Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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35810 Publications

Secondary particle yields from 400 MeV/u carbon ion and 250 MeV proton beams incident on thick targets

Ferrari, A.; Ferrarini, M.; Pelliccioni, M.

The double differential particle yield produced by hadron beams striking thick targets of copper, tungsten and ICRU tissue have been determined by means of the Monte Carlo transport code FLUKA (version FLUKA 2008.3b.1). 400 MeV/u carbon ions and 250 MeV proton pencil beams have been considered. Secondary neutrons, photons and protons have been scored. In order to validate the obtained data, a few simulations have been also repeated with MCNPX 2.6.0. The calculated results are presented and compared with the experimental data reported in literature. They should be very useful to solve a number of problems related to technological aspects of hadrontherapy.

Keywords: particle yields; hadrontherapy; FLUKA; MCNPX

Publ.-Id: 14974

PEGylierung von radioaktiv markierbaren L-Oligonukleotiden zur Optimierung ihrer Pharmakokinetik

Förster, C.

keine Abstract verfügbar

  • Lecture (Conference)
    18. Jahrestagung der Arbeitsgemeinschaft Radiochemie/Radiopharmazie der DGN, 16.-18.09.2010, Bad Boll, D

Publ.-Id: 14973

Matching of fluid flow observations in geological material (GeoPET, mm3 resolution) with lattice Boltzmann simulations in μm resolved structures

Kulenkampff, J.; Wolf, M.; Enzmann, F.; Gründig, M.; Richter, M.; Lippmann-Pipke, J.

Scaling is a fundamental problem in groundwater hydrology. A typical challenge is the verification that hydrodynamic parameters obtained in laboratory experiments well represent a situation on the field scale. Here were propose that for reactive transport modeling, the prediction of the large scale fluid dynamics and concentration distributions should be based on the characteristics of hydromechanic and geochemical parameter sets on the millimeter and micrometer scale. While it is common sense that chemical reactions take place on the atomic scale, here we show with tomograhic process observation and modeling that also hydrodynamic processes are considerably influenced even by sub-µm scale characteristics of the geomaterial and thus determine the fate and dynamics of the system components in the fluid phase.

We applied the spatially highly resolving computer tomography (μXCT) on rock cores for determining the open pore structures. Based on these µm scaled structural information the lattice Boltzmann simulations were conducted. Column experiments were performed on the same samples while applying the process visualization method GeoPET that allows for the non-invasive, quantitative monitoring of e.g. dissolved positron-emitting radio tracers ([18F]KF, [124I]KI) added to the eluent. Visualizing and quantifying transport processes in geological material by means of GeoPET provides a high volume resolution of 1.5 μl (1.3 mm) and extreme high sensitivity for tracer concentrations (10−15 to 10−12 moles/ml). The matching of measured time resolved fluid flow patterns and simulated small scale fluid dynamics is conducted by means of geostatistic methods (variography). It allows validating the structurally related hydromechanical parameters like flow velocities derived from the simulation and offers insights to the linking between ongoing processes on the micrometer scale and its impact on the centimeter scale. The applied scale independent geostatistical tools provide scale independent parameters, like the correlation lengths. Such parameters are a suggested fundamentally important base for valid upscaling to the field scale.

We provide results from rock cores with both, relatively simple structured pathways as well as complex ones. In both types only a small part of the available pathway is passed through by the mobile fluid implying that only fractions of the inner surface were available for chemical reactions. Such findings should fertilize the concepts of reactive transport models aiming at larger scales.

Therefore we conclude: Microscale information is essential for improving reactive transport models.

  • Poster
    AGU 2010 Fall Meeting, 13.-17.12.2010, San Francisco, USA

Publ.-Id: 14972

Electroluminescence from Er and Yb co-doped silicon dioxide layers: The excitation mechanism

Prucnal, S.; Rebohle, L.; Skorupa, W.

The excitation mechanism of photo- (PL) and electroluminescence (EL) of erbium ions co-implanted with ytterbium into the SiO2 layer of light emitting MOS devices (MOSLED) was investigated. Ytterbium implanted and annealed samples exhibit the blue and near infrared electroluminescence. The blue electroluminescence at 470 nm appears due to cooperative up-conversion emission in the Yb3+-Yb3+ system, and the near infrared EL at 975 and 1025 nm corresponds to transitions from the multiple state 2F5/2 to the 2F7/2 ground state in the Yb3+ ions. The Er implanted SiO2 exhibits the luminescence in the blue-green and infrared region. The green and blue peaks correspond to radiative transitions from the 2H11/2 or 4S3/2 energy levels and from the 2H9/2 or 4F5/2 energy levels to the 4I15/2 ground state, respectively. We have found that the energy transfer from Yb3+ to Er3+ ions exists only during photoluminescence excitation. The electroluminescence investigation shows the cooperative up-conversion in the Er3+ - Yb3+ system.

Keywords: electroluminescence; rare earth; SiO2; MOSLED

Publ.-Id: 14971

Study of the processes of corium-melt retention in the reactor pressure vessel (INVECOR)

Zhdanov, V.; Baklanov, V.; Bottomley, P. D. W.; Miassoedov, A.; Tromm, W.; Journeau, C.; Altstadt, E.; Clement, B.; Oriolo, F.

Integral large-scale vessel retention experiments have been performed using up to 60 kg of prototypic corium melt (C-30) that is discharged from the electric melting furnace from a height of 1,7 m into a model RPV (Reactor Pressure Vessel) (40cm dia. x 60cm depth) with plasmatrons for decay heating of corium. The experiments on corium retention in the vessel were 1-2 hours. Specific power release in corium was 5-8 W.cm3 and the maximum temperature of the RPV wall was up to 1300°C. The following has been achieved during the project: 1) The technology of the protective coating on the graphite crucibles and on surfaces of plasmatron graphite nozzles has been developed. The plasmatron design now gives improved simulation of decay heat in corium. This has required numerous trials to set up the experimental systems. 2) Calculations of the corium pool and its heating efficiency, distributions of thermal fluxes and temperatures in the RPV have been performed. Validation of the models for the large-scale integral experiments has been conducted by means of specific tests. 3) 4 large-scale experiments with sustained energy release into the molten corium pool in the model RPV using oxidic corium (C-30) and oxidic-metallic corium (C-30+10 wt% stainless steel) have been conducted. 4) Post-test analysis of corium samples and RPV steel has been done. This included sectioning of corium ingot and the RPV wall, XRD, optical metallography and element analysis. It was found during the post-test examination that solidified corium exists both in the form of a continuous, massive ingot and in the form of small fragments located above the ingot. There was insignificant erosion of the steel surface of the RPV wall at the impact point of the corium jet. The results lead us to the following preliminary conclusions: 1) The relatively low thermal fluxes through a RPV model wall could be explained as follows: firstly, the thermal insulation on the RPV external surface results in the redistribution of thermal fluxes normal to, and along, the RPV wall; secondly, there is incomplete dissolution of uranium dioxide by the metallic zirconium melt in the melting furnace and this endothermic dissolution of UO2 continues during the decay heat generation in the corium retained in the RPV; thirdly, the gap caused by differential expansion between the corium crust and the RPV wall reduces heat transfer; fourthly, the layered character of the corium crust effectively reduces the crust's thermal conductivity. 2) Steady-state phenomena during corium retention in the reactor vessel are highly dependent on the previous transient processes of the melt speed dropping onto the lower head, the corium pool formation and the configuration of this pool. The presence of a fragmented debris layer over a massive corium ingot suggests that an optimistic prediction about corium coolability can be made. Here, with a large area for "corium/water" interaction on the top layer of debris, internal flooding & cooling seems probable.

Keywords: In-vessel melt retention; reactor core melt; severe accident research

  • Contribution to proceedings
    International Congress on Advances in Nuclear Power Plants (ICAPP 2011) Paper 11375, 02.-05.05.2011, Nice, France
    Study of the processes of corium-melt retention in the reactor pressure vessel (INVECOR)

Publ.-Id: 14970

Spin reorientation transition and phase diagram in an He+ ion irradiated ultrathin Pt/Co(0.5 nm)/Pt film

Bergeard, N.; Jamet, J.-P.; Ferré, J.; Mougin, A.; Fassbender, J.

A rich thermal magnetic phase diagram has been determined by magneto-optical Kerr magnetometry and microscopy in a He+ ion irradiated ultrathin Pt/Co0.5 nm/Pt film. The components of the net magnetization and the evolution and disappearance of the ribbonlike magnetic domain pattern have been studied in the perpendicular to in-plane spin reorientation transition temperature region. As observed in a dipolar frustrated ferromagnet, the ribbon pattern blurs progressively with increasing the temperature as due to efficient spatial fluctuations. We emphasize the limitation of present theories for interpreting such a type of dynamic transition.

Keywords: magnetism; ion irradiation; spin reorientation

Publ.-Id: 14969

Measuring the Saturation Magnetization in Samples With Unknown Magnetic Volume

Markó, D.; Lenz, K.; Strache, T.; Kaltofen, R.; Fassbender, J.

Measuring the Saturation Magnetization in Samples With Unknown Magnetic Volume

Keywords: magnetism; ion irradiation; saturation magnetization

Publ.-Id: 14968

Model spectra of organic phosphate-U(VI) and sulfurU(VI) complexes: a study of P k-edge and S k-edge XANES

Li, B.; Foerstendorf, H.; Raff, J.; Rothe, J.; Dardenne, K.

Phoshorylation of serine threonine and tyrosine residues, oxidation of methionine residues and sulfation on the sugar residues are common post translational modification to proteins. And study of protein-U(VI) interaction at mo-lecular level deepens the knowledge of U(VI) influence in the biosphare. P/S k-edge XANES were applied in the protein-U(VI) complexation in order to detect the tiny amount of phosphate-/sulfoxide-/sulfate-U(VI) com-plexes. In this study, model spectra were set up using phosvitin-U(VI), DMSO-U(VI) and sulfate-U(VI) for phosphate-/ sulfoxide-/sulfate-U(VI) complexes, respec-tively. Spectra features to represent the U(VI) complexa-tion have been found.

Keywords: phosphorus sulfur Uranyl protein complexation DMSO XANES

  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2010
    ISSN: 1437-322X

Publ.-Id: 14967

Spectroscopic Characterization of Am(III)-Complexes with small organic acids

Müller, M.; Acker, M.; Bernhard, G.

Americium contributes to the activity of radioactive waste to a certain extend. Until now, a little is known about the complexation with organic molecules. We analyzed the complexation of the trivalent ion (Am(III)) with Pyromellitic acid (Pyr) using UV-vis spectroscopy with a Liquid Waveguide Capillary Cell (LWCC) at different temperatures.

Keywords: UV-Vis; LWCC; americium; complex formation

  • Poster
    5th FZD Ph.D. Seminar, 22.-24.09.10, Krögis, Deutschland

Publ.-Id: 14965

Bestimmung der mikrobiellen Diversität von Biofilmen mit Schwerpunkt des Nachweises von eukaryonten Mikroorganismen in Uran kontaminierten Milieus

Zirnstein, I.

The Eukaryote diversity of the biofilms from two uranium-contaminated habitats (uranium mine Königstein in Saxony and the “Gessenhalde” next to the former leaching dump Ronneburg inThuringia) were studied by microscopical and molecular analysis (18S rDNA PCR). In the sulphate- and heavy metal-rich acidic mine drainage water of the uranium mine Königstein biofilms are formed as gelatinous filaments in the drainage channels and as stalactite-like snotites hanging from the ceilings. The results showed a low Eukaryote diversity of the sampled biofilms. Amoebozoa, Ciliophora and Heterolobosea were the dominants species. Fungi, Rotatoria und Flagellates were only present in minor amounts. Apikomplexa and Acari were detected sporadically. The creek of the Gessenwiese next to the former uranium field Ronneburg is characterized by an acid pH and a high concentration of sulphate. The biofilms are formed as thick algae mats are in form of filamentous stream biofilms. The diversity of Eukaryotes of these biofilms is higher in comparison to the biofilms of the uranium mine Königstein due to the environmental influence. The green algae Microspora and Klebsormidium dominated the sampled biofilm of the creek. In addition, Diatoms, Flagellates, Ciliates, Rotatoria, Amoebozoa, Fungi, Cryptophycaea, Heliozoa, Gastrotricha, Chrysophycaea, Insecta and Bryophyta were analyzed. The season-dependent differences between the diversity of Eukaryote groups were low. Algae cultures of the biofilms and algae single-culture of Klebsormidium sp. were analyzed with analytical methods and TEM/EDX using different pH. An accumulation of uranium at the algae cell wall of the single-culture of Klebsormidium sp. was only determined using a pH of 6.57 of the culture medium.

Keywords: microbial diversity; biofilms; uranium

  • Diploma thesis
    TU Dresden, FZD, 2010
    115 Seiten

Publ.-Id: 14964

Sorption of U(VI) onto mineral model systems -An in situ ATR FT-IR spectroscopic study

Gückel, K.; Foerstendorf, H.; Brendler, V.

Motivation, Ziele und erste Ergebnisse der Doktorarbeit

  • Poster
    Doktorandenseminar im Rahmen des Kompetenzzentrums Ost für Kerntechnik, 16.12.2010, Dresden, Deutschland

Publ.-Id: 14963

Report and evaluation of the redox potential measurements during the Intercomparison Exercise of ReCosy

Krawczyk-Bärsch, E.

The redox potential measurements were performed in systems with simple organics, near-natural, and natural ground- and porewater samples (NAT) and in microbial systems (MIC). As an example the results of the natural ground- and porewater samples (NAT), which were obtained by 10 to 19 groups or redox sensors, were interpreted in detail. Nine groups or redox sensors, including FZD, showed similar results. In most cases the measurements varied insignificantly. In some samples a difference of approximately 100 mV between the lowest and the highest value was determined. Accepting the measurement ranges, the results will give a good overview of the redox conditions in the near-natural, and natural ground- and porewater samples. However, in special cases further studies are needed for the interpretation of the measured redox potential, e.g. Extended X-ray Absorption Fine Structure (EXAFS), TEM in combination with Electron Energy Loss Spectroscopy (EELS) and Confocal Laser Scanning Microscopy (CLSM)/Laser Induced Fluorescence Spectroscopy (LIFS).
Unfortunately, only one of the microbial system samples was studied. The results showed clearly that redox potential measurements of microbial systems need much more time for getting a stable signal due to microbially influenced processes.

Keywords: Redox potential; natural ground- and porewater; microbial systems

  • Contribution to external collection
    M. Altmaier, G. Buckau, D. Fellhauer, X. Gaona: ReCosy ICE Report - Assessment of ReCosy Intercomparison Exercise. Intercomparison of Redox Determination Methods on Designed and Near-Natural Aqueous Systems. KIT Scientific Reports 7572., Karlsruhe: KIT Sientific Publishing, 2011, 1-24

Publ.-Id: 14962

Tomography measurements of gas-holdup in rotating foam reactors with Newtonian non-Newtonian and foaming liquids

Tschentscher, R.; Schubert, M.; Bieberle, A.; Nijhuis, T. A.; van der Schaaf, J.; Hampel, U.; Schouten, J. C.

Rotating solid foam reactors have already proven to show high mass transfer rates and to be a potential alternative to slurry reactors. The rotation of a foam block stirrer results in a high mass transfer and in the development of different reactor sections showing specific hydrodynamics and gas holdup distributions. In order to optimize the reactor system the hydrodynamics in a lab scale reactor are studied using gamma-ray tomography, a powerful method to measure the gas holdup in three-phase reactors. The influence of liquid properties, such as viscosity and surface tension, and the rotational speed on the gas/liquid distribution in the different reactor sections is investigated. Especially the viscosity has a strong effect on the entrapment of gas bubbles in the foam block structure, while the surface tension is the dominant parameter in the outer reactor section. The influence of these paramters on the inset of foaming and the collapse of the gas/liquid dispersion is investigated. Conclusions on the mass transfer performance are drawn and recommendations for further optimizations of the reactor design and the operational conditions depending on the liquid properties are developed.

Keywords: multiphase reactors; gas holdup; tomography; solid foam

Publ.-Id: 14961

Advanced Laser Particle Accelerator Development at LANL: From Fast Ignition to Radiation Oncology

Flippo, K. A.; Gaillard, S. A.; Kluge, T.; Bussmann, M.; Offermann, D. T.; Cobble, J. A.; Schmitt, M. J.; Bartal, T.; Beg, F. N.; Cowan, T. E.; Gall, B.; Gautier, D. C.; Geissel, M.; Kwan, T. J.; Korgan, G.; Kovaleski, S.; Lockard, T.; Malekos, S.; Montgomery, D. S.; Schollmeier, M.; Sentoku, Y.

Laser-plasma accelerated ion and electron beam sources are an emerging field with vast prospects, and promise many superior applications in a variety of fields such as hadron cancer therapy, compact radioisotope generation, table-top nuclear physics, laboratory astrophysics, nuclear forensics, waste transmutation, Special Nuclear Material (SNM) detection, and inertial fusion energy. LANL is engaged in several projects seeking to develop compact high-current and high-energy ion and electron sources. We are especially interested in two specific applications: ion fast ignition/capsule perturbation and radiation oncology. Laser-to-beam conversion efficiencies of over 10% are needed for practical applications, and we have already shown inherent efficiencies of >5% from flat foils, on Trident using only a 5th of the intensity [1] and energy of the Nova Petawatt laser [2]. With clever target designs, like structured curved cone targets, we have also been able to achieve major ion energy gains, leading to the highest energy laser-accelerated proton beams in the world [3]. These new target designs promise to help usher in the next generation of particle sources realizing the potential of laser-accelerated beams.

Keywords: laser; proton; ion; plasma; acceleration; pulse duration; intensity; scaling; inertial fusion; warm dense matter; oncology; radiotherapy; trident; cone target

  • Contribution to proceedings
    AIP Conference Proceedings 1299(1) of the 14th Advanced Accelerator Concepts Workshop, AAC2010, 13.-19.06.2010, Annapolis, MD, United States of America
    Advanced Laser Particle Accelerator Development at LANL: From Fast Ignition to Radiation Oncology: AIP, 693-698

Publ.-Id: 14960

Prospects For and Progress Towards Laser-Driven Particle Therapy Accelerators

Cowan, T. E.; Schramm, U.; Burris-Mog, T.; Fiedler, F.; Kraft, S. D.; Zeil, K.; Baumann, M.; Bussmann, M.; Enghardt, W.; Flippo, K.; Gaillard, S.; Harres, K.; Herrmannsdoerfer, T.; Kluge, T.; Nürnberg, F.; Pawelke, J.; Roth, M.; Schmidt, B.; Sobiella, M.; Sauerbrey, R.

Recent advances in laser-ion acceleration have motivated research towards laser-driven compact accelerators for medical therapy. Realizing laser-ion acceleration for medical therapy will require adapting the medical requirements to the foreseeable laser constraints, as well as advances in laser-acceleration physics, beam manipulation and delivery, real-time dosimetry, treatment planning and translational research into a clinical setting.

