Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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39109 Publications

Efficient light emitting diodes based on nanoscale silicon

Helm, M.; Sun, J.; Potfajova, J.; Winnerl, S.; Dekorsy, T.; Skorupa, W.; Schmidt, B.; Mücklich, A.

After giving an overview about various approaches for silicon based light emitters, we will present our results on Si light emitting diodes prepared by high-dose boron implantation. The electroluminescence (EL) increases with temperature, resulting in a wall-plug efficiency of 0.1% at room temperature. Extensive low-temperature EL measurements allow us to put forward a model, which is based on the interplay between free excitons/carriers and excitons localized at nanoscale boron doping spikes. Finally, we demonstrate an electrically driven resonant-cavity LED based on silicon.

Keywords: silicon; luminescence; implantation; microcavity

  • Physica Status Solidi (c) 2 (2005) 2941-2946

Publ.-Id: 6600

Efficient light emitting devices based on nanoscale silicon

Helm, M.

Recently some remarkable advances concerning silicon based light emission have been reported. This has triggered the hope that a practical Si based light source may soon become reality. Several approaches are pursued by research groups worldwide: porous Si, Si nanocrystals and Er3+ in SiO2, Er3+ doped Si, Si/Ge quantum cascade structures, and also more “conventional” Si pn diodes. I will give a survey on some of these approaches and then concentrate on Si pn diodes, fabricated by high-dose boron implantation.

In these structures the electroluminescence (EL) increases with temperature, resulting in a wall-plug efficiency of 0.1% at room temperature. Extensive low-temperature EL measurements allow us to put forward a model, which is based on the interplay between free excitons/carriers and excitons localized at nanoscale boron doping spikes, similar to delta-doped layers produced by MBE. Within this model we are able to explain the EL dependence on current and temperature, as well as an electrical bistability occurring at low temperature. Finally, we have fabricated what is, to our knowledge, the first electrically driven resonant-cavity LED based on silicon.

Keywords: silicon; luminescence; implantation; microcavity

  • Invited lecture (Conferences)
    Invited talk at the International Conference on Superlattices, Nanostructures, and Nanodevices (ICSNN'04), July 19-23, 2004, Cancun, Mexico

Publ.-Id: 6599

LEDs und Laserdioden: die Lichtrevolution

Winnerl, S.

Leuchtdioden (LED, engl.: light emitting diode) und Laserdioden haben bereits vielfältigen Einzug in unser Alltagsleben gefunden, beispielsweise im CD- und DVD-Spieler. In dem Vortrag werden die Funktionsweisen von LEDs und Laserdioden erklärt, insbesonder wird erklärt, wie die verschiedenen Farben erreicht werden und warum gerade die Farbe Blau so wichtig ist. Unterschiedliche Konzepte für Weißlichtquellen auf der Basis von LEDs werden vorgestellt. Die Wirkungsgrade von LEDs werden diskutiert und ein Ausblick auf die Palette von Anwendungen wird geboten.

Keywords: LED; Laserdiode

  • Lecture (others)
    Lehrertag, Forschungszentrum Rossendorf, 2004, Dresden, Deutschland

Publ.-Id: 6598

Die Stoppuhren der Forschung: Femtosekundenlaser

Winnerl, S.

Nach einer kurzen Darstellung des Laserprinzips wird erklärt, wie in einem Titan-Saphir-Laser durch passeive Modenkopplung ultrakurze Lichtpulse erzeugt werden. Weiterhin wird gezeigt, wie mit Hilfe von nichtlinearen optischen Effekten eine Veränderung der Laserwellenlänge möglich ist. Als typisches Beispiel des Einsatzes von Kurzpuls-Lasern in der Forschung wird die Anrege-Abfrage-Technik vorgestellt. Abschließend werden praktische Beispiele vom Einsatz von Kurzpuls-Lasern in der physikalischer Forschung, Materialbearbeitung und Medizintechnik gezeigt.

Keywords: Ultraschnelle Prozesse; Femtosekundenlaser

  • Lecture (others)
    Lehrertag, Forschungszentrum Rossendorf, 2004, Dresden, Deutschland

Publ.-Id: 6597

LEDs und Laserdioden: die Lichtrevolution

Winnerl, S.

Die faszinierende Erfolgsgeschichte von Leuchtdioden (engl.: light emitting diode, LED) und Laserdioden wird vorgestellt. Es wird erklärt, wie Licht unterschiedlicher Farbe mit LEDs erzeugt wird und welche besondere Rolle blaue LEDs spielen. Neben weißen Leuchtdioden, die ein hohes Potential für zukünftige Alltagsbeleuchtung besitzen, werden auch aktuelle Forschungsergebnisse zu siliziumbasierenden LEDs diskutiert.

