeNi films
Solina, D.; Liedke, M. O.; Tietze, U.; Fassbender, J.ORC; Schreyer, A.
X-ray reflectivity, neutron reflectivity studies and Magneto-Optic Kerr Effect measurements (MOKE) have been carried out on thermally evaporated FeMn-FeNi exchange biased films before and after ion irradiation by helium ions. MOKE shows that ion irradiation reduces the exchange bias characteristics of the samples with increased dose. Modelling of the reflectivity data infers that atoms from the buffer layer are displaced by the ions and imbedded into the substrate material. The correlation between these strong structural modifications by irradiation with the reduction of the exchange bias will be discussed.
Keywords: magnetism, exchange-bias, x-ray reflectivity, neutron scattering
  • Poster
    MML'04, Boulder, USA, 07.-11.06.2004

Publ.-Id: 6373 - Permalink


Application of light ion irradiation on the magnetic properties of exchange bias layers
Fassbender, J.ORC
Due to the reduced dimension, the magnetic properties of ultrathin magnetic films and multilayers, e.g. magnetic anisotropies and exchange coupling, often depend strongly on the surface and interface structure. In addition chemical composition, crystallinity, grain sizes and their distribution govern the magnetic behaviour. All these structural properties can be modified by light-ion irradiation in an energy range from 5 – 150 keV due to the energy loss of the ions in the solid along their trajectory. Consequently the magnetic properties can be tailored by ion irradiation. Similar effects can also be observed using Ga+ ion irradiation, which is the common ion source in focused ion beam lithography. Examples for ion induced modifications of magnetic properties in exchange bias systems are presented.

Due to the local nature of the interaction, magnetic patterning without affecting the surface topography becomes feasible, which may have an applied interest. In contrast to topographic nanostructures the surrounding area of these nanostructures can be left ferromagnetic leading to new phenomena at their mutual interface.

Most of the material systems discussed here are important for technological applications. It will be shown that light-ion irradiation has many advantages for design of new material properties and the fabrication technology of actual devices.

Keywords: magnetism, exchange-bias, ion irradiation, thin magnetic films, magnetic patterning
  • Lecture (Conference)
    Sommerschule der EU-RTN Netzwerke "Nexbias" und "Ultraswitch", 13. – 17.09.2004, Biarritz, France

Publ.-Id: 6372 - Permalink


Ion beams for magnetic thin film applications
Fassbender, J.ORC
Due to the reduced dimension, the magnetic properties of ultrathin magnetic films and multilayers, e.g. magnetic anisotropies and exchange coupling, often depend strongly on the surface and interface structure. In addition chemical composition, crystallinity, grain sizes and their distribution govern the magnetic behaviour. All these structural properties can be modified by light-ion irradiation in an energy range from 5 - 150 keV due to the energy loss of the ions in the solid along their trajectory. Consequently the magnetic properties can be tailored by ion irradiation. Due to the local nature of the interaction, magnetic patterning without affecting the surface topography becomes feasible, which may have an applied interest. In contrast to topographic nanostructures the surrounding area of these nanostructures can be left ferromagnetic leading to new phenomena at their mutual interface. An overview over the present status will be presented.
Keywords: magnetism, ion irradiation, He, thin magnetic films, magnetic patterning
  • Lecture (Conference)
    14th International Conference on Ion Beam Modification of Materials (IBMM 2004), Abstract VIII A - Applications, 05. – 10.09.2004, Monterey, USA

Publ.-Id: 6371 - Permalink


Optical control of the magnetization on the picosecond timescale
Fassbender, J.ORC
A 9 ps long laser pulse (energy: 10 nJ, spot diameter: 25 µm) is used to thermally control the magnetization of a NiFe/FeMn exchange bias bilayer system on the ps-time scale. The short laser pulse creates a hot phonon/spin gas. Consequently the exchange coupling across the interface between the ferro- and antiferromagnetic layer is reduced within 20 ps. This fast reduction of the exchange coupling is sensed by a time delayed probe pulse and analyzed with longitudinal magneto-optic Kerr effect. The time evolution of the exchange bias field in the easy magnetization direction and of the zero field suszeptibility in the hard magnetization direction is investigated. Within the first 20 ps a reduction of the exchange bias field of about 50 percent is observed. In addition a drastic increase of the zero field suszeptibility is found. The results can be understood concerning that the crystal lattice is heated close to the blocking temperature of the FeMn film (150°C). After the fast thermal reduction of the exchange coupling a slow recovery of the exchange bias field and the zero field suszeptibility is observed with a time constant of 170 ps.
Keywords: magnetism, exchange-bias, pump-probe, MOKE
  • Lecture (Conference)
    Academy Colloquium "Ultrafast spin and magnetization dynamics in magnetic nanostructures", 15. – 17.06.2004, Amsterdam, Niederlande

Publ.-Id: 6370 - Permalink


Optical control of the magnetization on the picosecond timescale
Fassbender, J.ORC
A 9 ps long laser pulse (energy: 10 nJ, spot diameter: 25 µm) is used to thermally control the magnetization of a NiFe/FeMn exchange bias bilayer system on the ps-time scale. The short laser pulse creates a hot phonon/spin gas. Consequently the exchange coupling across the interface between the ferro- and antiferromagnetic layer is reduced within 20 ps. This fast reduction of the exchange coupling is sensed by a time delayed probe pulse and analyzed with longitudinal magneto-optic Kerr effect. The time evolution of the exchange bias field in the easy magnetization direction and of the zero field suszeptibility in the hard magnetization direction is investigated. Within the first 20 ps a reduction of the exchange bias field of about 50 percent is observed. In addition a drastic increase of the zero field suszeptibility is found. The results can be understood concerning that the crystal lattice is heated close to the blocking temperature of the FeMn film (150°C). After the fast thermal reduction of the exchange coupling a slow recovery of the exchange bias field and the zero field suszeptibility is observed with a time constant of 170 ps.
Keywords: magnetism, exchange-bias, pump-probe, MOKE
  • Lecture (Conference)
    International Workshop on "Magneto-optics of magnetic thin films and multilayers", 29. – 30.04.2004, Duisburg, Deutschland

Publ.-Id: 6369 - Permalink


Betriebshandbuch für die Mehrzweck-Thermohydraulikversuchsanlage TOPFLOW
Beyer, M.; Carl, H.; Schütz, P.; Pietruske, H.; Lenk, S.;
Ausgehend von einer ausführlichen Beschreibung der Mehrzweckversuchsanlage TOPFLOW und der zu ihrem Betrieb erforderlichen Bedien- und Sicherheitshinweise enthält dieses Betriebshandbuch Anweisungen zur Inbetriebnahme der Versuchsanlage sowie für die Vorbereitung und Durchführung von Experimenten zur Untersuchung von transienten Zweiphasenströmungen mit den Medien Dampf-Wasser bzw. Luft-Wasser. Erläuterungen über Sonderbetriebsfälle schließen das Betriebshandbuch ab. Eine Auflistung der technologischen Verriegelungen und Visualisierungen der Anlagenschemata aus dem Prozessleitsystem sind als Anlagen angefügt.
Keywords: Thermohydraulik, TOPFLOW, Zweiphasenströmungen, Betriebshandbuch
  • Open Access LogoWissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-405

