Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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35836 Publications

Recent and future liquid metal experiments on dynamo action and related magnetohydrodynamic instabilities

Stefani, F.; Gellert, M.; Gerbeth, G.; Giesecke, A.; Gundrum, T.; Rüdiger, G.; Seilmayer, M.

Starting with the Riga and Karlsruhe liquid sodium experiments, the last decade has seen great progress in the laboratory modeling of dynamo action and related magnetohydrodynamic instabilities such as the magnetorotational instability (MRI) and the Tayler instability (TI). We summarize the recent experimental achievements, and we present our plans for a large scale precession dynamo experiment and a combined MRI/TI experiment in the framework of the DRESDYN project at Helmholtz-Zentrum Dresden-Rossendorf.

  • Open Access Logo Abstract in refereed journal
    Geophysical Research Abstracts 13(2011), 2011-2352
  • Invited lecture (Conferences)
    European Geosciences Union, General Assembly 2011, 03.-08.04.2011, Vienna, Austria

Publ.-Id: 15101

Relativistic expansion of electron-positron-photon plasma droplets and photon emission

Yaresko, R.; Mustafa, M. G.; Kaempfer, B.

The expansion dynamics of hot electron-positron-photon plasma droplets is dealt with within relativistic hydrodynamics. Such droplets, envisaged to be created in future experiments by irradiating thin foils with counterpropagating ultraintense laser beams, are sources of flashes of gamma radiation. Warm electron-positron plasma droplets may be identified and characterized by a broadened 511 keV line. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3499410]

Keywords: hot electron-positron-photon plasma droplets; relativistic hydrodynamics

Publ.-Id: 15100

Why are depth profiles promising?

Braucher, R.; Bourlès, D. L.; Merchel, S.; Léanni, L.; Chauvet, F.; Arnold, M.; Aumaître, G.; Keddadouche, K.

Cosmonuclide concentrations in surface samples are function of both duration of exposure to cosmic radiation and surface stability. From the measurement of a single cosmonuclide concentration in a single surface sample, it is thus impossible to quantify simultaneously the exposure duration and the denudation rate affecting the studied object. In the case of a simple exposure history, measurements of two cosmonuclide concentrations within the same surface sample may however theoretically be used to estimate both unknowns.
A more constraining approach to accurately quantify both exposure time and denudation rate is to take advantage of the fact that the effective production attenuation length of neutrons is significantly shorter than that of muons. The neutron-induced cosmonuclide concentrations thus reach steady-state with respect to denudational loss much more rapidly than the muon induced ones. Consequently, the near-surface cosmonuclide concentrations mainly resulting from interactions with neutrons might be used to estimate the denudation rates while the several meters depth concentrations mainly resulting from interactions with muons might be used to estimate the exposure duration. A unique well constrained depth profile thus permits determination of both the exposure time and the denudation rate. In the case of abandoned material, inheritance due to previous exposition to cosmic rays can be revealed.
Multi-nuclide depth profiles are also excellent tools for better deducing the physical parameters of the particles involved in the production of these cosmogenic radionuclides. This approach has been applied twice: 1.) 10Be and 26Al along a 25 meters pure quartz core from Galicia, Spain, and 2.) 10Be, 26Al and 36Cl along a ~11 meters carbonate and quartzose conglomerates core from La Ciotat, S.E. France. Both projects confirm that the density of the material is one of the most sensitive parameters. The latter study also allows to precise the still debatable spallation production rate of 36Cl from Ca.

Keywords: accelerator mass sepctrometry

  • Poster
    12th International Conference on Accelerator Mass Spectrometry (AMS-12), 20.-25.03.2011, Wellington, New Zealand

Publ.-Id: 15099

The role of chemistry in setting-up a new AMS facility

Merchel, S.; Munnik, F.; Andermann, C.; Arnold, M.; Aumaître, G.; Benedetti, L.; Bourlès, D. L.; Braucher, R.; Finkel, R. C.; Fuchs, M.; Gloaguen, R.; Klemm, K.; Martschini, M.; Steier, P.; Wallner, A.

Medium-energy AMS facilities such as the British 5 MV-NEC machine at “SUERC”, the French 5 MV-HVEE-machine “ASTER” and the two German 6 MV-HVEE-machines “DREAMS” at Dresden [1] and “Cologne AMS” have been recently installed in Central Europe. Of course, these machines need physicists to get them running but also scientists to establish AMS chemistry on-site. As it is not advisable to change simultaneously two ”things”, i.e. machine and chemistry, a close cooperation with the teams of ASTER and VERA helped to check the new sample preparation laboratories of DREAMS.

Generally, a ”good” AMS sample has two features: high stable isotope current and low isobar concentration. High chemical yields and low concentrations of other elements originating from the matrix or chemical products used are less important, but may play a role if e.g. a matrix contains Ti being introduced into BeO-targets as shown by µ-XRF [2] and recent µ-PIXE analyses of final AMS-targets at HZDR. A processing blank with low radionuclide/stable nuclide ratio is, however, essential for projects near the detection limit. For high sample throughput and reasonable costs a fast, easy and cheap chemical separation is also favourable.

The results from first samples processed at DREAMS are excellent: For 10Be-AMS-targets isolated from quartz-rich river sediments (Himalaya, erosion rate study) 9Be-currents had been as high as ASTER standards and machine blanks. Corresponding processing blanks were in the same order as the machine blank (1x10-15), thus, more than one order of magnitude lower than the lowest sample ratio. Chlorine-36 targets originating from calcite-rich boulders from a medieval rockfall had been well-measurable with blank-corrections lower than 8 %. Finally, 36Cl prepared from river terraces (Anatolian Plateau, Turkey, uplift rate study) showed also perfect performance at VERA.

References: [1] Akhmadaliev et al., this meeting. [2] S. Merchel et al., NIMB 266 (2008) 4921.

Acknowledgments: Thanks to T. Schildgen and C. Yildirim (Potsdam) performing 36Cl chemistry tests at DREAMS.

Keywords: accelerator mass spectrometry

  • Lecture (Conference)
    12th International Conference on Accelerator Mass Spectrometry (AMS-12), 20.-25.03.2011, Wellington, New Zealand

Publ.-Id: 15098

Status of the French accelerator mass spectrometry facility ASTER after 4 years

Arnold, M.; Bourlès, D. L.; Finkel, R. C.; Nottoli, E.; Aumaître, G.; Keddadouche, K.; Benedetti, L.; Braucher, R.; Merchel, S.

Since the acceptance tests of ASTER in March 2007, routine measurement conditions for the long-lived radionuclides 10Be and 26Al have been established. Sample throughput as high as over 3300 unknowns has been reached for 10Be in 2010. Steady numbers for 26Al within the last three years settle down ~300 real samples.

Unacceptable cross-contamination for volatile elements has been largely solved by an ion source upgrade [1]. Thus, the second frequent nuclide measured at ASTER in 2010 is 36Cl with ~480 sample targets. The enhancement with respect to immediate short-term sample to sample by the new ion source is impressive allowing minimal measurement time losses such as for initial burn-in periods of 5 min for virgin targets and waiting periods between data acquisition of two sample runs of 2 min. However, recent long-term tests using 35Cl/37Cl samples with strongly varying ratios have shown that identical targets lead to different 35Cl/37Cl results at the 2-4% level when being measured after a time gap of 24 hours while the source is running other samples. Reasons for this such as source memory, time dependent mass fractionation, drift of the Faraday-cup measurement system or something else are not yet clear.

Finally, after establishing quality assurance at ASTER by cross-calibration of secondary in-house 26Al and 41Ca standards [1] and taking part in round-robin exercises of 10Be and 36Cl, we performed a two-step cross-calibration of secondary in-house 129I standards. The two ampoules of NIST 3231 containing 129I/127I at 0.981x10-6 have been used for step-wise dilution with NaI (MERCK, suprapur, 99.99 %) to get gram-quantities of lower-level standards for every-day use. The material SM-I-9 (~1x10-9) has been measured vs. AgI produced from the two NIST ampoules with (0.982+0.012)x10-8 solution using minimum chemistry. In a second stage, SM-I-10 and SM-I-11 with ratios of ~1x10-10 and ~1x10-11, respectively, have been cross-calibrated against SM-I-9. Individual uncertainties of the traceable secondary standards are around 1.5 %, mainly originating from the given uncertainty of the primary NIST 3231 at 10-8.

References: [1] M. Arnold et al., NIMB 268 (2010) 1954.

Keywords: accelerator mass spectrometry

  • Poster
    12th International Conference on Accelerator Mass Spectrometry (AMS-12), 20.-25.03.2011, Wellington, New Zealand

Publ.-Id: 15097

Stable Isotope Measurements in Presolar Grains

Wallner, A.; Melber, K.; Merchel, S.; Ott, U.; Forstner, O.; Golser, R.; Kutschera, W.; Priller, A.; Steier, P.

The most abundant presolar grains in primitive meteorites are nanodiamonds. These grains survived the formation of the solar system and kept their own individuality. However, existing measurements of their isotopic pattern do not allow for a consistent understanding of their origin. The reasons are mainly due to their small size (nm) and the low abundance of trace elements. Most likely, such nanodiamonds were formed in a supernova environment. Positive measurements of trace-element isotopic signatures will help understanding heavy element nucleosynthesis in massive stars and dust formation from their ejecta. They will also enable testing predictions of r-process nucleosynthesis and understanding the uniformity of the “main r-process”.
A first approach applying AMS to measure Pt in nanodiamonds was performed at TU Munich [1]. We have continued our search for stable isotope anomalies in nanodiamonds at VERA. Recent experience showed that AMS provides the required measurement precision together with a low Pt machine background. Moreover, we observed for the first time enhancements of 198Pt/195Pt isotope ratios in two diamond residues from the Allende meteorite, which were prepared by different separation techniques. Variations in other isotopic ratios were within analytical uncertainty, and no anomaly was identified in a third diamond fraction. An enhanced 198Pt/195Pt ratio is predicted by models that either include or exclude a simultaneous negative anomaly in 194Pt/195Pt. This negative anomaly, however, was not observed via AMS and is in contrast to data obtained for tellurium.
The robustness of these first results needs to be verified by detailed and systematic studies of possible mass-fractionation effects and potential scatter due to inhomogeneities within samples. We will present our measurement approach and plans for other isotopes. The latter is part of the Eurogenesis programme organized by the European Science foundation.

[1] S. Merchel et al., Geochim et Cosmochim Acta 67 (2003) 4949.

Keywords: accelerator mass spectrometry; r-process; supernova; astrophysics

  • Poster
    12th International Conference on Accelerator Mass Spectrometry (AMS-12), 20.-25.03.2011, Wellington, New Zealand

Publ.-Id: 15096

TOPFLOW-PTS air-water experiments on the stratification in the ECC nozzle and the ECC water mixing during PTS scenarios

Seidel, T.; Beyer, M.; Hampel, U.; Lucas, D.

Pressurized Thermal Shock (PTS) occurs in Pressurized Water Reactors (PWR) during hypothetical Small Break Loss of Coolant Accidents (SBLOCA) with the action of the Emergency Core Cooling System (ECCS). For reactor designs where cold water is injected into the cold leg, insufficient mixing can lead to critical thermal stress in the hot RPV wall. The mixing process involves several thermal hydraulic phenomena, such as direct contact condensation (DCC), entrainment of steam bubbles and multi-scale momentum transfer, and is therefore difficult to model. Conventional 1D system codes are not designed to resolve flows, which are governed by 3D multiphase phenomena. Therefore Computational Fluid Dynamics (CFD) codes are currently being qualified for such problems.
For the validation of CFD models and simulations related to PTS problems, the TOPFLOW-PTS project is running in cooperation with AREVA, CEA, EDF, ETHZ, IRSN and PSI. There, an experimental installation was implemented inside the TOPFLOW pressure tank to run scaled down steam-water and air-water experiments. The PTS assembly is a model of a French 900 MWe PWR including downcomer, cold leg with ECC injection and a pump simulator - all scaled at 1:2.5. (See [1] for details) The setup is highly instrumented with sophisticated special instrumentation. Temperature profiles in the flow domain are measured with about 200 distributed thermocouples and the cold leg wall is observed by an infrared camera system. Furthermore high-speed camera observation and wire mesh sensors record the flow structure in the cold leg.
In a first project phase several air-water experiments have been carried out which were intended to give a general insight into the flow structures for some parameter variations. These experiments indicated the flow stratification phenomena in the ECC line and thermal mixing in the cold leg in dependence on the ECC mass flow rate, the system pressure (gas density) and other parameters. One of the most important determinants for thermal mixing is the jet momentum at the impact position, which mainly depends on the jet’s mass flow rate. It turned out that there can be different flow regimes in the ECC line which need to be considered in boundary condition definitions for CFD calculations. Therefore this effect was particularly studied utilizing an ultrasound sensor together with simultaneous high-speed camera observations of the impingement point and the water surface. A digital bubble tracking velocimetry method was then applied to compare different flow regimes in the cold leg.

Keywords: pressurized thermal shock; experiment; steam; subcooled water; direct contact condensation; entrainment; impinging jet; emergency core cooling; model development; model validation

  • Contribution to proceedings
    NURETH-14, 25.-29.09.2011, Toronto, Canada
    Proceedings of The 14th International Topical Meeting on Nuclear Reactor Thermalhydraulics (NURETH-14)
  • Lecture (Conference)
    NURETH-14, 25.-29.09.2011, Toronto, Canada

Publ.-Id: 15095

Nano particles of iron oxides in SiO2 glass prepared by ion implantation

Nomura, K.; Reuther, H.

Quartz (SiO2) glass was implanted with 5-9x1016 57Fe ions/cm2 at a substrate temperature of 500°C, and annealed at temperatures between 700 and 950°C. The implanted and annealed plates were characterized by conversion electron Mössbauer spectroscopy (CEMS), and measured by a Kerr effect magnetometer or a vibration sample magnetometer. Kerr effect measurement of as-implanted SiO2 glass showed ferromagnetism at room temperature. CEM spectrum of the as-implanted glass consisted of magnetic relaxation peaks of finely dispersed metallic Fe species, and paramagnetic doublets of Fe3+ and Fe2+ species. The sample heated at 700°C contained large grains of metallic Fe and a lot of oxidation products of Fe2+ species. After oxidation at temperatures higher than 800°C, the samples showed also ferromagnetism, which was attributed mainly to ferromagnetic e-Fe2O3 precipitated in SiO2 matrix. Small amounts of a-Fe2O3 were produced at 950°C. The results suggest that ion implantation and oxidation make a transparent ferromagnetic glass possible.

Publ.-Id: 15094

Prospects of laser accelerated particle beams

Schramm, U.

Talk on the applicability of laser accelerated proton beams for radiotherapy and current status of the HZDR activities.

Keywords: laser accelerated proton beams; radiotherapy

  • Invited lecture (Conferences)
    470. WE Hereaeus Seminar on Particle Accelerators and High Intensity Lasers, 13.-16.12.2010, Bad Honnef, Deutschland

Publ.-Id: 15093

Laser Proton Acceleration from Mass Limited Targets

Zeil, K.; Metzkes, J.; Kraft, S.; Kluge, T.; Bussmann, M.; Schmidt, B.; Zier, M.; Schramm, U.; Cowan, T.; Sauerbrey, R.

We present recent studies on laser proton acceleration experiments using mass limited silicon targets. Small micro machined silicon foils with 2 µm thickness and 20x20 µm2 to 100x100µm2 size mounted on very tiny stalks were shot with ultra short laser pulses 30 fs of the new 150 TW DRACO Laser facility of the Research Centre Dresden-Rossendorf. The experiments were carried out using high contrast level 10-10. Proton spectra have been measured with magnetic spectrometers and radio chromic film stacks. The scaling of the maximum proton energy as function of the target size was investigated and strong influence of the stalk as well as the target edges was found.

  • Lecture (Conference)
    Superstrong Fields in Plasmas, 04.10.2010, Varenna, Italia

Publ.-Id: 15092

Dose-dependent biological damage of tumour cells by laser-accelerated proton beams Center for high power radiation sources

Kraft, S.; Zeil, K.; Richter, C.; Beyreuther, E.; Bussmann, M.; Enghardt, W.; Karsch, L.; Kluge, T.; Kraft, S.; Laschinsky, L.; Metzkes, J.; Naumburger, D.; Pawelke, J.; Sauerbrey, R.; Cowan, T. E.; Schramm, U.

Overview of the cell irradiation measuremants and the new Center for high power radiation sources

  • Lecture (Conference)
    TR 18 Meeting Advances in relativistic laser plasma physics, 03.-07.10.2010, Frauenwörth, Deutschland

Publ.-Id: 15091

Laser Proton Acceleration from Reduced Mass Targets

Metzkes, J.; Zeil, K.; Kraft, S.; Kluge, T.; Bussmann, M.; Schmidt, B.; Zier, M.; Schramm, U.; Cowan, T.; Sauerbrey, R.

In the last years, high power laser systems in the 100 TW range with ultrashort pulses (~30 fs) and repetition rates of up to 10 Hz have come into operation. In order to investigate the laser proton acceleration in this laser regime we have performed a series of experiments using plain few-micron-thick metal targets and mass-limited silicon targets. The targets were irradiated with 30 fs pulses from the new 150 TW DRACO Laser facility at the Forschungszentrum Dresden-Rossendorf which show a contrast level of 10-10 in the picosecond and 10-9 up to 10-10 in the nanosecond range.

For both target types, proton spectra have been measured with a magnetic spectrometer and radiochromic film stacks. In addition, two magnetic electron spectrometers at different angles have yielded information on the electrons emitted from the non-irradiated target rear side.

Using plain metal foil targets, we have observed a linear scaling of the maximum proton energy with laser power and could show that this behaviour is explained consistently by Schreiber’s analytical scaling model [1] in the limiting case of ultrashort laser pulses [2]. Despite the high laser contrast we have found that a slight deformation of the target rear side results in a predictable deflection of the emission of energetic protons away from the target normal direction [2].

The mass limited targets tested in the experiment were micromachined silicon foils with lateral sizes of 20x20 µm2 to 100x100 µm2 mounted on tiny stalks. Their thickness of 2 μm corresponded to the optimum target thickness for proton acceleration at DRACO. Depending on the size of the targets strong influences of the stalks as well as the target edges were found which could both increase or decrease the maximum proton energy in comparison to a plain foil.

[1] J. Schreiber et al., PRL 97, 045005 (2006)
[2] K. Zeil et al., NJP 12, 045015 (2010)

  • Lecture (Conference)
    TR 18 Meeting - Advances in relativistic laser plasma physics, 03.-07.10.2010, Frauenchiemsee, Deutschland

Publ.-Id: 15090

The 14N(p,gamma)15O reaction studied at 0.6 - 2 MeV

Marta, M.; Bemmerer, D.; Beyer, R.; Broggini, C.; Caciolli, A.; Erhard, M.; Fülöp, Z.; Hannaske, R.; Junghans, A. R.; Menegazzo, R.; Nair, C.; Schwengner, R.; Szücs, T.; Trompler, E.; Vezzù, S.; Wagner, A.

The 14N(p,γ)15O is the bottleneck reaction of the hydrogen burning CNO cycle. Although it contributes only 0.8% to energy production in our Sun, the CNO cycle is responsible for neutrino fluxes from the decay of 13N and 15O, which can be detected by underground laboratories as Borexino and SNO+. The interpretation of these data requires more precise nuclear reaction cross section data.
Recent experiments on 14N(p,γ)15O focused mainly in the region 70 < E < 500 keV. Only one set of data extends up to 2 MeV. However, also high energy data are important for the S-factor extrapolation towards lower energies.
The present experiment re-studied the reaction cross section in the energy region from E = 0.6 to 2 MeV, including the resonance at E = 0.987 MeV. New resonance strength and off-resonance S-factors will be presented.

Keywords: Nuclear astrophysics; CNO cycle; Tandetron; TiN solid target

  • Poster
    11th Symposium on Nuclei in the Cosmos, 19.-23.07.2010, Heidelberg, Germany

Publ.-Id: 15089

Superconducting RF Photo Injectors for Future Light Sources

Anders, W.; Jankowiak, A.; Kamps, T.; Knobloch, J.; Michel, P.; Neumann, A.; Sekutowicz, J.; Stephan, F.; Teichert, J.; Will, I.