  • Lecture (others)
    Lange Nacht der Wissenschaften, 2004, Dresden, Deutschland

Publ.-Id: 6596

THz-time-domain spectroscopy of doped semiconductor superlattices

Winnerl, S.; Dreyhaupt, A.; Dekorsy, A.; Helm, M.

The search for Bloch gain in doped semiconductor superlattices is still a field of considerable interest. Our approach is to obtain the dynamic conductivity of doped biased GaAs/AlGaAs superlattices by measuring the transmission in the terahertz frequency range. We present experimental results on the electric transport in a wide temperature range and preliminary results for the THz transmission through superlattice mesa elements.

Keywords: Bloch gain; seminconductor superlattice; THz spectroscopy

  • Lecture (others)
    Workshop on Quantum Heterostructures and THz Electronics, 2004, Regensburg, Deutschland

Publ.-Id: 6595

Time-domain THz spectroscopy of electronic excitations in GaAs/AlGaAs superlattices

Dreyhaupt, A.; Winnerl, S.; Dekorsy, T.; Helm, M.

We investigate the electronic properties of weakly and more strongly coupled n-doped GaAs/AlGaAs superlattices by time domain spectroscopy in the low frequency range from 0.5 to 3.5 THz. The sample properties are modulated either by applying an AC voltage via a gate electrode or by injecting a vertical current. The transmission signal is detected at the frequency of the modulation voltage with a lock-in amplifier leading to a signal-to-noise ratio superior compared to conventional FTIR spectroscopy in this frequency range. We will discuss the observation of inter-miniband transitions, confined donor transitions, plasmon excitations and the measurement of intra-miniband THz conductivity in a wide range of lattice temperatures.

Keywords: THz time-domain spectroscopy; semiconductor superlattice; interminiband-transition; intra-miniband THz conductivity

  • Lecture (Conference)
    DPG Frühjahrstagung, Fachverband Halbleiterphysik, HL 9.2, Regensburg 2004

Publ.-Id: 6594

Large-area high-power THz emitter based on interdigitated electrodes

Dreyhaupt, A.; Winnerl, S.; Dekorsy, T.; Helm, M.

We present a planar large-area photoconductor based on GaAs for impulsive generation of THz radiation. The device consists of an interdigitated electrode metal-semiconductor-metal structure (MSM) with 5 µm electrode spacing. The MSM structure is masked by an opaque metallization layer isolated from the MSM electrodes in a way that optical excitation takes place only in regions with unidirectional electric field. Constructive interference of the THz emission from accelerated carriers leads to amplitudes that are 104 times higher than from the same MSM structure without second metallization layer.

Keywords: THz emmitter

  • Lecture (Conference)
    The Joint 29th International Conference on Infrared and Millimeter Waves and 12th International Conference on Terahertz Electronics (IRMMW2004/THz2004); Karlsruhe
  • IEEE Conference digest IRMMW2004/THz2004

Publ.-Id: 6593

THz emission from GaSb samples with modified surface stoichiometry

Winnerl, S.; Dekorsy, T.; Helm, M.

The optimization of emitters for THz radiation pulses has gained a lot of interest in recent years. They can be based on accelerating charge carriers, which are photoexcited by fs-laser pulses in the surface field of a semiconductor. We studied the THz emission from GaSb samples that were annealed at different temperatures in the range from 300 °C to 700 °C. Without annealing, no THz emission was observed under fs excitation at 800 nm. This is consistent with the fact, that for GaSb no surface states in the band gap have been found [1]. The THz emission was strongest for the annealing temperature of 450 °C with amplitudes comparable to that of an InGaAs emitter. We attribute the THz emission to a surface field caused by decomposition of the surface of the annealed samples. The decomposition is confirmed by measuring the surface stoichiometry using Auger electron spectroscopy. We suggest that the decrease of THz intensity for the higher annealing temperatures is due to a lowering of the carrier mobility. Our experiment demonstrates the possibility to modify materials for THz emission in a very simple way.

[1] P.W. Chey, I.A. Babalado, T. Sukegawa, and W.E. Spicer, Phys. Rev. Lett. 35, 1602 (1975).

  • Lecture (Conference)
    DPG Frühjahrstagung, Fachverband Halbleiterphysik, HL 4.4, 2004, Regensburg, Deutschland

Publ.-Id: 6592

Structural analysis of ion irradiated polycrystalline NiFe/FeMn exchange bias systems

Blomeier, S.; Mcgrouther, D.; Mcvitie, S.; Chapman, J. N.; Weber, M. C.; Hillebrands, B.; Fassbender, J.