Keywords: radiotherapy; proton; laser; acceleration; cancer; compact; plasma; dosimetry; beamline

  • Contribution to proceedings
    14th Workshop on Advanced Accelerator Concepts, AAC2010, 13.-19.06.2010, Annapolis, MD, United States of America
    AIP Conference Proceedings, Advanced Accelerator Concepts, 1299: AIP, 721-726
  • Invited lecture (Conferences)
    AAC2010, 15.06.2010, Annapolis, USA

Publ.-Id: 14959

Cluster dynamics study of neutron irradiation induced defects in Fe-12.5at%Cr alloy

Gokhman, A.; Ulbricht, A.; Birkenheuer, U.; Bergner, F.

Cluster dynamics (CD) is used to study the evolution of the size distributions of vacancy clusters (VC), self-interstitial atom (SIA) clusters (SIAC) and Cr precipitates in neutron irradiated Fe-12.5at%Cr alloys at T = 573 K with irradiation doses up to 12 dpa and a flux of 140 ndpa/s. Transmission electron microscopy (TEM) and small angle neutron scattering (SANS) data on the defect structure of this material irradiated at doses of 0.6 and 1.5 dpa are used to calibrate the model. A saturation behavior was found by CD for the free vacancy and free SIA concentrations as well as for the number density of the SIAC and the volume fraction of the Cr precipitates for neutron exposures above 0.006 dpa. The CD simulations also indicate the presence of VC with radii less than 0.5 nm and a strong SIAC peak with a mean diameter of about 0.5 nm, both invisible in SANS and TEM experiments. A specific surface tension of about 0.028 J/m2 between the α matrix and the Cr-rich α' precipitate was found as best fit value for reproducing the long-term Cr evolution in the irradiated Fe-12.5%Cr alloys observed by SANS.

Publ.-Id: 14958

Comparative studies on electrochemical and laser-based fiber-optic oxygen microsensors applied to uranium contaminated biofilms.

Krawczyk-Bärsch, E.; Steinbrück, D.; Arnold, T.; Schmälzlin, E.; Kumke, M.

Dissolved oxygen is one of the key parameters in biofilm systems and may show different O2 concentrations within the biofilm. The O2 concentration may also be influenced by the microbial response to the exposure of heavy metals. Oxygen sensor measurements in such biofilms are a useful tool in interpreting oxygen microprofiles, which are influenced by the microbial respiratory activity. Consequently microsensors help to evaluate on redox processes in biofilms induced by heavy metals. To compare the applicability of electrochemical and laser-based fiber-optical microsensors for microbial ecology studies, oxygen microprofiling measurements in uranium free biofilms and in biofilms exposed to ecologically relevant uranium concentration were performed. The data obtained from both microsensor methods were in good agreement up to a depth of 680 and 480 µm. To avoid the risk of destroying the sensor tip, electrochemical sensor measurements had to be stopped at this depth. In contrast, laser-based sensor measurements were acquired over an additional range of 1 mm down to the biofilm/solid glass slide interface since optodes offer a high stability against consolidated materials. Thus, additional information on the oxygen concentration in lower zones of biofilms were obtained.

Keywords: electrochemical sensor; fiber-optic sensor; biofilm; uranium

  • Contribution to external collection
    in: 2nd Annual Workshop Proceedings of the Collaborative Project "Redox Phenomena Controlling Systems" (7th EC FP CP RECOSY), KIT Scientific Reports 7557, 139-146, 2010., Karlsruhe: KIT Scientific Publishing, Karlsruhe, 2010

Publ.-Id: 14956

Second reporting of the scientific-technical outcome of WP 4.2: - chemical and redox behavior of the investigated radionuclides in the different systems through microbial mediated processes.

Krawczyk-Bärsch, E.

First studies have been carried out in stalactite-like biofilms from the uranium mine Königstein (Germany), where the mining activities had been stopped in 1990 and the uranium mine has been partially flooded for remediation. In the acidic, sulphate-rich waters with high concentration of heavy metals and radionuclides (uranium) as contaminants, biofilms are formed and occur as gelatinous filaments, and as stalactite-like snotites. The analyses of the bacterial diversity of these biofilms showed a dominance of Ferrovum myxofaciens, an acidophilic, autotrophic, iron oxidizing bacteria, which belongs to the Betaproteobacteria. Ferrous iron is oxidized strongly catalyzed by Fe(II)-oxidizing bacteria with the consequence of producing oxidizing conditions within the biofilm with high oxygen concentration. Fiber-optic oxgyen microprofiles, carried out in these snotites are in a good agreement with electro-chemical measurements. The oxygen concentration is decreasing slowly from the edge versus center of the snotite biofilm. Electrochemical redox potential micoprofilings were carried out in these snottite-biofilms by a miniaturized platinum redox electrode with a tip diameter of 10 µm, too. In the bulk solution a redox potential of 728 mV +- 9.5 mV was measured in comparison to an increased redox potential of 834.5 mV +- 10.21 mV within the snottite-biofilm. We guess that the different geochemical conditions are due to the oxidation of ferrous iron catalyzed by Fe(II) oxidizing bacteria and that they will have an influence on the uranium speciations. A pH-Eh diagram for the U-S-O-H-C system at 15 °C was constructed using the geochemical speciation code “Geochemist´s Workbench” Version 8.0.8 / ACT2 Version 8.0.8 and the most recent NEA database for Uranyl Silicates and solid Uranates (Guillaumont et al., 2003), supplemented with solubility data for Uranophane (Nguyen et al., 1992) and CaU2O7.3H2O(s) (Altmaier et al., 2006) and the analytical data of the bulk water for the calculation of the field stability boundaries of different uranium species. The plotting of the measured pH and Eh values into this diagram showed that the theoretical stability fields of U species are defined in areas characterized by higher pH or lower Eh. The measured values indicate that aqueous Uranium(VI) Sulfate Complexations were formed in the biofilm as well as in the bulk solution. Only the changing of the local conditions (e.g. closure of the underground galleries) will lead to substantial changes and the formation of solid uranium(IV) species.

Keywords: Biofilm; uranium; redox potential; sensor measurements

  • Contribution to proceedings
    2nd Annual Workshop Proceedings of the Collaborative Project "Redox Phenomena Controlling Systems" (7th EC FP CP RECOSY), KIT Scientific Reports 7557, 51-54, 2010., 16.-19.03.2010, Larnaca, Cyprus: KIT Scientific Publishing, Karlsruhe

Publ.-Id: 14955

Development of redox determination methods

Steinbrück, D.; Kumke, M.; Neck, V.; Fellhauer, D.; Runke, J.; Grambow, B.; Landesman, C.; Ribert, S.; Krawczyk-Bärsch, E.; Perdicakis, M.

To compare the applicability of an electrochemical and a laser-based fiber-optical microsensor for oxygen determination in biofilm samples, microprofiling measurements in uranium free biofilms and in biofilms exposed to ecologically relevant uranium concentration were performed. For our studies we used a commercial available Clark-type microelectrode and a custom fiber-optic instrument, which was optimized for tip probes < 10 µm (optodes) by use of a diode laser and the so-called two frequency phase modulation technique, to mask interfering background fluorescence.
The data obtained from both microsensor methods in uranium free and uranium contaminated biofilms were in good agreement. Fiber-optic and electrochemical microsensor measurements showed high concentrations of oxygen over the total thickness of the uranium free biofilms. In contrast, biofilms exposed to uranium revealed a much lower oxygen concentration in the upper layers of the biofilm. At a biofilm depth of approximately 750 µm no oxygen was detectable at all.

Keywords: Electrochemical microsensor; fiber-optic sensor; biofilms; uranium

  • Contribution to proceedings
    2nd Annual Workshop Proceedings of the Collaborative Project "Redox Phenomena Controlling Systems" (7th EC FP CP RECOSY) Kit Scientific Reports 7557, 11-26, 2010, 16.-19.03.2010, Larnaca, Cyprus, KIT Scientific Publishing, Karlsruhe

Publ.-Id: 14954

Automated radiosynthesis of the thiol-reactive labeling agent N-[6-(4-[18F]fluorobenzylidene)aminooxyhexyl]maleimide ([18F]FBAM)

Knieß, T.; Kuchar, M.; Pietzsch, J.

The two-step radiosynthesis of N-[6-(4-[18F]fluorobenzylidene)aminooxyhexyl]maleimide ([18F]FBAM) was adapted to a remotely controlled synthesizer module. After optimization of reaction conditions as well as solid phase extraction based purification steps, the final [18F]FBAM was obtained in a decay-corrected radiochemical yield of 29±4% (related to [18F]fluoride, n=12) within a total synthesis time of 40min. The radiochemical purity of [18F]FBAM was in the range of 94-98%, the specific activity was determined with 13.4-17.2 GBq/µmol.

Keywords: 18F-labeling; Positron emission tomography (PET); Michael addition; Maleimide

Publ.-Id: 14953

Neue Ergebnisse zur Komplexbildung und Sorption im System Am(III)/Eu(III)-Modellligand-Opalinuston

Müller, M.; Acker, M.; Schott, J.; Barkleit, A.; Taut, S.; Bernhard, G.

Die Komplexbildung von Am(III) und Eu(III) wurde mittels UV-vis Spektroskopie und TRLFS untersucht. Mit steigender Temperatur konnte ein Anstieg der Eu(III)-Lactat Komplexierung festgestellt werden. Die Stabilität logb des Am(III)-Lactat Komplexes konnte bei Raumtemperatur mit TRLFS, UV-vis mit LWCC und UV-vis mit Standardpfadlänge übereinstimmend auf 2,2 ± 0,2 beziffert werden und liegt damit in der Nähe des Eu(III)-Lactat Komplexes. Vorgestellt wurde außerdem der Einfluss von Temperatur, pH-Wert und Organik auf die Sorption von Eu(III) am Opalinuston im Porenwasser. Es konnte gezeigt werden, dass die Sorption mit steigender Citrat und Tartat Konzentration abnimmt. Über TRLFS konnten Eu(III)-Citrat Spezies im Überstand der Batch-Sorptionsversuche, nicht jedoch am Opalinuston nachgewiesen werden. Die Erhöhung der Temperatur führt zwischen 20 und 60°C zu einer Verstärkung der Sorption. Ein Einfluss der Liganden auf die Sorptionsenthalpie konnte nicht nachgewiesen werden.

Keywords: complex formation; sorption; americium; europium; opalinus clay; UV-vis; TRLFS; temperature dependence

  • Lecture (others)
    9. Workshop zum Verbundprojekt "Wechselwirkungen und Transport von Actiniden im natürlichen Tongestein", 05.-06.10.10, Mainz, Deutschland

Publ.-Id: 14952

On the use of the Serpent Monte Carlo code for few-group cross section generation

Fridman, E.; Leppänen, J.

Serpent is a recently developed 3D continuous-energy Monte Carlo (MC) reactor physics burnup calculation code. Serpent is specifically designed for lattice physics applications including generation of homogenized few-group constants for full-core core simulators.
Currently in Serpent the few-group constants are obtained from the infinite lattice calculations with zero neutron current at the outer boundary. In this study, in order to account for the non-physical infinite-lattice approximation, B1 methodology, routinely used by deterministic lattice transport codes, was considered for generation of leakage-corrected few-group cross sections in the Serpent code. A preliminary assessment of applicability of the B1 methodology for generation of few-group constants in the Serpent code was carried out according to the following steps. Initially, two-group constants generated by Serpent were compared with those calculated by Helios deterministic lattice transport code. Then, 3D analysis of a Pressurized Water Reactor (PWR) core was performed by a nodal diffusion code DYN3D employing two-group cross section sets generated by Serpent and Helios. At this stage thermal-hydraulic (T-H) feedback was neglected. The DYN3D results were compared with those obtained from the 3D full core Serpent MC calculations. Finally, the full core DYN3D calculations were repeated taking into account T-H feedback and assuming Hot Full Power (HFP) conditions.
B1 two-group cross sections and diffusion coefficients generated by the Serpent and Helios codes agree within less than 2.5%. The results of the DYN3D calculations with the Serpent B1 cross-section sets agree very well with those of the Serpent full core MC calculations. The relative difference in keff is below 300 pcm. The maximum and root mean square (RMS) difference in the radial power distribution is 2.7% and 1.1% respectively. The results of the DYN3D full core calculations with T-H feedback obtained with Helios and Serpent generated cross-section libraries show an excellent agreement as well. The estimated critical boron concentration agrees within 5 ppm. The maximum and RMS difference in the core radial power peaking factors is 0.8% and 0.4% respectively.
In this study a Matlab script was used for calculation of the leakage-corrected few-group cross sections, however the B1 methodology has recently been implemented directly in the Serpent code.

Keywords: Monte Carlo; Lattice physics; Few-group cross section generation; Critical spectrum; 3D core analysis

Publ.-Id: 14951

Overview of the FZD and of the Institute of Safety Research / Counter-current flow limitation experiments in a model of the hot leg of a PWR

Vallée, C.

First, an overview of the FZD and of the Institute of Safety Research was presented. This includes in particular the departments of the institute, the main tests facilities and some examples of current research topics.
The second part of the presentation was focused on counter-current flow limitation experiments performed in a model of the hot leg of a pressurised water reactor. The background of this issue in the nuclear safety was introduced. Furthermore, the hot leg model of the TOPFLOW test facility and its particular operation in the pressure chamber were presented. The main results of the counter-current flow limitation experiments were shown in details, including the experimental procedure, a typical experiment performed with steam and saturated water at 50 bar, the method used to determine the flooding characteristics, the length characteristics in the Wallis parameter for channels with rectangular cross-sections and a comparison between air and steam experiments.

Keywords: FZD; hot leg; pressurised water reactor; PWR; counter-current flow limitation; CCFL

  • Lecture (others)
    Young scientists seminar, 08.11.2010, Mihama-cho (Fukui), Japan

Publ.-Id: 14950

Simulation of 1+1 dimensional surface growth and lattices gases using GPUs

Schulz, H.; Ódor, G.; Ódor, G.; Nagy, M. F.

Restricted solid on solid surface growth models can be mapped onto binary lattice gases. We show that efficient simulation algorithms can be realized on GPUs either by CUDA or by OpenCL programming. We consider a deposition/evaporation model following Kardar-Parisi-Zhang growth in 1+1 dimensions related to the Asymmetric Simple Exclusion Process and show that for sizes, that fit into the shared memory of GPUs one can achieve the maximum parallelization speedup (~ ×100 for a Quadro FX 5800 graphics card with respect to a single CPU of 2.67 GHz). This permits us to study the effect of quenched columnar disorder, requiring extremely long simulation times. We compare the CUDA realization with an OpenCL implementation designed for processor clusters via MPI. A two-lane traffic model with randomized turning points is also realized and the dynamical behavior has been investigated.

Keywords: surface growth model; lattice gas; GPU; parallel algorithm

Publ.-Id: 14949

S-layer based concept for the development of nano-structured biosensors

Weinert, U.; Günther, T.; Pollmann, K.; Raff, J.

The challenge for new sensor systems is to make them smaller and to achieve higher specifity and sensitivity to one analyte. Most promising, therefore, are bio molecules due to their typical sizes in the nano-meter range, self-assembly properties and the high affinity of biological binding molecules. Here we present a method to construct a nano-structured sensor device, using surface layer (S-layer) proteins as templates, aptamers (short oligonucleotides) as receptors and a dye pair for fluorescence resonance energy transfer (FRET) to detect the binding of one analyte . S-layer proteins are structural proteins, forming the outermost cell envelope of numerous bacteria and almost all archaea. They feature a lot of functions for the microbial cell such as protection, adhesion, filtration or framework. Their oblique, square or hexagonal structure, high content of regular arranged functional groups and the ability of the monomers to self-assembly in aqueous solution qualifies them for various nanotechnological purposes. The functional groups of the side chains of the S-layer proteins can be used for the sequential coupling of aptamers and organic or inorganic fluorescent dyes. In combination with an optical device for detection, a nano-structured sensor system is constructible. Aptamers were chosen as receptor system due to their high specificity, comparable to that of antibodies, and their higher stability against environmental changes. Currently we are working on a sensor system which is able to detect pharmaceuticals in water. Therefore different S-layer, aptamer and dye conformations will be tested to optimize the binding of a target molecule and to maximize the interference of the initial FRET between the two fluorescent dyes. It is expected to generate an optical signal that will allow the detection of very low analyte concentrations. An additional aim for the future is to bind two or more different aptamers to the S-layers, to develop a multiple detection system. Beside the construction of sensory devices, the aptamer-S-layers combination can be used for the construction of filter materials for the specific binding and degradation of toxic organic substances in water. For this purpose, photocatalytic active nanoparticles and aptamers are regularly arranged on the S-layer protein. Bound organic substances can be eliminated by the formation of reactive oxygen species induced by light irradiation.

Keywords: S-layer; aptamers; fluorescence dyes; FRET; biosensor

  • Poster
    Nanofair 2010 - 8th International Nanotechnology Symposium, 06.-07.07.2010, Dresden, Deutschland

Publ.-Id: 14947

Self assembling proteins as matrix for the construction of optical devices

Weinert, U.; Günther, T.; Raff, J.; Pollmann, K.

Self-assembling biomolecules are widespread in nature and attractive for technical purposes due to their size and highly ordered structures in nanometer range. Surface-layer (S-layer) proteins are one of those self-assembling molecules and their chemical and structural properties make them quite attractive for nanotechnical purposes. They possess a high content of functional groups so a sequential coupling of functional devices is possible and their ability to self assemble in aqueous solutions or on surfaces, e. g. SiO2 wafers, qualifies them for nanotechnical applications. In this work, first experiments were done in order to construct a sensory device containing S-layer proteins as matrix to bind optical elements and analytes for detection of specific substances. The S-layer proteins were isolated from the Lysinibacillus sphaericus strain JG-A12 recovered from a uranium mining waste pile in Germany. As optical elements fluorescent dyes or quantum dots can be used. Three different fluorescent dyes which are able to perform a Fluorescence resonance energy transfer (FRET) were used and coupled to the S-layer proteins. As receptor molecule aptamers were chosen due to their high specifity and stability towards many chemicals. Aptamers are short oligonucleotides which are able to bind specific molecules via their three dimensional structure. In this work, a model aptamer was used that is specific towards human thrombin. The aim was to construct a sensor system which is able to detect specific substances in very low concentration ranges in aqueous solutions.

Keywords: S-layer proteins; fluorescent dyes; aptamers

  • Lecture (Conference)
    Design and Nature 2010 Fifth International Conference on Comparing Design in Nature with Science and Engineering, 28.-30.06.2010, Pisa, Italia

Publ.-Id: 14946

Morphology Induced Magnetic Anisotropy of Thin Films Deposited on Nanoscale Ripple Substrates

Körner, M.; Liedke, M. O.; Lenz, K.; Ranjan, M.; Fritzsche, M.; Facsko, S.; Fassbender, J.; von Hörsten, U.; Krumme, B.; Wende, H.