Future accelerator based radiation sources need high-brightness electron beams with high average currents and pulse repetition rates in the megahertz rage. Electron guns which can provide beams with these properties are still under development. The most promising candidate is the superconducting photo electron injector (SRF gun). The SRF gun operation basic principle is the direct production of short electron pulses by laser pulse irradiation of a photocathode which is placed in radio frequency resonator. The RF field ensures an instantaneous and strong acceleration of the electron bunches. The superconducting RF resonator has an extremely high quality factor of about 1010 and thus very low thermal losses which allows for a continuous operation. The SRF gun has highest gradient and acceleration voltages combination of all CW electron guns. Due to the low RF power losses, nearly all of the RF power is consumed for beam acceleration. The SRF gun vacuum is ideal with an enormous cryopumping speed to provide best lifetime for sensitive photo cathodes.

Recently a collaboration network has been formed by the four German Institutes DESY, FZD, HZB, and MBI, called SRF gun cluster, with the aim to commonly work in the research and development of SRF guns. The guns are needed for accelerator projects in Germany as the extension of the ELBE accelerator as a part of the high-intensity radiation center at FZD, the construction of compact energy recovery linac at HZB, and the continuous wave upgrade of the European XFEL at DESY. The involved institutes has longstanding experiences, know-how and capacities in the fields of superconducting radio frequency, electron gun and photo cathode development, cryogenics, electron beam diagnostics, and UV laser development. Specific common activities which have been started are focused on the SRF gun cavity development and test with a superconducting lead cathode, the cryomodule development for SRF guns, and on the semiconductor photo cathode preparation and characterization.

  • Poster
    Particle Accelerators and High Intensity Lasers, 13.-17.12.2010, Physikzentrum Bad Honnef, Deutschland

Publ.-Id: 15088

Ultra fast electron beam X-ray CT for two phase flow phenomena

Barthel, F.; Hoppe, D.; Szalinski, L.; Beyer, M.; Hampel, U.

Two-phase flows are of primary importance in the understanding of thermal hydraulic phenomena in nuclear light water reactors. The qualification of CFD codes for the simulation of stationary and even transient two-phase flows in complex three dimensional geometries requires extending our knowledge toward the details of the flow structure under various thermal hydraulic conditions. At Helmholtzzentrum Dresden-Rossendorf the thermal hydraulic test facility TOPFLOW is currently extensively used to conduct two-phase flow experiments which aim at the disclosure of fine flow structure details in generic and also more complex geometries. Especially for upward adiabatic two-phase pipe flows in the bubbly to churn turbulent flow regimes valuable experimental data of the flow structure was generated in the past using the wire-mesh sensor technology. A known drawback of the wire mesh sensor is its intrusiveness.
Consequently, we have extended our measurement technology for two-phase flow to high-speed X-ray tomography which offers non-intrusive flow measurement at a speed comparable to the wire-mesh sensor. X-ray CT as an imaging modality is highly advantageous due to its non-invasiveness and its ability to penetrate opaque wall materials. One essential disadvantage of classical CT systems is the requirement for a rotating object or source-detector setup. To measure multiphase flows in a flow velocity range of up to 1 meter per second frame rates of up to 1000 frames per second are required. To achieve this, mechanically rotating parts are to be avoided. Scanned electron beam technology provides a mean to circumvent this problem. Instead of mechanical rotation of scanner components an electron beam is rapidly swept across an X-ray target using deflection coils. This technology has been introduced in medicine more than two decades ago were it is mainly used for cardiovascular diagnostics. However, medical systems with frame rates of about 20 fps are still too slow for technical flow diagnostic problems. For that reason we have developed a scanned electron beam X-ray CT apparatus “ROFEX” which can today already perform cross-sectional imaging at high frame rates and will in the future be extended to other measurement features, such as multi-plane tomography, phase velocity measurement, higher scanning diameters and high energy X-rays. This paper introduce the scanner design, discuss major performance parameters along with the application example of air-water two-phase flow measurements in a DN50 vertical test section of the TOPFLOW facility. Furthermore preparations for steam-water-experiments @70bar/286°C will be discussed.

Keywords: Thermal hydraulics; safety research; electron beam X-ray CT; TOPFLOW

  • Lecture (Conference)
    NURETH-14, 14th International Topical Meeting on Nuclear Reactor Thermalhydraulics, 25.-29.09.2011, Toronto, Canada
  • Contribution to proceedings
    NURETH-14, 14th International Topical Meeting on Nuclear Reactor Thermalhydraulics, 25.-29.09.2011, Toronto, Canada
    Proceedings of the 14th International Topical Meeting on Nuclear Reactor Thermalhydraulics (NURETH-14), CD-ROM

Publ.-Id: 15087

Experimentelle Untersuchung geschichteter Zweiphasenströmungen in rechteckigen Kanälen

Vallée, C.

Die Entwicklung und Validierung von CFD-Codes (computational fluid dynamics) für zweiphasige Anwendungsfälle, wie sie in der Reaktorsicherheit vorkommen, erfordern dedizierte experimentelle Daten. Mit diesem Hintergrund wurden im Helmholtz-Zentrum Dresden Rossendorf (HZDR) geschichtete Gas/Flüssikeits-Strömungen in verschiedenen Versuchsanlagen mit horizontalen Teststrecken untersucht. Diese Kanäle wurden mit rechteckigem Querschnitt konstruiert, um optimale Beobachtungsbedingungen für die Anwendung optischer Messmethoden zu erreichen. So wurde die lokale Strömungsstruktur mit einer Hochgeschwindigkeitskamera visualisiert, welche die für die CFD-Validierung benötigten Daten mit hoher räumlicher sowie zeitlicher Auflösung liefert.

Generische Gleichstromexperimente wurden im Strömungskanal HAWAC (Horizontal Air/Water Channel) mit Luft und Wasser bei Atmosphärendruck und Raumtemperatur durchgeführt. Der hydraulische Sprung, der den quasistationären diskontinuierlichen Übergang zwischen schießender und Flussströmung darstellt, wurde in diesem geschlossenen Kanal studiert. Zudem wurde das zur Schwallströmung führende instabile Wellenwachstum vom Teststreckeneintritt an untersucht. Für quantitative Analysen der optischen Messungen wurde ein Algorithmus entwickelt, um die stratifizierte Phasengrenze in den Kamerabildern zu erkennen und damit statistische Auswertungen für den Vergleich mit Ergebnissen aus CFD-Rechnungen zu ermöglichen. Anwendungsnahe Untersuchungen erfolgten im Heißstrangmodell der TOPFLOW-Anlage. Dieser Versuchsaufbau wurde in einer Druckkammer installiert, um es auch bei reaktortypischen Bedingungen im Druckgleichgewicht mit der Innenatmosphäre des Druckbehälters betreiben zu können. Die Teststrecke bildet ein flaches Modell des heißen Strangs eines deutschen Konvoi Druckwasserreaktors (DWR) im Maßstab 1:3 nach und ist für visuelle Beobachtungen mit großflächigen Glaswänden ausgestattet. Die Experimente erfolgten mit Luft und Wasser bei Raumtemperatur und Drücken von maximal 3 bar sowie mit Dampf und Wasser bei bis zu 50 bar und 264°C. Es werden Ergebnisse aus Gleichstromexperimente präsentiert, die den zweiphasigen Naturumlauf im Primärkreislauf eines DWRs simulieren. Zudem wird detailliert auf das Flutverhalten des Heißstrangmodells eingegangen, welches bei speziellen Gegenstrombegrenzungsexperimenten untersucht wurde. Nicht zuletzt weisen verschiedene Vergleichsbeispiele zwischen Experiment und Simulation nach, welche Möglichkeiten die Daten für die Unterstützung der CFD-Entwicklung und Validierung bieten.

Keywords: stratified two-phase flows

  • Lecture (others)
    Seminar der GRS, 17.01.2011, Garching, Deutschland

Publ.-Id: 15086

Mechanisms in passive synchronization of erbium fiber lasers

Walbaum, T.; Loeser, M.; Gross, P.; Fallnich, C.

The optical synchronization of two passively mode-locked erbium fiber lasers in a master-slave geometry is presented. Both lasers were mode-locked via nonlinear polarization rotation and used a shared semiconductor saturable absorber mirror for synchronization. Tight synchronization and transfer of the repetition frequency stability were shown. The contributions of several physical mechanisms to the change in roundtrip time during synchronization are investigated by analyzing the change of different pulse parameters in synchronized state, including carrier envelope offset frequency and spectrum.

Publ.-Id: 15085

Integration of DYN3D inside the NURESIM Platform

Gomez Torres, A.; Sanchez Espinosa, V.; Kliem, S.; Gommlich, A.; Rohde, U.

The NURISP project (Nuclear Reactor Integrated Simulation Project) is focused on the further development of the European NUclear REactor SIMulation (NURESIM) platform for advanced numerical reactor design and safety analysis tools. NURESIM is based on an open source platform – called SALOME – that offers flexible and powerful capabilities for pre- and post processing as well as for coupling of multi-physics and multi-scale solutions. The developments within the NURISP project are concentrated in the areas of reactors physics, thermal hydraulics, multi-physics, and sensitivity and uncertainty methodologies. The aim is to develop experimentally validated advanced simulation tools including capabilities for uncertainty and sensitivity quantification. A unique feature of NURESIM is the flexibility in selecting the solvers for the area of interest and the interpolation and mapping schemes according to the problem under consideration. The Sub Project 3 (SP3) of NURISP is focused on the development of multi-physics methodologies at different scales and covering different physical fields (neutronics, thermal hydraulics and pin mechanics). One of the objectives of SP3 is the development of multi-physics methodologies beyond the state-of-the-art for improved prediction of local safety margins and design at pin-by-pin scale. The Karlsruhe Institute of Technology (KIT) and the Research Center Dresden-Rossendorf (FZD) are involved in the integration of the reactor dynamics code DYN3D into the SALOME platform for coupling with a thermal hydraulic sub-channel code (FLICA4) at fuel assembly and pin level. In this paper, the main capabilities of the SALOME platform, the steps for the integration process of DYN3D as well as selected preliminary results obtained for the DYN3D/FLICA4 coupling are presented and discussed. Finally the next steps for the validation of the coupling scheme at fuel assembly and pin basis are given.

  • Contribution to proceedings
    17th Pacific Basin Nuclear Conference, 24.-30.10.2010, Cancun, Mexico
    Proc. 17th Pacific Basin Nuclear Conference - CDROM
  • Lecture (Conference)
    17th Pacific Basin Nuclear Conference, 24.-30.10.2010, Cancun, Mexico

Publ.-Id: 15084

Ultrafast carrier capture and THz resonances in InGaAs quantum posts

Stehr, D.; Morris, C. M.; Talbayev, D.; Wagner, M.; Kim, H. C.; Taylor, A. J.; Schneider, H.; Petroff, P. M.; Sherwin, M. S.

Semiconductor quantum posts (QPs) – nanowire-like InGaAs heterostructures [1] in a GaAs matrix – resemble many properties of regular self-assembled quantum dots (QDs), to which they are closely related. Due to their increased size as compared to QDs, QPs have proven to be suitable for very low threshold interband lasers [2]. However, their well controllable height makes them attractive for precise tuning of the interband energy spacing that in QDs can only be achieved via post-growth annealing. Specifically, the 1s – 2p transition energy is expected to drop below LO-phonon energies at post heights of more than 30 nm, making them attractive as frequency-agile structures at terahertz frequencies.
In the work presented here we explore the capture dynamics of QP structures after photoexcitation into the GaAs matrix. While the combined electron-hole dynamics are studied using time-resolved photoluminescence spectroscopy, optical pump - THz probe experiments were performed in order to solely study the electron dynamics. The results of the THz experiment show that after ultrafast excitation, electrons relax within a few picoseconds into the quantum posts, which act as efficient traps [3]. The saturation of the quantum post states, probed by photoluminescence, was reached at approximately ten times the quantum post density in the samples. Recently, we also studied the response of possible electronic resonances after direct photoexcitation into QPs where a broad absorption around 1.5 THz was observed.
[1] J. He et al., Nano Lett. 7, 802 (2007).
[3] D. Stehr et al. Appl. Phys. Lett. 95, 251105 (2009).

Keywords: Nanostructure; Semiconductor; Terahertz; THz; Quantum Well; Intersubband; Quantum Dots

  • Invited lecture (Conferences)
    SPIE Photonics West 2011, 20.-27.01.2011, San Francisco, USA
  • Contribution to proceedings
    SPIE Photonics West 2011, 20.-27.01.2011, San Francisco, USA

Publ.-Id: 15083

Tunable narrowband THz pulses from a large-area photoconductive emitter

Krause, J.; Wagner, M.; Helm, M.; Stehr, D.

THz radiation in the range from 0.2 – 10 THz, generated by ultrafast lasers via optical rectification or highly excited dipole antennas typically consist of 0.5 to 2 cycles of light and therefore cover a broad frequency range. This makes these pulses ideal for broadband spectroscopic applications, where spectra can be acquired at the highest possible time-resolution. For applications such as free space imaging or intense driving of a specific resonance, such THz pulses are less useful due to water absorption and quantitative studies become difficult. In these cases narrowband sources such as Free-Electron Lasers proved to be the ideal light source, as well as cw-sources (e.g. quantum cascade lasers) and photomixers for imaging. However, utilizing the broad spectra of ultrafast lasers, one technique to generate narrowband THz radiation is based on mixing two time- delayed linearly chirped pulses in a nonlinear media: Narrowband, tunable THz pulses up to 780 GHz were successfully generated in a dipole antenna [1] via a fast oscillating current induced by the beat frequency of the mixed laser pulses. Higher frequency pulses were recently demonstrated in ZnTe, where a second order process is utilized [2]. In this work we show that an Auston-switch based emitter concept can serve as a high frequency emitter for narrowband THz pulses. We use our recently introduced concept of a large area emitter with interdigitated electrodes, based on low-temperature grown and semi-insulating substrates. The two pulses for THz generation as well as the probe pulses for electro-optic sampling were taken from a 250 kHz regenerative Ti:sapphire amplifier, whose output was positively chirped to a pulse duration of 3.3 ps. These pulses were split into three beams, where one, undergoing compression in a grating compressor, serves as the probe pulse for field-resolved detection. The other two pulses are sent – with an adjustable optical delay in one of the arms – to a combining beamsplitter and to the THz emitter. The beat frequency – set by the delay in the interferometer – now controls the emission frequency of the emitter, while the length of the input pulse gives the spectral width. The normalized spectra of the pulses for different delays are presented in Fig. 1 and show a frequency coverage from 0.35 to 2.3 THz. All spectra have a spectral width (FWHM) of about 200 GHz which could be further reduced by employing longer driving pulses. Fig. 2 demonstrates the excellent tuning behavior of the emitter, which is perfectly linear with the given time-delay (black dots). The maximum power is obtained around 1 THz and steeply drops towards higher frequencies (red dots) and resembles the power distribution of the emitter excited with a single 50 fs pulse.
[1] A. S. Weling and D. H. Auston, J. Opt. Soc. Am. B 13, 2783 (1996). [2] J. R. Danielson et al., J. Appl. Phys. 104, 033111 (2008).

Keywords: THz; Terahertz; Spectroscopy; Nonlinear Optics

  • Lecture (Conference)
    OTST 2011: International Workshop on Optical Terahertz Science and Technology 2011, 13.-17.03.2011, Santa Barbara, USA

Publ.-Id: 15082

AC Stark effect of the intraexciton 1s-2p quantum well transition

Wagner, M.; Stehr, D.; Schneider, H.; Winnerl, S.; Andrews, A. M.; Schartner, S.; Strasser, G.; Helm, M.

Driving a two-level system with intense light can lead to non-perturbative phenomena like Rabi oscillations. The frequency-domain equivalent is the Autler-Townes or AC Stark effect where dressed light-matter states of a resonantly driven two-level scheme occur. Known from molecular spectroscopy [1] the effect exhibits an absorption line splitting whose magnitude is proportional to the field strength, whereas the symmetry of the splitting depends on the detuning from the resonance case. For atomic or molecular resonances the line widths are much smaller compared to solid state systems like semiconductors, making it much easier to observe the effect in the first case. While only recently the effect has been investigated in quantum well intersubband transitions [2, 3], we present strong evidence for the THz induced intraexciton AC Stark effect using the hydrogen-like 1s and 2p levels of the quantum well heavy-hole exciton [4]. Despite its similarity to the hydrogen problem (except the energy is scaled down by a factor of 1000) we can reach a regime where the Keldysh parameter is comparable to the transition energy and the Rabi energy, a regime not easily accessible for atomic systems.In the experiment an undoped GaAs/AlGaAs multiple quantum well (substrate etched away) is illuminated under normal incidence with intense THz pulses from the Dresden Free-Electron Laser FELBE. At the same time we probe the low-temperature transmission with a broadband femtosecond laser. Tuning the THz photon energy around the measured 1s-2p resonance energy of 9 meV (at 3 meV line width) we observe characteristic absorption changes at the hh(1s) exciton line (see Fig. 1). Below resonance (6.1 meV) an additional high-energy peak or Rabi sideband occurs, while above resonance (14 meV) a low-frequency shoulder is seen. On resonance we have a nearly symmetric splitting. From the two-line fit of Fig. 1 we find an anticrossing for various THz photon energies around the undriven exciton line (marked as dashed vertical line). Moreover, with increasing THz field strength the splitting on resonance increases linearly up to a field of 10 kV/cm with a Rabi energy as large as 60% of the resonance energy. We explain our findings qualitatively on the basis of a simple two-level model, though the underlying rotating-wave approximation breaks down in this regime. Observed deviations would have to be addressed within a full many-body theory. Shifting the NIR probe pulses in time with respect to the THz pulses has been found to change the transmission adiabatically on a time scale of several picoseconds only during the THz pulse. In connection with the observed up to 20-fold transmission change at the hh(1s) exciton, this temporal behavior promises ultrafast optical modulation based on the AC Stark effect.
In conclusion, our experimental data provides the first unambiguous evidence for the intraexciton Autler-Townes effect with its characteristic intensity-dependent line splitting on resonance and its frequency-dependent anticrossing behavior.

[1] S. H. Autler and C. H. Townes, Phys. Rev. 100, 703 (1955).
[2] S. G. Carter et al., Science 310, 651 (2005).
[3] J. F. Dynes et al., Phys. Rev. Lett. 94, 157403 (2005).
[4] M. Wagner et al., Phys. Rev. Lett. 105, 167401 (2010).

Keywords: Terahertz; Semiconductor; Nonlinear Optics; Light-Matter Coupling; Free Electron Laser

  • Lecture (Conference)
    OTST 2011: International Workshop on Optical Terahertz Science and Technology 2011, 13.-17.03.2011, Santa Barbara, USA

Publ.-Id: 15081

Terahertz Ionization of Highly Charged InGaAs Quantum Posts

Morris, C. M.; Stehr, D.; Kim, H. C.; Truong, T. A.; Pryor, C.; Petroff, P. M.; Sherwin, M. S.

Over the last decade, the optoelectronic properties of self-assembled semiconductor quantum dots (QDs) have been studied intensely. Quantum dots confine carriers in the conduction and valence band three dimensionally, acting as artificial atoms with energy levels that are tunable based on the dot’s geometry and material parameters. We have recently introduced a novel quantum dot based nanostructure grown by molecular beam epitaxy [1] called the quantum post (QP). These structures - whose height can be controlled with nanometer precision - form short cylindrical structures (average composition In.4Ga.6As) aligned along the growth direction. The QPs are embedded in an In.1Ga.9As quantum well (QW) of the same height as the QPs. The three dimensional confinement of the post structures produces quantized states. The energies of these states relative to the Fermi level of the structure are strongly dependent on the amount of charge in the QPs. This strong dependence is due to the similarity of the electron-electron Coulomb repulsion energies in the QPs and the spacing between their electronic states (~10 meV).
Here we report absorption from electrons transitioning from three dimensionally confined QP states to one dimensionally confined states of the two dimensional electron gas in the surrounding QW. For the absorption experiments, a single QP layer was embedded between an n-doped back- gate and a Schottky-contact, allowing for controllable loading of electrons into the QPs and surrounding quantum well. The electronic loading behavior was studied by capacitance- voltage spectroscopy, which shows distinct loading-features of electrons into the QPs and the QW. The absorption measurements were performed at a voltage where the posts are completely filled (~6 electrons per post) and the well is filled to a sheet charge density of ~2.4x1011/cm2. Absorption measurements were taken in an FTIR with the THz light polarized parallel to the growth direction to couple only to states arising from vertical confinement. Comparison of spectra from a sample containing quantum posts and a reference quantum well sample show an absorption feature due to the QPs entirely absent in the QW sample.
Temperature dependent absorption measurements show that this absorption is due to a transition that begins in the QP. Using 8 band k.p calculations of post and well energies as a function of the number of electrons in the posts, the absorption is determined to be from an ionizing transition from the posts to the well. The highest filled state in the posts absorbs a terahertz photon, making a transition to a weakly bound unfilled post state ~20 meV higher. From this state, the electron quickly scatters into the quantum well matrix. The Coulomb repulsion of the QP electrons locally depopulates the quantum well states, leaving open states for the electrons to scatter into despite the well’s large average charge density. These results represent a promising structure for investigation of Coulomb blockade physics, as well as the physics of ionizing transitions that occur when Coulomb effects are on the same energy scale as the ionizing transition, a regime only accessible in these artificial atom systems. Additionally, these ionizing transitions hold promise for use in terahertz infrared photodetectors with their three dimensional confinement giving reduced sensitivity to temperature compared to their quantum well based equivalents.
This work is supported by the NSF NIRT grant No. CCF 0507295 and the Alexander von Humboldt Foundation.
References [1] Krenner, H. J. & Petroff, P. M., Sol. St. Comm. 149, 1386 (2009).