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Publ.-Id: 6368 - Permalink


Intersubband semiconductor light sources: history, status, and future
Helm, M.;
After the first theoretical proposal in 1971, it took more than two decades until the first laser based on intersubband transitions in semiconductor quantum structures was demonstrated. The progress since then has indeed been spectacular with lasers now spanning the range from 3.5 to 140 microns. The physics of these devices will be elucidated and some crucial achievements will be highlighted. Finally the perspectives for closing the still existing wavelength gap (25-60 microns) and extending the covered range to even longer as well as shorter wavelength will be discussed.
Keywords: intersubband transitions, quantum cascade laser, semiconductor light emitters, infrared
  • Lecture (Conference)
    Plenary talk at The Joint 29th International Conference on Infrared and Millimeter Waves and 12th International Conference on Terahertz Electronics (IRMMW 2004/THz 2004), Karlsruhe, Germany
  • Contribution to proceedings
    Infrared and Millimeter Waves, 2004 and 12th International Conference on Terahertz Electronics, 2004. Conference Digest of the 2004 Joint 29th International Conference on 27 Sept.-1 Oct. 2004 Page(s): 57 - 58

Publ.-Id: 6367 - Permalink


In-depth optical and structural study of silver-based low-emissivity multilayer coatings for energy-saving applications
Martín-Palma, R. J.; Gago, R.; Vinnichenko, M.; Martínez-Duart, J. M.;
Low-emissivity coatings on glass, which provide highly efficient heat isolation, are nowadays extensively used in windows in offices and residential buildings for the purpose of saving energy in air conditioning. In this paper, multilayer low-emissivity coatings with the structure glass/SnO2/Ni–Cr/Ag/Ni–Cr/SnO2 were deposited onto large glass substrates in an industrial sputtering system. The extremely low thickness of the layers which compose such structures, as well as the large substrate area, causes the structure and impurity content of the films and interfaces to play an important role in providing efficient energy-saving performance as well as high optical transmittance. Thus, in-depth characterization was performed by spectroscopic ellipsometry, Rutherford backscattering spectroscopy and Auger electron spectroscopy. The use of these characterization techniques provides the great advantage of allowing the investigation of the whole multilayer system, including the interfaces. However, the properties of the individual layers (density, composition and optical behaviour) were also determined independently and compared to the values of corresponding bulk materials. Special attention is devoted to inter-diffusion of species.

Publ.-Id: 6366 - Permalink


Structure analysis and speciation of actinide systems using XAFS
Hennig, C.;
Invited Lecture
  • Lecture (others)
    16.02.2004, CEA Marcoule, France

Publ.-Id: 6363 - Permalink


Combining EXAFS and X-ray powder diffraction to solve structures containing heavy atoms
Hennig, C.; Reich, T.; Kraus, W.; Reck, G.; Prokert, K.; Schell, N.;
Determination of structures using X-ray powder diffraction is complicated if the reflection intensities are mainly influenced by the scattering from heavy atoms and the atomic coordinates of light atoms remain uncertain. A method like EXAFS, which is sensitive to short range order, gives reliable atomic distances in the surroundings of heavy atoms with a precision of ± 0.02 Å. The probability for obtaining the complete structure from X-ray powder diffraction increases if one includes parameters derived from EXAFS measurements as restraints during the procedure of structure solving. We demonstrate the potential of combining EXAFS and X-ray powder diffraction by solving the structure UO2[H2AsO4]2×H2O. The procedure starts with the determination of space group and cell parameters from XRD powder data. In a second step the absolute values of the structure factor |F| are separated by iterating a decomposition formula. The heavy atom positions are determined by direct methods. In the third step atomic distances of coordination polyhedra are estimated using EXAFS. Subsequently, the complete coordination geometries around the heavy atoms including reliable distances are used as restraints in the structure solving and refinement procedure.
  • Physica Scripta T115(2005), 352-355

Publ.-Id: 6362 - Permalink


Spéciation des complexes du technétium par couplage SAX - Electrochimie
Poineau, F.; Fattahi, M.; Grambow, B.; Den Auwer, C.; Hennig, C.;
Le couplage SAZ - Elèctrochimie s'avère être une méthode pour la spéciation des espèces réduites du technetium. Cette méthode pourra être étendue à l'étude des espèces instables des actinides et des produits de fission.
  • Poster
    Journée de Radiochimie, 5.-6.2.2004, Avignon, France

Publ.-Id: 6361 - Permalink


Polarized EXAFS of uranium L1 and L3 edges - A comparison
Hennig, C.; Scheinost, A.;
Uranium(VI) consists predominantly of two double bonded oxygen atoms in axial direction (Oax) and 4, 5 or 6 oxygen atoms in the equatorial plane (Oeq). This anisotropic coordination leads to a strong polarization dependence of XAFS. Responsible for the polarization dependence are the electron waves with different impulse-momentum at the L1 and L3 edges. We investigated this effect by comparing experimental data with the theory. The Theory and experiment are in good agreement. The polarization dependence of L1 edge is significantly stronger that of L3 edge.
  • Poster
    14th ESRF User Meeting, 10./11.02.2004

Publ.-Id: 6360 - Permalink


Pre-irradiation memory effect on the photoluminexcence intensity of Ge-implanted SiO2 layers
Lopes, J. M. J.; Zawislak, F. C.; Fichtner, P. F. P.; Behar, M.; Rebohle, L.; Skorupa, W.;
  • Nuclear Instruments and Methods in Physics Research B 218(2004), 438-443

Publ.-Id: 6358 - Permalink


Evidence for the Interaction of Technetium colloids with humic substances by X-ray absorption spectroscopy
Maes, A.; Geraedts, K.; Bruggeman, C.; Vancluysen, J.; Rossberg, A.; Hennig, C.;
Spectroscopic extended X-ray absorption fine structure (EXAFS) evidence was obtained on the chemical environment of 99Tc(IV) atoms formed upon introduction of TcO4 - into four types of laboratory-scale synthetic and natural systems which mimic in situ natural reducing conditions in humicrich geochemical environments: (a) magnetite/pyrite in synthetic groundwater in the absence of humic substances (HSs), (b) magnetite/pyrite in natural Gorleben groundwater in the presence of HSs, (c) Boom clay sediment mixed with synthetic groundwater, and (d) Gorleben sand mixed with natural Gorleben groundwater. The investigated systems obey to pH 8-9 conditions, and all measured samples show similar EXAFS spectra for Tc, which could be fitted by a hydrated TcO2¿ xH2O phase. The results are interpreted as follows: upon introduction of high concentrations (millimolar to micromolar) of TcO4 to chemically reducing environments, small Tc(IV) oxidic polymers are formed, which either may aggregate into larger units (colloids) and finally precipitate or may interact in their polymeric form with (dissolved and immobile) humic substances. This latter type of interactionsTc(IV) colloid sorption onto HSssdiffers significantly from the generally accepted metal-humate complexation and therefore offers new views on the possible reaction pathways of metals and radionuclides in humic-rich environments.
  • Environmental Science & Technology 38(2004), 2044-2051