Magnetic properties of thin films are influenced by the morphology of substrates with periodically modulated patterns on the nanometer scale [1]. These well ordered surface modulations (ripple) can be produced by low energy ion beam erosion and are tuneable over a wide range [2]. Thin magnetic films deposited on these ripple surfaces repeat the surface profiles of these patterns and thus an additional uniaxial magnetic anisotropy is induced. This is shown for thin films of Fe, Co as well as the quasi-Heusler compound Fe3Si. The magnetic anisotropy is determined by means of angular- as well as frequency-dependent ferromagnetic resonance measurements using a vector network analyzer. We find a strong uniaxial magnetic anisotropy induced by the ripple surface, which is superimposed on the cubic anisotropy in the case of single crystalline films.
This work is supported by DFG grant FA 314/6-1.
[1] M. Körner et al., Phys. Rev. B 80, 214401 (2009).
[2] J. Fassbender et al., New Journal of Physics 11, 125002 (2009).

  • Poster
    DPG Frühjahrstagung der Sektion AMOP (SAMOP) und der Sektion Kondensierte Materie (SKM) 2011, 13.-18.03.2011, Dresden, Germany

Publ.-Id: 14945

Novel biosensors by combining bacterial S-layers, fluorescent dyes and aptamers

Weinert, U.; Günther, T.; Pollmann, K.; Raff, J.

Bacterial surface layer (S-layer) represent the outermost cell envelope of various bacteria and feature a lot of functions such as protection, binding, filtration or framework. From the uranium mining waste pile “Haberland” near Johanngeorgenstadt in Saxony different Bacilli were isolated and their S-layer proteins were investigated. These S-layer proteins are able to self assemble in aqueous solution and form sheets or tubes with a hexagonal, square or oblique paracrystalline structure. By combining S-layers with polyelectrolytes, many technical surfaces, e.g. SiO2 wafers, can be covered with a closed and highly regular protein layer. The so modified materials are very promising for the construction of multifunctional materials, particularly new biosensors. Their high content of different and regularly arranged functional groups such as hydroxyl, amino and carboxyl groups provide a convenient platform for the sequential and defined coupling of sensor elements. So the idea is to construct a sensor system on the basis of S-layer proteins by combining fluorescence dyes with aptamers. Possible fluorescence dyes have an overlapping emission and excitation spectra and therefore the ability to induce a fluorescence resonance energy transfer (FRET). FRET describes a non radiation energy transfer between two chromophores. One chromophore (donor) in its excitated state will transfer energy to another chromophore (acceptor) and as a result the donor emission will decrease while the acceptor emission increases. The latter is detectable and can be used for the construction of the optical device of a sensor. One requirement therefore is the spatial proximity of both chromophores (<10nm), what can be easily achieved by using S-layers as nano-structured polymer with a high density of functional groups. Further components of the sensors are aptamers, working as specific receptors for analytes. Aptamers are short oligonucleotides with a high binding capacity towards a specific analyte, in present case specific pharmaceuticals and biocides. The combination of all three parts will lead to a selective, sensitive and adaptable sensory layer system useful for environmental technologies.

Keywords: S-layer; aptamers; fluorescence dyes; biosensor

  • Poster
    Nanocoatings-International Conference on Functional Nanocoatings, 28.-31.03.2010, Dresden, Deutschland

Publ.-Id: 14944

Entwicklung von sensorischen Schichten auf Basis bakterieller Hüllproteine

Weinert, U.; Günther, T.; Pollmann, K.; Raff, J.

Hüllproteine sind Strukturproteine, welche die äußerste Abgrenzung von zahlreichen Bakterien und fast allen Archaeen darstellen. Isoliert bilden diese Proteine in wässrigen Lösungen und an Grenzflächen regelmäßige parakristalline Gitterstrukturen aus, auf deren Oberfläche sich zahlreiche modifizierbare Carboxyl- und Aminogruppen befinden. Hüllproteine sind daher eine ideale Matrix zur Konstruktion von nanoskaligen sensorischen Schichten, bestehend aus Aptameren und Fluoreszenzfarbstoffen. Aptamere sind kurze Oligonukleotide, welche ähnlich wie Antikörper an Strukturmerkmale anderer Moleküle binden können, jedoch deutlich stabiler sind. Mittels dieser chemischen Antikörper lassen sich eine Vielzahl verschiedener Targetmoleküle spezifisch an die sensorische Schicht binden. Alle Komponenten des Detektionssystems befinden sich durch die regelmäßige Proteinmatrix in räumlicher Nähe, was einen Fluoreszenz Resonanz Energie Transfer zwischen den Fluoreszenzfarbstoffen ermöglicht. Dieser Energietransfer kann durch Binden eines Analyten an das Aptamer gestört werden (Quenching), was die Realisierung eines hochempfindlichen Sensors ermöglicht.

Keywords: S-layer; aptamers; fluorescence dyes; FRET; biosensor

  • Poster
    7. Max-Bergmann-Symposium, 16.11.2010, Dresden, Deutschland

Publ.-Id: 14943

Experimental characterisation of the interfacial structure during counter-current flow limitation in a model of the hot leg of a PWR

Vallée, C.; Nariai, T.; Futatsugi, T.; Tomiyama, A.; Lucas, D.; Murase, M.

In order to investigate the two-phase flow behaviour during counter-current flow limitation in the hot leg of a pressurised water reactor, dedicated experiments were performed in a scaled down model of Kobe University. The structure of the interface was observed from the side of the channel test section using a high-speed video camera. An algorithm was developed to recognise the stratified interface in the camera frames after background subtraction. The evolution of the water level along the hot leg is analysed in function of the liquid and gas flow rates.

Keywords: counter-current flow limitation; CCFL; hot leg; stratified two-phase flow; interfacial structure; image processing

  • Contribution to proceedings
    The 14th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-14), 25.-29.09.2011, Toronto, Canada
    Proceedings of the 14th International Topical Meeting on Nuclear Reactor Thermalhydraulics (NURETH-14), paper 175

Publ.-Id: 14942

1 J white-light continuum from 100 TW laser pulses

Petit, Y.; Henin, S.; Nakaema, W.; Béjot, P.; Jochmann, A.; Kraft, S.; Bock, S.; Schramm, U.; Stelmaszczyk, K.; Rohwetter, P.; Kasparian, J.; Sauerbrey, R.; Wöste, L.; Wolf, J.-P.

We experimentally measured the supercontinuum generation by 3 J, 30 fs laser pulses and measured white-light generation at the unprecedented level of 1 J. Such high energy is allowed by a strong contribution to the continuum by the photon bath, as compared to the self-guided filaments. This contribution due to the recently observed congestion of the filament number density in the beam profile at very high intensity, also results in a wider broadening for positively chirped pulses rather than for negatively chirped ones, similarly to broadening in hollow-core fibers.

Keywords: Beam trapping; Frequency conversion; harmonic generation; including higher-order harmonic generation; self-phase modulation; self focusing and defocusing

Publ.-Id: 14941

Mn-doped Ge and Si: A Review of the Experimental Status

Zhou, S.; Schmidt, H.

Diluted ferromagnetic semiconductors (FMS) are in the focus of intense research due to their potential applications in spintronics and their striking new physical properties. So far Mn-doped III-V compound semiconductors such as GaMnAs are the most important and best understood ones, but they are ferromagnetic only at well below room temperature. An interesting alternative could be magnetic semiconductors based on elemental semiconductors, also owing to their compatibility with Si microelectronics. In the last decades, considerable amount of work has been devoted to fabricate Mn-doped Ge and Si FMS. In this article, the structural, magnetic and magneto-transport properties of Mn-doped Ge and Si will be reviewed.

Keywords: Diluted ferromagnetic semiconductor; Mn doped Si; Mn doped Ge

Publ.-Id: 14940

XAS versus IC-ICP-MS for arsenic-sulfur speciation analysis

Planer-Friedrich, B.; Suess, E.; Scheinost, A. C.; Wallschläger, D.

Modern analytical methods have demonstrated that soluble arsenic-sulfur species constitute a major fraction of dissolved arsenic in sulfidic waters. However, an intense debate is going on about the exact chemical nature of these compounds, with X-ray absorption spectroscopy (XAS) data suggesting thioarsenites, ion chromatographic and mass spectroscopic (IC-ICP-MS) data suggesting thioarsenates. We show here by XAS that thioarsenites are the primary reaction products of arsenite and sulfide which convert rapidly to thioarsenates when exposed to atmospheric oxygen. The predominance of dithioarsenate determined by IC-ICP-MS in arsenite-sulfide solutions is thus an analytical artifact. When eliminating oxygen by conducting the chromatographic separation under anaerobic conditions, thioarsenites convert to arsenite as they are instable in the acidic and alkaline pH-range and undergo competitive dissociation at decreasing SH-:OH- ratios when diluting a sample. As thioarsenites were found to be necessary precursor species for thioarsenate formation, their analysis at environmentally relevant concentrations is a necessity for modern arsenic speciation analysis.

Keywords: arsenic; sulfur; XAS

  • Invited lecture (Conferences)
    11th International Conference on the Biogeochemistry of Trace Elements, 03.-07.07.2011, Florence, Italy

Publ.-Id: 14939

Experimentelle Untersuchung polydisperser Dampf/Wasser-Strömungen mit Phasenübergang in einer vertikalen DN200-Teststrecke

Beyer, M.; Lucas, D.; Szalinski, L.; Pietruske, H.; Schütz, P.; Lindner, K.

Zweiphasenströmungen spielen eine wichtige Rolle bei der Sicherheitsbewertung bzw. Optimierung verfahrenstechnischer Prozesse. Ein wichtiger Anwendungsbereich für Zweiphasenströmungen sind die konventionelle und die Kernkraftwerkstechnik, bei denen in Dampfkesseln, Dampferzeugern aber auch in Siedewasserreaktoren schon im Normalbetrieb unterschiedliche Dampfgehalte und damit verschiedene Strömungsformen auftreten. Für die nukleare Sicherheitsforschung ist die zuverlässige Vorhersage charakteristischer Parameter von Zweiphasenströmungen in Störfallanalysen wichtig. Bei hypothetischen Leck- bzw. Reaktivitätsstörfällen in Siedewasser- oder Druckwasserreaktoren können auf Grund der teilweisen Verdampfung des Kühlmittels komplexe Zweiphasenströmungen auftreten. Auch für solche Fälle muss die Abfuhr der im Kern generierten Nachzerfallswärme nachgewiesen werden.
Eine zweckmäßige Methode zur Modellierung von Mehrphasenströmungen ist die dreidimensionale rechnergestützte Simulation der Strömungsvorgänge mittels CFD (Computational Fluid Dynamics) Programmen. Während diese Programme gegenwärtig für einphasige Strömungen gute Ergebnisse liefern, besteht im Zweiphasenbereich noch Entwicklungsbedarf. Zur Modell-entwicklung und –validierung sind qualitativ hochwertige Daten von Experimenten in praxisrelevanten Geometrien bei hohen Drücken und Temperaturen erforderlich. Ein wesentlicher Teil dieser Datenbasis wird im Rahmen der TOPFLOW Projekte bereitgestellt.
Der Vortrag erläutert die Durchführung von Experimenten mit polydispersen nicht adiabaten Dampf/Wasser-Strömungen und beschreibt die speziell für den Hochtemperaturbereich entwickelte Messtechnik. Ein wesentlicher Bestandteil des Seminars ist die Diskussion von ausgewählten Ergebnissen der Kondensationsversuche, die die Entwicklung der Strömungsstruktur über die Höhe eines vertikalen Rohres sowie die dabei auftretenden Energie- und Stofftrans-portprozesse zwischen den Phasen veranschaulichen.

Keywords: Steam/Water-flow; non adiabatic flow; wire-mesh sensor; gas velocity profiles; bubble size distributions; vertical pipe

  • Lecture (others)
    Institutsseminar, 09.12.10, FZD, Germany

Publ.-Id: 14938

Atom probe characterization of solute atom clustering in decommissioned Greifswald unit 4 weld metal

Nishida, K.; Soneda, N.; Nomoto, A.; Dohi, K.; Bergner, F.; Viehrig, H.-W.

Characterization of irradiation-induced microstructural features in reactor pressure vessel (RPV) materials is essential for the understanding of the mechanisms of RPV embrittlement. Particularly, analyses of materials from decommissioned RPVs are very important because the simulated irradiations in material testing reactors can never be the same as that of actual commercial reactors. In this study, we performed microstructural characterization of the weld metals cut from the decommissioned Greifswald Unit 4 RPV. Atom probe tomography technique was used to characterize the microstructural features in terms of solute atom clustering and segregation. The following conclusions can be drawn: (1) Carbides containing large amount of vanadium with a small size of ~10nm in diameter were observed. (2) Clusters which consist of Cu, Ni, Mn, Si, Cr and P are formed at Layer 4 and 6. (3) At Layer 4, two kinds of solute atom clusters i.e. with and without Cu atoms are observed, whereas almost all clusters contain Cu at Layer 6. (4) Concentrations of Ni, Mn, Si and Cr in the clusters are independent of cluster size. Si concentration is different between Layer 4 and 6. (5) There is a large scatter in the concentrations of Cu and P. However, the sum of Cu and P concentrations in the clusters is almost constant.

Keywords: Reactor pressure vessel steel; Atom probe tomography

  • Lecture (Conference)
    MRS Fall Meeting, 29.11.-03.12.2010, Boston, USA

Publ.-Id: 14937

Exceptional points: Mathematical aspects and Krein space related physics (I + II)

Günther, U.

In the first one-hour lecture some basic mathematical facts are presented on an introductory level. Specifically, we discuss spectral singularities of various types, projective Hilbert space structures at exceptional points (EPs), Krein spaces and PT quantum mechanics (PTQM) as well as gauged PTQM and PTQM Jaynes-Cummings models.
The second one-hour lecture is devoted to Krein space related physics and some concrete applications. The topics of this lecture are higher-order EPs in PT-symmetric Bose-Hubbard models, the PTQM brachistochrone and entanglement dynamics. Additionally, we briefly comment on EPs in MHD dynamo setups.

Keywords: exceptional points; non-Hermitian operators; PT quantum mechanics; Krein space; projective Hilbert space; Bose-Hubbard model; Bose-Einstein condensate; gauged PT quantum mechanics; Jaynes-Cummings models; quantum brachistochrone; entangled states; MHD dynamos

  • Invited lecture (Conferences)
    The physics of exceptional points., 02.-05.11.2010, Stellenbosch, South Africa

Publ.-Id: 14936

Krein spaces and PT quantum mechanics

Günther, U.

In 1998, 1999 it was shown by Bender and collaborators that there are certain classes of Hamiltonians which at a first glance seem not selfadjoint in Hilbert spaces, but which nevertheless are having real spectra. Examples are Hamiltonians of the type H=p2+x2(ix)μ. For parameters μ ∈ [0,1] these Hamiltonians have positive real eigenvalues with square integrable eigenfunctions defined over the real line. It was found that the reality of the eigenvalues was connected with an underlying PT-symmetry of the Hamiltonians and their eigenfunctions, i.e. the systems are in a sector of unbroken PT-symmetry. There exist other sectors like μ ∈ (-1,0) where this PT-symmetry is spontaneously broken: although the Hamiltonian remains PT-symmetric, part of its eigenfunctions loose PT-symmetry and the corresponding eigenvalues are coming in complex conjugate pairs. A PT phase transition occurs at μ=-0.
It turns out that the PT-symmetry of the Hamiltonian H induces a natural indefinite metric structure in Hilbert space and that H, instead of being selfadjoint in a usual Hilbert space (with positive definite metric), is selfadjoint in a generalized Hilbert space with an indefinite metric --- a so called Krein space. Similar to time-like, space-like and light-like vectors in Minkowski space a Krein space has elements of positive and negative type as well as neutral (isotropic) elements. Moreover in analogy to passing via Wick-rotation from Minkowski space to Euclidian space, in the sector of exact PT-symmetry there exists an operator which allows to pass from a Krein space description of the system to a description in a Hilbert space with a highly nontrivial metric operator. At the PT phase transition point this operator becomes singular and the corresponding mapping breaks down.
In the talk, on an introductory level, some of the basic structures of PT-symmetric quantum mechanics and their relation to corresponding Krein-space setups are sketched. For gaining some rough intuition, the facts are illustrated by simple matrix models. The richness of the systems is demonstrated on the simple example of a PT-symmetric two-mode Bose-Hubbard model, PT-symmetric brachistochrone setups and gain-loss-balanced PT-symmetric optical waveguide systems.

Keywords: PT quantum mechanics; PT-symmetry; non-Hermitian operators; Krein space; indefinite metric; phase transition; quantum brachistochrone; Bose-Hubbard model; PT-symmetric optical waveguides; gain-loss balanced systems

  • Lecture (others)
    seminar of the Computational Physics Group, Institute of Theoretical Physics, TU Dresden, 26.11.2010, Dresden, Deutschland

Publ.-Id: 14935

Three models of Krein-space-related physics: PT-symmetric Quantum Mechanics, Squire equation and the MHD α²-dynamo

Günther, U.

PT-symmetric Quantum Mechanics, the Squire equation of hydrodynamics and the spherically symmetric α²-dynamo of magnetohydrodynamics (MHD) can be structurally linked and treated in a unified way as spectral problems in Krein spaces. We demonstrate their interrelation explicitly and provide examples for specific parameter dependencies of their spectra. Special emphasis is laid on the physical relevance of transitions between real and complex spectral branches in connection with phase transitions between sectors of exact PT-symmetry and spontaneously broken PT-symmetry in Quantum Mechanics as well as with possible polarity reversals of dynamo maintained magnetic fields of planets. We briefly comment on third order spectral branch points with geometric multiplicity one and algebraic multiplicity three as well as on a dynamo related resonant unfolding of diabolical points (spectral intersection points of geometric and algebraic multiplicity two). Finally, we sketch the general technique of versal deformations as specific unfolding of Jordan-block related singularities.

Keywords: operator theory; Krein space; PT-symmetric Quantum Mechanics; dynamo theory; Squire equation; spectral branch points; diabolical points; phase transition; perturbation theory; resonances; Puiseux expansions; versal deformations; Jordan block; unfolding of singularities

  • Lecture (others)
    weekly seminar of the Krein space group, 05.05.2010, Ilmenau, Deutschland

Publ.-Id: 14934

PT-symmetry, Cartan decompositions, Lie triple systems and Krein space related Clifford algebras

Günther, U.; Kuzhel, S.

Gauged PT quantum mechanics (PTQM) and corresponding Krein space setups are studied. For models with constant non-Abelian gauge potentials and extended parity inversions compact and noncompact Lie group components are analyzed via Cartan decompositions. A Lie-triple structure is found and an interpretation as PT-symmetrically generalized Jaynes-Cummings model is possible with close relation to recently studied cavity QED setups with transmon states in multilevel artificial atoms. For models with Abelian gauge potentials a hidden Clifford algebra structure is found and used to obtain the fundamental symmetry of Krein space related J-selfadjoint extensions for PTQM setups with ultra-localized potentials.