Keywords: Nanostructure; Semiconductor; Terahertz; THz; Quantum Well; Intersubband; Quantum Dots

  • Lecture (Conference)
    OTST 2011: International Workshop on Optical Terahertz Science and Technology 2011, 13.-17.03.2011, Santa Barbara, USA

Publ.-Id: 15080

Anomalous Autler-Townes splitting in terahertz-driven quantum wells: interplay of Coulomb interactions, non-rotating wave effects and Stark shifts...

Zaks, B.; Stehr, D.; Truong, T. A.; Petroff, P. M.; Hughes, S.; Sherwin, M. S.

Semiconductor heterostructures are attractive for investigating intense light-matter interactions since the quantum energy levels and effective masses can be engineered at the growth stage, while the dimensionality of the system can be uniquely controlled by quantum confinement. With intense electromagnetic fields tuned resonantly to a two-level quantum transition, Autler and Townes found that absorption of a weak probe from a third state split into two symmetric peaks. Autler-Townes splitting has been studied in atomic and molecular systems for decades but has only recently been observed in the solid state in low dimensional semiconductors and in superconducting quantum systems. In semiconductors, effects such as Coulomb interactions between electrons and holes offer an interesting deviation from effects in atomic systems.
The Autler-Townes effect in a three-level system can be exactly calculated within the rotating wave approximation (RWA). In this model, the splitting in the absorption spectrum is symmetric when the driving frequency ωTHz is resonant with the transition frequency ω2-1. With a non-resonant driving frequency, the symmetry of the absorption spectrum is broken and the relative peak amplitudes are determined by the detuning, ωTHz - ω2-1. A positive (negative) detuning results in a larger (smaller) peak at higher energy. In the RWA model, ω2-1 is independent of the strength of the driving field.
The effect of abandoning the RWA was first studied by Bloch and Siegert in a two-level system. They found a blue shift to the energy of the two-level system ħω2-1 with increasing EM field amplitude. As the E-M field amplitude is increased in the three-level system, the amplitude of the higher energy Autler-Townes peak will decrease as a result of the Bloch-Siegert blue shift. A systematic investigation of Autler-Townes splitting for several quantum well systems demonstrated that the amplitude of the peak at higher energy increases with increasing terahertz intensity, indicating a red shift of the two-level transition energy ω2-1. At first sight this reverse Bloch-Siegert shift is a complete surprise. Only by solving a full four-subband semiconductor Bloch model outside the RWA are we able to achieve theoretical results that are in good agreement with experiment (Figure 1).We have observed large red shifts - exceeding 12% of the transition energy - in THz driven quantum wells. A non-RWA four subband model based on the full semiconductor Bloch equations including Coulomb effects was developed and calculations are shown to have excellent agreement with experimental data. We demonstrate that terahertz-driven quantum wells are an ideal system to investigate strong light-matter interactions on a quantitative level and to controllably explore the regime outside the three-level model and beyond the RWA.

Keywords: Terahertz; Semiconductor; Nonlinear Optics; Light-Matter Coupling; Free Electron Laser

  • Lecture (Conference)
    OTST 2011: International Workshop on Optical Terahertz Science and Technology 2011, 13.-17.03.2011, Santa Barbara, USA

Publ.-Id: 15079

Fast scanning terahertz spectrometer based on synchronized fiber lasers

Stehr, D.; Morris, C. M.; Schmidt, C.; Sherwin, M. S.

Mostly common for THz spectroscopy systems is the use of a single ultrafast laser where the original laser beam is split into a part for THz generation and a part for probing. The latter can be mechanically delayed in time to sample the electric field of the THz pulses which is usually realized by a motorized translation stage. An alternative to this approach is the use of fast-scanning devices such as loudspeaker membranes or spring- loaded stages that can achieve scan rates of 10-30 Hz, barely reaching video-rate operation. However, having two synchronized lasers with high timing precision available, it is possible to overcome this mechanical limit. With the technique called asynchronous optical sampling (ASOPS), where one laser has a constant offset in the repletion rate, it was shown that kHz scan rates are possible [1], enabling high speed THz spectroscopy.
Here we demonstrate the operation of a robust and true turnkey fiber-laser based ASOPS system that is used for THz spectroscopy. The laser system consists of two amplified Er-doped fiber lasers (TWIN M-Fiber A, Menlo Systems), emitting 80 fs pulses centered at 1550 nm at a repetition rate fREP of 250 MHz. The repetition rates of both lasers are frequency stabilized and a frequency synthesizer can add an adjustable (1 Hz- 10 kHz) frequency offset Δf to Laser A for ASOPS operation. In ASOPS operation, the entire time between adjacent pulses (1/fREP=4 ns) is scanned during the inverse of the offset frequency, a fact that often has been criticized as for THz spectroscopy usually only a few tens of picoseconds are needed. To circumvent this, the technique called electronically controlled optical sampling (ECOPS) has been proposed: here, both lasers are set to the same repetition rate and one of the laser’s phase relative to the timing stabilization gets modified. By this, a smaller time-window can be selected and electronically sampled.
For efficient THz generation, we excite an interdigitated large-area photoconductive antenna (PCA) with the second harmonic output of one of the lasers and employ electro-optic sampling in a 500 μm thick <110> ZnTe crystal for detection. The result of a single ECOPS scan at a rate of 2.5 kHz is presented in Fig. 1 and shows a signal-to-noise ratio of the electric field of greater than 50. When averaging is applied, the noise floor drops (Fig. 2, solid line) and the dynamic range reaches 50 dB. To compare the ASOPS with the ECOPS technique, the spectrum of an ASOPS scan with identical acquisition time (Δf=2.5 kHz, 1000 averages) is shown as the dashed line. It can be seen that ECOPS reaches a higher signal-to-noise ratio in this case. Also shown in Fig. 2 is the amplitude spectrum of a 680 μm thick DAST crystal excited at 1550 nm as the dotted curve. It demonstrates that using the fundamental wavelength of the lasers can actually enhance the accessible frequency range, a feature only available for fiber-laser based systems.
[1] A. Bartels et al., Rev. Sci. Instrum. 78, 35107 (2007).

Keywords: THz; Terahertz; Spectroscopy; Fiber Laser; Nonlinear Optics

  • Poster
    OTST 2011: International Workshop on Optical Terahertz Science and Technology 2011, 13.-17.03.2011, Santa Barbara, USA

Publ.-Id: 15078

Terahertz Ionization of Highly-Charged Quantum Posts in a Perforated Electron Gas

Morris, C. M.; Stehr, D.; Kim, H. C.; Truong, T. A.; Pryor, C.; Petroff, P. M.; Sherwin, M. S.

“Quantum posts” are roughly-cylindrical semiconductor nanostructures that are embedded in an energetically shallower “matrix” quantum well of comparable thickness. We report measurements of voltage-controlled charging and THz absorption of 30 nm thick InGaAs quantum wells and posts. Under flat-band (zero-electric field) conditions, the quantum posts each contain ~6 electrons, and an additional ~2.4x1011 cm-2 electrons populate the quantum well matrix. In this regime, absorption spectra show peaks at 3.5 and 4.8 THz (14 and 19 meV) whose relative amplitude depends strongly on temperature. These peaks are assigned to intersubband transitions of electrons in the quantum well matrix. A third, broader feature has a temperature-independent amplitude and is assigned to an absorption involving quantum posts. Eight-band k·p calculations incorporating the effects of strain and Coulomb repulsion predict that the electrons in the posts strongly repel the electrons in the quantum well matrix, “perforating” the electron gas. The strongest calculated transition, which has a frequency close to the center of the quantum post-related absorption at 5 THz (20 meV), is an ionizing transition from a filled state to a quasi-bound state that can easily scatter to empty states in the quantum well matrix.

Keywords: Nanostructure; Semiconductor; Terahertz; THz; Quantum Well; Intersubband; Quantum Dots

Publ.-Id: 15077

THz-driven quantum wells: Coulomb interactions and Stark shifts in the ultrastrong coupling regime

Zaks, B.; Stehr, D.; Truong, T. A.; Petroff, P. M.; Hughes, S.; Sherwin, M. S.

We investigate the near infrared interband absorption of semiconductor quantum wells driven by intense terahertz (THz) radiation in the regime of ultrastrong coupling, where the Rabi frequency is a significant fraction of the frequency of the strongly driven transition. With the driving frequency tuned just below the lowest frequency transition between valence subbands, a particularly interesting phenomenon is observed. As the THz power increases, a new peak emerges above the frequency of the undriven exciton peak, which grows and eventually becomes the larger of the two. This reversal of relative peak intensity is inconsistent with the Autler–Townes effect in a three-state system while within the rotating wave approximation (RWA). In the samples investigated, the Bloch–Siegert shift (associated with abandoning the RWA), exciton binding energy, the Rabi energy, and non-resonant ac Stark effects are all of comparable magnitude. Solution of a semiconductor Bloch model with one conduction and multiple valence subbands indicates that the ac Stark effect is predominantly responsible for the observed phenomenon.

Keywords: Terahertz; Semiconductor; Nonlinear Optics; Light-Matter Coupling; Free Electron Laser

  • Open Access Logo New Journal of Physics 13(2011), 083009

Publ.-Id: 15076

Instrumentation for liquid metal flows

Eckert, S.; Gerbeth, G.; Buchenau, D.; Stefani, F.

The safe and reliable operation of liquid metal systems requires corresponding measuring systems and control units, both for the liquid metal single-phase flow as well as for bubble-laden liquid metal two-phase flows. However, velocity measurements in opaque liquid metal flows still represent a challenging task as commercial measuring systems are not available for such fluids.
The paper reports on established methods and new developments in the field of measuring techniques for liquid metal flows. The presentation is focussed on measurements of the flow rate and the local velocity field as well as on the characterization of liquid metal two-phase flows. During the last two decades considerable effort was spent by miscellaneous researcher groups to provide new solutions for measurements of flow fields in liquid metals. This paper intends to summarize different approaches and tempts to account on perspectives, particularly in view of some recent developments (ultrasonic techniques, magnetic tomography).

Keywords: sodium cooled fast reactor; instrumentation; flow rate measurements; velocity measurements; inductive tomography; ultrasound Doppler method

  • Lecture (others)
    ESFR Workshop-Education and Training, 15.-19.11.2010, Cadarache, France

Publ.-Id: 15075

Ultra-trace analysis of 36Cl by accelerator mass spectrometry: An interlaboratory study

Merchel, S.; Bremser, W.; Alfimov, V.; Arnold, M.; Aumaître, G.; Benedetti, L.; Bourlès, D. L.; Caffee, M.; Fifield, L. K.; Finkel, R. C.; Freeman, S. P. H. T.; Martschini, M.; Matsushi, Y.; Rood, D. H.; Sasa, K.; Steier, P.; Takahashi, T.; Tamari, M.; Tims, S. G.; Tosaki, Y.; Wilcken, K. M.; Xu, S.

A first international 36Cl interlaboratory comparison has been initiated. Evaluation of the final results of the eight participating accelerator mass spectrometry (AMS) laboratories on three synthetic AgCl samples with 36Cl/Cl ratios at the 10-11, 10-12, and 10-13 level shows no difference in the sense of simple statistical significance. However, more detailed statistical analyses demonstrate certain interlaboratory bias and underestimation of uncertainties by some laboratories. Subsequent remeasurement and reanalysis of the data from some AMS facilities, the round-robin data indicates that 36Cl/Cl data from two individual AMS laboratories can differ by up to 17%. Thus, the demand for further work on harmonising the 36Cl-system on a world-wide scale and enlarging the improvement of measurements is obvious.

Keywords: accelerator mass spectrometry; round-robin; quality assurance; AMS; radionuclides

Publ.-Id: 15074

The influence of silica to stabilize actinide(IV) colloids at neutral pH

Hennig, C.; Weiss, S.; Banerjee, D.; Zänker, H.

The solubility and environmental mobility of the actinides under reducing conditions is a widely discussed issue. Tetravalent actinides show a strong tendency towards hydrolysis followed by the formation of oligomers and colloids. However, An(IV) oxohydroxide colloids easily precipitate already at low pH values. We found that silica is able to stabilize such colloids at neutral pH through modification of the inner structure and by influencing the surface charge [1]. The internal structure of the particles can be characterized by An-O(H)-Si bonds that replace the An-O(H)-An bonds of the oxyhydroxide structure. The structure and the formation process were investigated with EXAFS, LIBD, XPS, HEXS, UV-Vis and NMR. The results suggest that the assessment of actinide behaviour in the aquatic environment should take the possible existence of An(IV)-silica colloids into consideration.

Keywords: Actinides; silica; colloids; EXAFS

  • Invited lecture (Conferences)
    Actinide-XAS 2011, 02.-4.3.2011, Hyogo, Japan

Publ.-Id: 15073

Overview of optics-related projects at the Dresden High Magnetic Field Laboratory

Pronin, A.

es hat kein Abstract vorgelegen!

  • Lecture (others)
    Einladung an die Universität Stuttgart, 16.11.2010, Stuttgart, Deutschland

Publ.-Id: 15072

Magnetic Breakdown in the Electron-Doped Cuprate Superconductor Nd2-xCexCuO4: The Reconstructed Fermi Surface Survives in the Strongly Overdoped Regime

Helm, T.; Kartsovnik, M. V.; Sheikin, I.; Bartkowiak, M.; Wolff-Fabris, F.; Bittner, N.; Biberacher, W.; Lambacher, M.; Erb, A.; Wosnitza, J.; Gross, R.

We report on semiclassical angle-dependent magnetoresistance oscillations and the Shubnikov–de Haas effect in the electron-overdoped cuprate superconductor Nd2-xCexCuO4. Our data provide convincing evidence for magnetic breakdown in the system. This shows that a reconstructed multiply connected Fermi surface persists, at least at strong magnetic fields, up to the highest doping level of the superconducting regime.

Publ.-Id: 15071

Highly anisotropic energy gap in superconducting Ba(Fe0.9Co0.1)2As2 from optical conductivity measurements

Fischer, T.; Pronin, A. V.; Wosnitza, J.; Iida, K.; Kurth, F.; Haindl, S.; Schultz, L.; Holzapfel, B.; Schachinger, E.

We have measured the complex dynamical conductivity, sigma = sigma1+i sigma2, of superconducting Ba(Fe0.9Co0.1)2As2 (Tc = 22 K) at terahertz frequencies and temperatures 2–30 K. In the frequency dependence of sigma1 below Tc, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of sigma1 demonstrates a pronounced coherence peak at frequencies below 15 cm−1 (1.8 meV). The temperature dependence of the penetration depth, calculated from sigma2, shows power-law behavior at the lowest temperatures. Analysis of the conductivity data with a two-gap model, gives the smaller isotropic s-wave gap of DeltaA = 3 meV, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is Delta0 = 8 meV. Overall, our results are consistent with a two-band superconductor with an s± gap symmetry.

Publ.-Id: 15070

The Fulde–Ferrell–Larkin–Ovchinnikov state in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 as observed in magnetic-torque experiments

Bergk, B.; Demuer, A.; Sheikin, I.; Wang, Y.; Wosnitza, J.; Nakazawa, Y.; Lortz, R.

We present magnetic-torque experiments on the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 for magnetic fields applied parallel to the 2D superconducting layers. The experiments show a crossover from a second-order to a first-order transition when the upper critical field reaches 21 T. Beyond this field, which we interpret as the Pauli limit for superconductivity, the upper critical field line shows a pronounced upturn and a phase transition line separates the superconducting state into a low- and a high-field phase. We interpret the data in the framework of a Fulde–Ferrell–Larkin–Ovchinnikov state.

Publ.-Id: 15069

Impact of uranium (U) on the cellular glutathione pool and resultant consequences for the redox status of U

Viehweger, K.; Geipel, G.; Bernhard, G.

Uranium (U) as a redox-active heavy metal can cause various redox imbalances in plant cells. Measurements of the cellular glutathione/glutathione disulfide (GSH/GSSG) by HPLC after cellular U contact revealed an interference with this essential redox couple. The GSH content remained unaffected by 10 µM U whereas the GSSG level immediately increased. In contrast, higher U concentrations (50 µM) drastically raised both forms. Using the Nernst equation, it was possible to calculate the half-cell reduction potential of 2GSH/GSSG. In case of lower U contents the cellular redox environment shifted towards more oxidizing conditions whereas the opposite effect was obtained by higher U contents. This indicates that U contact causes a consumption of reduced redox equivalents such as GSH, NAD(P)H. Artificial depletion of GSH by chlorodinitrobenzene and measuring the cellular reducing capacity by tetrazolium salt reduction underlined the strong requirement of reduced redox equivalents.
An additional element of cellular U detoxification mechanisms is the complex formation between the heavy metal and carboxylic functionalities of GSH. Because two GSH molecules catalyze electron transfers each with one electron forming a dimer (GSSG) two UO2 2+ are reduced to each UO2 + by unbound redox sensitive sulfhydryl moieties. UO2 + subsequently disproportionates to UO2 2+ and U4+. This explains that in-vitro experiments revealed a reduction to U(IV) of only around 33 % of initial U(VI). Cellular U(IV) was transiently detected with the highest level after 2 hours of U contact. Hence, it can be proposed that these reducing processes are an important element of defense reactions induced by this heavy metal.

Keywords: uranium; GSH/GSSG; half-cell reduction potential; redox process; complex formation

Publ.-Id: 15068

The steady magnetic field impact on the mould flow at the continuous casting process of steel

Timmel, K.; Eckert, S.; Gerbeth, G.

The influence of a steady magnetic field on the spatio-temporal structure of the mold flow in a physical model of the continuous casting process is studied. The velocity field is measured by an array of ultrasound sensors. The application of the magnetic field provokes a strong recirculation flow at the upper part of the nozzle outlet. The inclination of the jet becomes flatter, and its up- and downturn leads to large scale oscillations of the local velocity.

Keywords: continuous casting; mould flow; liquid metal model; ultrasound doppler velocimetry; electromagnetic brake

  • Poster
    13th MHD-days, 22.-23.11.2010, Dresden, Deutschland

Publ.-Id: 15067

An analytical solution for the consideration of the effect of adjacent fuel elements

Merk, B.; Rohde, U.

A new analytical method is described to deal with the Leakage Environmental Effect. The method is based on the analytical solution of the two group diffusion equation for two adjacent fuel elements. The quality of the results for this highly efficient method is demonstrated on a homogeneous test case and on a heterogeneous combination of two fuel elements described in the PWR MOX/UO2 CORE TRANSIENT BENCHMARK.

Keywords: Analytical Solution; Cross Section Preparation; Leakage Environmental Effect

  • Contribution to proceedings
    International Conference on Mathematics and Computational Methods Applied to Nuclear Science and Engineering (M&C 2011), 08.-12.05.2011, Rio de Janeiro, Brazil, ISBN 978-85-63688-00-2
  • Lecture (Conference)
    International Conference on Mathematics and Computational Methods Applied to Nuclear Science and Engineering (M&C 2011), 08.-12.05.2011, Rio de Janeiro, Brazil

Publ.-Id: 15066

Nonlinear Compton Scattering of Ultrashort Intense Laser Pulses

Seipt, D.; Kämpfer, B.

The scattering of temporally shaped intense laser pulses off electrons is discussed by means of manifestly covariant quantum electrodynamics. We employ a framework based on Volkov states with a time dependent laser envelope in light-cone coordinates within the Furry picture. An expression for the cross section is constructed unambiguously with respect to the pulse length. A broad distribution of scatted photons with a rich pattern of subpeaks like that obtained in Thomson scattering is found. These broad peaks may overlap at sufficiently high laser intensity, rendering inappropriate the notion of individual harmonics. The limit of monochromatic plane waves as well as the classical limit of Thomson scattering are discussed. As a main result, a scaling law is presented connecting the Thomson limit with the general result for arbitrary kinematics. In the overlapping regions of the spectral density, the classical and quantum calculations give different results, even in the Thomson limit. Thus, a phase space region is identified where the differential photon distribution is strongly modified by quantum effects.