Publ.-Id: 6357 - Permalink


X-ray absorption spectroscopy studies of reactions of technetium, uranium and neptunium with mackinawite
Livens, F.; Jones, M.; Hynes, A.; Charnock, J.; Mosselmans, J.; Hennig, C.; Steele, H.; Collins, D.; Vaughan, D.; Et, A.;
Technetium, uranium and neptunium may all occur in the environment in more than one oxidation state (IV or VII, IV or VI and IV or V respectively). The surface of mackinawite, the first-formed iron sulfide: phase in anoxic conditions, can promote redox changes so a series of laboratory experiments were carried out to explore the interactions of Tc, U and Np with this mineral. The products of reaction were characterised using X-ray absorption spectroscopy. Technetium, added as TcO4-, is reduced to oxidation state IV and forms a TcS2-like species. On oxidation of the mackinawite in air to form goethite, Tc remains in oxidation state IV but in an oxide, rather than a sulfide environment. At low concentrations, uranium forms uranyl surface complexes on oxidised regions of the-mackinawite surface but at higher concentrations, the uranium promotes surface oxidation and forms a mixed oxidation state oxide phase. Neptunium is reduced to oxidation IV and forms a surface complex with surface sulfide ions. The remainder of the Np coordination sphere is filled with water molecules or hydroxide ions.
  • Journal of Environmental Radioactivity 74(2004), 211-219

Publ.-Id: 6356 - Permalink


Structure of uranium sorption complexes at montmorillonite edge sites
Hennig, C.; Reich, T.; Dähn, R.; Scheidegger, A. M.;
Extended X-ray absorption fine structure (EXAFS) spectroscopy at the uranium LIII-edge was used for determining the structural environment of aqueous uranyl sorbed onto montmorillonite. The study reveals that uranyl uptake at pH ~ 5- ~ 7 and at an initial uranyl concentration of 5×10-5 M takes place at amphoteric surface hydroxyl sites as inner-sphere complex. The measured bond distances between uranium and the equatorial oxygen atoms are in the range of 2.34 Å and 2.37 Å indicating an inner-sphere coordination. At ~ 3.4Å the presence of a U-Al backscattering pair was determined. This backscattering pair indicates that the binding of the uranyl unit to amphoteric surface hydroxyl sites occurs preferred as a bidentate inner-sphere complex on aluminol groups.
Keywords: Montmorillonite, Uranium, Sorption, EXAFS
  • Radiochim. Acta 90, 653-657 (2002)

Publ.-Id: 6355 - Permalink


EXAFS and XRD investigations of zeunerite and meta-zeunerite
Hennig, C.; Reck, G.; Reich, T.; Rossberg, A.; Kraus, W.; Sieler, J.;
In this paper EXAFS was used to determine bond lengths in the structures of zeunerite and meta-zeunerite. The atomic distances between heavy and light scatterers observed using EXAFS in meta-zeunerite deviate approximately 0.1 _A from literature data of single-crystal X-ray diffraction measurements. Because this difference is significant higher than the error limits of EXAFS measurements, the complete crystal structure of meta-zeunerite, Cu[UO2AsO4]2 _ 8 H2O, is revised by X-ray structure analysis. The bond length determinations by EXAFS and the revised XRD data agree within the experimental error limits. In this study EXAFS spectroscopy has proven to be an useful tool for determining precise local bond lengths in the environment of heavy atoms. Moreover, the crystal structure of zeunerite, Cu[UO2AsO4]2 _ 12 H2O, hitherto not been described in the literature, was investigated. Reflex broadening effects and intergrowth relationship between zeunerite and meta-zeunerite show that meta-zeunerite grows in nature due to dehydration of zeunerite. The structural transition from zeunerite to meta-zeunerite is connected with a change in the uranyl arsenate layer arrangement and the crystal water content.
  • Z. Kristallogr. 218 (2003) 37-45

Publ.-Id: 6354 - Permalink


An EXAFS and TRLFS study of the sorption of trivalent actinides onto smectite and kaolinite
Stumpf, T.; Hennig, C.; Bauer, A.; Denecke, M. A.; Fanghänel, T.;
The structure parameters of the Am3+ aquo ion and of Am(III) sorbed onto smectite and kaolinite at varying pH are analyzed using EXAFS. An Am-O distance of 2.47-2.49 Å is found and a coordination number of 8-9 oxygen atoms is observed for the Am3+ and its hydration sphere. Combined TRLFS (Cm(III)) and EXAFS (Am(III)) results show that An(III) sorbs onto smectite at pH 4, forming an outer-sphere complex and retains its complete primary hydration sphere. With increasing pH, inner-sphere sorption onto smectite and kaolinite occurs. The overall number of oxygen atoms coordinating the actinide ion remains about the same at pH 6, with four water molecules being replaced by oxygen atoms from the mineral surface during inner-sphere complex formation. The coordination number of sorbed Am(III) at pH 8 exhibits an apparent decrease, which may be affected by the formation of ternary OH-/Am/clay mineral surface species.
Keywords: Actinides, Clay minerals, EXAFS, Surface complexation, TRLFS
  • Radiochimica Acta 92(2004), 133-138

Publ.-Id: 6352 - Permalink


"Programme & Projekte - Beispiele für das Management in einer Forschungseinrichtung"
Joehnk, P.; Keywords: -
  • Lecture (others)
    Vortrag Technische Universität, Bergakademie Freiberg, 27.05.2004

Publ.-Id: 6351 - Permalink


Grundlagen der Gestaltungslehre als Voraussetzung für gute (wissenschaftliche) Präsentationen
Joehnk, P.; Keywords: -
  • Lecture (others)
    Doktorandenklausur, TU Szezcin, TU Lódz, IFW Dresden, Sklarska, Poreba, Polen. - 17.05.2004

Publ.-Id: 6350 - Permalink


n-type conductivity in high-fluence Si-implanted diamond
Weishart, H.; Heera, V.; Skorupa, W.;
Epitaxial SiC nanocrystals are fabricated by high-fluence Si-implantation into natural diamond at elevated temperatures between 760 °C and 1100 °C. Fluences under investigation range from 4.5 to 6.2 x 1017 Si cm-2 . This implantation scheme yields a buried layer rich of epitaxial aligned SiC- nanocrystals within slightly damaged diamond. The generation of a small fraction of graphitic sp2-bonds of up to 15 % in the diamond host matrix can not be avoided. Unintentional coimplantation with nitrogen results in a very high doping level of more than 1021 cm-3. Resistivity and Hall measurements in van der Pauw geometry reveal a high, thermally stable n-type conductivity with electron concentrations exceeding 1020 cm-3and mobilities higher than 2 cm2/Vs. It is supposed, that both the SiC regions as well as the diamond matrix exhibit n-type conductivity and that the electron transport occurs across the low-resistivity SiC nanograins. In the SiC nanocrystals the electrons originate from nitrogen donors whereas in diamond defects are responsible for the electron conductivity. The formation of disordered graphite, which leads to low electron mobility, is substantially reduced.
Keywords: Ion implantation, thin films, Heterostructures, electronic properties, Ion radiation effects
  • Journal of Applied Physics 97(2005), 103514.