Structural investigations of the ion irradiated polycrystalline NiFe/FeMn exchange bias bilayer system were carried out by means of transmission electron microscopy. Key structural parameters like average grain size, lattice constant, and texture, as well as their dependence upon ion irradiation could be determined. For this purpose, a detailed analysis of a series of bright field images, dark field images, and diffraction patterns was performed. Also in this context, a previously established model was tested which ascribes changes in the magnetic properties upon irradiation with 5 keV He+ to the creation of point defects within the antiferromagnetic layer and to the intermixing between the ferromagnetic and antiferromagnetic layers. The obtained results indirectly support this model by excluding the aforementioned structural parameters as a possible source of the observed changes of the magnetic properties.

Keywords: magnetism; exchange-bias; ion irradiation; ion implantation; focused ion beam; TEM

  • European Physical Journal B 45(2005), 213

Publ.-Id: 6591

All-optical probe of magnetization dynamics in exchange biased bilayers on the picosecond time-scale

Weber, M. C.; Nembach, H.; Blomeier, S.; Kaltofen, R.; Schumann, J.; Carey, M.; Fassbender, J.

All-optical control of the magnetization of polycrystalline exchange bias bilayer systems is achieved using short picosecond laser pulses. Due to the photoexcitation, the spin temperature across the interface between the ferromagnetic and antiferromagnetic layer is elevated, resulting in a collapse of the interfacial exchange coupling. Thus, within the first 10 ps, a fast reduction of both the exchange bias field and the coercive field is observed for three different exchange bias systems comprising both different ferromagnets and antiferromagnets. The fast thermal unpinning is followed by a slower heat diffusion dominated relaxation process, which strongly depends on the thermal conductivity of the used buffer layers and substrates. The fast optical unpinning can be understood in terms of an internal anisotropy pulse field capable of triggering ultrafast precessional magnetization dynamics of the ferromagnetic layer, which makes heat-assisted coherent magnetization rotation feasible.

Keywords: magnetism; exchange bias; magnetic films; pump-probe; time-resolved techniques

  • European Physical Journal B 45(2005)2, 243-248

Publ.-Id: 6590

THz emission from GaSb samples with modified surface stoichiometry

Winnerl, S.; Sinning, S.; Dekorsy, T.; Helm, M.

We prepared efficient THz emitters based on photoexcited (fs-laser pulses) carriers in GaSb. Annealing (range 300 °C - 700 °C) provided a simple way for optimization. This is due to a surface field caused by a decomposition of the surface induced by the annealing. The decomposition is confirmed by Raman spectroscopy.

Keywords: THz emission; GaSb; Raman spectroscopy

  • Lecture (Conference)
    The Joint 29th International Conference on Infrared and Millimeter Waves and 12th International Conference on Terahertz Electronics (IRMMW2004/THz2004), 28.09.-1.10.2004, Karlsruhe, Deutschland
  • IEEE Conference digest IRMMW2004/THz2004

Publ.-Id: 6588

Detecting irradiation induced damage in RPV steels by SANS

Ulbricht, A.; Bergner, F.

It is well known that reactor pressure vessel steels suffer a degradation of the mechanical properties caused by neutron irradiation. Unfortunately, the ultra-fine microstructural features produced by irradiation cannot be directly detected by means of microscopic imaging techniques. Indirect evidence can, however, be obtained on the basis of Positron Annihilation Spectroscopy (PAS) and Small-Angle Neutron Scattering (SANS). The latter method has the advantage of high sensitivity to particle sizes from less than 1 to more than 10 nm in conjunction with integrating over a macroscopic volume (Fig. 1). Volume integration is important, because the difference between SANS intensities for neutron-irradiated material and unirradiated reference becomes independent of local fluctuations of both composition and microstructure. Furthermore, both size distribution and volume fraction of irradiation-induced defects can be calculated from measured SANS intensities. The ratio of SANS intensities for magnetic and nuclear scattering reveals also information on the composition of defects. In the Institute of Safety Research at the Forschungszentrum Rossendorf SANS experiments on Reactor Pressure Vessel (RPV) steels have been systematically and successfully designed, performed and analysed for a period of more than 10 years. In the present paper a short overview of the main results obtained by the Rossendorf group is given. Some open questions are summarized. On the other hand, one will not be able to obtain optimum information on the basis of a single experimental technique alone in most cases. From this viewpoint, supplementary experimental methods and numerical simulation techniques are considered.