Keywords: high-intensity laser pulses; Compton scattering; Volkov states

Publ.-Id: 15065

MRPC prototyping for NeuLAND at Dresden

Bemmerer, D.; Elekes, Z.; Heidel, K.; Hutsch, J.; Kempe, M.; Sobiella, M.; Stach, D.; Wagner, A.; Yakorev, D.; Röder, M.; Zuber, K.

We discuss the progress of the prototyping effort for an MRPC-based solution for the NeuLAND detector for 1 GeV neutrons at FAIR.

  • Lecture (Conference)
    R3B collaboration meeting on technical issues, 14.-16.12.2010, Darmstadt, Deutschland

Publ.-Id: 15064

A possible accelerator in the Dresden Felsenkeller

Bemmerer, D.

Due to the suppression by the Coulomb barrier, the cross sections of astrophysically relevant nuclear reactions are very low at the stellar energy. Therefore they can only be directly measured in a low-background environment. For more than a decade now, the LUNA collaboration has pursued this approach with a 0.4 MV accelerator in the Gran Sasso underground laboratory in Italy. It was highly successful in studying the nuclear physics of the Sun and of the Big Bang.

However, the energy range of LUNA is not sufficient to address the nuclear reactions of stellar helium and carbon burning and the neutron source reactions for the astrophysical s-process. Therefore, in the 2010 NuPECC Long Range Plan for nuclear physics in Europe, it is recommended to install one or more accelerators with higher energy underground.

I will discuss the feasibility and science program of a possible accelerator to be placed in the Felsenkeller underground facility in Dresden/Germany

  • Lecture (others)
    Seminar, 16.12.2010, Livermore, CA, USA

Publ.-Id: 15063

Measuring stellar fusion cross sections underground

Bemmerer, D.

Due to the suppression by the Coulomb barrier, the cross sections of astrophysically relevant nuclear reactions are very low at the stellar energy. Therefore they can only be directly measured in a low-background environment. For more than a decade now, the LUNA collaboration has pursued this approach with a 0.4 MV accelerator in the Gran Sasso underground laboratory in Italy. It was highly successful in studying the nuclear physics of the Sun and of the Big Bang.

However, the energy range of LUNA is not sufficient to address the nuclear reactions of stellar helium and carbon burning and the neutron source reactions for the astrophysical s-process. Therefore, in the 2010 NuPECC Long Range Plan for nuclear physics in Europe, it is recommended to install one or more accelerators with higher energy underground.

I will discuss the methodology and main results of LUNA, and then review projects for new accelerators that are under discussion in Italy (Gran Sasso), Spain (Canfranc), Britain (Boulby), and recently also in Germany (Felsenkeller). Analogous efforts are being made in the US (DUSEL).

Keywords: underground accelerator; Felsenkeller

  • Lecture (others)
    Institute for Nuclear and Particle Astrophysics Journal Club, 17.12.2010, Berkeley, USA

Publ.-Id: 15062

Kinetic Modelling

van den Hoff, J.

kein Abstract verfügbar

  • Contribution to external collection
    F. Kiessling, B. Pichler: Small Animal Imaging, Heidelberg: Springer Verlag, 2010, 978-3-642-12944-5, 387-403

Publ.-Id: 15061

PET Imaging of Cerebral Nicotinic Acetylcholine Receptors (nAchRs) in Early Alzheimer's Disease (AD) Assessed with the New Radioligand (-)-[18F]-Norchloro-Fluoro-Homoepibatidine (NCFHEB)

Sabri, O.; Wilke, S.; Graef, S.; Schoenknecht, P.; Habermann, B.; Becker, G.; Luthardt, J.; Patt, M.; Kendziorra, K.; Meyer, P.; Hesse, S.; Wagenknecht, G.; Hoepping, A.; Hegerl, U.; Brust, P.

Post mortem studies have shown a degeneration of cholinergic neurons in the brain of AD-patients. Further evidence suggests that the loss of nAChRs is a major contributor to the cognitive deterioration in AD, whereby the alpha4beta2-nAChR subtype is thought to be the most severely reduced in the onset of AD. Using 2-[18F]F-A85380 PET we showed a significant decline in alpha4beta2-nAChRs in early AD-patients which correlated significantly with the loss of cognitive function (1, 2). However, this tracer was not well suited as a biomarker in a routine clinical set-up for early AD-diagnosis because of unfavourable properties (slow kinetics, long acquisition times up to 7 hours, limited alpha4beta2-receptor selectivity). We, therefore, developed the new radiotracer [18F]NCFHEB (epibatidine derivative without toxicity in humans) with significantly improved brain uptake, nAChR affinity and selectivity (3). Here, we present the results of the worldwide first ongoing NCFHEB-PET study in humans.
6 mild AD-patients (NINCDS-ADRDA, age 76.7±5.9, MMSE 23.8±3.0) and 5 age-matched healthy controls (HC, age 71.1±5.3, MMSE 28.4±1.1) underwent NCFHEB-PET (370 MBq, 3D-acquisition, ECAT Exact HR+). All were nonsmokers and naïve for central acting medication. In each subject, 4 scans (41 frames) were acquired from 0-270 min post injection and motion correction was performed with SPM2. Kinetic modelling was applied to the VOI-based tissue-activity curves generated for 29 brain regions (irregularly anatomically defined via MRI co-registration) using a one tissue compartment model with measured arterial input-function. Total distribution volume (DV) and binding potential (BP, reference region: corpus callosum) werde used to characterize specific binding. Additionally, parametric images of DV were computed (Logan plot).
Image quality of NCFHEB scans was clearly superior to 2-[18F]F-A85380, and a 20 minutes scan already adequate for visual analysis. All 29 regions were well described with one tissue compartment. PET data acquired over only 90 minutes were sufficient to estimate all kinetic parameters precisely indicating a fast receptor binding kinetic (much faster than for 2-[18F]F-A85380). DVs in HCs increase as expected with receptor density: Corpus callosum (DV: 4.81±0.32), posterior cingulate (8.92±0.66), temporal (9.03±0.44), pons (11.00±1.19), thalamus (24.32±2.96). The AD-patients showed extensive BP reductions in frontal, parietal, temporal, anterior and posterior cingulated cortices, caudate, and hippocampus (all p<0.05) compared to HCs.
Due to the significant shorter acquisition time and superior image quality NCFHEB appears to be a much more valuable tracer than 2-[18F]F-A85380 to image alpha4beta2-nAChRs in humans. Early AD-patients show significant declines of alpha4beta2-nAChRs in brain regions typically affected by AD-pathology. These results indicate that NCFHEB-PET has a great potential to be tested as a biomarker for early AD-diagnosis.
1. O. Sabri,..P. Brust: Acetylcholine receptors in dementia and mild cognitive impairment. Eur J Nucl Med Mol Imaging 2008; 35 (Suppl. 1): 30-45.
2. K. Kenziorra,..O. Sabri: Decreased cerebral α4β2 nicotinic acetylcholine receptors in living patients with mild cognitive impairment and Alzheimer disease with positron emission tomography.
Eur J Nucl Med Mol Imaging 2010; DOI 10.1007/s00259-010-1644-5.
3. P. Brust,..O. Sabri: In-vivo measurement of nicotinic acetylcholine receptors with [18F]norchloro-fluoro-homoepibatidine (NCFHEB). Synapse 2008; 62: 205-218.

  • Lecture (Conference)
    XXVth International Symposium on Cerebral Blood Flow, Metabolism and Function & Xth International Conference on Quantification of Brain Function with PET, 24.-28.05.2011, Barcelona, Spain

Publ.-Id: 15060

Defect characterization of Er implanted, Ge-rich SiO2 layers using slow positron implantation spectroscopy

Anwand, W.; Kanjilal, A.; Wagner, A.; Brauer, G.; Cowan, T.; Rebohle, L.; Cherkouk, C.; Skorupa, W.

The electroluminescence properties of rare earth–doped SiO2 layers are known to deteriorate markedly at room temperature due to rare earth clustering. The key challenge is therefore to probe ongoing processes at microscopic level and the subsequent impact on the optical response with increasing rare earth concentration. Positron annihilation spectroscopy, especially Doppler Broadening Spectroscopy, was applied in order to clarify the structural changes in Ge and Er doped SiO2 layers in dependence on the Er concentration. The obtained results will be compared with Transmission Electron Microscopy investigations and with conclusions in recently published papers. Finally, an outlook about future applications of luminescent SiO2 will be given.

Keywords: Ge; Er doped SiO2; electro-luminescence; positron annihilation spectroscopy

  • Invited lecture (Conferences)
    International Workshop on Advanced Positron Beam Technology for Material Sciences - APSB 2010, 15.-18.03.2010, Algiers, Algeria

Publ.-Id: 15059

Microorganisms Relevant to Nuclear Waste Repositories and Their Influence on Radionuclide Speciation – an Exemplary Investigation on an Äspö Bacterium

Frost, L.

Granite is the aspired potential host rock material for future nuclear waste storage in Sweden. The Äspö Hard Rock Laboratory (Äspö HRL) today is an interdisciplinary research facility for geological disposal in granite rock. The research within the Äspö HRL programme involves international cooperation with various countries including Germany.
This presentation drafts the operations done at Äspö and the contribution by the IRC. Here the objectives and results of the study on the interaction of uranium(VI) with Pseudomonas fluorescens, a strain that has been isolated at the Äspö site, are presented. Explicitly results about the accumulation capability of the strain and TRLFS investigations are shown.

Keywords: Uranium(VI); speciation; Pseudomonas fluorescens; Äspö; nuclear waste repository

  • Lecture (others)
    Doktorandenseminar - Kompetenzzentrum Ost für Kerntechnik, 16.12.2010, Dresden, Deutschland

Publ.-Id: 15058

Depth profiling of ultra-shallow boron implants by high depth resolution ERD

Zier, M.; Kosmata, M.; Munnik, F.

The increase of integration density in modern nano-electronics demands the shrinking of device dimensions in lateral directions as well as in depth. Ultra-shallow p-n junctions in semiconducting materials are vital for this technology [1].
Low-energy ion implantation is a feasible way to incorporate dopant elements such as boron, phosphorous or antimony in the near-surface region of the substrate material. Characterisation of these implants, especially the depth profile of the implant, is important for evaluating the specific process parameters. However, there are not many analysis techniques that can provide depth profiles of nanometre resolution at the very surface of a sample. Secondary Ion Mass Spectrometry (SIMS) is a well established technique for such measurements, but in the near-surface region SIMS encounters problems due to the so-called transient effects, which distort the measured atomic concentrations in that area. [2]. Therefore, complementary techniques capable of providing accurate depth profiles in the upper few nanometres are required.
High depth resolution Elastic Recoil Detection (ERD) is able to provide these depth profiles. ERD in conjunction with a magnetic spectrometer for high depth resolution (setup developed at HZDR, see. Fig. 1) has been performed to evaluate boron implant profiles in silicon for different implantation parameters [3]. Experimental conditions have been optimised to minimise sample damage while measuring. Boron recoil ions are detected using 6.5 MeV chlorine ions. Under those conditions no beam damage could be detected during measurement. Undistorted near-surface depth profiles of the boron implants can be reconstructed.
Acknowledgments: We thank B. Beckhoff (PTB Berlin) for providing the samples and W. Möller for very helpful discussions.

[1] L. Shao, J.R. Liu, Q.Y. Chen, W.K. Chu, Materials Science & Engineering R-Reports 42 (2003) 65-114.
[2] D. Avau,W. Vandervorst, H.E. Maes, Surf. Interf. Anal. 11 (1988) 522–528.
[3] P. Hönicke, B. Beckhoff, M. Kolbe et al., Anal. Bioanal. Chemistry 396 (2010) 2825-2832.

Keywords: Ultra-shallow junction; High-resoulution Elastic recoil detection; Boron; Silicon

  • Poster
    20th International Conference on Ion Beam Analysis, 10.-15.04.2011, Itapema - Santa Catarina, Brazil

Publ.-Id: 15057

Influence of Fe buffer thickness on the crystalline quality and the transport properties of Fe/Ba(Fe1-xCox)(2)As-2 bilayers

Iida, K.; Haindl, S.; Thersleff, T.; Haenisch, J.; Kurth, F.; Kidszun, M.; Huehne, R.; Moench, I.; Schultz, L.; Holzapfel, B.; Heller, R.

The implementation of an Fe buffer layer is a promising way to obtain epitaxial growth of Co-doped BaFe2As2 (Ba-122). However, the crystalline quality and the superconducting properties of Co-doped Ba-122 are influenced by the Fe buffer layer thickness, d(Fe). The well-textured growth of the Fe/Ba-122 bilayer with d(Fe) = 15 nm results in a high J(c) of 0.45 MA cm(-2) at 12 K in self-field, whereas a low J(c) value of 61 000 A cm(-2) is recorded for the bilayer with d(Fe) = 4 nm at the corresponding reduced temperature due to the presence of grain boundaries. (C) 2010 American Institute of Physics. [doi:10.1063/1.3509418]

Keywords: barium compounds; buffer layers; cobalt; grain boundaries; high-temperature superconductors; iron; iron compounds; pulsed laser deposition; superconducting epitaxial layers

Publ.-Id: 15056

Hybrid magnetic materials created by local ion irradiation

Strache, T.; Roshchupkina, O.; Fritzsche, M.; Basith, M. A.; McCord, J.; Fassbender, J.

Lateral patterning of thin ferromagnetic films allows the modification of the magnetic parameters below certain intrinsic magnetic length scales like the domain wall width. In contrast to the creation of isolated micro- and nanostructures, magnetic patterning by means of local ion irradiation results in direct exchanged coupled regions in the thin film, which have different magnetic properties. At the lateral interfaces of these regions magnetic domain walls can be trapped and thus easily investigated. The modification of the material properties, e.g. saturation magnetization and magnetic anisotropy, also directly affects the intrinsic length scales. As a model system periodic patterns of Ni80Fe20 stripes with alternating saturation magnetization value located either inside a Ni80Fe20 matrix or isolated are used. During the magnetic reversal with the external field parallel to the stripes, magnetic domain walls are created between adjacent stripes due to a noneparallel alignment of the stripe’s magnetization
(Fig. 1). Fig. 1: Magnetization distribution in periodic pattern of 10 μm wide stripes with alternating saturation magnetization value. The magnetization directions are indicated by arrows. (Non-) irradiated regions with (unchanged) decreased saturation magnetization are indicated with (gray) white shaded areas at the bottom. The location of a 180° domain wall is indicated by the green rectangle. As the wall profile for stripe widths in the sub-μm regime is not easily accessible in experiment, changes of the possible orientations of the magnetization inside the stripes with respect to the stripe orientation are used to obtain informations about the domain walls. For special pattern configurations these domain walls are able to mediate an exchange spring behavior through the storage of energy [1]. By scaling the stripes width down, a crossing between the patterning size and the intrinsic length scales associated with the domain walls is expected. This is combined with a transformation to an effective magnetic medium, showing neither the properties of the fully irradiated nor the unirradiated material (so-called hybrid material). Local ion irradiation is accomplished by irradiation in combination with a lithographically defined mask or the by the use of a focused ion beam. For comparision the ion energies for both methods are choosen to be 30 keV. For a Ni80Fe20 film thickness of 20 nm the resulting penetration depths of the ions cover the complete film as well as the interface to the seed layer. In order to understand the ion irradiation induced changes in the magnetic parameters of the film, ions of different species (Ar+, Ga+, Cr+, Ni+, Co+, Si+, O+) as well as different seed layers are used. Changes of the magnetic and structural properties are studied as function of ion species, fluence and seed layer material. The uniaxial magnetic anisotropy of the Ni80Fe20 film is not affected by the implanted ion species, but by the species of the recoils originating from the seed layer. Using SiO2 as seed layer material, the anisotropy can be successively annihilated with increasing fluence. Irradiation using a Ga+ focused ion beam with its high current density induces a grain growth to the material [2], limiting therefore also the minimum patterning size (Fig. 2). Fig. 2: Bright field plane view TEM image of 200 nm wide stripes. The focused ion beam irradiated areas are observable by the increased grain size.
References: [1] McCord J. et al. Advanced Materials 20 (2008) 2090. [2] Ozkaya D.L. et al. Journal of Applied Physics 91 (2002) 9937.

  • Poster
    Workshop Ionenstrahlphysik, 29.03.2010, Dresden, Deutschland

Publ.-Id: 15055

Saturation magnetization modulated stripes embedded in a ferromagnetic matrix

Strache, T.; Wintz, S.; Basith, M. A.; Martin, N.; Fritzsche, M.; Mönch, I.; Liedke, M. O.; Körner, M.; Markó, D.; Raabe, J.; Mcvitie, S.; McCord, J.; Fassbender, J.

Lateral patterning of thin magnetic films allows structuring on dimensions below certain intrinsic length scales like the domain wall width. By means of magnetic patterning using local ion irradiation periodic patterns of stripes with alternating saturation magnetization value were created in a ferromagnetic Ni80Fe20 matrix. The domain configuration during magnetization reversal was investigated using Kerr microscopy, scanning transmission x-ray microscopy, as well as Lorentz microscopy. The reversal mechanisms in the stripe panels are influenced by the domain configuration of the surrounding film. Starting from 1 um stripe width, changes in the micromagnetic behavior with respect to decreasing width are investigated. Special emphasis is put on the formation of 180∘ walls between adjacent stripes with different saturation magnetization, on the orientation of the magnetization inside the stripes with respect to the stripe axis orientation, as well as on the transition of the patterned material to an effective medium.

  • Lecture (Conference)
    DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2010, 24.03.2010, Regensburg, Deutschland
  • Poster
    Joint European Magnetic Symposia (JEMS) 2010, 27.08.2010, Kraków, Poland

Publ.-Id: 15054

Influence of implantation induced Ni-doping on structural, optical, and morphological properties of nanocrystalline CdS thin films

Chandramohan, S.; Strache, T.; Sarangi, S. N.; Sathyamoorthy, R.; Som, T.

Ni-doped CdS thin films were prepared by 90 keV Ni+ implantation at room temperature. Ni-ion implantation induced modifications in structural, optical, and morphological properties are studied for a wide range of impurity concentrations (1.86–10.19 at.%). Addition of Ni+ ions does not lead to any structural phase transformation or formation of metallic clusters or secondary phase precipitates. However, it induces structural disorder leading to a reduction in the optical band gap from 2.39 to 2.28 eV following Ni implantation up to 3 × 1016 ions cm−2. This is addressed on the basis of band tailing due to the creation of localized energy states and implantation induced grain growth. Moreover, Ni-doping is found to modify the luminescence properties by creating shallow acceptor states.

Publ.-Id: 15053

Magnetic properties of granular CoCrPt:SiO2 films as tailored by Co+ irradiation

Tibus, S.; Strache, T.; Springer, F.; Makarov, D.; Rohrmann, H.; Schrefl, T.; Fassbender, J.; Albrecht, M.

We report on an approach to tailor the magnetic exchange in a conventional granular CoCrPt:SiO2 recording medium by irradiation with Co+ ions. Irradiation at low fluences enhances the intergranular exchange resulting in a narrowing of the switching field distribution (SFD). The modification of magnetic exchange coupling was evidenced by measuring the angular dependence of the switching field and is supported by an increase in the magnetic domain size. Further, micromagnetic simulations of a granular magnetic medium confirm the correlation between intergranular exchange and SFD. At high fluences, however, the irradiation damages lead to the degradation of the magnetic layer as magnetic anisotropy and saturation magnetization decrease. Ion irradiation simulations suggest that this is caused by strong intermixing at the grain and layer interfaces.

Keywords: chromium alloys; cobalt alloys; discontinuous metallic thin films; granular materials; ion beam effects; magnetic anisotropy; magnetic domains; magnetic recording; platinum alloys; silicon compounds

Publ.-Id: 15052

A Development Strategy for Creating a Suite of Reference Materials for the in-situ Microanalysis of Non-conventional Raw Materials

Renno, A. D.; Merchel, S.; Michalak, P. P.; Munnik, F.; Wiedenbeck, M.