Keywords: PT quantum mechanics; non-Hermitian Hamiltonians; gauge theory; Abelian gauge field; non-Abelian gauge field; Cartan decomposition; compact and noncompact components; Lie triple system; Clifford algebra; ultra-localized potential; Krein space; J-selfadjoint extension; Jaynes-Cummings system; multilevel artificial atoms; cavity QED; transmon states

Publ.-Id: 14933

PT symmetry, Cartan decompositions, Lie triple systems and Krein space related Clifford algebras

Günther, U.; Kuzhel, S.

Gauged PT quantum mechanics (PTQM) and corresponding Krein space setups are studied. For models with constant non-Abelian gauge potentials and extended parity inversions compact and noncompact Lie group components are analyzed via Cartan decompositions. A Lie-triple structure is found and an interpretation as PT-symmetrically generalized Jaynes-Cummings model is possible with close relation to recently studied cavity QED setups with transmon states in multilevel artificial atoms. For models with Abelian gauge potentials a hidden Clifford algebra structure is found and used to obtain the fundamental symmetry of Krein space related J-selfadjoint extensions for PTQM setups with ultra-localized potentials.
The talk is based on arXiv:1006.1134[math-ph]

Keywords: PT quantum mechanics; non-Hermitian Hamiltonians; gauge theory; Abelian gauge field; non-Abelian gauge field; Cartan decomposition; compact and noncompact components; Lie triple system; Clifford algebra; ultra-localized potential; Krein space; J-selfadjoint extension; Jaynes-Cummings system; multilevel artificial atoms; cavity QED; transmon states

  • Lecture (Conference)
    21st International Workshop on Operator Theory and its Applications, 12.-16.07.2010, Berlin, Deutschland

Publ.-Id: 14932

PTQM related involution structures

Günther, U.; Kuzhel, S.

Gauged PT quantum mechanics (PTQM) and corresponding Krein space setups are studied. For models with constant non-Abelian gauge potentials and extended parity inversions compact and noncompact Lie group components are analyzed via Cartan decompositions. A Lie-triple structure is found and an interpretation as PT-symmetrically generalized Jaynes-Cummings model is possible with close relation to recently studied cavity QED setups with transmon states in multilevel artificial atoms. For models with Abelian gauge potentials a hidden Clifford algebra structure is found and used to obtain the fundamental symmetry of Krein space related J-selfadjoint extensions for PTQM setups with ultra-localized potentials. Based on: arXiv:1006.1134[math-ph]

Keywords: PT quantum mechanics; non-Hermitian Hamiltonians; gauge theory; Abelian gauge field; non-Abelian gauge field; Cartan decomposition; compact and noncompact components; Lie triple system; Clifford algebra; ultra-localized potential; Krein space; J-selfadjoint extension; Jaynes-Cummings system; multilevel artificial atoms; cavity QED; transmon states

  • Invited lecture (Conferences)
    Pseudo-Hermitian Hamiltonians in Quantum Physics IX, 21.-24.06.2010, Hangzhou, China

Publ.-Id: 14931

PTQM related Cartan and Clifford structures

Günther, U.

Gauged PT quantum mechanics (PTQM) and corresponding Krein space setups are studied. For models with constant non-Abelian gauge potentials and extended parity inversions compact and noncompact Lie group components are analyzed via Cartan decompositions and an underlying Lie-triple structure is described. For models with Abelian gauge potentials a hidden Clifford algebra structure is found and used to obtain the fundamental symmetry of Krein space related J-selfadjoint extensions for PTQM setups with ultra-localized potentials.

Keywords: PT quantum mechanics; non-Hermitian Hamiltonians; gauge theory; Abelian gauge field; non-Abelian gauge field; Cartan decomposition; compact and noncompact components; Lie triple system; Clifford algebra; ultra-localized potential; Krein space; J-selfadjoint extension

  • Invited lecture (Conferences)
    Analytic and algebraic methods VI, 10.-11.05.2010, Prague, Czech Republic

Publ.-Id: 14930

Redox-dependent suphate coordination of neptunium in aqueous solutions

Hennig, C.; Ikeda-Ohno, A.; Tsushima, S.; Scheinost, A.

A direct determination of actinide coordination in aqueous solution under controlled redox conditions is an important issue to understand the specific chemical reactions and to predict the behavior in nuclear waste repositories. We developed several techniques to study actinide solutions under dedicated redox conditions at the ESRF. The methods to control the actinide redox state are based on potentiostatic or galvanostatic electrolyses. The solutions can be investigated either ex situ or in situ with a specific spectroelectrochemical cell where the electrochemical process is combined with EXAFS spectroscopy. As example we show the redox reaction of 0.05 M neptunium in presence of 2.0 M sulfate. The neptunium species occur preferred with one monodentate (Np5+) and two bidentate (Np6+) sulfate ligands. Np4+ forms very strong complexes with up to fife sulfate ligands. The EXAFS signal reveals a unique ligand rearrangement during the redox reaction. This provides the explanation why the cyclic voltammogram show irreversible reaction steps.

Keywords: Neptunium; EXAFS; elektrochemistry

  • Contribution to external collection
    G. Admans: ESRF Highlights 2009, Grenoble: Müllerdruck Mannheim, 2010, 84-85

Publ.-Id: 14929

Structure, energetics and thermodynamics of copper-vacancy clusters in bcc-Fe: An atomistic study

Al-Motasem, A. T.; Posselt, M.; Bergner, F.; Birkenheuer, U.

A combination of on-lattice simulated annealing based on Metropolis Monte Carlo simulations and off-lattice relaxation by Molecular Dynamics is applied in order to determine the structure and energetics of coherent copper-vacancy clusters in bcc-Fe. The most recent interatomic potential for Fe-Cu alloys is used. About 150 clusters consisting of up to 200 monomers (vacancies or copper atoms) are investigated. The atomic structure and the formation energy of the most stable configurations as well as their total and monomer binding energy are calculated. All clusters show facets which correspond to the main crystallographic planes. In the case of mixed clusters a core-shell structure is found where Cu atoms coat the outer surface of vacancy clusters. These findings are in agreement with previous theoretical results and with indications from measurements. For small clusters the total binding energy determined in this work shows a good agreement with literature data obtained by first-principle calculations. For further application in rate theory and object kinetic Monte Carlo simulations compact and physically-based fit formulae are derived from the atomistic data for the total and the monomer binding energy. The fit is based on the classical capillary model. An atomistic nucleation model is established, and for typical irradiation conditions the nucleation free energy of pure vacancy and pure copper as well as the critical size for cluster formation are estimated.

Keywords: Diluted iron alloy; Copper-vacancy clusters; Atomistic simulation

Publ.-Id: 14928

Das Arcanum von Meissner Porzellan: Beharrung oder Wandel?

Neelmeijer, C.; Pietsch, U.; Ulbricht, H.

Das Arcanum zur Herstellung von Hartporzellan in Europa geht auf ein Patent von Johann Friedrich Böttger im Jahre 1708 zurück. Seitdem ist die Porzellanmanufaktur Meißen Hersteller und Vertreiber des „Weißen Goldes“. Wie streng wird auf dem dort hinterlegten Geheimnis seiner Rezeptur beharrt? Gibt es durch Variationen in den Einsatzstoffen entscheidende Modifikationen in der chemischen Zusammensetzung?
Die zerstörungsfreie Materialanalyse mittels Protonenstrahl an Luft gestattet es, dieser Frage nachzugehen. Vor der Restaurierung war die nicht glasierte Porzellanmasse an Defekten von 34 gesicherten Originalen aus dem 18. Jahrhundert für den Rossendorfer Protonenstrahl (5 MV Tandembeschleuniger) und das Arrangement verschiedener, simultan arbeitender Nachweisgeräte zugänglich.
Über den Untersuchungszeitraum von knapp 100 Jahren erweist sich die Porzellanzusammensetzung als erstaunlich stabil. Unikate von höchstem Wert sind in die Untersuchungen eingebunden, auch deren Glasuren und Malfarben.

Keywords: Porcelain; chemical analysis; Ion beam Analysis; PIXE; PIGE

  • Invited lecture (Conferences)
    DGZfP Arbeitskreis – 09. Dezember 2010, 09.12.2010, Residenzschloss Dresden, Deutschland
  • Invited lecture (Conferences)
    Das Arcanum von Meissener Porzellan: Beharrung oder Wandel?, 09.12.2010, Residenzschloss Dresden, Deutschland
  • Communication & Media Relations
    DGZfP Dach-Zeitschrift 06.01.2011
    1 Seiten

Publ.-Id: 14927

Tuning the shape and damage in ion-beam induced ripples on silicon

Biermanns, A.; Hanisch, A.; Grenzer, J.; Metzger, T. H.; Pietsch, U.

We investigate the influence of ion beam parameters on the ripple formation on Si(001) surfaces after bombardment with Xe+ ions of 25 keV kinetic energy using a scanning ion beam system. By combining grazing incidence x-ray diffraction, small angle scattering and x-ray reflectivity, we show that during ion irradiation with 70◦ off-normal angle of incidence, changing the size of the irradiated area leads to an increased number of defects at the interface towards crystalline material. At 65◦ angle of incidence, the ripple amplitude grows.

Keywords: ion beam erosion; x-ray diffraction

Publ.-Id: 14926

In-situ grazing incidence scattering investigations during magnetron sputtering deposition of FePt/Ag nanocomposite layers

Cantelli, V.; Grenzer, J.; Jeutter, N. M.; von Borany, J.

We report on an in-situ GISAXS study of the evolution of nanocomposite magnetic L10-FePt/Ag layers as a function of the Ag amount. Simultaneously, the formation of the hard ferromagnetic L10 phase has been detected by x-ray diffraction at grazing incidence. The methodology applied is a sequential magnetron sputtering deposition of FePt and Ag at 400°C on an a-SiO2 substrate: the deposition chamber is equipped with two Be windows to allow x-ray penetration and mounted on the six-circle goniometer of ROBL – BM 20 at the ESRF.

[1] V. Cantelli, J. von Borany, N.M. Jeutter, J. Grenzer, Adv. Eng. Mat. 11, 478 (2009).

Keywords: In-situ sputtering; X-ray scattering

  • Poster
    10th Biennial Conference on High Resolution X-Ray Diffraction and Imaging, 20.-23.09.2010, Warwick, England

Publ.-Id: 14925

X-ray scattering and diffraction from Xe-beam induced ripples in crystalline Si

Biermanns, A.; Hanisch, A.; Grenzer, J.; Facsko, S.; Metzger, T. H.; Pietsch, U.

In recent years, the creation of surface-nanostructures due to ion-beam sputtering has gained much interest due to the possibility to pattern large surface areas with tunable morphologies in a short time. One kind of those nanostructures are wave-like patterns (ripples) produced by an interplay between a roughening process caused by ion beam erosion (sputtering) of the surface and strengthening processes caused by surface diffusion [1]. In this contribution we report on investigations of patterned Si (001) surfaces after irradiation with Xe-ions using ion-energies up to 70keV. During the sputtering, an amorphous surface-layer is formed followed by an interface towards crystalline material, showing the same morphology as the surface. The structure and morphology of the amorphous layer and the amorphous-crystalline (a/c) interface were studied using synchrotron-radiation. Whereas the combination of grazing-incidence small angle scattering (GISAXS) and X-ray reflectivity can be used to study the surface morphology with large statistical sampling (Fig. 1), grazing incidence diffraction (GID) gives access to the buried crystalline material and probes both the morphology and the crystal structure of the a/c interface.
We found that for non-optimal sputter conditions, defects are created at the a/c interface, leading to a net expansion of the crystal along the ripples [2]. This inclusion of defects can be reduced if optimal sputter conditions are chosen, leading to an increasing ripple amplitude rather than the formation of defects.
[1] M. A. Makeev et al., Nucl. Instrum. Methods Phys. Res., Sect. B 197, 185 (2002)
[2] A. Biermanns et al., J. Appl. Phys. 104, 044312 (2008)

Keywords: ion beam erosion; X-ray diffraction and scattering

  • Lecture (Conference)
    Deutsche Tagung für Forschung mit Synchrotronstrahlung, Neutronen und Ionenstrahlen an Großgeräten (SNI2010), 24.-26.02.2010, Berlin, Deutschland

Publ.-Id: 14924

Growth of quantum dot crystals in amorphous matrix on rippled substrates

Buljan, M.; Grenzer, J.; Keller, A.; Radić, N.; Cornelius, T.; Metzger, T. H.; Holý, V.

The formation of quantum dot crystals by multilayer deposition has been reported and explained satisfactorily only in crystalline materials, so far. Here we demonstrate a method for the growth of quantum dot crystals in amorphous matrices. The ordering of the positions of quantum dots is induced by the deposition of a multilayer on a periodically rippled substrate at an elevated substrate temperature. During the deposition, the quantum dots self-arrange following the morphology of the substrate. The result is a formation of well ordered lattice of Ge quantum dots in amorphous silica matrix. We have investigated the ordering of the dots by grazing-incidence small-angle X-ray scattering and we found that the distance of the dots in the multilayer interfaces close to the rippled surface indeed equals the ripple period. However, in more distant interfaces the dot-dot distance approaches the value for non-rippled substrate and the dot ordering is slightly less pronounced. This finding confirms the beneficial influence of the rippled substrate on the ordering of quantum dots in an amorphous matrix.

Keywords: x-ray diffraction and scattering

  • Lecture (Conference)
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM), 21.-26.03.2010, Regensburg, Deutschland

Publ.-Id: 14923

In-situ grazing incidence scattering investigations during magnetron sputtering deposition of FePt/Ag thin films

Grenzer, J.; Cantelli, V.; Jeutter, N. M.; von Borany, J.

We report on an in-situ study on the evolution of granular magnetic L10 -FePt/Ag layers deposited by magnetron sputtering on an amorphous SiO2 substrate. Using synchrotron radiation we investigated the nanostructure growth during deposition as function of the Ag thickness by the simultaneous detection of the cluster growth and of the formation of the hard ferromagnetic L10 -phase applying grazing incidence small-angle X-ray scattering (GISAXS) and X-ray diffraction, respectively. FePt/Ag nanoparticle were prepared using a dual magnetron deposition chamber, equipped with two Be windows to allow X-ray penetration, that was mounted on the six-circle goniometer of the Rossendorf beam line (ROBL BM20) at the ESRF (European Synchrotron Radiation Facility). The possibility to tune X-ray beam energy, to reduce air scattering and absorption, together with the high brilliance of the synchrotron source had made it possible to obtain a reliable GISAXS signal and to control the cluster morphology the initial stage [1].

[1] V. Cantelli, J. von Borany, N.M. Jeutter, J. Grenzer, Adv. Eng.Mat. 11, 478 (2009).

Keywords: in-situ x-ray diffraction

  • Lecture (Conference)
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM), 21.-26.03.2010, Regensburg, Deutschland
  • Lecture (Conference)
    "Deutsche Tagung für Forschung mit Synchrotronstrahlung, Neutronen und Ionenstrahlen an Großgeräten“ (SNI2010), 24.-26.02.2010, Berlin, Deutschland

Publ.-Id: 14922

In-situ X-ray Scattering: a Tool to Investigate the Formation of Nanostructures

Grenzer, J.

Nowadays, the development of new materials is often associated with specific properties of functionalized nanostructures. X-ray investigations are a very important tool to find the link between the functional (magnetism, luminescence) and the corresponding structural properties (size, orientation etc.) that are generating this function and to explain the underlying physical processes. This knowledge makes it possible to design new materials with specific properties. We report on (in-situ) X-ray studies that are focused to ion-beam sputtering (IBS) processes creating nanostructures either by ion beam erosion or by sputter deposition processes.

Keywords: in-situ x-ray diffraction; nano structures

  • Invited lecture (Conferences)
    SLS Seminar, 19.02.2010, Villigen, Schweiz

Publ.-Id: 14921

X-ray Investigations on CoSi2 nano wires manufactured by focused ion beam synthesis

Grenzer, J.; Biermanns, A.; Akhmadaliev, C.; Bischoff, L.

Nano wires and chains of nano particles are of emerging interest in nanoelectronics, nano-optics and plasmonics as well as for their monolithic integration into microelectronic devices; CoSi2 is a promising material due to its CMOS-compatibility that shows metallic behavior with low resistivity and high thermal stability. It is well known that cobalt disilicide films can be formed in silicon by implanting Co in stoichiometric concentration and a subsequent annealing procedure. It has been shown that ion beam synthesis allows the fabrication of epitaxial buried or surface CoSi2 layers on silicon. Submicron patterns can be directly produced by a writing focused ion beam (FIB) cobalt implantation. The formation of continuous nano wire structures follows always the <110> direction [1].
We have studied the strain of the Si host lattice in the surrounding area of a single nanostructures depending on their crystallographic orientation using high resolution X-ray diffraction in combination with a highly focused (~1µm) X-ray beam at the beam line ID01 at the ESRF. The crystalline nano wire is embedded into the Si matrix and shows a tensile strain of about 1.4%. This feature can be only found if the beam focused on the nano wire itself. We will show that it is possible to investigate a single nano wire of a size of below 50 nm. The CoSi2 peak intensity is strongly modulated if we move from one wire to an other. Moreover the diffuse scattered intensity around the Si bulk reflection is increased and is getting even more enhanced between the wires (see Figure 1). A possible mechanism, the formation of stable dislocation loops, lying behind the formation of CoSi2 wires will be discussed.

[1] Ch. Akhmadaliev, B. Schmidt and L. Bischoff, Appl. Phys. Lett. 89, 223129 (2006); Ch. Akhmadaliev, L. Bischoff and B. Schmidt; Mat. Sci. & Eng., C26, 818 (2006).

Keywords: Nanostrukturen; focused x-ray beams

  • Lecture (Conference)
    Science with X-ray Nano-beams, 09.-12.02.2010, Grenoble, Frankreich
  • Poster
    10th Biennial Conference on High Resolution X-Ray Diffraction and Imaging, 20.09.2010, Warwick, England

Publ.-Id: 14920

Structural characterization of buried superconducting Ga rich films in Si

Fiedler, J.; Heera, V.; Skrotzki, R.; Herrmannsdörfer, T.; Voelskow, M.; Mücklich, A.; Schmidt, B.; Skorupa, W.; Gobsch, G.; Helm, M.; Wosnitza, J.