Publ.-Id: 6349 - Permalink


Nonlinear Charakteristics of Gas-Liquid Two-phase Flow and Verification of Extended Two-Fluid Model
Misawa, M.; Suzuki, A.; Morikawa, Y.; Minato, A.; Prasser, H.-M.;
Void fraction distributions of air-water two-phase flow in 3 different diameter vertical tubes (0.042 m, 0.1 m, and 0.2 m) were measured by 16x16 wire-mesh sensors with a sampling frequency of 1 kHz. Also, predictions under the same flow conditions were made by a 3D extended two-fluid model, generating a time-series of cross-section averaged void fraction at the same frequency with the experiment. Fluctuation characteristics from experiments and simulations were analysed in view of the probability density function, frequency spectrum, and Trajectory Parallel Measures (TPM). The analyses of TPM showed that dynamics is well-established for large nunnles whose fiameter is on the order of Taylor wave length in an air-water system, although small bubble void fraction signal is almost random in dynamical point of view. It is shown that the fluctuation component in void fraction time series can be used as one of criteria for validation of numerical models.
Keywords: two-fluid model, gas-liquid flow, wire-mesh sensor, statistical analysis, trajectory parallel measures, code validation
  • Lecture (Conference)
    5th International Conference on Multiphase Flow, ICMF'04, 30.05.-04.06.2004, Yokohama, Japan
  • Contribution to proceedings
    5th International Conference on Multiphase Flow, ICMF'04, 30.05.-04.06.2004, Yokohama, Japan, paper 213.

Publ.-Id: 6348 - Permalink


NuclearReactor - Reaktorphysik populär im Internet - ein Versuch, den Internetauftritt der KTG attraktiver zu machen
Prasser, H.-M.;
Die Ortssektion Sachsen der Kerntechnischen Gesellschaft bemüht sich kontinuierlich um eine aktuelle und attraktive Präsens im Internet. Mit der eigenen Internetseite "www.ktgsachsen.de", die seit Oktober 2001 besteht, wird versucht, neben der Information für die Mitglieder - in erster Linie Ankündigungen und Auswertungen von Veranstaltungen - mit Diskussionsbeiträgen zur Kernenergie ein breiteres Publikum zu erreichen. Dabei werden unter der Rubrik ARGUMENTE Themen, wie Nachhaltigkeit, CO2-Emissionen und Energieressourcen berührt. Unter der Überschrift NEWS wird über aktuelle Ereignisse informiert. Insgesamt
bleibt die Resonanz auf diese Internetseite jedoch gering. Nur sehr selten trifft Post ein, die auf die publizierten Argumente eingeht oder auch Widerspruch anmeldet. Um die Attraktivität der Internetseite zu erhöhen und dabei gleichzeitig einen Beitrag zum allgemeinen Informationsstand über die physikalischen Grundlagen der Reaktortechnik in der Bevölkerung zu leisten, wurde im November 2003 ein neuer Weg beschritten. Es wurde ein Programm zur On-line Simulation der Kettenreaktion in einem Kernreaktor entwickelt und ins Netz gestellt. Die Ortssektion Sachsen strebt damit die folgenden Ziele an:
(1) Durch eine attraktive Darstellung der physikalischen Prozesse in einem Kernreaktor, die durch interaktive Eingriffe über die Steuerorgane beeinflusst werden können, entsteht eine Art Computerspiel, mit dem es gelingen soll, interessierten Personen - insbesondere Kindern und Jugendlichen - Einblicke in die Reaktorphysik zu ermöglichen. Dabei wird Wert darauf gelegt, die Grundprozesse qualitativ möglichst korrekt und umfassend darzustellen, um einen hohen Bildungseffekt zu erreichen.
(2) Durch die spielerische Erschließung des Stoffes soll die emotionale Barriere, die häufig gegenüber der Kerntechnik besteht, abgebaut werden, um die Besucher der Internetseite an weitergehenden Informationen zu interessieren und die Aufgeschlossenheit für weitere Diskussionen und Argumente zu erhöhen.
(3) Durch ständige Weiterentwicklung sollen in monatlichen Abständen jeweils neue Versionen des Programms mit erweiterten Möglichkeiten angeboten werden, um eine Art Fangemeinde entstehen zu lassen, die der Internetseite ihre Treue hält. Das Programm wurde in der Programmiersprache DELPHI erstellt. Der unter WINDOWS ausführbare Code (EXE) kann zusammen mit einem Manual, welches die physikalischen
Hintergründe in populärwissenschaftlicher Weise beschreibt und dem Anwender numerische Experimente vorschlägt, von der Webseite www.ktg-sachsen.de heruntergeladen werden.

Keywords: Education, Reactor Physics, Public Relation, Monte-Carlo Simulation
  • Lecture (Conference)
    Annual Meeting on Nuclear Technology 2004, 25. - 27. Mai 2004, Düsseldorf, Germany
  • Contribution to proceedings
    Annual Meeting on Nuclear Technology 2004, 25. - 27. Mai 2004, Düsseldorf, Germany, paper 495-500

Publ.-Id: 6347 - Permalink


Effect of excess vacancies in ion beam synthesis of SiC nanoclusters
Kögler, R.; Eichhorn, F.; Mücklich, A.; Skorupa, W.; Serre, C.; Perez-Rodriguez, A.;
SiC nanoclusters are synthesized in Si by simultaneous dual implantation using two ion beams, of C and Si ions. This implantation mode is associated with extra excess vacancy generation. The effect of vacancies on SiC synthesis is investigated in this study. The amount of synthesized SiC is compared for different implantation modes, simultaneous and sequential ones. Sequential pre-deposition of vacancy defects in Si before the C implantation is performed by additional Si and He implantation. The simultaneous dual beam implantation is found to be the only method to improve SiC synthesis. The introduction of vacancies, both excess vacancies and He induced vacancies, by a sequential implantation process is disadvantageous for SiC nanocluster formation. The pre-deposition of vacancy defects is accompanied with higher crystal damage and/or the defects are annealed out during the subsequent C implantation at temperatures above 400oC. Vacancies must be created “in-situ” during C implantation to achieve enhanced output of SiC.
Keywords: Ion implantation, Simultaneous dual implantation, SiC, Si, Ion beam synthesis
  • Vacuum 78(2005)2-4, 177-180

Publ.-Id: 6346 - Permalink


Excess vacancies induced by ion beam implantation into silicon
Kögler, R.; Eichhorn, F.; Peeva, A.; Voelskow, M.; Mücklich, A.; Serre, C.; Skorupa, W.;
The process of formation of excess vacancies by ion implantation is explained. Moreover, the simulation of the process, the experimental detection of excess vacancies and their application for defect engineering is described.
Keywords: Ion implantation, Silicon, Excess vacancies, Electron microscopy, Positron annihilation spectrometry
  • Lecture (Conference)
    V th international conference on ion implantation and other applications of ions and electrons, ION2004, June 14-17, 2004, Kazimierz Dolny, Poland