Keywords: Small-angle neutron scattering; Neutron irradiation; Reactor pressure vessel steel

  • Lecture (Conference)
    30th MPA-Seminar in conjunction with 9th German-Japanese Seminar 'Safety and Reliability in Energy Technology', 06.-07.10.2004, Stuttgart, Germany
  • Contribution to proceedings
    Proceedings of the 30th MPA-Seminar in conjunction with 9th German-Japanese Seminar 'Safety and Reliability in Energy Technology', 06.-07.10.2004, Stuttgart, Germany, MPA-Seminar, Stuttgart, (2004)1, 10.1-10.13

Publ.-Id: 6587

Increased terahertz emission from thermally treated GaSb

Winnerl, S.; Sinning, S.; Dekorsy, T.; Helm, M.

We report on the terahertz emission from GaSb surfaces with modified surface stochiometry. While very weak emission is observed from virgin GaSb wafers, the emission is significantly increased by a single thermal treatment of the wafers. Optimum emission is observed for 500 °C thermal annealing. The reason for the THz emission is a surface electric field induced by thermal decomposition of the surface as corroborated by Raman spectroscopy.

Keywords: Terahertz emission; Raman spectroscopy; surface field; GaSb

Publ.-Id: 6586

Recents Developments in Nuclear Safety Research

Weiß, F.-P.

The presentation starts from sustainability considerations of different energy carries like oil, hoard cool, or nuclear energy.
It is highlighted, that in terms of sustainability nuclear energy suffers from the danger of proliferation of weapons grade material, the maximum damage of a conceivable severe accident, and the extremely long isolation period of the nuclear waste in the final disposal. Discussing the safety analysis of so called boron dilution transients in PWRs, the behaviour of the reactor pressure vessel during a severe core melt accident, and the possibilities of the transmutation of long lived radio-nuclides it is shown how modern nuclear safety research can contribute to improved sustainability performance of nuclear energy.

  • Invited lecture (Conferences)
    Conference on Multi-Phase Flow: Simulation Experiment and Application, 28.-30.06.2004, Dresden, Germany

Publ.-Id: 6585

Boron dilution Transients in PWRs

Weiß, F.-P.; Kliem, S.

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

  • Book (Authorship)
    Unterrichtsmaterial für die Frederic Joliot/Otto Hahn Summer School, Karlsruhe,August 20-29, 2003

Publ.-Id: 6584

Boron dilution Transients in PWRs

Weiß, F.-P.

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

  • Lecture (Conference)
    Vortrag zur Frederic Joliot/Otto Hahn Summer School, Karlsruhe, August 20-29, 2003

Publ.-Id: 6583

Charged pion production in 9644Ru + 9644Ru collisions at 400A and 1528 A MeV

Hong, B.; Kim, Y. J.; Herrmann, N.; Stockmeier, M. R.; Alard, J. P.; Andronic, A.; Barret, V.; Basrak, Z.; Bastid, N.; Benabderrahmane, L.; Caplar, R.; Crochet, P.; Dupieux, P.; Dzelalija, M.; Fodor, Z.; Gobbi, A.; Grishkin, Y.; Hartmann, O. N.; Hildenbrand, K. D.; Kecskemeti, J.; Kirejczyk, M.; Koczon, P.; Korolija, M.; Kotte, R.; Kress, T.; Lebedev, A.; Leifels, Y.; Lopez, X.; Mangiarotti, A.; Merschmeyer, M.; Neubert, W.; Pelte, D.; Petrovici, M.; Rami, F.; Reisdorf, W.; Schüttauf, A.; Seres, Z.; Sikora, B.; Sim, K. S.; Simion, V.; Siwek-Wilczynska, K.; Smolyankin, V.; Stoicea, G.; Tyminski, Z.; Wagner, P.; Wisniewski, K.; Wohlfarth, D.; Xiao, Z. G.; Yushmanov, I.; Zhilin, A.

We present the transverse momentum and rapidity spectra of charged pions in central Ru + Ru collisions at 400A and 1528A MeV. The data exhibit enhanced productions at low transverse momenta compared to the expectations from the thermal model that includes the decay of Delta(1232)-resonances and thermal pions. Modification of the Delta-spectral function and the Coulomb interaction are necessary to describe the detailed shape of the transverse momentum spectra. Within the framework of the thermal model, the freeze-out radii of pions are similar at both beam energies. The IQMD model also reproduces the shapes of the transverse momentum and rapidity spectra of pions, but the predicted absolute yields are larger than in the measurements, especially at lower beam energy.

  • Physical Review C 71(2005), 034902

Publ.-Id: 6582

Prüfung duktiler Werkstoffe mit dem Small Punch Test

Kuna, M.; Abendroth, M.