Recently it has been shown that heavily p-doped group-IV semiconductors such as diamond, silicon and germanium can become superconducting at low temperatures. Here, we present a study of Ga-implanted Si that becomes superconducting due to precipitation after annealing. Ion implantation allows introducing a high Ga dose (4E16cm-2) in Si that leads to peak concentrations far beyond the solid solubility limit. Rapid thermal annealing (RTA) causes redistribution of the Ga and re-crystallization of the amorphous implanted Si layer. After annealing at temperatures up to 850°C the implanted layers are polycrystalline and contain Ga-rich precipitates. Structural investigations by means of RBS/C measurements and TEM demonstrate a high density of precipitates at the interface of a protective SiO2 layer and the silicon substrate. At optimized annealing conditions (600-700°C) such samples become superconducting with critical temperatures up to 7 K [1].
[1] Skrotzki R. et al. , Appl. Phys. Lett. 97 (2010) 192505

  • Lecture (Conference)
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2011, 13.-18.03.2011, Dresden, Deutschland

Publ.-Id: 14919

Acceptor deactivation in individual silicon nanowires: From thick to ultrathin

Ou, X.; Geyer, N.; Kögler, R.; Werner, P.; Skorupa, W.

An individual free standing spicular silicon nanowire (NW) fabricated by Ag assisted chemical etching was doped to 1018 dopants/cm-3 by boron ion implantation and annealed at 1000°C. The presented NW is around 900 nm in length and consists of two connected parts, a thicker basis with a diameter of about hundred nm and of a thin top part with an average diameter as thin as 16 nm. This NW was analyzed by scanning spreading resistance microscopy (SSRM). The paper demonstrates that SSRM is capable to spatially and electrically resolve such an ultrathin NW with the thinnest part down to 12 nm in diameter. Experimental results show the dependence of the acceptor deactivation on the diameter of the NW cross section. The doping efficiency of the NW dramatically decreases as the diameter is below 25 nm. Deactivation mechanisms are discussed.

Keywords: nanowire; SSRM; doping; implantation

Publ.-Id: 14918

Three-dimensional carrier profiling of individual Si nanowires by scanning spreading resistance microscopy

Ou, X.; Das Kanungo, P.; Kögler, R.; Werner, P.; Gösele, U.; Skorupa, W.; Wang, X.

Individual silicon nanowires (Si NWs) grown by molecular beam epitaxy and in-situ doped with boron were investigated by scanning spreading resistance microscopy (SSRM), a technique based on the conductive atomic force microscopy. The carrier profiles of the NWs were derived from the measured spreading resistance values and calibrated with the known carrier concentration of the underlying epi-layer. The three-dimensional SSRM profile of a NW was obtained by measuring the NW cross sections at different depths along the radial direction. Scanning the same NW with a controlled force on the SSRM tip can abrade material from the cross-sectional surface and the tip moves deeper into the volume of the NW after each image scan. Repeated stripping of the material from the NW results in a "thinning" of the remaining NW segment and a corresponding increase of its resistance which can be addressed by an appropriate data correction. The achieved three-dimensional carrier profile reveals a multi-shell structure of the carrier distribution across the NW diameter which consists of a lower doped core region, a higher doped shell region and a carrier depleted sub-surface region.

Keywords: nanowire; doping; three-dimensional; carrier profile; SSRM

Publ.-Id: 14917

Aqueous coordination chemistry and photochemistry of uranium(VI)

Tsushima, S.

We have been exploring aqueous coordination chemistry of uranium(VI) by combining different spectroscopic techniques and computational chemistry, i.e. EXAFS and FTIR spectroscopy and DFT calculations. In uranyl(VI) sulfate system we found sulfate may bound to uranium in both unidentate and bidentate modes.
Whether ligand bind to uranium in unidentate or bidentate might seem to be irrelevant. However the EXAFS and DFT study on uranyl(VI) oxalate system proved that this is entirely not the case and the mode of ligand coordination plays decisive role to the photoreactivity of uranyl(VI) oxalate. Only the uranyl(VI) oxalate having unidentate coordination was found to be photoreactive. The result is consistent with recent UV-Vis absorption spectroscopic study by Görller-Walrand and Servaes.
The validity of the use of DFT on uranium complexes is often disputed. Sophisticated theory such as CASPT2 is proved to be requisite for getting accurate excitation energies of uranyl(VI) complexes. However, DFT calculations were also proved to provide accurate geometries as long as the ground states and the lowest lying triplet states are concerned. Photoluminescence characters of uranium(VI) were also found to be well reproduced by cost-effective DFT calculations.

  • Invited lecture (Conferences)
    Seminar at Katholieke Universiteit Leuven, 14.01.2011, Leuven, Belgium

Publ.-Id: 14916

Improved vortex nucleation in truncated soft magnetic cones

Martin, N.; Mönch, I.; Schäfer, R.; Schultz, L.; McCord, J.

Magnetic vortex structures are promising for data storage applications due to the possibility to switch vortex chirality and core orientation independently. Subsequently two bits per element can be stored, which is favorable for high data storage densities. The magnetic vortex state of soft-magnetic nano-disks is stable over a large range of thickness and radii. Nevertheless the formation of a vortex state has to overcome an energy barrier in order to nucleate the vortex at the element’s edge. In the presented work the influence of a large edge tilt on the behavior of vortex nucleation is studied experimentally and by numerical simulations. Single elements and arrays of closely packed elements with a diameter of approx. 300 nm were fabricated by means of nanosphere lithography, by which an element edge tilt of 45° could be achieved. From the simulations it was found that with decreasing edge tilt from 90° (cylindrical dot) to 45° the probability to nucleate a vortex during magnetization reversal increases. For cylindrical elements at the onset of vortex nucleation the out of plane component of magnetization near the edge has opposite signs at the top and bottom of the element. For truncated cones, however, the magnetization points only in one direction and favors the nucleation of a magnetic vortex. Thus, the vortex formation in smaller elements is facilitated by engineering the shape of the dots. For narrowly packed structures it is shown that despite of strong dipolar interactions the magnetization reversal involves a vortex nucleation and annihilation process.

Keywords: magnetic vortex; nanosphere lithography; vortex nucleation

  • Lecture (Conference)
    55th Annual Conference on Magnetism & Magnetic Materials, 14.-18.11.2010, Atlanta, Georgia, USA

Publ.-Id: 14915

Competition of dipolar interactions and lateral exchange spring effect in NiFe elements

Martin, N.; Strache, T.; Mönch, I.; Schultz, L.; Fassbender, J.; McCord, J.

Conventional exchange spring systems consist of directly exchange coupled hard and soft magnetic layers, which characteristic behaviour is related to an interlayer domain wall that is formed during reversal of magnetization. Recently, a similar exchange spring effect was achieved in lateral hard/soft magnetic stripe structures, which were fabricated by means of ion implantation of single layer films [1]. This technique offers the possibility to directly investigate the changes of magnetization in and between the hard/soft phases.

In the presented work, lateral exchange spring structures were prepared on patterned samples to investigate the interplay between structuring and additional dipolar fields. In contrast to extended films, the lateral exchange spring effect only occurs for a higher difference in saturation magnetization Ms between the two magnetic phases. The measured collective magnetization reversal of hard and soft phase is attributed to strong dipolar fields at the element edges that cause a hysteresis behaviour which is comparable to that of a magnetic homogeneous square element. The exchange spring behaviour, occurring with increasing difference in Ms between the two phases is related to an increase in effective shape anisotropy in the high Ms stripes. The magnetization reversal approaches an individual switching of the stripes, with the low Ms stripes switching first. This results in an antiparallel alignment of magnetization of the individual stripes with lateral domain walls in between, which configuration is stabilized by the inter-stripe flux closure. The resulting two-step reversal is modelled, taking the demagnetization and domain wall energy terms into account.

Keywords: ion implantation; thin films; Permalloy; magneto-optical imaging

  • Lecture (Conference)
    55th Annual Conference on Magnetism & Magnetic Materials, 14.-18.11.2010, Atlanta, Georgia, USA

Publ.-Id: 14914

Herstellung und Charakterisierung magnetisch heterogener, dünner Schichten

Martin, N.; Schäfer, R.; Fassbender, J.; Schultz, L.; McCord, J.

In der Präsentation werden Methoden vorgestellt, die das Hystereseverhalten dünner magnetischer Schichten gezielt beeinflussen. Insbesondere geht es dabei um lokale Eigenschaftsveränderungen durch Strukturierung mittels lithografischer Techniken und anschließender Ionenimplantation. Der Einfluss der Strukturierung auf das Ummagnetisierungsverhalten der gesamten Schicht wird anhand von magneto-optisch gemessenen Hysteresekurven und Domänenaufnahmen dargestellt und mittels geeigneter Modelle erklärt.

Keywords: thin films; magnetic microstructure; magneto-optical imaging

  • Invited lecture (Conferences)
    VAC-Kolloqium, 09.-10.12.2010, Hanau, Deutschland

Publ.-Id: 14913

Lipid Protein Interactions underlie an Evolutionary Conserved Proton Switch in a Transmembrane Peptide derived from Helix-3 of Class-1 GPCRs

Eichler, S.; Madathil, S.; Fahmy, K.

The visual photoreceptor rhodopsin is a proto¬typical class-I (rhodopsin-like) G protein-coupled receptor (GPCR). Photoisomerization of the covalently bound ligand 11-cis-retinal leads to restructuring of the cytosolic face of rhodopsin. The ensuing protonation of Glu-134 in the class-conserved D(E)RY motif at the C-terminal end of trans¬membrane helix-3 promotes the formation of the G-protein-activating state. Glu-134 acts as an autonomous proton switch also in synthetic transmembrane peptides, where lipid protein interactions couple protonation to helix extension and hydrophobic burial of the side chain resulting in an elevated side chain pKa. The implied change in "helical-end-solvation" by interfacial water has been investigated by pulsed hydration experiments. we observed within several seconds (i) lipid/peptide conformational changes by time-resolved Fourier transform infrared (FTIR) difference spectroscopy and (ii) water penetration into the lipidic phase using the simultaneous recording of the fluorescence of a tryptophan (trp) inserted 3 amino acids N-terminally from the ERY motif. Cross-correlation of both monitors reveals the linkage of lipid ester carbonyl hydration to helical end unwinding followed by water penetration into the bilayer one helical turn N-terminally of the conserved ERY motif. Trp-emission further shows that the indole ring is more water-exposed in the ionized than in the protonated state of the adjacent ERY motif evidencing a shift of the lipid/water phase boundary relative to the transmembrane peptide upon proton uptake. The proton-dependent reorganisation of the lipid/peptide/water microdomain N-terminally of the ERY motif is further supported by a pH-sensitive Förster-resonance-energy-transfer from the peptidic trp to 5-(dimethylamino)naphthalene-1-sulfonyl (DANSYL)-labelled lipids. In conclusion, the data reveal a conserved hydration site at the membrane water interface of GPCRs that attracts or repels water in response to protonation, thereby, linking proton uptake to protein conformation and rearrangement of the lipid/peptide/water phase boundary independently of helix-helix interactions.

Keywords: G protein; infrared; spectroscopy; peptide; rhodopsin

  • Contribution to proceedings
    Annual Meeting of the German Biophysical Society 2009, 03.-06.10.2009, Bochum, Deutschland
    Book of Abstracts

Publ.-Id: 14912

Zum Komplexbildungsverhalten ausgewählter Actiniden (U, Np, Cm) mit mikrobiellen Bioliganden

Glorius, M.

Diese Arbeit war ein eigenständiger Teil eines Projektes, welches sich mit der Aufklärung des Einflusses von Mikroorganismen auf die Ausbreitung von Actiniden bei einer Freisetzung dieser aus dem Endlager beschäftigt. Dabei wurde der Einfluss von mikrobiell produzierten Substanzen auf die Mobilisierung ausgewählter Actiniden untersucht. Die in diesem Projekt untersuchten mikrobiell produzierten Substanzen, sogenannte Bioliganden, wurden von Bakterien des Genus Pseudomonas unter speziellen Bedingungen produziert. Die von den Pseudomonaden freigesetzten Bioliganden, hier Siderophore vom Pyoverdin-Typ, haben ein hohes Potential, Metalle, insbesondere Eisen(III), zu komplexieren und so zu transportieren. Es wurde untersucht, in welcher Weise und unter welchen Bedingungen diese Bioliganden in der Lage sind, auch radioaktive Schadstoffe zu komplexieren und damit zu mobilisieren. Für die Untersuchungen wurden die α-strahlenden Actiniden Uran, Curium und Neptunium ausgewählt, weil diese auf Grund ihrer Langlebigkeit und Radiotoxizität von besonderem Interesse sind.
Diese Arbeit beschäftigte sich mit der Wechselwirkung der Actiniden U(VI), Np(V) und Cm(III) mit Modellliganden, die die Funktionalitäten der Pyoverdine simulieren. Für die Metallbindung der Pyoverdine sind die Katecholgruppe des Chromophors und die funktionellen Gruppen der Peptidkette (Hydroxamsäuregruppen und α-Hydroxysäurereste) verantwortlich. Für die Simulation der Hydroxamsäuregruppen kamen dabei die Monohydroxamate Salicylhydroxamsäure (SHA) und Benzohydroxamsäure (BHA) und das natürliche Trihydroxamat Desferrioxamin B (DFO) zum Einsatz und für die Katecholgruppe das 6-Hydroxychinolin (6HQ) und 2,3-Dihydroxynaphthalin (NAP). Als Vergleichsligand wurde außerdem Benzoesäure (BA) untersucht. Für die Bestimmung der Stabilitätskonstanten zur Einschätzung der Stärke der gebildeten Komplexe, die Aufklärung der Struktur der Actinid-Ligand-Verbindungen und die Verfolgung der Änderung der Speziation der Actiniden vor und nach der Wechselwirkung mit den Modellliganden kamen verschiedene spektroskopische Verfahren wie Absorptionsspektroskopie, Laserfluoreszenzspektroskopie,
Röntgenabsorptionsspektroskopie und Schwingungsspektroskopie zum Einsatz. Außerdem wurden erstmals theoretische Modellierungen zur Aufklärung der Struktur der Actinid-Modellligand-Komplexe durchgeführt.
Die Ziele dieser Arbeit waren also die spektroskopische Charakterisierung und Bestimmung der Speziation und Komplexbildungskonstanten sowohl der ausgewählten Modellliganden als auch der gebildeten Actinid-Modellligand-Komplexe, die Aufklärung möglicher Strukturen der Komplexe sowie ein Vergleich der Ergebnisse mit denen der Pyoverdine.
Der Vergleich der Stabilitätskonstanten der untersuchten Liganden mit den drei Actiniden U(VI), Cm(III) und Np(V) ergab im Wesentlichen folgende Reihenfolge der Komplexstärke:
PYO ≥ DFO > NAP > 6HQ > SHA ≥ BHA > BA.
Benzoesäure besitzt als einziger Ligand eine Carboxylfunktionalität und weist mit 103 die geringste Stabilitätskonstante auf. Die beiden Monohydroxamate SHA und BHA bilden mit allen drei Actiniden ähnlich starke 1:1-Komplexe. Bei den 1:2-Komplexen besitzt SHA mit Cm(III) und Np(V) etwas höhere Stabilitätskonstanten als BHA, wahrscheinlich verursacht durch einen stabilisierenden Einfluss der zusätzlichen phenolischen OH-Gruppe. Dieser Trend wurde auch in den theoretischen Modellierungen für die U(VI)-Komplexe beobachtet. Die natürlichen Siderophore DFO und PYO bilden die stärksten Komplexe mit den Actiniden (Stabilitätskonstanten von 1012 bis 1034). Dies liegt in der Struktur und der hohen Anzahl an funktionellen Gruppen begründet; DFO besitzt drei Hydroxamatgruppen, das Pyoverdinmolekül neben den Hydroxamatgruppen noch die Katecholgruppen der Chromophorfunktionalität. Die Modellliganden für die Chromophorfunktionalität, NAP und 6HQ, bilden stärkere Komplexe als die Monohydroxamate SHA und BHA, aber schwächere Komplexe als DFO und PYO. Daraus lässt sich schlussfolgern, dass die Chromophorfunktionalität eine wichtige Rolle bei der Anbindung der Actiniden an die Pyoverdine spielt.
Der Vergleich der Stabilitätskonstanten der Komplexe der Liganden SHA, BHA und 6HQ mit den drei untersuchten Actiniden U(VI), Cm(III) und Np(V) untereinander zeigte, dass die Stärke der Komplexe von U(VI) über Cm(III) zu Np(V) abnimmt. Der Grund dafür liegt in den unterschiedlichen Ladungsdichten der Actinidionen. Während das UO22+-Ion mit einer Koordinationszahl von 5 und einem Ionenradius von ~ 0.6 eine effektive Ladung von + 3.3 besitzt, hat das Cm3+-Ion eine effektive Ladung von + 2.6 und das NpO2+-Ion von + 2.3. Damit besitzt das NpO2+-Ion die geringste Ladungsdichte der untersuchten Actinidionen und bildet damit auch die schwächsten Komplexe mit den niedrigsten Stabilitätskonstanten. Die Stärke der Komplexe der Liganden NAP, DFO und PYO nimmt von Cm(III) über U(VI) zu Np(V) ab. Obwohl Cm(III) eine geringere effektive Ladung als U(VI) hat, bildet es stärkere Komplexe als U(VI). Eventuell sind dafür strukturelle Behinderungen der Koordination durch die lineare O=U=O Einheit verantwortlich.
In einer Kooperation mit dem Institut für Theoretische Chemie der Universität zu Köln wurden für die 1:1- und 1:2-Komplexe der wässrigen U(VI)-SHA-, U(VI)-BHA- und U(VI)-BA-Systeme erstmals theoretische Modellierungen durchgeführt. Dabei wurden die Strukturen der Komplexe sowohl in der Gasphase als auch unter Berücksichtigung der Solvatation optimiert und die relativen Stabilitäten und Anregungsspektren berechnet. Die mit DFT berechneten Bindungsenergien bestätigen die experimentell anhand der Stabilitätskonstanten log β ermittelte Reihenfolge der Komplexstabilitäten (SHA ≥ BHA > BA). Außerdem zeigen die höheren Bindungsenergien der 1:2-Komplexe, dass diese stabiler sind als die 1:1-Komplexe. Dies lässt sich auch anhand der experimentell ermittelten Stabilitätskonstanten nachweisen. Für den 1:1-Komplex des U(VI)-SHA-Systems konnte mit Hilfe der theoretischen Modellierung die strukturelle Anbindung des Uranylions an die Hydroxamsäuregruppe aufgeklärt werden. Der Vergleich der berechneten Strukturen, Bindungsenergien, Bindungslängen und Anregungsspektren der beiden möglichen Anbindungsmodi [O,O] und [N,O’] zeigte deutlich, dass das Uranylion bevorzugt über die beiden Sauerstoffatome der Hydroxamsäuregruppe, also den [O,O]-Modus, gebunden wird. Die Methode der DFT konnte also dazu beitragen, Defizite in der experimentellen Aufklärung der Komplexstruktur im Fall des U(VI)-SHA-Systems zu beheben.
Die Modellliganden und deren Komplexe mit U(VI), Cm(III) und Np(V) wurden zum größten Teil erstmals spektroskopisch charakterisiert sowie deren bisher weitgehend unbekannten Stabilitätskonstanten bestimmt. Außerdem konnte die Struktur der U(VI)-Hydroxamat-Komplexe mit Hilfe der ATR-FTIR-Spektroskopie und der theoretischen Modellierung aufgeklärt werden. Im Vergleich der Ergebnisse der Modellliganden mit denen der Pyoverdine konnte festgestellt werden, dass die Katecholfunktionalität der Pyoverdine eine große Rolle bei der Komplexierung mit den Actiniden spielen wird. Weiterhin ließen sich aus den Ergebnissen Schlussfolgerungen zur Stärke der gebildeten Actinid-Modellligand- und Actinid-Pyoverdin-Komplexe ziehen. Die Pyoverdine bildeten mit U(VI) Komplexe mit Stabilitätskonstanten bis 1030, mit Cm(III) bis 1032 und mit Np(V) bis 1020. Die wichtigsten, in höheren Konzentrationen vorkommenden anorganischen Komplexbildner in natürlichen Wässern sind das Hydroxidion OH- sowie das Carbonation CO32-. Diese besitzen eine hohe Komplexierungsfähigkeit und bilden mit den drei Actiniden U(VI), Cm(III) und Np(V) Komplexe mit Stabilitätskonstanten von 102 bis 1020. Der Vergleich der Konstanten von OH und CO32- mit denen der organischen, mikrobiellen Pyoverdin-Liganden zeigt, dass die Pyoverdine ähnlich starke bzw. teilweise stärkere Komplexe mit den Actiniden bilden als die anorganischen Komplexbildner. Daraus lässt sich ableiten, dass die Pyoverdine selbst in niedrigeren Konzentrationen ein hohes Potential besitzen, Actiniden in natürlichen Wässern zu binden und damit zu transportieren. Die untersuchten Bioliganden sind also in der Lage, bei Anwesenheit in der Natur in bestimmten Konzentrationen im Grundwasser Actiniden, z.B. durch Herauslösen aus Festphasen, zu mobilisieren. Damit können solche Bioliganden das Verhalten der Actiniden in der Umwelt entscheidend beeinflussen. Die Ergebnisse dieser Arbeit tragen dazu bei, den Einfluss der mikrobiellen Liganden auf die Mobilisierung und Ausbreitung der Actiniden besser einschätzen zu können. Damit können die Ergebnisse zur Quantifizierung des Mobilisierungseffekts der Actiniden durch freigesetzte Bioliganden im Nahfeld genutzt werden.