Publ.-Id: 6345 - Permalink


Generation eines Pseudomonas aeruginosa Biofilms auf Biotit und dessen mikroskopische Charakterisierung.
Großmann, K.; Arnold, T.; Bernhard, G.; Garske, B.; Krawietz, R.;
Zum Studium der Wechselwirkungen von Schwermetallen mit Biofilmen in der Umwelt ist die reproduzierbare Präparation und detaillierte Charakterisierung von Biofilmen auf Mineral- und Gesteinsoberflächen notwendig. Die initialen Aufwachsstadien von auf Biotit erzeugten Pseudomonas aeruginosa Biofilmen wurden mit Rasterelektronenmikroskopie (SEM) und Rasterkraftmikroskopie (AFM) visualisiert. Zum Studium des reifen Biofilms wurden Fluoreszenzmikroskopie und konfokale Fluoreszenzmikroskopie (CLSM) eingesetzt. Die Unterscheidung zwischen lebenden und toten Bakterien erfolgte mit Hilfe eines LIVE/DEAD - Kit. Zur Visualisierung von Polysacchariden innerhalb der EPS wurden Lektine gekoppelt mit Fluoreszenzfarbstoffen verwendet. Hierbei konnten die einzelnen Polysaccharide der EPS unterschieden werden in solche, die direkt an Zellwände der Bakterien gebunden sind und weitere, die eine diffuse Verteilung der EPS im Biofilm aufzeigen. Die Dicke der erzeugten Biofilme auf den Biotitoberflächen wurde mit CLSM bestimmt. Sie schwankte zwischen 5,3 µm für Biofilme, die unter Nährstoffmangel angezogen wurden, und 16,6 µm bei ausgereiften Biofilmen.
Keywords: Biofilm, Pseudomonas aeruginosa, Mikroskopie, Fluoreszenzfarbstoffe, Lectine
  • GWF-Wasser/Abwasser 146(2005) 2, 113-118

Publ.-Id: 6344 - Permalink


Validation of models for bubble forces
Lucas, D.; Krepper, E.; Prasser, H.-M.;
Closure laws are needed for the qualification of CFD codes for two-phase flows. In case of bubbly and slug flow, the momentum transfer between the phases is usually modelled by forces acting on the bubbles. These forces depend on the liquid flow field as well as on the size and the shape of the bubbles. To validate models for these forces a detailed database for vertical pipe flow is used. It contains not only the information on the radial distribution of bubbles of different size, but also information on the bubble size distribution. Measurements were done for a large number of combinations of gas and liquid superficial velocities and for two different pipe diameter (DN50 and DN200). An one-dimensional model, which resolves the variables in radial direction and which considers a large number of bubble size classes to account for the size effect, is used for an analysis of models for these forces. The results obtained by this model were compared to 3D results obtained by the commercial code CFX-5.7 for simplified cases with only one bubble class. The effect of the number of bubbles classes as well as the effect of the lateral extension of the bubbles were analysed. For the validation of the bubble forces measured bubble size distributions were taken as an input for the model. On basis of the assumption of an equilibrium of the lateral bubble forces, radial volume fraction profiles were calculated separately for each bubble class. In the result of the validation of different models for the bubble forces, a set of Tomiyama lift and wall force and the Favre averaged turbulent dispersion force was found to reflect the experimental data with best agreement. Some discrepancies remain at high liquid superficial velocities.
Keywords: Two-phase flow, bubble, bubble forces, modelling, CFD
  • Lecture (Conference)
    42nd European Two-Phase Flow Group Meeting, 23.-25.06.2004, Genua, Italy
  • Contribution to proceedings
    42nd European Two-Phase Flow Group Meeting, 23.-25.06.2004, Genua, Italy, paper E1

Publ.-Id: 6342 - Permalink


Atomic computer simulations of defect migration in 3C and 4H-SiC
Gao, F.; Weber, W. J.; Posselt, M.; Belko, V.;
Knowledge of the migration of intrinsic point defects is crucial to understand defect recovery, various annealing stages and microstructural evolution after irradiation or ion implantation. Molecular dynamics (MD) and the nudged-elastic band method have been applied to investigate long-range migration of point defects in SiC over the temperature range from 0.36 to 0.95 Tm , and the defect diffusion coefficient, activation energy and defect correlation factor have been determined. The results show that the activation energies for C and Si interstitials in 3C-SiC are about 0.74 and 1.53 eV, respectively, while it is about 0.77 eV for a C interstitial in 4H-SiC. The minima energy paths reveal that the activation energies for C and Si vacancies are about 4.1 and 2.35 eV, respectively. Finally, the results are discussed and compared with experimental observations and available ab initio data.
Keywords: Defect diffusion, activation energy, molecular dynamics, nudged-elastic band method
  • Contribution to external collection
    Materials Science Forum, 457-460; Part 1 (2004), 457-460

Publ.-Id: 6341 - Permalink


Development of a SAXS-Method to characterize the damage zone around a crack tip in metals
van Ouytsel, K.; Müller, G.;
The aim was to characterize the damage zone ahead a crack tip in technical aluminium alloys using Small Angle X-ray Scattering. Experiments were conducted at the Austrian beamline at ELETTRA in Trieste from 18th-24th of February 2003. Four specimens of 2024-T351 aluminium alloy, different in shape, deformation condition and grain size, were investigated. Anisotropic grain boundary scattering of the CT-specimen was observed.
Keywords: Small Angle X-ray Scattering (SAXS), damage zone, crack tip
  • Contribution to external collection
    Austrian Small Angle X-ray Scattering (SAXS) Beamline at ELETTRA, Annual Report 2003, published 2004, 51-52

Publ.-Id: 6340 - Permalink


Simultaneous Determination of 226Ra, 233U and 237Np by Liquid Scintillation Spectrometry
Nebelung, C.; Baraniak, L.;
A method has been developed for the simultaneous determination of 226Ra, 233U and 237Np by liquid-scintillation (LS) spectrometry. This method consists in the evaluation of strongly overlapped multi peak spectra by a special peak fit function, considering the deviation of the LS-peak shape from the pure Gaussian distribution. First 237Np is determined using its daughter 233Pa by analyzing the ß-spectrum. 226Ra follows from the α-spectrum recorded 6 weeks after sample preparation. The peak of the three short-lived daughters 222Rn, 218Po and 214Po corresponds to the 226Ra activity under radioactive equilibrium conditions. Finally the 233U activity results from the joint α-peak at 4.8 MeV of the deconvoluted spectrum by subtracting the activity of 226Ra and 237Np. A second option uses the characteristic double peak of 237Np near 4.8 MeV. Due to the fact that the position is accurately known the pure α-spectrum can be analyzed for the determination of the given nuclides.
Keywords: Liquid scintillation, Alpha-Beta- Separation, Deconvolution of Multielement spectra
  • Applied Radiation and Isotopes, 65 (2007) 209-217