Recent economic trends regarding the supply of rare metals readily justify scientific research into non-conventional raw materials, where a particular need is a better understanding of the relationship between mineralogy, microstructure and the distribution of key metals within ore deposits (geometallurgy). Achieving these goals will require an extensive usage of in-situ microanalytical techniques capable of spatially resolving material heterogeneities which can be key for understanding better resource utilization. The availability of certified reference materials (CRMs) is an essential prerequisite for (1) validating new analytical methods, (2) demonstrating data quality to the contracting authorities, (3) supporting method development and instrument calibration, and (4) establishing traceability between new analytical approaches and existing data sets. This need has led to the granting of funding by the European Union and the German Free State of Saxony for a program to develop such reference materials . This effort will apply the following strategies during the selection of the phases: (1) will use exclusively synthetic minerals, thereby providing large volumes of homogeneous starting material. (2) will focus on matrices which are capable of incorporating many ‘important’ elements while avoid exotic compositions which would not be optimal matrix matches. (3) will emphasise those phases which remain stable during the various microanalytical procedure. This initiative will assess the homogeneity of the reference materials at sampling sizes ranging between 50 and 1 µm; it is also intended to document crystal structural homogeneity too, as this too may potentially impact specific analytical methods. As far as possible both definitive methods as well as methods involving matrix corrections will be used for determining the compositions of the of the individual materials. A critical challenge will be the validation of the determination of analytes concentrations as sub-µg sampling masses. It is planned to cooperate with those who are interested in the development of such reference materials and we invite them to take part in round-robin exercises.

Keywords: reference materials; geometallurgy

  • Poster
    American Geophysical Union, Fall Meeting 2010, 13.-17.12.2010, San Francisco, USA


Publ.-Id: 15051

Energy oscillations and a possible route to chaos in a modified Riga dynamo

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

Starting from the present version of the Riga dynamo experiment with its rotating magnetic eigenfield dominated by a single frequency we ask for those modifications of this set-up that would allow for a non-trivial magnetic field behaviour in the saturation regime. Assuming an increased ratio of azimuthal to axial flow velocity, we obtain energy oscillations with a frequency below the eigenfrequency of the magnetic field. These new oscillations are identified as magneto-inertial waves that result from a slight imbalance of Lorentz and inertial forces. Increasing the azimuthal velocity further, or increasing the total magnetic Reynolds number, we find transitions to a chaotic behaviour of the dynamo.

  • Astronomische Nachrichten 332(2011), 4-10

Publ.-Id: 15050

DRESDYN – A new platform for thermo-hydraulic studies and the development of measurement techniques for liquid sodium

Stefani, F.; Eckert, S.; Buchenau, D.; Gerbeth, G.

At Forschungszentrum Dresden-Rossendorf the large-scale liquid sodium facility DRESDYN (DREsden Sodium facility for DYNamo and thermo-hydraulic studies) is under construction that will comprise experiments for thermo-hydraulic studies and for the development and the test of measurement techniques for liquid sodium flows as well as experiments with geo- and astrophysical background. This paper summarizes previous achievements in these fields, and gives an overview about the planned activities with relevance to sodium-cooled fast reactors.

  • Lecture (Conference)
    ICAPP 2011 "Performance & Flexibility: The Power of Innovation", 02.-05.05.2011, Nice, France
  • Contribution to proceedings
    ICAPP 2011 "Performance & Flexibility: The Power of Innovation", 02.-05.052011, Nice, France
    Proceedings of ICAPP 2011, 631-638

Publ.-Id: 15049

Application of DYN3D/ATHLET and DYN3D/RELAP5 coupled codes to simulations of RUTA-70 reactor with CERMET fuel

Kozmenkov, Y.; Rohde, U.

Computer models of the RUTA-70 reactor facility for simulations with the coupled code systems DYN3D/ATHLET and DYN3D/RELAP5 have been developed. The main specific features of the RUTA reactor concept are low pressure in the primary circuit and natural circulation of the primary coolant at reactor power levels less then 30% of the rated power. According to the RUTA facility design requirements the saturated boiling of primary coolant must be excluded for all regimes of normal reactor operation. Only a small degree of subcooled boiling is allowed in the reactor core. However, the steam generation in the primary circuit can occur during accidents, for example, as a result of the pump failure event. Transition from the single-phase to the two-phase flow in natural circulation systems can lead to unstable system behavior, and this possibility should be properly analysed. For this reason, the thermal-hydraulic model of RELAP5 code was validated against the CIRCUS experiments conducted under low pressure conditions. The comparison between calculated and measured data proved the ability of RELAP5 to model the flashing-induced instability in natural circulation systems. The RELAP5 code predicts measured frequencies and amplitudes of oscillations with a high degree of accuracy.
The transient with a positive reactivity insertion and activation of SCRAM was simulated for the RUTA facility using ATHLET and RELAP5 codes. These calculations were aimed at verification of the RUTA thermal-hydraulic model and were performed with the fixed profile of power distribution in the core and the core power history calculated with the point neutron kinetics model (the same for both compared models). Under these assumptions the ATHLET and RELAP5 models of RUTA give close results for the rated steady-state parameters and transient behavior of the reactor. A BDB accident scenario for the RUTA reactor was simulated using the coupled codes DYN3D/ATHLET and DYN3D/RELAP5 to verify the code systems under conditions of the two-phase flow and possible unstable reactor behavior. The failure of all primary circulation pumps (initial event) and the failure of reactor SCRAM were postulated. The predictions for initial and final reactor states given by the codes are in a good agreement. However, the process of transition between these two states shows a qualitative difference. The DYN3D/RELAP5 code predicts unstable transient behavior of the reactor, while in the DYN3D/ATHLET simulation a smooth change of reactor parameters is observed during the whole accident. It was found that due to differences between the subcooled boiling models of ATHLET and RELAP5 there is a delay in transition to the two-phase flow in the core in the DYN3D/RELAP5 simulation compared to the DYN3D/ATHLET results. This fact leads to a less decrease of the reactor power due to the core coolant density feedback during the initial phase of the DYN3D/RELAP5 calculation. As a result, a higher energy is transferred to the core coolant during first seconds of the accident. Then a sharp transition from the single-phase to the two-phase flow in the core occurs, with a significantly higher volume of steam generated in the most heated fuel assemblies compared to the DYN3D/ATHLET prediction. A strong perturbation of the core coolant density transfers the reactor to unstable state in the case of the DYN3D/RELAP5 simulation.
Despite the different performance of RUTA in the DYN3D/ATHLET and DYN3D/RELAP5 simulations of the BDB accident the obtained results confirm a high intrinsic safety level for this reactor concept. In both compared calculations the allowed safety margins have not been reached.

Keywords: pool-type reactor; light water reactor; heat supply; flashing-induced instability; natural circulation experiments; code validation; inherent safety; beyond design basis accident

  • Other report
    Rossendorf: FZD: Internal Report FZD\FWS\2010\11, 2010
    37 Seiten

Publ.-Id: 15048

Adaptation of the DYN3D code for calculation of transients in graphite-moderated channel-type reactors - Analysis of reactivity accidents

Kozmenkov, Y.; Matveev, Y.; Pivovarov, V.; Editor: Rohde, U.

The adaptation of the reactor dynamics code DYN3D to the analysis of EGP-6 type reactors (graphite-moderated channel-type reactor) has been performed. Test calculations of EGP-6 transients with an unauthorized movement of control rods as well as simulations of reactor experiments with control rod movement and short-time steam valve opening have been performed. Calculated results were compared with recorded reactivity measurement curves.

Keywords: reactor dynamics; graphite-moderated channel-type reactor; reactor experiments; code validation

  • Other report
    Rossendorf: FZD Internal Report FZD\FWS\2010\10, 2010
    92 Seiten

Publ.-Id: 15047

Adaptation of multi-group version of DYN3D code to analytical studies on boiling water reactor with reduced neutron moderation - Analysis of reactivity accidents

Matveev, Y.; Pivovarov, V.; Editor: Rohde, U.

Adaptation of multi-group version of DYN3D code was made to make analytical studies on boiling water reactor with tight fuel element lattice and fast neutron spectrum. For this purpose, the code was linked to nuclear cross section data generated by the lattice code IPPE-WIMS. Library of 5-group nuclear data was prepared for analytical studies on boiling water fast reactor. In order to confirm correctness of the preparation of the nuclear data and code performance for this type of reactor, series of test and demonstration calculations of steady state modes were made including calculations with base constants (ignoring thermal hydraulics feedbacks), as well as combined analysis of reactor neutronics and thermal hydraulics for the beginning and the end of the core lifetime. Analytical studies were performed on two reactivity accidents caused by ejection of highest-worth control rod and unauthorized withdrawal of control rods group in the reactor operating on rated power.

Keywords: boiling water reactor; high conversion; fast spectrum; core design; transient analyses; rod ejection accident

  • Other report
    Rossendorf: FZD\FWS\2010\09, 2010
    45 Seiten

Publ.-Id: 15046

WTZ mit Russland - Transientenanalysen für Kernreaktoren - Abschlussbericht

Rohde, U.; Kozmenkov, Y.; Matveev, Y.; Pivovarov, V.

Der Reaktordynamikcodes DYN3D wurde in der neu entwickelten Mehrgruppen-Version DYN3D-MG für die Anwendung auf wassergekühlte Reaktoren alternativ zu industriellen DWR und SWR ertüch-tigt. Es wurde die Anwendbarkeit für den graphitmoderierten Druckröhrenreaktor EGP-6 (KKW Bilibi-no), eine Konzeptstudie eines fortgeschrittenen Siedewasserreaktors mit schnellem Neutronenspekt-rum (RMWR) und das Reaktorkonzept RUTA-70 zur Wärmeversorgung nachgewiesen. Beim RUTA-Reaktor geht es vor allem um die Modellierung des Naturumlaufs des Kühlmittels bei niedrigen Sys-temdrücken. Zur Validierung wurden Experimente zu flashing-induzierten Naturumlaufinstabilitäten an der Versuchsanlage CIRCUS der TU Delft mit RELAP5 nachgerechnet. Für die Anwendung von DYN3D auf die alternativen Reaktorkonzepte wurden Modellerweiterungen und Anpassungen vorgenommen, u. a. Modifikationen in den Wärmeleitungs- und -übergangsmodellen. Vergleichsrechnungen mit dem stationären russischen Feingitter-Diffusionscode ACADEM ergänzen die Verifikationsdatenbasis von DYN3D-MG. Zur Validierung wurden zwei reak-tordynamische Experimente am Reaktor EGP-6 nachgerechnet. Für Reaktoren EGP-6, RMWR und RUTA wurden verschiedene Transienten mit Ausfahren von Re-gelstäben mit und ohne Reaktorschnellabschaltung gerechnet. Weiterhin wurden Analysen für den ATWS-Störfall "Abschalten aller Hauptkühlmittelpumpen bei Vollleistung" für den RUTA-Reaktor mit den gekoppelten Programmkomplexen DYN3D/ATHLET und DYN3D/RELAP5 durchgeführt. Der Reaktor geht in einen sicheren Zustand mit reduzierter Leistung bei Naturumlauf des Kühlmittels über. Die Ergebnisse von Analysen zum unkontrollierten Ausfahren einer Regelgruppe für den RMWR lassen dagegen eine belastbare Schlussfolgerung bezüglich der Beherrschbarkeit des Aus-fahrens einer Regelgruppe nicht zu. Abschließend wurde der Nutzen der Programmertüchtigung von DYN3D für die Anwendung auf GenIV -Konzepte und LWR mit hohem Konversionsfaktor bewertet.

Keywords: Light water reactors; reactor physics; thermal hydraulics; transients; high conversion; validation

  • Open Access Logo Wissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; FZD-543 2010
    ISSN: 1437-322X


Publ.-Id: 15045

Nanomachining of freestanding Si nanowires

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
    Dresdner Barkhausen-Poster-Preis 2010 für Studenten und Nachwuchswissenschaftler, 16.12.2010-15.01.2011, Dresden, Deutschland

Publ.-Id: 15044

Magnetic-field-induced crossover of vortex-line coupling in SmFeAsO0.85 single crystal

Lee, H.-S.; Bartkowiak, M.; Kim, J. S.; Lee, H.-J.

We take the resistivity (rho) and the current-voltage (I-V) characteristics of a SmFeAsO0.85 single crystal as a function of temperature (T) for various magnetic fields up to 18 T along the c axis. The tail region of rho(T) well fits the three-dimensional (3D) vortex-glass critical behavior. The critical exponents for the vortex-glass scaling of (d ln rho/dT)-1-T, I-V curves, and the thermal activation energies of vortices in magnetic fields show a crossover for magnetic fields around 3 T. The crossover behavior results from the change in the strength of the vortex pinning and the entanglement of vortex lines in the stacked superconducting structure of SmFeAsO0.85 material with the c-axis vortex-line coupling lying in between highly anisotropic Bi-based cuprates with decoupled two-dimensional vortices and less-anisotropic YBa2Cu3O7-delta with coupled 3D vortices.

Publ.-Id: 15043

Irradiation Effects in FeCrAl ODS Alloys

Chen, C. L.; Richter, A.; Talut, G.; Kögler, R.

Results are presented for the FeCrAl ODS alloy containing nanoparticles with mean size of 21 nm. Structural analysis including HRTEM shows that the chemical composition of the Y2O3 oxide is modified with perovskite YAlO3 (YAP) and Y2Al5O12 garnet (YAG). Irradiation of these alloys was performed both with 400 keV Fe+ ions and a fluence Φ of 5x1016 cm-2 and a dual beam irradiation with 2.5 MeV Fe2+ (65 dpa) and 400 keV He+ (12 appm/dpa). He ions preferentially occupy the oxide-metal interfaces. He accumulation can occur around large nanoparticles but is not observed around small ones. In addition to structural changes, irradiation can also alter the mechanical properties such as hardness and indentation modulus. Nanoindentation experiments are suitable measurements to investigate these changes since the irradiation damage produced by the ion beam is in a thin surface layer. An increase in hardness with increasing radiation dose has been observed.

Keywords: Irradiation hardened steel; nanointendation; ion implantation

  • Lecture (Conference)
    1st International Conference on Materials for Energy, 04.-08.07.2010, Karlsruhe, Deutschland

Publ.-Id: 15042

Optical and microstructural properties of self-assembled InAs quantum structures in silicon

Prucnal, S.; Turek, M.; Drozdziel, A.; Pyszniak, K.; Wójtowicz, A.; Zhou, S. Q.; Kanjilal, A.; Shalimov, A.; Skorupa, W.; Zuk, J.

The InAs quantum structures were formed in silicon by sequential ion implantation and subsequent thermal annealing. Two kinds of crystalline InAs nanostructures were successfully synthesized: nanodots (NDs) and nanopyramids (NPs). The peaks at 215 and 235 cm−1, corresponding to the transverse optical (TO) and longitudinal optical (LO) InAs single-phonon modes, respectively, are clearly visible in the Raman spectra. Moreover, the PL band at around 1.3 μm, due to light emission from InAs NDs with an average diameter 7 ± 2 nm, was observed. The InAs NPs were found only in samples annealed for 20 ms at temperatures ranging from 1000 up to 1200°C. The crystallinity and pyramidal shape of InAs quantum structures were confirmed by HRTEM and XRD techniques. The average size of the NPs is 50 nm base and 50 nm height, and they are oriented parallel to the Si (001) planes. The lattice parameter of the NPs increases from 6.051 to 6.055 Å with the annealing temperature increasing from 1100 to 1200°C, due to lattice relaxation. Energy dispersive spectroscopy (EDS) shows almost stoichiometric composition of the InAs NPs.

Keywords: InAs; nanodots; flash lamp annealing; photoluminescence

Publ.-Id: 15041

Occurrence and origin of non-martensitic acicular artifacts on NiTi

Undisz, A.; Reuther, K.; Reuther, H.; Rettenmayr, M.

Acicular structures on NiTi are often interpreted as martensite or martensitic surface relief. These structures are identified as artifacts and form during etching using hydrofluoric acid based etching solutions. Their origin is clarified in the present work. After standard metallographic preparation, the acicular structures cannot be distinguished from bulk material by standard analysis, e.g. energy dispersive X-ray analysis and Auger electron spectroscopy. By investigating samples after etching, but previous to cleaning with water, rod-like crystals on the NiTi surface were detected. The crystals are identified as NiTiF66H2O by X-ray diffraction. During etching, the crystals develop on the surface and locally prevent contact of etching solution and NiTi base material. Further etching solely removes NiTi at adjacent uncovered areas. During the final cleaning with water the crystals completely dissolve and the surface relief with a shape according to that of the dissolved crystals remains. The surface relief does not represent the bulk microstructure.

  • Acta Materialia 59(2011), 216-224

Publ.-Id: 15040

Bridging frustrated-spin-chain and spin-ladder physics: Quasi-one-dimensional magnetism of BiCu2PO6

Tsirlin, A. A.; Rousochatzakis, I.; Kasinathan, D.; Janson, O.; Nath, R.; Weickert, F.; Geibel, C.; Läuchli, A. M.; Rosner, H.

We derive and investigate the microscopic model of the quantum magnet BiCu2PO6 using band-structure calculations, magnetic susceptibility and high-field magnetization measurements, as well as exact diagonalization (ED) and density-matrix renormalization group (DMRG) techniques. The resulting quasi-one-dimensional spin model is a two-leg antiferromagnetic ladder with frustrating next-nearest-neighbor couplings along the legs. The individual couplings are estimated from band-structure calculations and by fitting the magnetic susceptibility with theoretical predictions, obtained using full diagonalizations. The nearest-neighbor leg coupling J1, the rung coupling J4, and one of the next-nearest-neighbor couplings J2 amount to 120-150 K while the second next-nearest-neighbor coupling is J'2 similar or equal to J2/2. The spin ladders do not match the structural chains, and although the next-nearest-neighbor interactions J2 and J'2 have very similar superexchange pathways, they differ substantially in magnitude due to a tiny difference in the O-O distances and in the arrangement of nonmagnetic PO4 tetrahedra. An extensive ED study of the proposed model provides the low-energy excitation spectrum and shows that the system is in the strong rung coupling regime. The strong frustration by the next-nearest-neighbor couplings leads to a triplon branch with an incommensurate minimum. This is further corroborated by a strong-coupling expansion up to second order in the inter-rung coupling. Based on high-field magnetization measurements, we estimate the spin gap of Delta-32 K and suggest the likely presence of antisymmetric Dzyaloshinskii-Moriya anisotropy and interladder coupling J(3). We also provide a tentative description of the physics of BiCu2PO6 in magnetic field, in the light of the low-energy excitation spectra and numerical calculations based on ED and DMRG. In particular, we raise the possibility for a rich interplay between one- and two-component Luttinger liquid phases and a magnetization plateau at 1/2 of the saturation value.

Publ.-Id: 15039

Doping & Annealing Issues of Electronic Materials

Skorupa, W.

Overview on basics and advances in doping of semicondutors

Keywords: ion implantation; annealing; boron; phosphorus; arsenic; nanowires; diluted material; light emitter

  • Invited lecture (Conferences)
    SPIRIT Tutorial "Ion Implantation and Irradiation", 13.-14.12.2010, Dresden, Deutschland

Publ.-Id: 15038

Structural and charge trapping properties of two bilayer (Ge+SiO2)/SiO2 films deposited on rippled substrate

Buljan, M.; Grenzer, J.; Holý, V.; Radić, N.; Mišić-Radić, T.; Levichev, S.; Bernstorff, S.; Pivac, B.; Capan, I.

We report on structural properties and charge trapping in [(Ge+SiO2)/SiO2]×2 films deposited by magnetron sputtering on a periodically corrugated-rippled substrate and annealed in vacuum and forming gas. The rippled substrate caused a self-ordered growth of Ge quantum dots, while annealing in different environments enabled us to separate charge trapping in quantum dots from the trapping at the dot-matrix and matrix-substrate interfaces. We show that the charge trapping occurs mainly in Ge quantum dots in the films annealed in the forming gas, while Si–SiO2 interface trapping is dominant for the vacuum annealed films.

Publ.-Id: 15037

Formation of void lattice after annealing of Ge quantum dot lattice in alumina matrix

Pinto, S. R. C.; Rolo, A. G.; Gomes, M. J. M.; Ivanda, M.; Bogdanović-Radović, I.; Grenzer, J.; Mücklich, A.; Barber, D. J.; Bernstorff, S.; Buljan, M.

We report on the formation of a regularly ordered void lattice with a void size of about 4 nm in an alumina matrix. The voids were formed by thermal treatment of a well-ordered three-dimensional Ge quantum dot lattice formed earlier by self-assembled growth in an alumina matrix during magnetron sputtering codeposition of Ge+Al2O3. During the subsequent annealing the germanium atoms were lost from the film and so voids were produced. The positions of the voids are ordered in the same way as the Ge quantum dots that were present before annealing, while their sizes can be controlled by the deposition parameters.

Publ.-Id: 15036

Tuning the quality of nanoscale ripple patterns by sequential ion-beam sputtering

Keller, A.; Facsko, S.