Keywords: Actiniden; Hydroxamsäure; Pyoverdin; Spektroskopie; Komplexierung; actinides; hydroxamic acids; pyoverdin; spectroscopy; complex formation

  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2009
    152 Seiten
    ISSN: 1437-322X


Publ.-Id: 14911

Fluence dependence of small-angle neutron scattering in neutron-irradiated Fe-Cr

Bergner, F.; Heintze, C.; Ulbricht, A.

Four Fe-Cr alloys with chromium concentrations ranging from 2.5 to 12.5 at% in three irradiation conditions (0.06, 0.6 and 1.5 displacements per atom) were investigated by means of small-angle neutron scattering. For Fe-12.5 at% Cr, alpha‘–phase particles were unambigiously identified. The size distribution of these particles was estimated and the kinetics of formation were compared with a rate theory model which allows to extrapolate the evolution of the nanostructure to still lower dpa-levels. For 2.5, 5 and 9 at% Cr, the results of the SANS investigation are more complex and the input from other methods, such as APT and TEM, is needed in order to explore the irradiation effects in detail.

Keywords: Fe-Cr alloy; neutron irradiation; SANS

  • Lecture (Conference)
    17th Workshop on Multiscale Modelling and Basic Experiments of Iron-Chromium Alloys for Nuclear Applications, 03.-04.11.2010, Stockholm, Schweden

Publ.-Id: 14910

Investigation of wear-out phenomena in Eu-implanted metal-oxide semiconductor light emitting devices

Lehmann, J.; Rebohle, L.; Kanjilal, A.; Voelskow, M.; Skorupa, W.; Helm, M.

The anomalous wear-out phenomenon of Eu-implanted MOS based light emitting devices (MOSLED’s) was investigated intensively by many different techniques, on samples exposed to different annealing temperatures and times. It will be shown, that in contrast to other rare earth elements the EL intensity of Eu-implanted SiO2 layers can rise under constant current injection before the known EL quenching will start. Under certain circumstances this rise may amount up to two orders of magnitude. The EL behaviour will be correlated with the microstructural and electrical properties of the devices. Transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS) were used to trace the growth of Eu / Eu oxide clusters and the diffusion of Eu to the interfaces of the gate oxide layer, which are induced by the annealing process. Current-voltage (I(V)) characteristics, EL decay times (τ) and the evolution of the voltage under constant current injection (Vcc) as well as evolution of the EL spectrum with injected charge (EL(Qinj)) were studied with respect to charging and trapping phenomena in the oxide layer in order to reveal details of the EL excitation mechanism. A qualitative model for the anomalous wear-out phenomenon is proposed.

Keywords: Electroluminescence; rare earth; charge trapping; ion implantation; Europium; Si-based light emission; Electroluminescence wear-out; quenching

  • Poster
    Nanofair 2010 - 8th International Nanotechnology Symposium, 06.-07.07.2010, Dresden, Deutschland

Publ.-Id: 14909

Wear-out phenomena in Si-based light emitting devices with ion beam implanted europium

Lehmann, J.; Rebohle, L.; Kanjilal, A.; Voelskow, M.; Helm, M.; Skorupa, W.

Recently, it was shown that it is possible to obtain efficient electroluminescence (EL) from UV to IR by implanting lanthanides into the oxide layer of metal-oxide semiconductor (MOS) structures [1]. The best efficiencies could be achieved on a Tb-implanted light emitter with an external quantum efficiency of 16% and a corresponding power efficiency of 0.3%. With these electrically driven Si-based light emitting devices, also called MOSLEDs, it is possible to build an integrated biosensor for the detection of organic molecules like estrogene, e.g. see Ref. [2]. For this purpose, the intensity and stability of the emitted EL are the most critical properties of the light emitters. Usually, lanthanide implanted MOSLEDs show a quenching of the EL-signal with time, which can be attributed to charge trapping in the oxide layer. In contrast to this normal wear-out phenomenon, Eu-implanted MOSLEDs can show a rise in the EL-signal during the operation time of the device [3]. Due to this anomalous wear-out phenomenon, Eu-implanted MOSLEDs offer the possibility to build a device with an extremely stable EL if the occurring processes can be better understood. For this reason, an intensive investigation was performed on Eu-implanted MOSLEDs exposed to different annealing temperatures and times. Transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS) were used to trace the growth of Eu / Eu oxide clusters and the diffusion of Eu to the interfaces of the gate oxide layer, respectively. Both of them were induced by the annealing process. Resonant nuclear reaction analysis (rNRA) was used to measure the hydrogen depth profile in the dielectric. Current-voltage (I-V) characteristics, the EL decay times (τ) and the evolution of the voltage under constant current injection (Vcc) as well as the evolution of the EL spectrum with injected charge (EL-Qinj) were studied with respect to charging and trapping phenomena in the oxide layer in order to reveal details in the occurring wear-out mechanism of the EL. It will be shown, that for certain annealing conditions the EL intensity of Eu-implanted SiO2 layers can increase during constant current injection which is followed by the known EL quenching. In extreme cases this rise may amount up to two orders of magnitude. The EL behaviour will be correlated with the microstructural and electrical properties of the devices. A qualitative model for the anomalous wear-out phenomenon is proposed.

Keywords: Electroluminescence; rare earth; charge trapping; ion implantation; Europium; Si-based light emission; Electroluminescence wear-out; quenching

  • Lecture (Conference)
    Ion Implantation and Other Applications of Ions and Electrons (ION 2010), 14.-17.06.2010, Kazimierz Dolny, Poland

Publ.-Id: 14908

Influence of Al concentration on structure and electrical properties of polycrystalline and epitaxial Al-doped ZnO films grown by reactive pulsed magnetron sputtering

Cornelius, S.; Vinnichenko, M.; Kolitsch, A.; Möller, W.

Al-doped ZnO (AZO) films which combine maximum carrier mobility (μe), moderate free electron densities (Ne) and high surface roughness are of special interest for application as transparent front electrode in thin film solar cells. They posses high transmission in the near infrared region, close to the bandgap energy of absorber materials like Si (Eg =1.11 eV), and enable a superior light trapping behaviour. A key to tailor AZO film properties is understanding the mechanisms and effects of the Al-dopant incorporation into the ZnO matrix. The present work focuses on investigation of the influence of Al concentration on the electrical properties of AZO and on establishing performance limits with respect to carrier mobility and resistivity (ρ). Polycrystalline and epitaxial AZO films are grown on fused silica and c-axis oriented sapphire substrates, respectively, by reactive pulsed magnetron sputtering using several sets of Zn/Al alloy targets with an Al concentration (cAl) between 0.7 and 8.7 at%. A systematic variation of process parameters such as substrate temperature (Ts) and oxygen partial pressure results in polycrystalline films with μe>45 cm2V-1s-1 and ρ<2.3x10-4 Ωcm at optimum conditions, whereas μe~55 cm2V-1s-1 could be obtained for epitaxial films. It is observed that cAl has a strong influence on the optimum value of Ts, the maximum μe and Ne values and also on film structure and surface roughness. Extensive XRD investigations reveal that Al incorporation in the ZnO matrix has a detrimental effect on in-plane orientation and texture of epitaxial films, even for the lowest cAl used. Furthermore RBS and ERDA confirm a considerable Al enrichment in the films which correlates with deterioration of μe, when Ts is increased above its optimum value. The observed dependence of carrier mobility on Ne in ZnO:Al is discussed in the framework of ionized impurity scattering and clustering as well as grain boundary limited transport which predicts a fundamental physical limit of μe.

Keywords: reactive sputtering; zinc oxide; AZO; mobility; density; resistivity; RBS; ERDA; transparent conductive oxides; ionized impurity scattering

  • Poster
    Twelfth International Conference on Plasma Surface Engineering (PSE 2010), 13.-17.09.2010, Garmisch Partenkirchen, Deutschland

Publ.-Id: 14907

A Compton Imager for in-vivo Dosimetry of Proton Beams - A Design Study

Kormoll, T.; Fiedler, F.; Schöne, S.; Wüstemann, J.; Zuber, K.; Enghardt, W.

In-beam SPECT during therapeutic proton beam irradiation is a novel method for three dimensional in-vivo dose verification. For this purpose a Compton camera design is evaluated with respect to the special requirements and conditions that arise from this application. Different concepts are studied by means of simulation concerning the angular resolution and the efficiency. It was found that a cadmium zinc telluride system can perform sufficiently well. For further evaluation the construction of a semiconductor scintillator hybrid system is under way.

Keywords: in-beam SPECT; in vivo dosimetry; ion beam therapy; proton therapy

Publ.-Id: 14906

Lateral straggling and its influence on lateral diffusion in implantation with a focused ion beam

Batabyal, R.; Roy, A.; Roy, S.; Bischoff, L.; Dev, B. N.

Ion implantation plays an important role in semiconductor industry. Different kinds of ions are introduced into semiconductors by implantation in order to form n-type and p-type semiconductors. Nanoscale doped structures are required to be fabricated, usually by a focused ion beam (FIB), for applications in nanoelectronics. However, how closely these structures could be fabricated for an effective device performance would be determined by the limitation imposed by the lateral diffusion of the implanted species. For ion implantation, lateral straggling of the ion beam also produces defects surrounding the implanted region. This would produce radiation enhanced lateral diffusion. It is important to understand the lateral diffusion in FIB-fabricated nanostructures. We present a method based on photoemission electron microscopy (PEEM) for the determination of lateral diffusion coefficient in nanostructures. As an example, we present the investigation of lateral diffusion of Ga in FIB-fabricated structures of Ga-implanted n-type Si. For a set of parallel Ga implanted stripes, the diffusion profiles at the end of the stripes along their length obey the standard diffusion equation and the diffusion coefficient is extracted. However, across the stripes the diffusion profile is more complex, due to the presence of defects due to lateral straggling in the neighbouring stripes.

Keywords: photoemission; focused ion beam; diffusion in nanostructures

  • Lecture (Conference)
    24th International Conference on ATOMIC COLLISIONS IN SOLIDS, ICACS-24, 18.-23.07.2010, Krakow, Poland

Publ.-Id: 14905

Spectroscopic studies of arsenic retention onto biotite

Chakraborty, S.; Bardelli, F.; Mullet, M.; Greneche, J.-M.; Varma, S.; Ehrhardt, J.-J.; Banerjee, D.; Charlet, L.

Biotite is a constituent Fe-bearing mineral of Delta subsoils in India and Bangladesh and has been hypothesized as a primary source of arsenic (As). The adsorption behavior of As onto structural Fe(II,III)-bearing biotite fractions (<50 µm) was investigated in the pH range 4-8 under a CO2-free, anoxic condition (O2 <1 ppmv) using X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). The batch experiments indicate that As adsorption is strongly pH dependent and As(V) adsorbs more efficiently than As(III). X-ray Absorption Near-Edge Structure (XANES) spectra show no oxidation or reduction of As by biotite after 3 days reaction with As(III) or As(V) solutions. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic results suggest that As(III) forms bidentate mononuclear edge-sharing (2E) and bidentate binuclear corner-sharing (2C) surface complexes at pH 7.7 as indicated by the average As-Fe bond distances at 3.00 ± 0.02 Å and 3.37 ± 0.03 Å respectively. The surface speciation retrieved by XPS does not provide any evidence of reduction of As(V) on biotite after 30 days further confirming the thermodynamic prediction and the XANES results. This study has therefore significant environmental implications for As contaminated areas, where biotite retards the release of As into reducing groundwater. The changes in soil redox condition and weathering of biotite may likely contribute to the occurrence of high As in groundwater.

Keywords: Arsenic; adsorption; biotite; surface complex; reduction

  • Chemical Geology 281(2011), 83-92

Publ.-Id: 14904

On the "yl" bond weakening in uranyl(VI) coordination complexes

Tsushima, S.

The U–Oyl triple bonds in the UO22+ aquo ion are known to be weakened by replacing the first shell water with organic or inorganic ligands. Weakening of the U–Oyl bond may enhance the reactivity of “yl” oxygens and uranyl(VI) cation–cation interactions. Density functional theory calculations as well as previously published vibrational spectroscopic data have been used to study the origin of the U–Oyl bond weakening in uranyl(VI) coordination complexes. Natural population analyses (NPA) revealed that the electron localization on the Oyl 2p orbital is a direct measure of the U–Oyl bond weakening, indicating that the bond weakening is correlated to the weakening of the U–Oyl covalent bond and not that of the ionic bond. The Mulliken analysis gives poor results for uranium to ligand electron partitioning and is thus unreliable. Further analyses of molecular orbitals near the highest occupied molecular orbital (HOMO) show that both the sigma and pai donating abilities of the ligands may account for the U–Oyl bond weakening. The mechanism of the bond weakening varies with coordinating ligand so that each case needs to be examined independently.

Publ.-Id: 14903

Influence of humic acids on the actinide migration in the environment: Suitable humic acid model substances and their application in studies with uranium – a review

Sachs, S.; Bernhard, G.

Humic acids (HA) can influence the speciation of metal ions, e.g., actinide ions, and thus their migration in the environment. Therefore, knowledge of the impact of HA on the actinide migration is required to assess their transport in natural systems. However, due to the complex and heterogeneous nature of HA, there are a lot of difficulties in the thermodynamic description of their geochemical interaction behavior. A more basic understanding of the interaction processes of HA can be obtained by investigations applying HA model substances with more defined and specific properties. This work gives a review of selected types of HA model substances (HA-alike melanoidins, synthetic HA with pronounced redox functionality, modified HA with blocked phenolic/acidic OH groups, synthetic humic substance-clay-associates), their synthesis, isotopic labeling, and characterization in comparison to isolated natural HA. Examples for their application in various geochemical studies, such as complexation, redox, sorption and migration studies with uranium as representative for actinides are presented.

Keywords: Humic acids; model substances; synthesis; characterization; labeling; modification; application; uranium; actinides

Publ.-Id: 14902

On the use of different analytical solutions for recalculation of the YALINA-Booster experiment SC3A

Merk, B.; Glivici-Cotruţă, V.; Weiß, F.-P.

The SC3A experiment in the YALINA-Booster facility in Belarus is described and investigated. For this investigation the very special configuration of YALINA-Booster core, consisting of a fast and a thermal zone, decoupled with a neutron ‘valve’ is analyzed in detail based on a full HELIOS model for the calculations. The two region design causes unexpected results in the experiments The special problems for the analysis of the experiments are shown. For an improved representation, a recently developed two group analytical solution for the time dependent diffusion equation obtained by Green’s function method without separation of space and time is used. To model the streaming of neutrons from the thermal area into the fast area an analytical solution for the space-time dependent neutron flux with two sources has been developed from the available Green’s functions for two groups. The space-time dependent thermal and fast neutron flux distributions are discussed. The new analytical solution shows very good agreement in the comparison with the experimental results, even for the unexpected behavior at the outermost detector. Thus analytical solutions without separation of space and time are a very promising tool to develop a new method for the analysis of ADS experiments

Keywords: Yalina; experimental analysis; Green’s function; Two group diffusion equation

Publ.-Id: 14901

Constraining the S factor of 15N(p,g)16O at Astrophysical Energies

Leblanc, P. J.; Imbriani, G.; Goerres, J.; Junker, M.; Azuma, R.; Beard, M.; Bemmerer, D.; Best, A.; Broggini, C.; Caciolli, A.; Corvisiero, P.; Costantini, H.; Couder, M.; Deboer, R.; Elekes, Z.; Falahat, S.; Formicola, A.; Fulop, Z.; Gervino, G.; Guglielmetti, A.; Gustavino, C.; Gyurky, G.; Kaeppeler, F.; Kontos, A.; Kuntz, R.; Leiste, H.; Lemut, A.; Li, Q.; Limata, B.; Marta, M.; Mazzocchi, C.; Menegazzo, R.; O'Brien, S.; Palumbo, A.; Prati, P.; Roca, V.; Rolfs, C.; Rossi Alvarez, C.; Somorjai, E.; Stech, E.; Straniero, O.; Strieder, F.; Tan, W.; Terrasi, F.; Trautvetter, H. P.; Uberseder, E.; Wiescher, M.

The 15N(p,g)16O reaction represents a break out reaction linking the first and second cycle of the CNO cycles redistributing the carbon and nitrogen abundances into the oxygen range. The reaction is dominated by two broad resonances at Ep = 338 keV and 1028 keV and a Direct Capture contribution to the ground state of 16O. Interference effects between these contributions in both the low energy region (Ep < 338 keV) and in between the two resonances (338

Publ.-Id: 14900

Blitzlampentemperung für die Photovoltaik

Skorupa, W.

A review regarding the application of flash lamp annealing for photovoltaic purposes was presented.