Publ.-Id: 6337 - Permalink


Molecular Bacterial Diversity in Water at the Deep-Well Monitoring Site Tomsk-7
Nedelkova, M.; Radeva, G.; Selenska-Pobell, S.;
Bacterial diversity was studied in a water sample collected from a depth of 290-324 m below the land surface from the monitoring site S15 near the city of Tomsk, Siberia, Russia. In order to overcome the problems of culturability of bacteria, we applied a direct molecular approach based on PCR amplification and cloning of the 16S rDNA of the members of the natural bacterial communities. 65% of the cloned sequences were almost identical to the 16S rRNA genes of Dechlorosoma sp. belonging to the Rhodocyclus group of b-subclass of Proteobacteria. Relatively high was the number of the sequences which shared high identities with the 16S rRNA genes of some cultured g-Proteobacteria (6.9%), a-Proteobacteria (4.6%) and with the sequences of mostly uncultured members of Cytophaga/Flavobacterium/Bacteroides (5.4%), Actinobacteria (4.6%) and Planctomycetales (2.3 %). The rest of the retrieved sequences were clustered in small groups, each containing about 1 % of the total number of clones, and they matched with 16S rDNA sequences of Bacillales, Holophaga/Acidobacteria, Nitrospira, and d-Proteobacteria, respectively.
  • Contribution to external collection
    Chin-Fu Tsang and John A. Apps: Developments in Water Science, Underground Injection Science and Technology, Netherlands: Elsevier, 2005, 978-0-444-52068-5, 521-536

Publ.-Id: 6336 - Permalink


Time-Dependent Kinematic Dynamos in Finite Cylinders as Treated with the Integral Equation Approach
Xu, M.; Stefani, F.; Gerbeth, G.;
Dynamo action is generally accepted as the source of cosmic magnetic fields, such as the fields of planets, stars and galaxies. The understanding of dynamos has considerably progressed during the last decades. Recently, the hydromagnetic dynamo effect has been demonstrated experimentally in large liquid sodium facilities in Riga and Karlsruhe.
The electrically conducting fluid is usually confined to a finite domain which is surrounded by an insulator. For only a few cases, such as spheres or cylinders with infinite length, the boundary conditions for the magnetic field are easily implementable. In most cases, including finite cylinders which are relevant for the liquid sodium experiments in Riga, Karlsruhe, Cadarache and New Mexico, the correct handling of the non-local boundary conditions is far from trivial. We have re-formulated Pre-Maxwell's equations to an integral equation system under the assumption that the velocity field is stationary.
The main advantage of this approach is that the numerical solution of the integral equation system does not require any discretization of the exterior region.
In the present work, the integral equation approach is performed on a group of Beltrami flows (and some modifications) with the topologies s2t2, s2t1 and s1t1 in a finite cylinder. Beltrami flows are characterized by maximum helicity for a given magnetic Reynolds number. Due to the axisymmetry of these flows, the integral equation system in the cylindrical system is reduced to a two-dimensional form.
The calculated results show a good agreement with those obtained by a differential equation approach. This integral equation approach exhibits attractive features such as robustness and stability.
The impact of a stagnant conducting layer surrounding the cylinder is also investigated. Such a layer can reduce the critical magnetic Reynolds number significantly, and it can even transform no-dynamos into dynamos. Interestingly, the presence of a layer can also change an oscillatory dynamo to a steady dynamo.
  • Lecture (Conference)
    7th MHD-Days, Technische Universität Ilmenau, 20.-21.09.2004, Ilmenau, Germany

Publ.-Id: 6335 - Permalink


Application of gamma tomography to the measurement of fluid distributions in a hydrodynamic coupling
Hampel, U.; Hoppe, D.; Diele, K.-H.; Fietz, J.; Höller, H.; Kernchen, R.; Prasser, H.-M.; Zippe, C.;
Gamma tomography has been used to investigate the fluid distribution within multiple axial planes of a hydrodynamic test coupling under operating conditions. The tomographic data was obtained by phase correlated averaging of projections from the homogeneously rotating parts of the coupling. By application of the filtered backprojection reconstruction technique to the data we reconstructed average fluid fraction distributions in the transversal planes at different operating points of the coupling.
Keywords: gamma tomography, hydrodynamic coupling, two-phase flow measurement
  • Flow Measurement and Instrumentation 16(2005), 85-90

Publ.-Id: 6334 - Permalink


Overview of the present status of the SRF gun design and construction
Teichert, J.; Evtushenko, P.; Janssen, D.; Lehmann, W.-D.; Lehnert, U.; Michel, P.; Schneider, C.; Stephan, J.; Volkov, V.; Will, I.;
A status report on the SRF photo-injector activities at FZR within the CARE/PHIN project will be given. The new SRF gun has been designed for CW operation at the ELBE linac with an average current of 1 mA, 77 pC bunch charge, and 10 MeV energy. The basic concept of the gun, operating a normal conducting, thermally insolated photo-cathode within a superconducting cavity, have been taken from the first SRF half-cell gun [1] which was successfully tested. In this report the design layout of the SRF photo-injector, the parameters of the superconducting cavity and the expected electron beam parameters are presented. The SRF gun will have a 31/2-cell niobium cavity working at 1.3 MHz and will be operated at 2 K. The cavity consists of three full cells with TESLA-like shapes and a half-cell in which the photocathode is situated.
Keywords: SRF photo-injector, electron source, superconductivity, niobium cavity, electron acceleration
  • Lecture (Conference)
    1st Workshop of ELAN, 04.-06.05. 2004, Frascati, Rom, Italy
  • Contribution to proceedings
    1st Workshop of ELAN, 04.-06.05.2004, Frascati, Italy CARE-Note-2004-010-ELAN

Publ.-Id: 6333 - Permalink


Study of e+,e production in elementary and nuclear collisions near the production threshold with HADES
Salabura, P.; Agakichiev, G.; Agodi, C.; Alvarez-Pol, H.; Balanda, A.; Bellia, G.; Belvers, D.; Bielcik, J.; Böhmer, M.; Bokemeyer, H.; Boyard, J.; Braun-Munzinger, P.; Chepurnov, V.; Chernenko, S.; Christ, T.; Coniglione, R.; Diaz, J.; Djeridi, R.; Dohrmann, F.; Duran, I.; Eberl, T.; Emeljanov, V.; Fabbietti, L.; Fateev, O.; Fernandez, C.; Finocchiaro, P.; Friese, J.; Fröhlich, I.; Fuentes, B.; Garzon, J.; Gernhäuser, R.; Golubeva, M.; Gonzalez, D.; Grosse, E.; Guber, F.; Hennino, T.; Hlavac, S.; Hoffmann, J.; Holzmann, R.; Ierusalimov, A.; Iorii, J.; Jaskula, M.; Jurkovic, M.; Kämpfer, B.; Kanakir, K.; Karavicheva, T.; Koenig, I.; Koenig, W.; Kolb, B.; Kotte, R.; Kotulic-Bunta, J.; Krücken, R.; Kugler, A.; Kühn, W.; Kulessa, R.; Kurepin, A.; Kurtukian-Nieto, T.; Lang, S.; Lehnert, J.; Maiolino, C.; Marn, J.; Markert, J.; Metag, V.; Montes, N.; Mousa, J.; Münch, M.; Müntz, C.; Naumann, L.; Novotny, J.; Otwinowski, J.; Pachmayer, Y.; Panebratsev, Y.; Pechenov, V.; Perez, T.; Pietraszko, J.; Pleskac, R.; Pospisil, V.; Przygoda, W.; Rabin, N.; Ramstein, B.; Reshetink, A.; Ritmanh, J.; Rodriguez-Prieto, G.; Roy-Stephan, M.; Rustamov, A.; Sabin-Fernandez, J.; Sadovsky, A.; Sailer, B.; Sanchez, M.; Smolyankin, V.; Smykov, L.; Spataro, S.; Spruck, B.; Stroebele, H.; Stroth, J.; C., S.; Sudol, M.; Titov, A.; Tlusty, P.; Toia, A.; Traxler, M.; Tsertos, H.; Vazquez, A.; Volkov, Y.; Wagner, V.; Walus, W.; Winkler, S.; Wisniowski, M.; Wojcik, T.; Wüstenfeld, J.; Zanevsky, Y.; Zumbruch, P.;
HADES is a second generation experiment designed to study dielectron production in proton, pion, and heavy ion induced reactions at the GSI accelerator facility in Darmstadt. The physics programme of HADES is focused on in-medium properties of the light vector mesons. In this contribution we present status of the HADES experiment, demonstrate its capability to identify rare dielectron signal, show first experimental results obtained from C+C reactions at 2 A GeV and shortly discuss physics programme of up-coming experimental runs.
  • Progress in Particle and Nuclear Physics 53(2004), 49