It is demonstrated that the quality of nanoscale ripple patterns on silicon surfaces can be substantially improved by applying sequential ion-beam sputtering. A flat silicon surface is sputtered at an intermediate incident angle which leads to the spontaneous formation of a periodic ripple pattern with 25 nm periodicity oriented normal to the direction of the incident ion beam. After rotating the sample by an azimuthal angle of 90, the surface is sputtered parallel to the ripples under grazing incidence. At the low fluences applied in this second step, no ripple pattern oriented parallel to the ion beam forms. However, due to geometrical shadowing and preferential erosion of pattern defects, grazing incidence sputtering enhances the order and regularity of the ripple pattern. The quality of the ripple pattern is assessed by evaluating its normalized density of topological defects which is determined from atomic force microscopy images. During grazing incidence ion sputtering, the normalized defect density is found to decrease exponentially with the applied ion fluence. It is shown that in this way the defect density of the initial ripple pattern can be reduced by at least 40% while keeping its periodicity approximately constant. Numerical integrations of the Kuramoto-Sivashinsky equation are in good qualitative agreement with the experimental results and suggest that the observed reduction in the density of pattern defects during sequential ion-beam sputtering is a universal effect present in a large variety of experimental systems.


Publ.-Id: 15035

Experiments on the transport, deposition and resuspension of nuclear aerosols

Barth, T.; Banowski, M.; Hampel, U.

Nuclear aerosol deposition and the assessment of its resuspension during a design basis accident in the primary circuit are a key issue in the development and certification of advanced pebble bed High Temperature Reactors (HTRs). Nuclear aerosols, in particular graphite dust in size of d = [0.1; 50] μm, are deposited during operation in the primary circuit (Moormann (2008)). It is of general interest how much of these aerosols escape from the primary circuit into the containment during a depressurization scenario. The knowledge about the amount of resuspended dust allows the detailed estimate of the dose escaping the primary circuit Flow conditions in the primary circuit range from laminar flows in the recuperator till turbulent high Reynolds number flows in the pipes and ducts. Considering the particle size distribution published by Moormann (2008), particle Stokes numbers will range from very small Stk << 1 till moderately high Stk > 1. In order to investigate the fluid mechanic behavior between the flow and the aerosol within the set of characteristic numbers, we designed a small scale gas aerosol test facility .
We found a turbulent square duct flow most suitable because all the flow features such as streamwise and spanwise velocity gradients, as well as vortical structures are apparent. Furthermore, there will be a wide range of experimental and numerical data for comparison.
The test facility is a small scale wind tunnel in total length of 6 m with a 10 x 10 cm² square duct section. A 500 W radial fan at the outlet of the channel accelerates the flow field from 0 up to 7.5 m/s which is equivalent to a Reynolds number of about Re = 50 k.
The inlet is equipped with a HEPA filter to clean the incoming air. A nozzle contracts the flow into a square duct which is divided into a flow formation zone (15 x d) and a test section (5 x d). In the beginning of the flow formation zone the dust feeder or the aerosol generator injects the aerosol. The length of 15 x d for the flow formation zone ensures that the test section is streamed by a well developed turbulent channel flow with an evenly distributed aerosol. Both, test section and flow formation zone, are made of transparent acrylic glass to allow optical flow field measurements, such as PIV, high speed camera imaging for the analysis of the flow field and microscopic imaging techniques for the surface particle detection.
A diffusor stage decelerates the flow before it enters the electrostatic filter for air cleaning purposes. Finally, the 500 W radial fan produces the pressure drop for the desired flow speed.
Preliminary measurements of total pressure drop in the square duct section and time averaged mean center velocity profiles for different Reynolds number will be presented on the poster.

Keywords: Nuclear Aerosol Particles; Particle Image Velocimetry; VHTR; Deposition; Resuspension

  • Poster
    Doktorandenseminar 2010 im Rahmen des Kompetenzzentrums Ost für Kerntechnik, 16.12.2010, Dresden, Deutschland

Publ.-Id: 15034

Surface protection of TiAl alloys and Ti against high-temperature environmental degradation

Yankov, R. A.; Kolitsch, A.; Munnik, F.; von Borany, J.; Donchev, A.; Schütze, M.

Ti and TiAl alloys are lightweight materials that hold great promise for advanced aerospace, automotive and power generation applications. They are, however, limited in applicability by their poor oxidation resistance at elevated temperatures. We have developed viable techniques for enhancing the high-temperature environmental durability of these materials. In the case of TiAl, the process has involved a single step, i.e. plasma immersion implantation (PIII) of fluorine relying on the so-called “halogen effect”. Optimum processing conditions have been established under which the F-implanted alloys acquire a stable, adherent and highly protective alumina scale upon subsequent high-temperature oxidation in air. The extent of oxidation protection has been evaluated by testing F-implanted TiAl laboratory coupons as well as machine components (e.g. turbine blades and turbochargers) at temperatures as high as 1050°C, and for times up to 6000 h under conditions of both isothermal and thermal cyclic oxidation. Analytical methods such as elastic recoil detection, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis have been used for materials characterization. In the case of Ti, an efficient protective coating that serves as a barrier to the oxygen in-diffusion has been developed. Specifically, the coating consists of a γ-phase TiAl formed by magnetron co-sputtering of Ti and Al onto the Ti substrate, and subsequent vacuum annealing. After PIII of F, the coating has proven capable of forming a protective alumina scale upon exposure to air for extended times at temperatures up to 600°C.

  • Poster
    Dresdner Werkstoffsymposium Werkstoffe der Energietechnik, 09.-10.12.2010, Dresden, Germany

Publ.-Id: 15033

Comparison of different fluorine-treatments for improved high temperature oxidation resistance of TiAl-alloys

Donchev, A.; Schütze, M.; Kolitsch, A.; Yankov, R.

Intermetallic TiAl-alloys can replace the heavier Ni-based superalloys in several high temperature applications with regard to their mechanical properties but they can not be used at temperatures above 800°C in oxidizing environments for longer times because of an insufficient oxidation resistance. Despite an Al-content of about 45 at.% in technical alloys no protective alumina layer is formed because the thermodynamic stabilities of titanium oxide and aluminum oxide are in the same order of magnitude. Therefore a mixed TiO2/Al2O3-scale is formed which is fast growing so that the metal consumption rate is quite high. On the other hand the formation of a slowly growing alumina layer is promoted by a fluorine treatment. This so-called fluorine effect leads to the preferential intermediate formation of gaseous aluminum fluorides at elevated temperatures if the fluorine content at the surface is being kept within a certain concentration range. These fluorides are then converted into solid Al2O3 due to the high oxygen partial pressure of the high temperature service environment forming a protective pure Al2O3 surface scale. In this paper results of high temperature oxidations tests of several technical TiAl-alloys will be presented. Different F-treatments such as dipping or spaying which are easy to apply or, alternatively, more sophisticated ion beam and plasma-based techniques have been used for surface modification, and their results will be compared. The weight change data of the F-treated specimens are always lower than those of the untreated ones. Analytical methods such as light microscopy, scanning electron microscopy and energy dispersive X-ray analysis reveal the formation of a thin alumina layer on the F-treated samples after optimization of the process while a thick mixed scale is found on the untreated samples. The results will be discussed in view of both the development of an optimized process, and the future use of TiAl-components in high temperature environments.

  • Lecture (Conference)
    2010 MRS Fall Meeting, 29.11.-03.12.2010, Boston, USA

Publ.-Id: 15032

Experiments on the transport, deposition and resuspension of nuclear aerosols

Barth, T.; Hampel, U.

Nuclear aerosol deposition and the assessment of its resuspension during a design basis accident in the primary circuit are a key issue in the development and certification of advanced pebble bed High Temperature Reactors (HTRs). Nuclear aerosols, in particular graphite dust in size of d = [0.1; 50] μm, are deposited during operation in the primary circuit (Moormann (2008)). It is of general interest how much of these aerosols escape from the primary circuit into the containment during a depressurization scenario. The knowledge about the amount of resuspended dust allows the detailed estimate of the dose escaping the primary circuit
Flow conditions in the primary circuit range from laminar flows in the recuperator till turbulent high Reynolds number flows in the pipes and ducts. Considering the particle size distribution published by Moormann (2008), particle Stokes numbers will range from very small Stk << 1 till moderately high Stk > 1. In order to investigate the fluid mechanic behavior between the flow and the aerosol within the set of characteristic numbers, we designed a small scale gas aerosol test facility (Figure 1).

We found a turbulent square duct flow most suitable because all the flow features such as streamwise and spanwise velocity gradients, as well as vortical structures are apparent. Furthermore, there will be a wide range of experimental and numerical data for comparison.
The test facility is a small scale wind tunnel in total length of 6 m with a 10 x 10 cm² square duct section. A 500 W radial fan at the outlet of the channel accelerates the flow field from 0 up to 7.5 m/s which is equivalent to a Reynolds number of about Re = 50 k.
The inlet is equipped with a HEPA filter to clean the incoming air. A nozzle contracts the flow into a square duct which is divided into a flow formation zone (15 x d) and a test section (5 x d). In the beginning of the flow formation zone the dust feeder or the aerosol generator injects the aerosol. The length of 15 x d for the flow formation zone ensures that the test section is streamed by a well developed turbulent channel flow with an evenly distributed aerosol. Both, test section and flow formation zone, are made of transparent acrylic glass to allow optical flow field measurements, such as PIV, high speed camera imaging for the analysis of the flow field and microscopic imaging techniques for the surface particle detection.
A diffusor stage decelerates the flow before it enters the electrostatic filter for air cleaning purposes. Finally, the 500 W radial fan produces the pressure drop for the desired flow speed.
Preliminary measurements of total pressure drop in the square duct section and time averaged mean center velocity profiles for different Reynolds number will be presented on the poster.

Keywords: Nuclear Aerosol Particles; Deposition; Resuspension; VHTR

  • Poster
    International Aerosol Conference – IAC 2010, 29.08.-03.09.2010, Helsinki, Finnland

Publ.-Id: 15031

Feasibility study of flow control by dynamic vane vortex generators based on piezoceramic actuators

Barth, T.; Scholz, P.; Wierach, P.

This investigation is an fundamental study about the perfomance of dynamic operated solid/mechanical vane vortex generators (VVGs). Fluidic vortex generators or so called vortex generator jets (VGJs) are more efficient in dynamic operation. Thus, the scope is to explore if VVGs are superior in dynamic operation, too. Besides, the influence of higher frequencies will be observed. The motion of the VVGs is generated by piezoceramic actuators or so called Marco Fiber Composites (MFC).The MFC actuators consist of bimoporh carbon fibre bars with applied piezoceramic face actuators on the top and downside. These MFC actuators generate a sinusoidal motion of the VVGs and were designed by analytical and experimental investigations. The MFC actuators are operated in resonance to gain the required displacement. Mass clamps applied on the bar tune the eigenfrequency into the operating point. Both, VVGs and MFC actuators, will bei integrated into a flat plate in a low speed wind tunnel. The VVGs penetrate through small plastic slits into a turbulent boundary layer. A stereo PIV system will record phase locked flow field pictures. The transient formation of the position and circulation of the vortex centers states that dynamic operated VVGs are supiror to static VVGs.

Keywords: vortex generator; Stereo PIV; active flow control; turbulent boundary layer

Publ.-Id: 15030

Zeitaufgelöste, Laser-induzierte Fluoreszenzspektroskopie am Beispiel von Untersuchungen zur Sekundärphasenbildung auf Geschossprojektilen aus abgereichertem Uran

Baumann, N.

Untersuchungen zur Speziationsaufklärung fluoreszierender Schwermetallionen mittels der zeitaufgelösten, Laser-induzierten Fluoreszenzspektroskopie (TRLFS) werden am Beispiel der Sekundärphasenanalyse auf Geschossprojektilen aus abgereichertem Uran erläutert, wobei die niedrigen Nachweisgrenzen der TRLFS und damit die Tauglichkeit der Methode zur Lösung von Problemen in umweltrelevanten Konzentrationsbereichen von Uran(VI) Spezies verdeutlicht werden. Zur Petrografie Bezug nehmend wird die Möglichkeit der Identifikation winzigster Uranylmineralmengen durch die TRLFS aufgezeigt.
Baumann N. et al. (2008) ES&T 42 8266-8269.
Geipel G. et al. (2000) Radiochim Acta 88 757-762.

  • Lecture (others)
    Jahrestreffen des petrographischen Studienkreises, 02.05.2010, Kulmbach, Deutschland

Publ.-Id: 15029

Characterization of Mesoporous ZnO:SiO2 Films Obtained by the Sol-gel Method

Martins, R. M. S.; Musat, V.; Mücklich, A.; Fortunato, E.

ZnO:SiO2 films are intensively investigated for optical and electronic applications. Additionally, porous ZnO:SiO2 films are of great interest as catalyst and gas sensing materials. The sol-gel method is an efficient and low-cost process for the deposition of meso- and microporous silica-based films.
The present paper studies the effect of the withdrawal speed on the microstructure and optical properties of mesoporous ZnO:SiO2 films obtained by the sol-gel method. The morphology of the films was investigated by atomic force microscopy and the overall structure was studied by x-ray diffraction. The structure and size of the zinc oxide nanoparticles embedded in the silica matrix was investigated in more detail by transmission electron microscopy. These techniques showed ZnO:SiO2 films with crack-free mesoporous morphology and highly efficient embedding of ZnO nanoparticles with (100) preferred orientation.
Furthermore, the optical transmittance (in the visible and near infrared regions) and the optical band gap value were observed to vary with withdrawal speed. It is shown that ZnO:SiO2 nanocomposites films which possess ZnO particles exhibiting a (100) orientation, with possible special applications in non-linear optics, could be prepared by the low-temperature crystallization sol-gel method.

  • Poster
    Materials Research Society Fall Meeting (Symposium MM: Transparent Conducting Oxides and Applications), 29.11.-03.12.2010, Boston, MA, USA

Publ.-Id: 15028

In-situ X-ray Diffraction Studies During Magnetron Co-sputtering of Ni-Ti Shape Memory Alloy Films

Martins, R. M. S.; Schell, N.; Mahesh, K. K.; Silva, R. J.; Braz Fernandes, F. M.

The study of Ni-Ti shape memory alloy films is of great technological interest for applications in the field of microengineering. They can work as sensors and actuators at the same time.
However, there are still important issues unresolved like formation of film texture and its control. Films exhibiting the two-way shape memory effect are also required.
A better understanding of the underlying growth mechanisms and their microstructural development requires sophisticated in-situ techniques. A two-magnetron sputter deposition chamber mounted into the six-circle diffractometer of the Rossendorf Beamline at the European Synchrotron Radiation Facility has been used for the processing of the Ni-Ti films. The in-situ x-ray diffraction studies enabled us to identify the different steps of the structural evolution during deposition with a set of parameters as well as to evaluate the effect of changing parameters (Ti target power) during film growth.
It has been found that the type of substrate plays an important role for the preferential orientation of sputtered Ni-Ti films. In some cases they exhibit a pronounced depth dependence of their preferential orientations. Amorphous SiO2 and TiN buffer layers have been used to successfully control their crystallographic orientations. This is an important achievement since the texture has a strong influence on the extent of the strain recovery of the Ni-Ti films. The deposition conditions leading to films mainly containing grains with (100) or (110) planes of the B2 phase parallel to the film surface are presented.
The deposition of graded Ni-Ti films by changing deliberately the Ti:Ni ratio, thereby altering microstructure and transformation temperatures across the film thickness, has also been performed. The aim has been the optimization of the deposition parameters in order to fabricate films with a “two-way” actuation (films with a combination of superelasticity and shape memory characteristics). It will lead to the development of smaller devices due to an optimal design of microdevices regarding size and weight (i.e., no consideration has to be paid to a resetting spring).

  • Lecture (Conference)
    Materials Research Society Fall Meeting (Symposium UU: Real-Time Studies of Evolving Thin Films and Interfaces), 29.11.-03.12.2010, Boston, MA, USA

Publ.-Id: 15027

Surface Modification of Ni-Ti for Biomedical Applications by Plasma-immersion Ion Implantation

Martins, R. M. S.; Barradas, N.; Alves, E.; Henke, D.; Reuther, H.; Carmezim, M. J.; Silva, T. M.; Fernandes, J. C. S.

The enormous elasticity of Ni-Ti is becoming integral to the design of a variety of new medical products. The wide spectrum of application in implantology imposes special requirements on the biocompatibility of Ni-Ti. The biological response to implant materials is a property directly related to their surface conditions and an optimum surface layer is thus desired.

The plasma-immersion ion implantation (PIII) technique was used to modify and improve the surface of a Ni-Ti alloy (~ 50.2 at.% Ni, superelastic at body temperature) for biomedical applications. The main goal has been the formation of a Ni-depleted surface, which should serve as a barrier to out-diffusion of Ni ions from the bulk material. Ion implantation of oxygen was carried out. The depth profiles of the elemental distribution in the alloy surface region, obtained by Auger electron spectroscopy (AES), confirm the formation of a Ti-rich oxide layer. The working plan also comprised ion implantation of nitrogen. In this case, the formation of titanium oxynitride (TiNxOy) was observed. The AES depth profiles clearly show a Ni-depleted fraction for experiments performed with 40 keV.

The deposition of a coating by a PVD technique would have disadvantages due to the interface between the coating and the bulk (lower adhesion). PIII creates a graded interface between the modified surface and the bulk. Techniques like thermal oxidation and nitriding could also lead to an improved corrosion resistance and Ni-depleted Ni-Ti surface. However, the high temperature necessary for the experimental procedure would lead to modification of the phase transformation characteristics and loss of specific mechanical properties of the alloy. Heat treatments tests performed at temperatures above 350ºC led to a shift of the transformation temperatures of the Ni-Ti alloy used in this work. Moreover, the R-phase is then present at body temperature, which is not the case for Ni-Ti samples modified by the PIII technique. This technique only changes the properties of the Ni-Ti alloy top layer.

  • Lecture (Conference)
    Materials Research Society Fall Meeting (Symposium N: Intermetallic-Based Alloys for Structural and Functional Applications), 29.11.-03.12.2010, Boston, MA, USA

Publ.-Id: 15026

Large per-shot numbers of photons from Thomson scattering with variable energy and bandwidth

Debus, A. D.; Bussmann, M.; Siebold, M.; Jochmann, A.; Irman, A.; Schramm, U.; Cowan, T. E.; Sauerbrey, R.

Finite bandwidth x-ray pulses generated in the interaction of a bunch of relativistic electrons with a high-intensity, ultra-short laser pulse are interesting for a variety of experiments that do not require the very small bandwidth delivered by x-ray free electron lasers.
We present a new scheme for Thomson scattering in which we use a grating setup similar to that found in a grating compressor as used in high-power short-pulse laser systems. The gratings are used to introduce a tilt of the laser pulse front and to compensate for dispersion and other unwanted effects on the laser pulse interacting with the electrons. Electron and laser beam are brought in overlap in a side scattering geometry for which the laser pulse is elongated in one spatial direction using an elliptical mirror so that the region of overlap between laser pulse and electron bunch is greatly increased compared to a simple head-on scenario for Thomson scattering..
This increase in size allows to increase the laser pulse energy while keeping the local intensity of the light seen by the electrons low, thus effectively increasing the number of scattered photons by up to three orders of magnitude.
By increasing the length of the overlap region one can decrease the bandwidth of the scattered x-ray photons for a given laser pulse energy, while changing the pulse front tilt angle allows to vary the wavelength of the x-ray photons without changing the electron beam energy.

Keywords: thomson scattering; electron; relativistic; laser; pulse; side-scattering; energy; bandwidth; traveling wave; twts

  • Poster
    470. Wilhelm and Else Heraeus Seminar on Particle Accelerator and High Intensity Lasers, 13.-17.12.2010, Bad Honnef, Deutschland

Publ.-Id: 15025

Scaling up the energy of laser-accelerated ions using novel target structures

Kluge, T.; Gaillard, S.; Flippo, K. A.; Gall, B.; Lockard, T.; Geissel, M.; Offermann, D. T.; Schollmeier, M.; Kraft, S. D.; Metzges, J.; Zeil, K.; Schramm, U.; Sentoku, Y.; Enghardt, W.; Sauerbrey, R.; Bussmann, M.; Cowan, T. E.