Keywords: Flash lamp annealing; doping; ion implantation; solar cells

  • Lecture (Conference)
    Topical meeting with members of the company Bosch, 10.02.2010, FZ Dresden-Rossendorf, Deutschland

Publ.-Id: 14899

Advances in Si & Ge millisecond processing: From SOI to superconducting Ge

Skorupa, W.; Wündisch, C.; Posselt, M.; Heera, V.; Herrmannsdörfer, T.; Buca, D.; Mantl, D.; Haeberlein, S.; Fendler, R.; Gebel, T.

Recently we could demonstrate that advanced SOI material can be treated in advantageous manner regarding USJ formation [1]. Especially, strained Si and SiGe/Si heterostructures on insulator are promising channel materials for future nanoelectronics devices. Their successful integration into new device architectures depends on the ability of forming ultra shallow and ultra steep junctions. We present results for dopant activation in SOI, sSOI, HOI and sHOI [2]. FLA allows complete suppression of diffusion while obtaining sheet resistances lower than 500 Ω/□ in both, SOI and sSOI. Strained and unstrained SiGe heterostructures indicated significant diffusional broadening of Sb implant profiles and low electrical activation. In contrast, B shows higher activation but significant dopant loss in the near surface region. Moreover, we demonstrate that, after diamond and silicon, the third elemental group-IV semiconductor, germanium, exhibits superconductivity at ambient pressure [3]. For the first time, techniques of the state-of-the-art semiconductor processing as ion implantation and FLA were used to fabricate such material, i.e. a highly Ga-doped Ge (Ge:Ga) layer in near-intrinsic Ge. It is shown that superconductivity can be generated and tailored in the Ge host at temperatures as high as 0.5 K. Results of critical-field measurements demonstrate the quasi-two-dimensional character of superconductivity in the 60 nm thick Ge:Ga layer.
[1] F. Lanzerath, D. Buca, H. Trinkaus, M. Goryll, S. Mantl, J. Knoch, U. Breuer, W. Skorupa, B. Ghyselen, J. Appl. Phys. 104 (2008), 044908
[2] R. A. Minamisava, W. Heiermann, D. Buca, H. Trinkaus, J. Hartmann, W. Skorupa, U. Breuer, B. Ghyselen, S. Mantl, Proc. 215th ECS Meeting, Vol. 19, Issue 1, May 24-29, 2009
[3] T. Herrmannsdörfer, V. Heera, O. Ignatchik, M. Uhlarz, A. Mücklich, M. Posselt, H. Reuther, B. Schmidt, K.-H. Heinig, W, . Skorupa, M. Voelskow, C. Wündisch, R. Skrotzki, M. Helm, J. Wosnitza,
Phys. Rev. Lett., 102 (2009) 217003, Supercond. Sc. & Techn., 23 (2010) 034007

Keywords: ion implantation; flash lamp annealing; SOI; superconduction; germanium; silicon

  • Invited lecture (Conferences)
    The VIII-th International Conference on Ion Implantation and other Applications of Ions and Electrons – ION 2010 will be held in Kazimierz Dolny, Poland, between 14-th and 17-th June 2010., 14.-17.06.2010, Kazimierz Dolny, Poland

Publ.-Id: 14898

Doping of Vertical Si Nanowires by Ion Implantation

Skorupa, W.

A review of recent work published in:
ADVANCED MATERIALS Volume: 22 Issue: 36 Pages: 4020-4024
NANO LETTERS Volume: 10 Issue: 1 Pages: 171-175
NANOSCALE RESEARCH LETTERS Volume: 5 Issue: 1 Pages: 243-246

Keywords: ion implantation; silicon nanowires; doping; scanning spreading resistance microscopy

  • Invited lecture (Conferences)
    43.Deutsches Nutzertreffen Ionenimplantation, 06.05.2010, Erlangen, Deutschland

Publ.-Id: 14897

Literaturübersicht Temperaturmessung in RTP Mit Augenmerk auf die Blitzlampenausheilung

Reichel, D.; Skorupa, W.; Lerch, W.; Gelpey, J.

No abstract available.

  • Lecture (Conference)
    25. Treffen der Nutzergruppe RTP, 07.05.2009, Erlangen, Deutschland

Publ.-Id: 14896

Application of Large-Eddy Simulation to Pressurized Thermal Shock: assessment of the accuracy

Loginov, M. S.; Komen, E.; Höhne, T.

Pressurized Thermal Shock (PTS) is identied as one of the safety issues where Computational Fluid Dynamics (CFD) can bring real benefits. The turbulence modeling may impact overall accuracy of the calculated thermal loads on the vessel walls, therefore advanced methods for turbulent fows are required. The feasibility and mesh resolution of LES for single-phase PTS are assessed earlier in a companion paper. The current investigation deals with the accuracy of LES approach with respect to the experiment. Experimental data from the Rossendorf Coolant Mixing (ROCOM) facility is used as a basis for validation. Three test cases with dierent ow rates are considered. They correspond to a buoyancy-driven, a momentum-driven, and a transitional coolant mixing pattern in the downcomer. Time- and frequency-domain analysis are employed for comparison of the numerical and experimental data.

Keywords: PTS; CFD; Coolant Mixing; ROCOM

Publ.-Id: 14895

Doping Issues of Si Nanowires

Skorupa, W.

A review of recent work published in:
ADVANCED MATERIALS Volume: 22 Issue: 36 Pages: 4020-4024,
NANO LETTERS Volume: 10 Issue: 1 Pages: 171-175,

Keywords: Doping; ion implantation; scanning spreading resistance profiling; silicon nanowire

  • Invited lecture (Conferences)
    27.Deutsches Nutzertreffen RTP und Heissprozesse, 05.05.2010, Erlangen, Deutschland

Publ.-Id: 14894

Biaxial optical anisotropy of self aligned silver nanoparticles and nanowires

Ranjan, M.; Facsko, S.; Möller, W.

In the present study ion beam sputtering has been used for prestructuring of a silicon substrate followed deposition of metal by ebeam evaporation. First a low energy ion beam (Ar+, 500 eV) is incident on the substrate surface at an angle of 67∘ to the surface normal to produce well ordered (20-50 nm) ripple patterns. Then physically vaporized Ag atoms are deposited at grazing angle of 70∘ to the surface normal and normal to the ripples direction. Varying deposition parameters, i.e. ripple periodicity, substrate temperature and atomic flux, we were able to produce well ordered nanoparticles and nanowires. Self-aligned Ag nanoparticles and nanowires deposited on pre-patterned ripple surfaces exhibit strong optical anisotropy. Generalised ellipsometry measurements show that off diagonal Jones matrix elements (Ψ_ps, Δ_ps, Ψ_sp, Δ_sp) are non zero and vary with Eulers angle \phi. This indicates that such a medium is biaxial in nature. A biaxial layer model approach is used to calculate dielectric functions for such a system. Tauc-Lorentz oscillators are used along x and y direction independently and Drude model along z-direction for nanoparticles. This approach provides a very good fitting with the measured Jones matrix element Ψ_pp, Δ_pp, Ψ_ps, Δ_ps, Ψ_sp, Δ_sp. Different cases for ordered nanoparticles and wires will be presented.

  • Lecture (Conference)
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2010, 21.-26.03.2010, Regensburg, Germany

Publ.-Id: 14892

Self-Organisation of metal nanoparticles and nanowires grown on ripple templates

Ranjan, M.; Oates, T. W. H.; Facsko, S.

Nobel metal nanoparticles exhibit distinct optical properties due to their localized surface plas-mon resonance. Potential applications of these structures can be found in solar cells, nonlinear optical devices, or sensors. Especially for nanoscale optics aligned equidistant chains of metal nanoparticles are favored.
Ion beam sputtered surfaces featuring self-organized ripple patterns have proven to be excellent templates for the alignment of these metal nanoparticles [1]. In the continuous sputtering proc-ess, induced by the bombardment with low-energy ions (100 – 2000 eV), periodic surface pat-terns appear in form of ripples or arrays of hexagonally ordered mounds, which show a very high degree of regularity [2]. The dimension of the pattern is related to the size of the typical collision cascade of a single ion event and lies in the range of ten to tens of nanometers. The periodicity and regularity of the pattern is established by the effective filtering of a narrow band of spatial frequencies on the surface, which results from the interplay between a surface instability caused by the sputtering and surface diffusion processes. Regular ripple morphologies have been pro-duced in this way on very different materials including semiconductors, isolators, and metals, demonstrating the universality of the mechanism [3].
Depending on deposition angle, substrate temperature, beam flux, and deposition time, the metal nanostructures align parallel to the ripples, eventually coalesce forming nanowires [4]. Due to the alignment the nanoparticles exhibit strongly anisotropic optical properties. The difference in the interparticle distance along the parallel and perpendicular direction, respectively, leads to different plasmonic coupling in the respective directions. Therefore a red shift of the plasmon-polariton resonance is observed for light polarized parallel to the ripple direction. In addition, the resonance shifts with the aspect ratio of the nanoparticles. Energy shifts of the plasmon reso-nance of 0.2 eV to 0.7 eV have been determined for aspect ratios in the range of 2 to 5.
This work is partly supported by DFG FOR 845.
1. T.W.H. Oates, A. Keller, S. Facsko , A. Mücklich , Plasmonics 2, 47 (2007).
2. A. Keller, S. Rossbach, S. Facsko, et al., Nanotechnology 19, 135303 (2008).
3. A. Keller, S. Facsko, and A. Moller, Jour. Phys. Cond. Matt. 21, 495305 (2009).
4. T.W. H.Oates, A.Keller, S.Noda, S.Facsko , Appl. Phys. Lett. 93, 063106 (2008

  • Poster
    Nanofair 2010, 06.07.-07.11.2010, Dresden, Germany

Publ.-Id: 14891

Self-Organisation of metal nanoparticles on ion beam produced ripple templates

Ranjan, M.; Facsko, S.; Möller, W.

Ion beam sputtering has been used for pre-structuring of the substrate. Low energy ion beam (Ar+, 500 eV) incident on the substrate surface (Si in our case) at an angle of 67o to the surface normal to produce well ordered (20-50 nm) ripple patterns [1, 2]. The periodicity and regularity of the pattern is estab-lished by the effective filtering of a narrow band of spatial frequencies on the surface, which results from the interplay between a surface instability caused by the sputtering and surface diffusion processes. Regular ripple morphologies have been produced in this way on very different materials including semiconductors, iso-lators, and metals, demonstrating the universality of the mechanism [2].
Depending on deposition angle, substrate temperature, beam flux, and deposition time, the metal nanostruc-tures align parallel to the ripples, eventually coalesce forming nanowires [3,4].A very high degree of align-ment not reported so far using the present technique has been achieved. Due to the alignment the nanoparti-cles exhibit strongly anisotropic optical properties. The difference in the interparticle distance along the paral-lel and perpendicular direction, respectively, leads to different plasmonic coupling in the respective direc-tions. Therefore a red shift of the plasmon-polariton resonance is observed for light polarized parallel to the ripple direction. In addition, the resonance shifts with the aspect ratio of the nanoparticles. Energy shifts of the plasmon resonance of 0.2 eV to 0.7 eV have been determined for aspect ratios in the range of 2 to 5.
[1]. Keller.A, Rossbach.S, Facsko.S, Nanotechnology 19(2008) 135303
[2]. Keller.A, Facsko.S,Möller.W, Jour. Phys. Cond. Matt. 21(2009) 495305
[3]. Oates.T.W.H, Keller.A, Facsko.S, A. Mücklich, Plasmonics 2(2007) 47
[4]. Oates.T.W.H, Keller.A, Noda.S, Facsko.S, Appl. Phys. Lett. 93(2008) 063106

  • Poster
    Workshop Ion Beam Physics, 29.-31.03.2010, Dresden, Germany

Publ.-Id: 14890

Self-assembled Ag nanoparticles on plasma sputtered hexagonally ordered GaSb nanodots

Ranjan, M.; Fritzsche, M.; Zhou, J.; Facsko, S.; Möller, W.

Low energy ion-irradiation is capable to produce self-organized ripple and dot pattern on surfaces [1, 2].Ion bombardment normal to surface generate hexagonally correlated dot pattern on semiconductor surfaces [2]. Metal film grown on such pre-patterned substrate may lead to highly ordered self-assembled nanoparticles or nanowires [3]. Ordered metal nanoparticles show unique optical properties [4, 5]. In this work negatively biased (-1.2 kV) GaSb substrate was immersed in an inductively coupled RF plasma. Plasma sheath formation around the sample maintained the normal ion incidence. Ion fluence equivalent to 1x1018 cm-2 is sufficient to produce well pronounced hexagonally ordered dot structures. Later oblique incidence PVD growth of silver on such substrate, produce self-assemble nanoparticles fallowing the dots periodicity. Reflection measurement shows a plasmonic splitting and results in two pronounce resonance peak. Such a splitting is arises due to hexagonal arrangement of nanoparticles, not appeared for non-ordered particles. More details study of this work will be presented.

[1] A. Keller, S. Facsko, W. Möller, New J. Phys. 10 (2008) 063004
[2] S.Facsko, T.Dekorsy, C. Koerdt, H. Kurz, A. Vogt, H. Hartnagel, Science 285 (1999)1551
[3] T.W. H.Oates, A.Keller, S.Noda, S.Facsko, Appl. Phy. Lett., 93(2008) 063106.
[4] T.W. H.Oates, A.Keller, S.Facsko, A.Mücklich, Plasmonics 2(2007) 47.
[5] A. Taleb, V. Russier, A. Courty, M. P. Pileni, Phys.Rev.B 59 (1999) 13350

  • Poster
    Plasma Surface Engineering (PSE), 13.-17.09.2010, Garmisch-Partenkirchen, Deutschland

Publ.-Id: 14889

Radioökologische Forschung als Basis zur Entwicklung neuer Materialien

Raff, J.

wird nachgereicht

  • Lecture (others)
    Besuch ehemaliger IRC-Mitarbeiter, 19.11.2010, Dresden, Deutschland

Publ.-Id: 14888

Vortex coupling in magnetic multilayer elements

Wintz, S.; Puzic, A.; Strache, T.; Bunce, C.; Körner, M.; Schönherr, T.; Neubert, A.; McCord, J.; Moench, I.; Mattheis, R.; Raabe, J.; Quitmann, C.; Erbe, A.; Fassbender, J.

Spin vortices have attracted much attention due to their chiral nature and the variety of dynamic phenomena associated with them. In this contribution we present experimental findings on vortex coupling in trilayer elements, where two ferromagnetic layers are separated by a nonmagnetic spacer. For such systems the relative configurations of the in-plane flux senses (circulations) as well as the core orientations (polarities) of layered vortices are identified by means of scanning transmission x-ray microscopy (STXM). The dominant coupling mechanisms here are the magneto-dipolar interaction and interlayer exchange coupling (IEC). Remarkably, a modification of the IEC, which can be induced by noble gas ion irradiation, allows to specifically set the circulation configuration of a layered vortex pair to be either an- tiferromagnetic or ferromagnetic. In addition, time-resolved measurements of the response of interlayer coupled vortices to an excitation by sinusoidal magnetic fields will be shown.

Keywords: magnetic vortex; coupling; STXM; interlayer exchange coupling

  • Poster
    DPG Frühjahrstagung der Sektion AMOP (SAMOP) und der Sektion Kondensierte Materie (SKM), 13.-18.03.2011, Dresden, Deutschland

Publ.-Id: 14887

Iodine in alluvial platinum–palladium nuggets

Cabral, A. R.; Radtke, M.; Munnik, F.; Lehmann, B.; Reinholz, U.; Riesemeier, H.; Tupinambá, M.; Kwitko-Ribeiro, R.

Synchrotron radiation-induced X-ray fluorescence (SR-XRF) spectrometry was applied to determine iodine concentrations in alluvial Pt–Pd aggregates with delicate morphological features from Córrego Bom Sucesso, Minas Gerais, Brazil. The millimetre-sized botryoidal and rod-shaped grains of alluvial Pt–Pd–Hg intermetallic compounds have surprisingly high concentrations of iodine, in the range from 10 to ~120 µg/g. Because iodine is a strongly biophile element, known to be enriched in peatlands and plant remains in soils by microbial activity, its concentration in the Pt–Pd nuggets suggests biogenic precious-metal fixation in the aqueous alluvial milieu.

Keywords: Iodine; Pt–Pd nuggets; Córrego Bom Sucesso; Minas Gerais; Brazil

Publ.-Id: 14886

Magnetically induced electric polarization in an organometallic magnet

Zapf, V. S.; Kenzelmann, M.; Wolff-Fabris, F.; Balakirev, F.; Chen, Y.

The coupling between magnetic order and ferroelectricity has been under intense investigation in a wide range of transition-metal oxides. The most direct coupling is obtained in so-called magnetically induced multiferroics where ferroelectricity arises directly from magnetic order that breaks spatial inversion symmetry. However, it has been difficult to find nonoxide-based materials in which these effects occur. Here we present a study of copper dimethyl sulfoxide dichloride (CDC), an organometallic quantum magnet containing S = 1/2 Cu spins, in which electric polarization arises from noncollinear magnetic order. We show that the electric polarization can be switched in a stunning hysteretic fashion. Because the magnetic order in CDC is mediated by large organic molecules, our study shows that magnetoelectric interactions can exist in this important class of materials, opening the road to designing magnetoelectrics and multiferroics using large molecules as building blocks. Further, we demonstrate that CDC undergoes a magnetoelectric quantum phase transition where both ferroelectric and magnetic order emerge simultaneously as a function of magnetic field at very low temperatures

Publ.-Id: 14885

Breaking Translational Invariance by Population Imbalance: The Fulde-Ferrell-Larkin-Ovchinnikov States

Zwicknagl, G.; Wosnitza, J.

An overview is given of our present understanding of superconductivity with spontaneously broken translation symmetry in polarized Fermi systems. The existence of “crystalline” superconducting phases is considered in a wide range of systems, prominent examples being conduction electrons in metals, ultra-cold atoms in a trap, nuclear matter and dense quark systems. The underlying physics is delineated and theoretical approaches to the inhomogeneous phases and their properties are discussed. From the experimental side, it is argued that superconductivity with imbalance-induced order parameters is realized in layered organic compounds and potentially in heavy-fermion systems.

Publ.-Id: 14884

On-chip superconductivity via gallium overdoping of silicon

Skrotzki, R.; Fiedler, J.; Herrmannsdörfer, T.; Heera, V.; Voelskow, M.; Mücklich, A.; Schmidt, B.; Skorupa, W.; Gobsch, G.; Helm, M.; Wosnitza, J.

We report on superconducting properties of gallium-enriched silicon layers in commercial (100) oriented silicon wafers. Ion implantation and subsequent rapid thermal annealing have been applied for realizing gallium precipitation beneath a silicon-dioxide cover layer. Depending on the preparation parameters, we observe a sharp drop to zero resistance at 7 K. The critical-field anisotropy proofs the thin-film character of superconductivity. In addition, out-of-plane critical fields of above 9 T and critical current densities exceeding 2 kA/cm2 promote these structures to be possible playgrounds for future microelectronic technology

Publ.-Id: 14883

Investigation of conducting nanostructures on ta-C films made by FIB lithography

Philipp, P.; Bischoff, L.