Publ.-Id: 6329 - Permalink


Uranium binding by the natural isolate Bacillus Sphaericus JG-A12 and its application for bioremediation
Raff, J.; Merroun, M.; Rossberg, A.; Foerstendorf, H.; Selenska-Pobell, S.;
Bacillus sphaericus JG-A12 is a natural isolate recovered from a uranium mining waste pile near the town of Johanngeorgenstadt in Saxony, Germany. Similarly to some other Bacillus isolates from this site, cells and spores of B. sphaericus JG-A12 bind selectively and reversibly large amounts of different toxic metals, such as uranium /1/. The cells of the B. sphaericus strain JG-A12 are covered by a surface layer (S-layer) protein. The latter forms a highly ordered lattice with distinct structural and chemical properties. Because it was shown that S-layers may work as protective coat, matrix for biomineralization, molecular sieve or as ion and molecule trap /2, 3, 4/ their special importance for the interaction of cells with radionuclides and heavy-metals in the surrounding environment is evident. As a consequence of the above mentioned properties, B. sphaericus JG-A12 was investigated with regard to its suitability to work as a binding matrix for bioremediation of uranium mining waste waters. A fundamental requirement for biotechnological application is the complete immobilization of the biomass in a porous structure. In this work, sol-gel techniques were used for embedding B. sphaericus JG-A12 cells, spores and S-layer protein in a SiO2-matrix in order to obtain a novel kind of biological ceramic (biocer) /5/. To select the appropriate filter material, uranium sorption and desorption experiments were carried out with free and immobilized cells, spores and S-layer protein of B. sphaericus JG-A12. Besides quantification of the available binding sites, the formed uranium complexes were investigated also by using infrared and extended X-ray absorption fine structure (EXAFS) spectroscopy. The latter revealed that the binding of uranium by free cells, spores and S-layer occurred via phosphate and carboxyl groups. The presence of phosphate groups was confirmed by different methods not only for cells and spores but also for the S-layer protein. The spectroscopic analysis of the uranium complexes formed by the biocer demonstrates the metal binding via phosphate groups and the formation of a uranium precipitate, what is of special interest for the binding capacity of the biocer. In case of the xerogel uranium was bound to silanol groups. Analyses of the uranium sorption by cells, spores and S-layer protein of B. sphaericus JG-A12 and of the xerogel itself (without biocomponent) showed highest binding capacity for spores followed by cells, S-layer protein and xerogel. In case of embedded biocomponents only the binding capacity of spores is significantly reduced and is lower than for cell- and S-layer-ceramics. Uranium bound to biocers can be completely removed by washing with citric acid.

1. Selenska-Pobell, S. et al. (1999) FEMS Microbiol. Ecol. 29, 59-67
2. Beveridge, T.J. 1979 J. Bacteriol. 139 (3): 1039-1048
3. Schultze-Lam, S. et al. (1992) J. Bacteriol. 174 (24): 7971-7981
4. Sára, M. & Sleytr, U.B. 1987. J. Bacteriol. 169 (6): 2804-2809
5. Raff, J. et al (2003) Chem. Mater 2003; 15(1):240-4
  • Lecture (Conference)
    Biometals 2004, 03.-05.09.2004, Garmisch-Partenkirchen, Germany
  • Contribution to proceedings
    Biometals 2004, 03.-05.09.2004, Garmisch-Partenkirchen, Germany
    Proceedings

Publ.-Id: 6328 - Permalink


Bacterial Diversity in a Soil Sample from a Uranium Mining Waste Pile as estimated via a Culture-Independent 16S rDNA Approach
Satchanska, G.; Golovinski, E.; Selenska-Pobell, S.;
Bacterial diversity was studied in a soil sample collected from a uranium mining waste pile situated near the town of Johanngeorgenstadt, Germany. As estimated by ICP-MS analysis the studied sample was highly contaminated with Fe, Al, Mn, Zn, As, Pb and U. The 16S rDNA retrieval, applied in this study, demonstrated that more than the half of the clones of the constructed 16S rDNA library were represented by individual RFLP profiles. This indicates that the composition of the bacterial community in the sample was very complex. However, several 16S rDNA RFLP groups were found to be predominant and they were subjected to a sequence analysis. The most predominant group, which represented about 13% of the clones of the 16S rDNA library, was affiliated with the Holophaga/Acidobacterium phyllum. Significant was also the number of the proteobacterial sequences which were distributed in one predominant alpha-proteobacterial cluster representing 11% of the total number of clones and in two equal-sized beta- and gamma-proteobacterial clusters representing each 6% of the clones. Two smaller group representing both 2% of the clones were affiliated with Nitrospira and with the novel division WS3. Three of the analysed sequences were evaluated as a novel, not yet described lineage and one as a putative chimera.
  • Comptes Rendues del Ácademie bulgare des Sciences 57(2004)5, 75-82

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Publ.-Id: 6327 - Permalink


Molecular analysis of bacterial populations in water samples from two uranium mill tailings by using a RISA retrieval
Radeva, G.; Selenska-Pobell, S.;
A ribosomal intergenic spacer amplification (RISA) retrieval was applied to analyze the natural bacterial communities in drain waters of two uranium mill tailings - Gittersee/Coschütz in Germany and Shiprock in the USA. About 35% of the clones from the RISA library constructed for the samples of the German tailings represented a microdiverse population of Planctomycetales. The rest of the clones were affiliated with rather diverse bacterial groups including g- and d- Proteobacteria, Cytophaga/Flavobacterium/Bacteroides (CFB), Nitrospira, Verrucomicrobia and Actinobacteria. 8% of the cloned sequences represented a novel bacterial lineage from the recently described division NC3.
Bacterial diversity in the Shiprock mill tailings was found to be significantly lower. The RISA library constructed for those samples contained only two larger groups of clones, representing b-proteobacterial species and one small group, which was affiliated with d-Proteobacteria.
  • Comptes Rendues de l´Academie bulgare des Sciences 57(2004)6, 111-115