We present results on laser acceleration of ions using various designs for micro-structured targets. Our analysis compares recent experimental results on ion acceleration at FZD and Los Alamos to extensive realistic large-scale particle-in-cell simulations.
We focus on the electron dynamics leading to an enhancement in ion energy and show how the interplay of target structure and laser pulse parameters such as temporal contrast, peak intensity and pulse duration can be used to increase the ion energy in a robust way.
As two prominent examples we firstly discuss the role of grazing laser interaction with the walls of a conical target structure on the generation of hot electrons and secondly analyse the effect of lateral target size on repeated electron recirculation for simple mass reduced targets.
While the ion acceleration mechanism itself is based on the well-know target normal sheath acceleration mechanism and is thus robust enough for future applications such as radio therapy of tumours, the transfer of laser energy to electron energy is shown to greatly depend on laser parameters and target structure and thus needs detailed analysis using simulations.
In order to connect the electron dynamics during the short time the laser interacts with the electrons to the dynamics of the ion acceleration that evolve on a time scale that is typically three orders of magnitude longer, particle-in-cell simulations up to a few picoseconds duration, equating to some hundred thousand time steps, have been used.

Keywords: laser; ion; acceleration; cone; target; laser pressure; mass-limited; particle-in-cell; pic; simulation; maximum energy

  • Poster
    470. Wilhelm and Else Heraeus Seminar on Particle Accelerator and High Intensity Lasers, 13.-17.12.2010, Bad Honnef, Deutschland

Publ.-Id: 15024

Simulating realistic laser wakefield acceleration scenarios on the fly using GPUs

Bussmann, M.; Burau, H.; Berninger, F.; Debus, A.; Irman, A.; Jochmann, A.; Hönig, W.; Widera, R.; Schmitt, F.; Juckeland, G.; Nagel, W.; Schramm, U.; Cowan, T. E.

In order to optimise internal and external injection of electrons into laser-driven wakefields generated and the subsequent acceleration it is necessary to get fast response from simulations. This means that when considering a number of parameters such as plasma density, laser intensity, laser focal position and size or plasma density gradients, the optimum set of parameters is not easy to obtain by realistic particle-in-cell (PIC) simulations.
PIC simulations take into account the full dynamics of the particles in the interaction of the laser pulse with the plasma and thus give a detailed and realistic view of the plasma dynamics that can be used for optimisation. However, such detailed simulations usually require a lot of computing power not easily available to small-scale laboratories and universities.
We present PIConGPU, a fully relativistic, scalable implementation of the PIC algorithm on graphic cards (GPUs) that gives a speed up of orders of magnitude compared to PIC implementations on CPUs. Our code allows for an execution time below one nanoseond per macroparticle simulated (2D3V). This gives the user “real-time” feedback from the simulation, seeing on the fly the influence of the chosen parameter set on the injection and acceleration of electrons into the laser-driven wakefield. With this tool, the optimisation of injection schemes for the laser wakefield acceleration of electrons using realistic particle-in-cell simulations is in reach.

Keywords: gpu; laser wakefield acceleration; electron; parallel; computing; particle-in-cell; pic

  • Poster
    470. Wilhelm and Else Heraeus Seminar on Particle Accelerator and High Intensity Lasers, 13.-17.12.2010, Bad Honnef, Deutschland

Publ.-Id: 15023

The spatial structure of longitudinal and transversal fields in focussed terahertz beams

Winnerl, S.; Hubrich, R.; Peter, F.; Schneider, H.; Helm, M.

While for many applications the terahertz (THz) frequency range is the least technologically developed region of the electromagnetic spectrum, which is often referred to as the THz gap, this region offers unique research opportunities. For example, coherent detection of single cycle and few cycle pulses has been developed in the THz range and is now extended to the full infrared range. Here we apply coherent detection via electro-optic sampling in ZnTe crystals of different orientation to study amplitude and phase of longitudinal and transversal field components in focused terahertz beams. The THz radiation is generated by two types of scalable photoconductive emitters. The electrode pattern is optimized for generation of linearly polarized beams in one case and radially polarized beams in the second case. We show that these beams can be described well as Bessel Gauss beams, which are solutions of the vector Helmholtz equation. Consistent with the theory we observe a larger amplitude and a smaller spot size for the longitudinal component of a radially polarized beam as compared to a similarly focused linearly polarized beam.

Keywords: Terahertz; radially polarized beams; longitudinal electromagnetic fields

  • Lecture (Conference)
    DPG Frühjahrstagung der Sektion Kondensierte Materie (SKM) 2010, 22.-26.03.2010, Regensburg, Deutschland

Publ.-Id: 15022

First experiments at the new 6 MV-Tandetron at HZDR

Akhmadaliev, S.; Heller, R.; Hanf, D.; Merchel, S.

The 6 MV tandem type accelerator replaces the old 5 MV machine in the beginning of 2011. Besides ion beam analysis (IBA) and material modification via high-energy ion implantation, the system is equipped for accelerator mass spectrometry (AMS). The first interest is in the radioisotopes 10Be, 26Al, 36Cl, 41Ca, and 129I [1]. An energy calibration of the machine and a test of ion beam energy stability have been carried out as a first experiment using the 1H(15N,alpha gamma)12C nuclear reaction with a sharp resonance at anenergy of 6385 keV. The total energy resolution for the system is about 12 keV. First tests of the Dresden AMS system (DREAMS) [2] have been performed and demonstrated background levels of 2E-16 for 10Be/9Be, 7E-16 for 26Al/27Al and 8E-15 for 41Ca/40Ca, respectively. The background of 2E-13 for 129I/127I originates from intrinsic 129I (MERCK KI), whereas the reasons for high 36Cl/35Cl background of 3E-15 are under discussion.

[1] Sh. Akhmadaliev et al., MS6.2, DPG Frühjahrstagung der Sektion AMOP, Hannover 2010.

Keywords: IBA; AMS; Tandetron

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

Publ.-Id: 15021

Time-resolved terahertz spectroscopy and imaging at the FELBE free-electron laser facility

Winnerl, S.; Schneider, H.; Stehr, D.; Helm, M.; Seidel, W.; Michel, P.; Zibik, E. A.; Carpenter, B. A.; Porter, N. E.; Wilson, L. R.; Grange, T.; Ferreira, R.; Bastard, G.; Liu, H. C.; Y. Song, C.; Bründermann, E.

Free-electron lasers are the only high-power, continuously tunable radiation sources in the terahertz range. Here we highlight some outstanding experiments performed at the FELBE free-electron laser at the Forschungszentrum Dresden-Rossendorf and discuss the influence of the beam parameters on the feasibility of these studies. FELBE is driven by a superconducting accelerator operating at a repetition rate of 13 MHz. The laser can be tuned in an extremely wide spectral range from 1.2 THz to 75 THz. The pulse duration is 1 – 25 ps, the pulse energy 10 nJ – 2 µJ, depending on the wavelength. Correspondingly, the average power can be up to several Watts. Most experiments at FELBE are studies in the field of condensed matter physics, with a special focus on semiconductor quantum structures. Here we present two time-resolved terahertz experiments, a study of two-photon currents in GaAs/AlGaAs quantum-well infrared photodetectors (QWIP) [1] and a pump-probe study of the relaxation dynamics in self-organized InGaAs quantum dots [2]. In a two-photon QWIP operated at 7.1 THz a quadratic dependence of the photocurrent on the THz intensity is observed over 3 orders of magnitude. Quadratic photocurrent signals were observed already at pulse energies of only few pJ. Fringe resolved autocorrelation traces were measured. An analysis showed that their duration is determined by the duration of the FELBE pulses rather than by intrinsic time constants. For the quantum dots a change of the relaxation time by 3 orders of magnitude was observed when the photon energy of the FELBE pulses was varied only by a factor of two (1.5 ns for 3.4 THz, 2 ps for 7 THz, respectively). All photon energies were below the longitudinal optical phonon energy in GaAs, hence the fast relaxation via those phonons was suppressed. The relaxation dynamics in these experiments is governed by polarons which decay into acoustic phonons. A theoretical description taking into account all possible acoustic phonon decay channels provides an excellent agreement with the measured relaxation times.
THz imaging in reflection geometry was demonstrated in an experiment, where an oil painting was scanned through the attenuated focused FELBE beam. The result is compared with earlier THz images, where broadband radiation from photoconductive antennas was applied. The higher spatial resolution and the improved contrast of the image can be explained by the higher frequency (3.4 THz) of FELBE compared to the spectrum of the THz antenna (below 1 THz). The wavelength was chosen in order to maximize the reflection from lead white pigments [3]. In general the spectral range from 1 – 15 THz seems more appropriate for art studies, since many pigments have spectral fingerprints in this region, while their transmission and reflection spectra are basically flat in the range below 1 THz [3].

[1] H. Schneider, H.C. Liu, S. Winnerl , C.Y. Song, M. Walther and M. Helm, Opt. Express, 17, 12279 (2009).
[2] E.A. Zibik, T. Grange, B.A. Carpenter, N.E. Porter, R. Ferreira, G. Bastard, D. Stehr, S. Winnerl, M. Helm, H.Y. Liu, M. S. Skolnick, and L. R. Wilson, Nature Mat., 8, 803 (2009).
[3] K. Fukunaga, Y. Ogawa, Sh. Hayashi, and I. Hosako, IEICE Electronics Express 4, 258 (2007).

Keywords: time resolves spectroscopy; terahertz; THz imaging; free-electron laser

  • Lecture (Conference)
    International Forum on Terahertz Spectroscopy and Imaging, 02.-03.03.2010, Kaiserslautern, Deutschland

Publ.-Id: 15020

Ion-assisted phase separation during the growth of carbon-transition metal nanocomposite thin films

Abrasonis, G.

Nanostructures determine material properties at the macroscale, and are therefore a key issue in materials science of thin films. This is of particular importance for nanoscale multiphase films due to their multifunctionality and the combination of properties which cannot be predicted from the constituents alone. Control over the morphology and spatial correlations at the nanoscale becomes one of the major challenges. In this context, the tools allowing the structure control at the nanoscale are of utmost importance. The ion-solid interactions in the hyperthermal ion energy range (~10 -100 eV) are confined in the nanometer scale, and are therefore of key relevance. In this talk I will summarize recent research activities of our group on the phase separation during the growth of carbon-transition metal thin films. These materials are relevant in the context of sensing, fusion, electrochemistry, tribology, information storage as well as spintronics. Different processes can be 'switched off/on' by external control of the experimental parameters such as temperature, substrate type, matrix/dispersed phase chemical affinity or incoming particle energy. This results in a large variety of lateral or vertical composition modulations, such as encapsulated nanoparticles, high aspect ratio nanocolumns or self-organized layered 3D nanoparticle arrays. The latter deserve a special interest. They occur during ionized-PVD conditions if the metal amount surpasses a critical value. In addition, the ion induced atomic mobility is not isotropic, as it would be in the case of thermally excited migration, but conserves to a large extent the initial direction of the incoming ions, resulting in a tilting of the periodic precipitation structures for oblique ion incidences. The metal nanopatterns no longer align with the advancing surface, but with the incoming ions. Such self-organization process is versatile as different carbon-transition metal systems show this effect. The observed tendencies will be discussed on the basis of the interplay of thermal and energetic ion induced phenomena. As the dominating driving force for the pattern formation is of neither thermal nor chemical origin, we believe that the presented results are applicable to other immiscible or partially miscible systems and can be used to sculpt (multi)functional nanomaterials.

  • Lecture (others)
    Talk at Département Physique et Mécanique des Matériaux, Institut P', UPR 3346 CNRS, Université de Poitiers, 07.12.2010, 86962 Futuroscope Chasseneuil Cedex, France

Publ.-Id: 15019

Environment Controlled De-wetting Kinetics of Rh-Pd Bilayer/Alloy Thin Films on Silica: A Physical Approach to Synthesize Core-Shell Nanoparticles

Abrasonis, G.; Wintz, S.; Liedke, M. O.; Aksoy, F.; Liu, Z.; Kuepper, K.; Krause, M.

The control of morphology and surface composition of nanoalloys is the key factor in order to tune or to extend the range of their optical, magnetic and chemical properties. Therefore it is one of the major tasks in nanoalloy materials science. As the chemical environment has a profound influence on the structure of supported metallic nanoparticles, it can be used as a powerful tool to tune their structure and properties. This study concerns the in-situ investigation of the oxidizing/reducing environment influence on the de-wetting dynamics and kinetics of a Rh-Pd bilayer/alloy thin film model system.
Thin films of ~3nm thickness were grown at RT by molecular beam epitaxy electron beam evaporation on thermally oxidized silicon substrates. These films were subsequently subjected to different combinations of heating and chemical environment (CO and NO) treatments. The film surface composition and the chemical state was determined in-situ by ambient pressure environmental x-ray photoelectron spectroscopy.
Independently on whether the initial state is an alloy or a bilayer with Rh on the top, the film surface shows an enrichment of Pd upon heating in vacuum. Exposure to NO or CO at ~250-300°C results in the surface enrichment with either Rh or Pd, respectively, and subsequent film rupture. The metallic islands, produced by the film de-wetting, also show a similar response to the environment changes. On the other hand, de-wetting caused by heating in NO or CO shows significant differences in the surface chemical composition evolution and, consequently, in the de-wetting onset temperature.
The results are discussed on the basis of the interplay between thermodynamic and kinetic factors, e.g. the energetics of the metallic surfaces in response to the NO/CO adsorbates, the atomic mobility of metallic and oxide surfaces, and the role of mobile Pd/Rh carbonyl or nitrosyl species. The study demonstrates the effect of the chemical environment on the morphology as well as on the composition of supported nanostructures. The nanoalloy/ bilayer approach presented here opens a new route for the fabrication of supported arrays of core-shell nanoparticles.

  • Lecture (Conference)
    2010 MRS Fall Meeting, 29.11.-03.12.2010, Boston, United States of America

Publ.-Id: 15018

Tilting of precipitation patterns in carbon-transition metal nanocomposite thin films by hyperthermal ion deposition

Abrasonis, G.; Oates, T. W. H.; Kovacs, G. J.; Tucker, M.; Grenzer, J.; Persson, P. O. A.; Heinig, K. H.; Martinavicius, A.; Jeutter, N.; Baehtz, C.; Bilek, M. M. M.; Möller, W.

The structure control, especially at the nanoscale, is of the primary importance in the field of the materials science of thin films. Here, the hyperthermal ion induced self-organization caused by phase separation during the carbon-transition metal (Ni, Cu) thin film growth is reported. The films have been grown by ionized physical vapour deposition using filtered cathodic vacuum arc. Influence of the metal type, film composition, ion energy and incidence angle is studied. The film morphology has been determined by transmission electron microscopy and grazing incidence small angle x-ray scattering.
At these growth conditions, atomic displacements are caused solely by impacting energetic ions, resulting in phase separation in an advancing surface layer. If the metal amount surpasses some critical value, this layer switches to an oscillatory mode and a nanoscale precipitation pattern emerges. The results show that for the perpendicular incoming depositing ion incidence the C:Ni film structure consists of alternating self-organized nickel carbide and carbon layer oriented parallel to the film surface. Moreover, the ion induced atomic mobility is not random, as it would be in the case of thermal diffusion, but conserves to a large extent the initial direction of the incoming ions, resulting in a tilting of the periodic precipitation structures for the oblique ion incidences. The metal nanopatterns no longer align with the advancing surface, but with the incoming ions. While both type of films show tilted structures, for C:Cu films the ‘tilted-lying’ transition is observed when increasing Cu content.
We establish a dependence of the nanopattern morphology on the growth parameters and demonstrate a method for controlling the nanopatterning. The results are discussed on the basis of the interplay between thermodynamically driven phase separation and energetic ion induced ballistic effects. Application of this concept opens new ways for the bottom-up nanostructure control for composite materials.

  • Lecture (Conference)
    2010 MRS Fall Meeting, 29.11.-03.12.2010, Boston, United States of America

Publ.-Id: 15017

The cyclotron resonance of holes in InGaAs/GaAs heterostructures with quantum wells in quantizing magnetic fields

Spirin, K. E.; Ikonnikov, A. V.; Gavrilenko, V. I.; Kozlov, D. V.; Drachenko, O.; Schneider, H.; Helm, M.

The spectra of the cyclotron resonance of holes in the InGaAs/GaAs selectively doped heterostructures with quantum wells are studied in pulsed magnetic fields as high as 50 T at 4.2 K. The previously observed effect of the inverted (compared with the results of the single-particle calculation of the Landau levels) ratio of the spectral weight of two split components of the line of the cyclotron resonance, which is attributed to the effects of the exchange interaction of holes, is confirmed. It is found that the ratios of intensities of the components of the line of the cyclotron resonance profoundly differ on the ascending and descending branches of the magnetic field pulse, which may be associated with a long time of the spin relaxation of holes between the two lowest Landau levels, which constitute tens of milliseconds.

Keywords: Cyclotron resonance; spin; lifetime; InGaAs

Publ.-Id: 15016

Noncovalent Tripeptidyl Benzyl- and Cyclohexyl-Amine Inhibitors of the Cysteine Protease Caspase-1

Löser, R.; Abbenante, G.; Madala, P. K.; Halili, M.; Le, G. T.; Fairlie, D. P.

Potent and noncovalent inhibitors of caspase-1 were produced by incorporating a secondary amine (reduced amide) isostere in place of the conventional electrophile (e.g., aldehyde) that normally confers high potency to cysteine protease inhibitors. Benzyl- or cyclohexylamines produced potent, reversible, and competitive inhibitors that were selective for caspase-1 (e.g., Ki=47 nM) over caspases 3 and 8with minimal cytotoxicity. Unlike most cysteine protease inhibitors, these compounds do not react covalently and indiscriminately with thiols.

Publ.-Id: 15015

Antimalarial activity of azadipeptide nitriles

Löser, R.; Gut, J.; Rosenthal, P. J.; Frizler, M.; Gütschow, M.; Andrews, K. T.

Azadipeptide nitriles—novel cysteine protease inhibitors—display structure-dependent antimalarial activity against both chloroquine-sensitive and chloroquine-resistant lines of cultured Plasmodium falciparum malaria parasites. Inhibition of parasite’s hemoglobin-degrading cysteine proteases was also investigated, revealing the azadipeptide nitriles as potent inhibitors of falcipain-2 and -3. A correlation between the cysteine protease-inhibiting activity and the antimalarial potential of the compounds was observed. These first generation azadipeptide nitriles represent a promising new class of compounds for antimalarial drug development.

Publ.-Id: 15014

Synthesis and 99mTc-Labeling of a PNA Oligomer Containing a New Ligand Derivative of 2,2´-Dipicolylamine

Jäger, K.; Gasser, G.; Zenker, M.; Bergmann, R.; Steinbach, J.; Stephan, H.; Metzler-Nolte, N.

The search for chelating ligands which can be efficiently labeled with radiometals, and which also contain a functional group allowing a facile conjugation to biomolecules is currently a hot topic in radiopharmacy. The 2,2’-dipicolylamine (Dpa) has already been found to be a good candidate for labeling with 186Re, 188Re and 99mTc1 while the Cu(I)-catalyzed [2+3] azide/alkyne cycloaddition, often referred to as Click Chemistry,2 has been shown to be an effective coupling method. With this in mind, we recently developed the facile synthesis of an azido derivative of Dpa (Dpa-N3, Figure 1).3 Furthermore, as a proof of principle of the possible functionalization of our ligand to a biomolecule, Dpa-N3 was successfully coupled, on the solid phase, to a ethinyl-substituted Peptide Nucleic Acid (PNA) oligomer employing the Click Chemistry methodology to give the expected Dpa-ethyl-triazol-PNA. Both Dpa-N3 and Dpa-ethyl-triazol-PNA could be efficiently labeled with 99mTc using the precursor [99mTc(H2O)3(CO)3]+ to afford [99mTc(CO)3(Dpa-ethyl-triazol-N3)]+ and [99mTc(CO)3(Dpa-ethyl-triazol-PNA], respectively. The radionuclide 99mTc was tightly bound by the Dpa-chelator avoiding the formation of pertechnetate for at least 24h. Partitioning experiments in a 1-octanol/water system confirmed that both [99mTc(CO)3(Dpa-N3)]+ and [99mTc(CO)3(Dpa-ethyl-triazol-PNA] are rather hydrophilic. Biodistribution studies of [99mTc(CO)3(Dpa-ethyl-triazol-PNA] in Wistar rats showed a fast blood clearance and only a modest accumulation in the kidneys. Similar results were found when a mouse model (NMRI nu/nu) was used.


1. T. Storr, C. L. Fisher, Y.Mikata, S. Yano, M. J. Adam, C. Orvig, Dalton Trans., 2005, 654-655.
2. M. V. Gil, M. J. Arévalo, Ó. López Synthesis, 2007, 11, 1589-1620.
3. G. Gasser, K. Jäger, M. Zenker, R.Bergmann, J. Steinbach, H. Stephan, N. Metzler-Nolte, 2010, submitted.

  • Poster
    5th International Symposium on Bioorganometallic Chemistry, 05.-09.07.2010, Bochum, D

Publ.-Id: 15013

Radiomarkierung eines PNA-DPA (2-2´-Dipicolylamin)-Konjugates mit 99mTc

Jäger, K.; Gasser, G.; Bergmann, R.; Stephan, H.; Steinbach, J.