Tetrahedral amorphous carbon (ta-C) films with high sp3 content produced by mass filtered vacuum arc deposition were modified by Ga+ FIB irradiation. Surface swelling occurs as a function of fluence, caused by ion induced conversion of sp3 to sp2 hybridized carbon atoms. A model [1] for diamond swelling was applied to ta-C films to estimate the swelling for fluences up to 1 x 1016 cm-2. For higher fluences data from TRIDYN simulations were included due to sputtering in a good agreement with the experiments. Van der Pauw structures were produced by means of Ga+ FIB lithography. A decrease of the sheet resistance with increasing fluence due to the evolution of graphitic regions was observed. The lowest value of 290 Ohm/sq was achieved at 1.6 x 1017 cm-2. Additionally, conducting graphitic wires were produced (length: 10 µm, width: 300 nm to 5 µm). The wire resistivity was measured within 130 kOhm (5 µm width) and 3 GOhm (300 nm width). Ion induced graphitization of ta-C films by FIB offers prospective applications in nano technology to fabricate conductive nanostructures in an insulating thin film.
[1] F. Bosia et al. Nucl. Instrum. Meth. B 268 (2010) 2991.

Keywords: tetrahedral amorphous carbon; ta-C; FIB lithography; graphitization; nano structures

  • Poster
    75. Jahrestagung der DPG und DPG Frühjahrstagung, 13.-18.03.2011, Dresden, Deutschland

Publ.-Id: 14882

Freestanding Si nanowires fabricated by Ga+ FIB implantation and subsequent anisotropic etching

Böttger, R.; Bischoff, L.; Schmidt, B.; Krause, M.

Localized Ga+ ion implantation in silicon-on-insulator substrates (top layer 2 µm) by focused ion beam and subsequent anisotropic and selective wet etching has been used to fabricate freely suspended nanowires with reproducible widths between 20 and 200 nm.
The dependence of the resulting nanowire width on the implanted fluence has been investigated and is supported by a numerical model reproducing the experimental data and enabling an a priori estimation of the nanowire width as a function of the implanted fluence. Moreover, the temperature dependence of the nanowires’ resistivity and the activation energy for electrical current flow were investigated before and after direct current annealing in air and in vacuum ambient. Annealed nanowires showed a decrease of their resistivity up to two orders of magnitude, indicating a partial recrystallization of the nanowires through self-heating and a change in the conduction mechanism. The assumption of recrystallization is supported by scanning electron microscopy and Raman spectroscopy.
The comprehension of the pinpointed fabrication of such Si nanostructures establishes a broad range of application in the field of nano-electro-mechanical systems.

Keywords: silicon-on-insulator; Ga focused ion beam implantation; nanowire; anisotropic etching; width; resistivity; phase change; annealing; Raman spectroscopy

  • Poster
    DPG Frühjahrstagung der Sektion AMOP (SAMOP) und der Sektion Kondensierte Materie (SKM), 13.-18.03.2011, Dresden, Deutschland

Publ.-Id: 14881

Complex formation of trivalent americium with salicylic acid at very low concentrations

Müller, M.; Acker, M.; Taut, S.; Bernhard, G.

For the first time, the complexation of americium(III) with salicylic acid was studied at trace metal concentrations using a 2.0 m Long Path Flow Cell for UV–vis spectroscopy. The detection limit of Am(III) in aqueous solution at pH 3.0 was found to be 5 9 10-9 M. Two Am(III)-salicylate complexes were formed at pH 5.0 in 0.1 M NaClO4, indicated by a clear red shift of the absorption maximum. The absorption spectra obtained from spectrophotometric titration were analyzed by means of factor analysis and complex stabilities were calculated to be log b110 = 2.56 ± 0.08 and log b120 = 3.93 ± 0.19.

Keywords: Americium; Long path flow cell; UV–vis; Detection limit; Complexation; Salicylic acid

Publ.-Id: 14880

Development of a high resolution and high dispersion Thomson Parabola

Jung, D.; Hoerlein, R.; Kiefer, D.; Letzring, S.; Gautier, D. C.; Schramm, U.; Huebsch, C.; Oehm, R.; Albright, B. J.; Fernandez, J. C.; Habs, D.; Hegelich, B. M.

Here we report on the development of a novel high resolution and high dispersion Thomson Parabola for simultaneously resolving protons and low-Z ions of more than 100 MeV/nucleon necessary to explore novel laser-ion-acceleration schemes. High electric and magnetic ¯elds enable energy resolutions of ¢E=E < 5% at 100 MeV/nucleon and impede premature merging of di®erent ion species at low energies on the detector plane. First results from laser driven ion acceleration experiments performed at the Trident Laser facility demonstrate high resolution and superior species and charge state separation of this novel Thomson parabola for ion energies of more than 30 MeV/nucleon.

Keywords: laser ion acceleration; ino spectrometer

Publ.-Id: 14879

Sicherheitstechnische Fragestellungen im Reaktorbetrieb am Beispiel eines DWR

Schäfer, F.; Tusheva, P.; Weiss, F.-P.

Der Vortrag erläutert grundlegende Prinzipien der nuklearen Sicherheit, gibt einen Überblick zu Betriebstransienten und hypothetischen Störfällen in einem DWR und diskutiert die verschiedenen Methoden, welche für Untersuchungen zur Reaktorsicherheit angewendet werden.

Keywords: Nuclear Reactor; Reactor Safety

  • Invited lecture (Conferences)
    3. VDI-KTG Kolloquium "Perspektiven der Kernenergie", 26.10.2010, Dresden, Deutschland

Publ.-Id: 14878

Spatial manipulation of magnetic damping in ferromagnetic-antiferromagnetic films by ion irradiation

McCord, J.; Strache, T.; Mönch, I.; Mattheis, R.; Fassbender, J.

The effective magnetic damping parameter in ferromagnetic-antiferromagnetic-ferromagnetic NiFe/IrMn/NiFe try-layer structures is set over a wide range. Ultra-thin antiferromagnetic systems in combination with low fluence Ni-ion irradiation are used to control the magnetic damping parameter by two independent mechanisms. Changing the ferromagnetic-antiferromagnetic interface the initially strongly increased damping parameter is reduced. Applying the method locally, results in a magnetic layer with spatially distributed regions of different damping. The overall relaxation time of the mixed property film is found to be a direct superposition of the individual relaxation time contributions and thus determined by the ratio of phases with individual damping parameter.

Publ.-Id: 14877

Realization of radiobiological in vitro cell experiments at conventional X-ray tubes and unconventional radiation sources

Beyreuther, E.

More than hundred years after the discovery of X-rays different kinds of ionizing radiation are ubiquitous in medicine, applied to clinical diagnostics and cancer treatment as well. Irrespective of their nature, the widespread application of radiation implies its precise dosimetric characterization and detailed knowledge of the radiobiological effects induced in cancerous and normal tissue. Starting with in vitro cell irradiation experiments, which define basic parameters for the subsequent tissue and animal studies, the whole multi-stage process is completed by clinical trials that translate the results of fundamental research into clinical application. In this context, the present dissertation focuses on the establishment of radiobiological in vitro cell experiments at unconventional, but clinical relevant radiation qualities.
In the first part of the present work the energy dependent biological effectiveness of photons was studied examining low-energy X-rays (≤ 50 keV), as used for mammography, and high-energy photons (≥ 20 MeV) as proposed for future radiotherapy. Cell irradiation experiments have been performed at conventional X-ray tubes providing low-energy photons and 200 kV reference radiation as well. In parallel, unconventional quasi-monochromatic channeling X-rays and high-energy bremsstrahlung available at the radiation source ELBE of the Forschungszentrum Dresden-Rossendorf were considered for radiobiological experimentation. For their precise dosimetric characterization dosimeters based on the thermally stimulated emission of exoelectrons and on radiochromic films were evaluated, whereas just the latter was found to be suitable for the determination of absolute doses and spatial dose distributions at cell position. Standard ionization chambers were deployed for the online control of cell irradiation experiments. Radiobiological effects were analyzed in human mammary epithelial cells on different subcellular levels revealing an increasing amount of damage for decreasing photon energy. For this reason, the assumed photon energy dependence was reconfirmed for a cell line other than human lymphocytes, an important finding that was discussed on the 2007 Retreat of the German Commission on Radiological Protection.
After successful finalization of the photon experiments the focus of the present dissertation was directed to the realization of in vitro cell irradiation experiments with laser-accelerated electrons. This research was carried out in the frame of the project onCOOPtics that aims on the development of laser-based particle accelerators, which promise accelerators of potentially compact size and more cost-effectiveness suitable for a widespread medical application, especially for high precision hadron therapy. The unique properties, i.e., the ultrashort bunch length and resultant ultrahigh pulse dose rate, of these unconventional particle accelerators demand for extensive investigations with respect to potential effects on the dosimetric and radiobiological characterization. Based on the experiences gained at ELBE first experiments on the radiobiological characterization of laser-accelerated electrons have been performed at the Jena Titanium:Sapphire laser system. After beam optimization, a sophisticated dosimetry system was established that allow for the online control of the beam parameters and for the controlled delivery of dose to the cell sample. Finally, worldwide first systematic in vitro cell irradiation experiments were carried out resulting in a reduced biological effectiveness for laser-accelerated electrons relative to the 200 kV X-ray reference, irrespectively on the biological effect and cell lines examined. These successful results are the basis for future in vivo studies and experiments with laser-accelerated protons.

Keywords: X-ray tubes; radiation sources

  • Open Access Logo Wissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; FZD-540 2010
    ISSN: 1437-322X


Publ.-Id: 14876

Time resolved spectroscopy on quantum dots and graphene at the FELBE free-electron laser

Winnerl, S.; Stehr, D.; Wagner, M.; Schneider, H.; Helm, M.; Seidel, W.; Michel, P.; Zibik, E. A.; Carpenter, B. A.; Porter, N. E.; Skolnick, M. S.; Wilson, L. R.; Grange, T.; Ferreira, R.; Bastard, G.; Orlita, M.; Plochocka, P.; Kossacki, P.; Potemski, M.; Sprinkle, M.; Berger, C.; de Heer, W. A.

The free-electron laser FELBE at the Forschungszentrum Dresden-Rossendorf enables experiments with spectral, temporal, and, by means of near-field microscopy, also high spatial resolution. FELBE delivers picosecond IR and THz pulses in a wavelength range from 5 µm to 280 µm. Operation in a continous pulsing regime and synchronization to various tabletop laser sources allows for two-color pump-probe experiments. Here we review the potential of the laser and focus on two highlight pump-probe experiments. In the first experiments, the relaxation dynamics in self assembled InGaAs quantum dots at energies below the Reststrahlen band is studied. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to 2 ps at 30 meV (7 THz). The results are in very good agreement with our microscopic theory of the carrier relaxation process, taking into account polaron decay via acoustic phonons [1]. In the second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 – 250 meV. For excitations below the phonon energy of the G-mode, long relaxation times (20 – 40 ps) are observed. While the pump-induced transmission is positive for E > 30 meV, pump-induced absorption occurred for E < 20 meV. The increased transmission for E > 30 meV is caused by bleaching of the interband transition. We attribute the induced absorption to heating of carriers by intraband free-carrier absorption for E < 2E_F (E_F: Fermi energy).

[1] E. A. Zibik et al. Nature Materials 8, 803 - 807 (2009).

Keywords: Quantum dots; graphene; relaxation dynamics; polarons; terahertz; infrared; pump-probe spectroscopy

  • Invited lecture (Conferences)
    SPIE Photonics West, 22.-27.01.2011, San Francisco, USA
  • Proceedings of SPIE 7937(2011), 793708
    DOI: 10.1117/12.873702


Publ.-Id: 14875

Relaxation dynamics in epitaxial graphene probed at low excitation energies

Winnerl, S.; Orlita, M.; Plochocka, P.; Schneider, H.; Helm, M.; Kossacki, P.; Potemski, M.; Sprinkle, M.; Berger, C.; de Heer, W. A.

The relaxation dynamics of graphene has been investigated in various degenerate as well as two-color pump-probe studies at excitation energies E = 1.6 eV [1-5]. We report on degenerate pump-probe spectroscopy at much lower energies (E = 20 – 250 meV; radiation source: free-electron laser) on graphene multilayers grown by thermal decomposition. Relaxation times between 20 and 40 ps are found for photon energies E < 70 meV, where only acoustic phonons can contribute to the relaxation dynamics. At 245 meV a much faster relaxation with two time constants (tau_1 ≈ 1.2 ps, tau_2 = 4 – 8 ps) is observed and attributed to contributions of optical phonons. While the pump-induced transmission is positive for E > 30 meV, pump-induced absorption occurred for E < 20 meV (Fig. 1). The increased transmission for E > 30 meV is caused by bleaching of the interband transition. We attribute the induced absorption to heating of carriers by intraband free-carrier absorption for E < 2E_F (E_F: Fermi energy).
[1] J.M. Dawlaty et al., Appl. Phys. Lett. 92, 042116 (2008).
[2] D. Song et al., Phys. Rev. Lett. 101, 157402 (2008).
[3] P.A. George et al., Nano Lett. 8 ,4248 (2008).
[4] H. Wang et al., Appl. Phys. Lett., 96, 081917 (2010).
[5] D. Sun et al., Phys. Rev. Lett. 104, 136802 (2010).

Keywords: Relaxation dynamics; graphene; infrared; terahertz

  • Poster
    2nd International Symposium on the Science and Technology of Epitaxial Graphene, 14.-17.09.2010, Amelia Island, USA

Publ.-Id: 14874

Compositional depth profiling of TaCN thin films

Adelmann, C.; Conard, T.; Franquet, A.; Brijs, B.; Munnik, F.; Burgess, S.; Witters, T.; Meersschaut, J.; Kittl, J. A.; Vandervorst, W.

The composition profiling of thin TaCN films was studied. For the composition profile determination using x-ray photoemission spectrometry (XPS) in combination with Ar sputtering, preferential sputtering effects of N with respect to Ta and C were found to lead to inaccurate elemental concentrations. Sputter yield calculations for the given experimental conditions allowed for the correction of a part of the error, leading to fair accuracy by reference-free measurements. Further improvement of the accuracy was demonstrated by the calibration of the XPS compositions against elastic recoil detection analysis (ERDA) results. For Auger electron spectrometry (AES) in combination with Ar sputtering, accurate results required the calibration against ERDA. Both XPS and AES allowed for a reliable and accurate determination of the compositional profiles of TaCN-based thin films after calibration. Time-of-flight secondary-ion mass spectrometry was also used to assess the composition of the TaCN films. However, the analysis was hampered by large matrix effects due to small unintentional oxygen contents in the films. Energy-dispersive x-ray spectrometry is also discussed, and it is shown that an accurate reference-free measurement of the average film concentration can be achieved.

Keywords: TaC; TaN; TaCN; XPS; AES; ToF-SIMS; ERDA; EDX; composition profile

Publ.-Id: 14873

Speciation of Uranium in Seepage and Pore Waters of Heavy Metal-Contaminated Soil

Baumann, N.; Arnold, T.; Lonschinski, M.

The uranium speciation in natural occurring seepage water samples, and in soil water samples, all samples from test site “Gessenwiese”, were analyzed by Time-resolved Laser-induced Fluorescence Spectroscopy (TRLFS). Test site Gessenwiese was installed as a part of a research program of the Friedrich Schiller University Jena for investigations within the area of a recultivated former uranium mining heap close to Ronneburg (Eastern Thuringia).
Those investigations revealed that the uranium speciation in that seepage water is dominated by the uranium (VI) sulfate species UO2SO4(aq). The analyses were performed to compare presented results in a later stage with the uranium speciation in plants, which grow on the grassland test site Gessenwiese. It was showed that TRFLS is a useful tool for clearing up the speciation of uranium in water, which is affiliated by plants.

  • Book chapter
    Kothe, Erika; Varma, Ajit: Bio-Geo Interactions in Metal-Contaminated Soils, Berlin: Springer, 2012, 978-3-642-23326-5, 131-142

Publ.-Id: 14872

Reversals and the turbulent alpha-effect in kinematic simulations of natural and experimental dynamos

Giesecke, A.; Stefani, F.; Gerbeth, G.

From paleomagnetic observations, numerical simulations and -- meanwhile -- also experiments, it is evident that reversing dynamos are a quite common phenomenon. An easy access to the examination of reversing dynamos is provided by simplifying mean field simulations where the induction effects of helical small scale turbulence are parametrized by the so called α-effect.

In a geodynamo-like α2-model reversing solutions are obtained only for a highly restricted class of radial α-profiles which are characterized by a proximity of oscillating and non-oscillating state. The required properties of the α-effect, indeed, have been found from local simulations of rotating magnetoconvection in a Cartesian box (Giesecke et al. 2005, PEPI 152, 90-102). Essential properties of the field reversals can be assigned to the behavior of so called exceptional points in the spectrum of the dynamo-operator where two different non-oscillatory field eigenmodes merge and continue as one oscillatory eigenmode (Stefani et al. 2005, PRL, 94, 184506). The connection between oscillating solutions and irregular occurring reversals arises from fluctuations of the α-effect, which causes a sporadic switching from steady to the oscillatory regime and vice versa. In this picture a reversal is an inevitable consequence of the behavior of the instantanous growthrate in conjuction with fluctuations of the driving source.

Recent results of the French von-Kármán-Sodium (VKS) dynamo offer a possibility for detailed investigations of reversing magnetic fields. In the VKS experiment a turbulent flow of liquid sodium is driven by two counter rotating soft iron impellers located close to the endplates of a cylindrical vessel. The geometric structure of the observed magnetic field is dominated by an axisymmetric dipole that exhibits regular and irregular reversals if the impellers rotate with different frequencies. Recent simulations show that the induction process is essentially determined by the material properties of the flow driving impellers (high permeability) because of internal boundary conditions for the magnetic field on the interface between fluid and impeller (Giesecke et al. 2010, PRL 104, 044503). However, only the combined interaction of soft iron disks and a comparatively small α-effect allows to explain the dominating axisymmetric field. Contributions of dipole and quadrupole type fields deduced from the equatorial symmetry show nearly the same growth rate supporting the reversal model presented by Pétrélis & Fauve (2009, PRL 102, 144503) that is based on the coupling between these two modes.

Keywords: Dynamo; Geodynamo; alpha-effect; Simulations

  • Poster
    AGU Fall Meeting 2010, 13.-17.12.2010, San Francisco, USA
  • Contribution to proceedings
    AGU Fall Meeting 2010, 13.-17.12.2010, San Francisco, USA
    Reversals and the turbulent α-effect in simulations of natural and experimental dynamos

Publ.-Id: 14871

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