Publ.-Id: 6326 - Permalink


Combined Analysis of Threshold-Near Production of ω and φ Mesons in Nucleon-Nucleon Collisions
Kaptari, L. P.; Kämpfer, B.;
Vector meson (V=ω , φ) production in threshold-near elementary nucleon-nucleon collisions pp to ppV, pn to pnV and pn to dV is studied within an effective meson-nucleon theory.
It is shown that a set of effective parameters can be established to explain fairly well the available experimental data on angular distributions and the energy dependence of the total cross sections without explicit implementation of the Okubo-Zweig-Iisuka rule violation. Isospin effects are considered in details and compared with experimental data whenever available.
Keywords: bector mesons, OZI rule, hadron reactions
  • Eur. Phys. Journal A 23 (2005)291-304

Publ.-Id: 6324 - Permalink


Bacterial diversity in water samples from uranium wastes as demonstrated by 16S rDNA and RISA retrievals
Radeva, G.; Selenska-Pobell, S.;
Bacterial diversity was assessed in water samples collected from several uranium mining wastes in Germany and in the USA by using 16S rDNA and RISA retrievals.
The results obtained using the 16S rDNA retrieval showed that the samples collected from the uranium mill tailings Schlema/Alberoda, Germany, were predominated by Nitrospina-like bacteria, whereas those from the mill tailings Shiprock, New Mexico, USA, were predominated by g-Pseudomonas and Frateuria spp. Additional smaller populations of CFB, a-, and d-proteobacteria were identified in the Shiprock samples as well. Proteobacteria and CFB were also predominant in the third uranium mill tailings studied, Gittersee/Coschütz in Germany, but the groups of the predominant clones were rather small. Most of the clones of the Gittersee/Coschütz samples represented individual sequences that indicated a high level of bacterial diversity. The samples from the fourth uranium waste studied, namely Steinsee Deponie B1 in Germany, were predominantly occupied by Acinetobacter spp.
The RISA retrieval provided results complementary to those obtained by the 16S rDNA analyses. For instance, in the Shiprock samples an additional predominant bacterial group was identified and affiliated to Nitrosomonas sp., whereas in the Gittersee/Coschütz samples Anammox-populations were identified which were not retrieved by the used 16S rDNA approach.
Keywords: bacterial diversity, 16S rDNA, ribosomal intergenic spacer amplification (RISA), uranium wastes
  • Canadian Journal of Microbiology 51(2005)11, 910 - 923

Publ.-Id: 6323 - Permalink


Migration of Uranium (IV)/(VI) in the Presence of Humic Acids in Quartz Sand: A Laboratory Column Study
Mibus, J.; Sachs, S.; Nebelung, C.; Bernhard, G.;
The migration behavior of U(IV)/U(VI) in the presence of humic acids was studied in a laboratory quartz sand system by column experiments. It was found that humic acid influences the transport of U(IV) and U(VI). In presence of humic acid both redox species migrate nearly as fast as the groundwater flow. Retardation factors of 1.11 ± 0.03 and 1.14 ± 0.03 as well as eluate recoveries of 0.83 ± 0.09 and 0.65 ± 0.07 were determined for U(VI) and U(IV), respectively. In case of U(VI) humic acid exhibits a clear mobilizing effect compared to the humic acid-free system. There are also strong indications for a similar effect on the U(IV) transport.
The present study demonstrates that humic acids can play an important role in the migration of tetravalent actinides. Consequently, the humic colloid-borne transport of actinides has to be taken into account in performance assessment.

Keywords: Uranium(IV), Uranium(VI), Humic Acids, Quartz Sand, Migration, Adsorption
  • Journal of Contaminant Hydrology 89 (2007) 199 - 217 DOI 10.1016/j.jconhyd.2006.08.005

Publ.-Id: 6322 - Permalink


Biotechnological potential of bacteria from extreme environments
Raff, J.;
  • Lecture (Conference)
    Internationales Sales-Meeting, 17.-18.6.2004, Lampertheim, Germany

Publ.-Id: 6320 - Permalink


Synthesis of a 11C-labelled Nonsteroisal Glucocorticoid Receptor Ligand as Potential Radiotracer for Imaging Brain Glucocorticoid Receptors with Positron Emission Tomography (PET)
Wüst, F.; Kniess, T.; Bergmann, R.;
  • Lecture (Conference)
    12th Workshop of the Central European Division e. V. of the International Isotope Society, 17.-18.06.2004, Bad Soden, Germany
  • Abstract in refereed journal
    Journal of Labelled Compounds and Radiopharmceuticals 47(2004), 1051-1052

Publ.-Id: 6318 - Permalink


18F-Labelling of a Potent Nonpeptide CCR1 Antagonist for the Diagnosis of the Alz-heimer’s Disease
Mäding, P.; Füchtner, F.; Hilger, C. S.; Halks-Miller, M.; Horuk, R.;
Objectives: PET imaging of cardiac reporter gene expression holds promise for noninvasive monitoring of gene therapy. Two approaches based on herpesviral thymidine kinase gene(HSV1-tk) habe been applied. Wild-type HSV1-tk was imaged with 2´-fluoro-2´-deoxy-5[124I]-iodo-arabinofuranosyluracil([124I]FIAU), and mutant HSV1-sr39tk was imaged with 9-[4-[18F]-fluoro-3-(hydroxymethylbutyl)guanine([18F]FHBG).
Methods: Adenovirus expressing wild-type HSV1-tk(Adtk), mutant HSV1-sr39tk (Adsr39tk), or control gene (AdLacZ) were directly injected into myocarcardium of 24 rats and 8 pigs. Two days later, dynamic PET imaging was performed for 120 min after injection of [124I]FIAU or [18F]FHBG with a clinical scanner. Imaging with [13N]-ammonia was performed to identify cardiac regions of interest. Pig hearts were sliced into 5 short-axis slices for ex vivo imaging, and regional tracer uptake was analyzed.
Results: For [124I]FIAU, the largest difference of cardiac uptake between Adtk rats and controls was found at 10-30 min after injection (1.7±0.25 vs 0.87±0.22 %dose/ml, p=0.035). No difference was observed at later imaging times due to [124I]FIAU washout. For [18F]FHBG, difference between Adsr39tk rats and controls continuously increased over time and was largest at 105-120 min (1.45±0.45 vs 0.35±0.05 %dose/ml, P=0.0066). Global cardiac reporter probe kinetics in rats was confirmed by regional myocardial analysis in pigs. Transgene expression was specifically visualized by both approaches. Highest target/background ratio of [124I]FIAU in Adtk infected myocardium was 1.50±0.20 vs 2.64±0.49 for [18F]FHBG in Adsr39tk infected areas (P=0.01). In vivo results of rats were confirmed by ex vivo counting and autoradiography. In vivo segmental reporter probe uptake in pigs correlated well with those in ex vivo images.
Conclusions: Both combinations were feasible for PET of cardiac transgene expression in different species. Specific probe kinetics suggests different myocardial handling of pyrimidine ([124I]FIAU) and acycloguanosine ([18F]FHBG) derivatives. Results are in favour of [18F]FHBG because of continuous accumulation over time and higher imaging contrast.
  • Lecture (Conference)
    12th Workshop of the Central European Division e. V. of the International Isotope Society, 17.-18.06.2004, Bad Soden, Germany
  • Abstract in refereed journal
    Journal of Labelled Compounds and Radiopharmaceuticals 47(2004), 1053-1054

Publ.-Id: 6317 - Permalink


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