Ein großes Problem bei der Verwendung radiomarkierter Antikörper in der Tumorbehandlung ist deren langsame Anreicherung im Zielgewebe. Daraus ergibt sich eine erhebliche Strahlenbelastung des ganzen Körpers. Um dieses Problem zu umgehen, wurden bereits verschiedene Strategien, die auf einem Pretargeting-Konzept beruhen, untersucht. Diese basieren auf dem Prinzip, erst den Antikörper um Tumor anreichern zu lassen, und anschließend eine radiomarkierte Substanz zu applizieren, die sich schnell am Antikörper anreichert. Hierzu ist die Anwendung von komplementären Verbindungen notwendig. Insbesondere die Systeme (Strept-)Avidin/Biotin und MORF wurden bisher intensiv erforscht. Es gibt jedoch auch Ansätze, weitere metabolisch stabile Oligonukleotide zu verwenden. Unser Ziel ist es, PNA(Peptidnukleinsäure)-Derivate zu entwickeln, die sich für einen Pretargeting-Einsatz eignen.

Es wurde eine 12mer PNA mittels Festphasensynthese hergestellt, an die mittels Click-Chemie der bereits für die Komplexierung von Technetium und Rhenium gut untersuchte DPA-Chelator angebracht wurde. Für die Anbindung des Chelators wurde die so genannte Huisgen-Reaktion verwendet, bei der ein Alkin und ein Azid kupfervermittelt einen Triazolring bilden. Es wurde PNA-Alkin und DPA-Azid eingesetzt.
Die Radiomarkierung des entstandenen PNA-DPA-Konjugates wurde mit [99mTc(H2O)3]+ durchgeführt, wobei die Markierungsbedingungen mittels Radio-HPLC optimiert wurden. Zudem wurde die Bioverteilung des 99mTc-markierten Konjugates in gesunden Wistar-Ratten untersucht.

Das Konjugat konnte mit sehr guten Ausbeuten >90% mit 99mTc markiert werden. Bioverteilungsstudien zeigten, dass das Konjugat schnell über die Nieren ausgeschieden wird. Es wurde keine erhöhte Aufnahme von Aktivität im Magen und der Schilddrüse gefunden.

Unsere Untersuchungen deuten darauf hin, dass PNA-DPA-Konjugate zur stabilen Radiomarkierung mit 99mTc geeignet sind.
Mit den verwendeten 99mTc-PNA-DPA-Konjugaten wurde eine hohe in vivo Stabilität gepaart mit einer schnellen Blutclearance erreicht. Die Verweilzeit des Konjugates im Blut ist aber noch zu niedrig, um eine ausreichende Hybridisierung mit dem komplementären Strang in höherem Maße zu gewährleisten. Eine längere Zirkulation des radiomarkierten Präparates wäre wünschenswert. Die schnelle und fortdauernde Anreicherung in den Nieren stellt dennoch eine Belastung für das Nierengewebe dar. Die Modelluntersuchungen mit dem diagnostischen Radionuklid 99mTc sollen mit dem Therapienuklid 188Re fortgesetzt werden.

[1] O. C. Boerman, F. G. van Schaijk, W. J. G. Oyen, F. H. M. Corstens, J. Nucl. Med. 2003, 44, 3, 400-411
[2] G. Liu, S. Dou, J. He, D. Yin, S. Gupta, S. Zhang, Y. Wang, M. Rusckowski, D. J. Hnatowich, Appl. Radiat. Isot. 2006, 64, 9, 971-978
[3] M. Rusckowski, T. Qu, F. Chang, D. J. Hnatowich, Cancer, 1997, 80, 12, 2699-2705
[4] G. Gasser, K. Jäger, M. Zenker, R. Bergmann, H. Stephan, J. Steinbach, N. Metzler-Nolte. J. Inorg. Biochem. 2010; doi:10.1016/j.jinorgbio.2010.06.011

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

Publ.-Id: 15012

Kinetics and thermodynamics of defects, impurities, nanostructures and interfaces

Posselt, M.

Computer simulations using classical interatomic potentials are an efficient and promising tool to investigate and understand atomic-level properties and processes in advanced materials. They allow the consideration of length and time scales which are often hardly accessible by experiments. However, the accuracy of the interatomic potentials employed in such type of simulations determines decisively the quality of the obtained results. Therefore, these potentials must be continuously improved and evaluated.
In the present contribution three applications of atomistic computer simulations are illustrated. The focus is on kinetics, energetics and thermodynamics of defects, impurities, nanostructures and interfaces in materials for micro- and nanoelectronics and in structural materials for fission reactors.

The first example deals with molecular dynamics simulations on basic migration mechanisms of mono- and di-(self-)interstitials in Si. Both the atomic mobility due to the presence of the defect and the defect mobility itself were determined. The mechanism of di-interstitial migration depends on temperature, in contrast to that of the mono-interstitial.

In the second example amorphous Si and Ge as well as their solid-phase epitaxial recrystallization (SPER) are considered. Results obtained by different interatomic potentials are compared. The molecular dynamics simulations yield amorphous material with realistic structural and thermodynamic properties, but the SPER rate is strongly overestimated. It is shown that a more realistic SPER rate can be obtained using a modified interatomic potential which yields a higher melting temperature of the amorphous phase. This is explained by the fact that both melting and SPER are essentially determined by the flexibility of atomic bonds.

The subject of the third example is the formation of coherent clusters containing vacancies, Cu and Ni in bcc-Fe. Using the most recent interatomic potential for Fe-Cu-Ni alloys, the structure, energetics and thermodynamics of the clusters were determined. Many clusters up to a size of 200 monomers (vacancies, Cu and Ni atoms) were studied. In the case of vacancy-Cu clusters a core-shell structure is found where Cu atoms coat the outer surface of vacancy clusters. In a vacancy-Cu-Ni cluster Ni atoms are never first nearest neighbors of vacancies, and the Ni atoms cover the Cu-surface of inner vacancy-Cu cluster. The total and monomer binding energy as well as the nucleation free energy of the clusters were calculated. The monomer binding energy is an important input parameter of rate theory and object kinetic Monte Carlo simulations. For further application in these calculations compact and physically-based fit formulae were derived from the atomistic data.

Keywords: atomistic computer simulation; kinetics; thermodynamics; defects; interfaces; nanostructures

  • Lecture (others)
    Seminarvortrag, SRMP, Department des Materiaux pour le Nucleaire, CEA Saclay, 10.12.2010, Saclay/Paris, France

Publ.-Id: 15011

Verification and validation of numerical models of the transport of insulation debris

Cartland Glover, G. M.; Kratzsch, A.; Krepper, E.; Renger, S.; Seeliger, A.; Zacharias, F.; Alt, S.; Kästner, W.; Kryk, H.; Weiss, F.-P.

Damage to insulation materials located near to a primary circuit coolant leak may compromise the operation of the emergency core cooling system (ECCS). Insulation material in the form of mineral wool fiber agglomerates (MWFA) maybe transported to the containment sump strainers, where they may block or penetrate the strainers. Though the impact of MWFA on the pressure drop across the strainers is minimal, corrosion products formed over time may also accumulate in the fiber cakes on the strainers, which can lead to a significant increase in the strainer pressure drop and result in cavitation in the ECCS.
An experimental and theoretical study performed by the Helmholtz-Zentrum Dresden-Rossendorf and the Hochschule Zittau/Görlitz is investigating the phenomena that maybe observed in the containment vessel during a primary circuit coolant leak. The study entails the generation of fiber agglomerates, the determination of their transport properties in single and multi-effect experiments and the long-term effect that corrosion and erosion of the containment internals by the coolant has on the strainer pressure drop.
The focus of this paper is on the verification and validation of numerical models that can predict the transport of MWFA. A number of pseudo-continuous dispersed phases of spherical wetted agglomerates represent the MWFA. The size, density, the relative viscosity of the fluid-fiber agglomerate mixture and the turbulent dispersion all affect how the fiber agglomerates are transported. In the cases described here, the size is kept constant while the density is modified. This definition affects both the terminal velocity and volume fraction of the dispersed phases. Note that the relative viscosity is only significant at high concentrations.
Three single effect experiments were used to provide validation data on the transport of the fiber agglomerates under conditions of sedimentation in quiescent fluid, sedimentation in a horizontal flow and suspension in a horizontal flow. The experiments were performed in a rectangular column for the quiescent fluid and a racetrack type channel that provided a near uniform horizontal flow. The numerical models of sedimentation in the column and the racetrack channel found that the sedimentation characteristics are consistent with the experiments. For channel suspension, the heavier fibers tend to accumulate at the channel base even at high velocities, while lighter phases are more likely to be transported around the channel.

Keywords: Mineral Wool Fiber Agglomerates; Loss of Coolant Accidents; Containment Sump; Computational Fluid Dynamics; Multiphase Flow

  • Contribution to proceedings
    14th International Topical Meeting on Nuclear Reactor Thermalhydraulics, 25.-29.09.2011, Toronto, Canada
    Proceedings of the 14th International Topical Meeting on Nuclear Reactor Thermalhydraulics (NURETH-14), CD-ROM
  • Lecture (Conference)
    14th International Topical Meeting on Nuclear Reactor Thermalhydraulics, 25.-29.09.2011, Toronto, Canada

Publ.-Id: 15010

Setting-up an accelerator mass spectrometry (AMS) facility: The role of chemistry

Merchel, S.; Munnik, F.; Andermann, C.; Bourlès, D.; Braucher, R.; Gloaguen, R.; Martschini, M.; Steier, P.

Medium-energy AMS facilities such as the 5 MV British SUERC and French ASTER or the two 6 MV German DREAMS at Dresden[1] and Cologne AMS have recently been installed. These machines need physicists to get them running but also scientists to establish AMS chemistry on-site. As it is not advisable to change simultaneously two ”things”, i.e. machine and chemistry, a cooperation with the teams of ASTER and VERA helped to check the new sample preparation of DREAMS. A ”good” AMS sample has two features: high stable isotope current and low isobar concentration. High chemical yields and low concentrations of other elements, from the matrix or chemical products used, are less important, but may play a role if e.g. a matrix contains Ti being introduced into BeO-targets as shown by µ-XRF [2] and recent µ-PIXE analyses of final AMS-targets at HZDR. A processing blank with low radionuclide/stable nuclide ratio is essential for projects near the detection limit. Finally, a fast, easy and cheap separation guarantees high sample throughput and reasonable costs. Ref.: [1] [2] S. Merchel et al., NIMB 266 (2008) 4921. Ackn.: Thanks to T. Schildgen, C. Yildirim (Potsdam), K. Klemm, M. Fuchs (TUBA), M. Arnold, G. Aumaître (ASTER), A. Wallner (VERA).

Keywords: accelerator mass spectrometry

  • Invited lecture (Conferences)
    DPG Frühjahrstagung der Sektion AMOP (SAMOP) 2011, 13.-18.03.2011, Dresden, Deutschland

Publ.-Id: 15009

The Rossendorf Beamline at ESRF - A unique facility to perform X-ray absorption spectroscopy of radionuclides

Scheinost, A. C.

The Rossendorf Beamline at ESRF - A unique facility to perform X-ray absorption spectroscopy of radionuclides

Keywords: XAS; Actindies

  • Invited lecture (Conferences)
    Seminar des Instituts fuer Energie- und Klimaforschung, FZ Juelich, 25.03.2010, Juelich, Germany

Publ.-Id: 15007

Probing redox reactions at the mineral/water interface by X-ray absorption spectroscopy: Reduction of Se, Sb and Pu by Fe(II)-bearing minerals

Scheinost, A. C.

Probing redox reactions at the mineral/water interface by X-ray absorption spectroscopy: Reduction of Se, Sb and Pu by Fe(II)-bearing minerals

Keywords: EXAFS; XANES; Redox; Antimony; Plutonium; selenium; mackinawite; magnetite; siderite; clays

  • Invited lecture (Conferences)
    Graduiertenkolleg "Spurenanalytik von Elementspezies: Methodenentwicklungen und Anwendungen", 23.03.2010, Mainz, Germany

Publ.-Id: 15006

Vorschlag für ein Aktivierungsbenchmark an WWER-440 Reaktordruckbehältern

Konheiser, J.

Bei der Stilllegung von KKW spielt die Radioaktivität des Reaktordruckbehälters (RDB) eine entscheidende Rolle. Geeignete Messdaten für die Validierung von Programmen für die Fluenz- und Aktivitätsberechnungen sind schwer zu finden bzw. nicht vorhanden. Die genommene Trepans vom RDB des KKW Greifswald bietet eine einzigartige Möglichkeit Daten zu diesem Zweck zu gewinnen. In der Präsentation wird deshalb ein Aktivierungbechmark auf Basis von experimentellen Daten der Trepans vorgeschlagen.

At the decommissioning of NPP, the radioactivity of the reactor pressure vessel (RPV) is of crucial importance. Generally, suitable measurement data for the validation of codes for the calculation of fluences and aktivities are not easy to be found. Trepans taken from dismantled VVER-440 RPVs of Greifswald NPP provide a unique opportunity for this purpose. In the presentation an activation benchmark to carry out is suggested based on the experimental data of trepans.

Keywords: decommissioning; benchmark; fluence calculation; aktivity calculation

  • Lecture (others)
    10. AAA Usergroup Meeting, 06.12.2010, Garching bei München, Deutschland

Publ.-Id: 15005

Kelvin probe force microscopy in the presence of intrinsic local electric fields

Baumgart, C.; Müller, A.-D.; Müller, F.; Schmidt, H.

Kelvin probe force microscopy (KPFM) is used to investigate the electrostatic force between a conductive probe and doped semiconductors. The observed frequency dependence of the probed KPFM bias is strongly related to sample-specific intrinsic local electric fields. Equilibrium drift and diffusion of excess charge carriers at low operation frequencies influence the characteristics of the asymmetric electric dipole in the surface region of the investigated semiconductors during the KPFM measurement. The sample-specific KPFM background signal does not influence the frequency-dependent lateral variation of the electrical signal. The KPFM bias probed on doped semiconductor nanostructures with high or small enough operation frequencies allows for quantitative dopant profiling or investigation of diffusion processes in internal electric fields, respectively.

Keywords: Kelvin probe force microscopy (KPFM); Semiconductors; Dopant profiling

Publ.-Id: 15004

Strangeness measurements at the HADES experiment

Schmah, A.; Agakishiev, G.; Balanda, A.; Belver, D.; Belyaev, A. V.; Blanco, A.; Böhmer, M.; Boyard, J. L.; Cabanelas, P.; Castro, E.; Chernenko, S.; Díaz, J.; Dybczak, A.; Epple, E.; Fabbietti, L.; Fateev, O. V.; Finocchiaro, P.; Fonte, P.; Friese, J.; Fröhlich, I.; Galatyuk, T.; Garzón, J. A.; Gil, A.; Golubeva, M.; González-Díaz, D.; Guber, F.; Hennino, T.; Holzmann, R.; Huck, P.; Ierusalimov, A.; Iori, I.; Ivashkin, A.; Jurkovic, M.; Kämpfer, B.; Karavicheva, T.; Koenig, I.; Koenig, W.; Kolb, B. W.; Kopp, A.; Korcyl, G.; Kornakov, G. K.; Kotte, R.; Kozuch, A.; Kraza, A.; Krizek, F.; Krücken, R.; Kuc, H.; Kühn, W.; Kugler, A.; Kurepin, A.; Kurilkin, A.; Kurilkin, P.; Kählitz, P.; Ladygin, V.; Lamas-Valverde, J.; Lang, S.; Lapidus, K.; Liu, T.; Lopes, L.; Lorenz, M.; Maier, L.; Mangiarotti, A.; Markert, J.; Metag, V.; Michalska, B.; Michel, J.; Müntz, C.; Naumann, L.; Pachmayer, Y. C.; Palka, M.; Parpottas, Y.; Pechenov, V.; Pechenova, O.; Pietraszko, J.; Przygoda, W.; Ramstein, B.; Reshetin, A.; Roskoss, J.; Rustamov, A.; Sadovsky, A.; Salabura, P.; Schmah, A.; Siebenson, J.; Sobolev, Y. G.; Spataro, S.; Ströbele, H.; Stroth, J.; Sturm, C.; Sudol, M.; Tarantola, A.; Teilab, K.; Tlusty, P.; Traxler, M.; Trebacz, R.; Tsertos, H.; Vasiliev, T.; Wagner, V.; Weber, M.; Wüstenfeld, J.; Yurevich, S.; Zanevsky, Y.

We report on high acceptance di-electron spectrometer (HADES) measurements of strange hadrons in the collision systems Ar(1.756 AGeV)+KCl and p+p at 3.5 GeV. Comparisons of K0s transverse mass and rapidity spectra to IQMD transport model calculations give a strong hint to a repulsive kaon–nucleon potential. The effect of the potential shows up strongest at very low transverse momenta, which were measured by HADES with high statistics. Statistical model fits show a fair agreement to the particle yields measured in the heavy-ion data, apart from the Ξ− yield which is off by more than an order of magnitude. Furthermore, first results from the Σ(1385)+ reconstruction in p+p reactions are presented, showing an excellent agreement with published data.

Publ.-Id: 15003

Study of elementary reactions with the HADES dielectron spectrometer

Ramstein, B.; Agakishiev, G.; Agodi, C.; Balanda, A.; Bellia, G.; Belver, D.; Belyaev, A. V.; Blanco, A.; Böhmer, M.; Boyard, J. L.; Braun-Munzinger, P.; Cabanelas, P.; Castro, E.; Christ, T.; Destefanis, M.; Díaz, J.; Dohrmann, F.; Dybczak, A.; Fabbietti, L.; Fateev, O. V.; Finocchiaro, P.; Fonte, P.; Friese, J.; Fröhlich, I.; Galatyuk, T.; Garzón, J. A.; Gernhäuser, R.; Gil, A.; Gilardi, C.; Golubeva, M.; González-Díaz, D.; Grosse, E.; Guber, F.; Heilmann, M.; Hennino, T.; Holzmann, R.; Ierusalimov, A.; Iori, I.; Ivashkin, A.; Jurkovic, M.; Kämpfer, B.; Kanaki, K.; Karavicheva, T.; Kirschner, D.; Koenig, I.; Koenig, W.; Kolb, B. W.; Kotte, R.; Kozuch, A.; Krasa, A.; Krizek, F.; Krücken, R.; Kühn, W.; Kugler, A.; Kurepin, A.; Lang, S.; Lange, J. S.; Lapidus, K.; Liu, T.; Lopes, L.; Lorenz, M.; Maier, L.; Mangiarotti, A.; Marin, J.; Markert, J.; Metag, V.; Michalska, B.; Michel, J.; Mishra, D.; Morinière, E.; Mousa, J.; Müntz, C.; Naumann, L.; Novotny, R.; Otwinowski, J.; Pachmayer, Y. C.; Palka, M.; Parpottas, Y.; Pechenov, V.; Pechenova, O.; Pietraszko, J.; Przygoda, W.; Reshetin, A.; Roy-Stephan, M.; Rustamov, A.; Sadovsky, A.; Sailer, B.; Salabura, P.; Schmah, A.; Sobolev, Y. G.; Spataro, S.; Spruck, B.; Ströbele, H.; Stroth, J.; Sturm, C.; Sudol, M.; Tarantola, A.; Teilab, K.; Tlusty, P.; Traxler, M.; Trebacz, R.; Tsertos, H.; Wagner, V.; Weber, M.; Wisniowski, M.; Wojcik, T.; Wüstenfeld, J.; Yurevich, S.; Zanevsky, Y. V.; Zhou, P.; Zumbruch, P.

Results obtained with the HADES dielectron spectrometer at GSI are discussed, with emphasis on dilepton production in elementary reactions.

  • Open Access Logo Acta Physica Polonica B 41(2010), 365

Publ.-Id: 15002

Molecular Dynamics Simulations of BMP-2 Adsorption on a Hydrophobic Surface

Oliveira, A. F.; Gemming, S.; Seifert, G.

In the present work, the adsorption of BMP-2 on a hydrophobic surface model is investigated by means of molecular-dynamics simulations within the NVT ensemble. The adsorption paths of the single monomer and the homodimer of BMP-2 have been compared. The nature of the residues interacting directly with the hydrophobic monolayer at different adsorption stages is shown. Conformational changes driven by the adsorption process, as well as the influence of aqueous environment on the stabilization of the BMP-2 secondary structure are also discussed.

Keywords: bone remodeling; bone replacement; biomineralization; molecular dynamics; force field; BMP


Publ.-Id: 15001

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