Der Small Punch Test (SPT) ist ein Kleinstprobenversuch, der sich zur Bestimmung des aktuellen lokalen Materialzustandes in technischen Anlagen unter Betriebsbedingungen eignet. Dieser Artikel stellt ein kombiniertes experimentell-numerisches Verfahren zur Identifikation von Verfestigungs- und Versagenseigenschaften duktiler Werkstoffe aus dem SPT vor. Ein schädigungsmechanisches Materialmodell dient zur Beschreibung des Probenverhaltens.
Die Einflüsse der Parameter des schädigungsmechanischen Materialmodells auf das Versuchsergebnis werden anhand von FEM-Simulationen des SPT dargestellt. Neuronale Netze werden mit diesen Simulationen trainiert und stellen so einen generalisierten Zusammenhang zwischen Materialparametern und Versuchsergebnis dar, der in einem Optimierungsalgorithmus zur Identifikation der unbekannten schädigungsmechanischen Materialparameter benutzt wird. Die identifizierten Materialparameter lassen sich auf andere Probengeometrien übertragen und ermöglichen eine Vorhersage von bruchmechanischen Werkstoffkennwerten.

Keywords: Small Punch Test; Schädigungsmechanik; Parameteridentifikation; Neuronale Netze

  • Materialprüfung 47 2005)1-2, 45-54

Publ.-Id: 6580

The effect of atomic hydrogen flow on electrical resistance of the transition metal films

Kagadei, V.; Nefyodtsev, E.; Proskurovsky, D.; Romanenko, S.; Shevchenko, V.; Grambole, D.; Groetzschel, R.; Herrmann, F.; Ivanov, Y.

On using of nuclear reaction analysis, X-ray analysis, electron microscopy, the mechanisms of structure and phase composition change of thin vanadium films at treatment in a flow consisting of molecular and atomic hydrogen (AH) have been studied. A comparative analysis on regularities of the film saturation with hydrogen and its resistance changing was carried out. It has been settled that the resistance dependence on hydrogen concentration in the case of thin film hydrogenised in AH flow differs from the analogous dependence for the bulk material hydrogenised in an atmosphere of molecular hydrogen in the conditions of thermodynamic equilibrium. Nevertheless, the found peculiarities of thin film resistance dependence on hydrogenation time, the same as in the case of bulk material, are determined by formation of vanadium hydride -phase in the film. It is shown that with transient metal films it is possible to measure AH flow densities under reduced pressure of gas (10-1 to 10-3 Pa). The principle of measurement is based on dependence analysis of the film resistance increment determined by hydrogen solubilization in the film versus exposure time in AH flow. The suggested measurement method is characterized by high selectivity to AH allowing measurement of partial density of atomic flow over the range from 5×1013 up to 1016 at /cm²/s in a mixed atomic–molecular flow. The sensor made on the basis of this method can find applications in microelectronic techniques where semiconductor treatment by atomic hydrogen flows is employed.

Keywords: Gas sensor; Atomic hydrogen; Electrical conductivity

  • Sensors and Actuators A 113(2004), 293-300

Publ.-Id: 6579

Determination of ductile material properties by means of the small punch test and neural networks

Abendroth, M.; Kuna, M.

This Paper compares two different methods for the identifikation of ductile properties of materials using the small punch test to measure the material response under loading. The finite element method is used to calculate the load displacement curve of the punch depending on the parameters of a hardening law. Via systematical variation of the material parameters a data base is built up, which is used to train neural networks. The second method allows the indentification of material parameters by using a conjugate directions algorithm, which minimizes the error between an experimental load displacement curve and one calculated by the network function.

Keywords: small punch test; damage mechanics; neural networks; parameter identifikation; finite elements

Publ.-Id: 6578

Deformation-induced small-angle scattering contrast in aluminium alloys

Bergner, F.; Müller, G.; Ulbricht, A.; van Ouytsel, K.; Blank, C.; Bras, W.; Dewhurst, C.

Small-angle neutron scattering experiments have been performed on a homogeneously deformed specimen (uniform elongation 18.5%) of commercial thin-sheet wrought aluminium alloy AA 6013 (Al-Mg-Si-Cu) aged at room temperature. Comparisons with undeformed material revealed characteristic deformation-induced anisotropy of the two-dimensional X-ray and neutron scattering pattern, which can be interpreted as being due to deformation-induced arrangements of preferentially aligned dislocation segments. The results turn out to be helpful for the understanding of anisotropic small-angle X-ray scattering patterns obtained from the crack-tip plastic zone of cracked specimens of the aluminium alloy AA 6013 tempered at 190°C to maximum hardness.

Keywords: Small-angle X-ray scattering; Small-angle neutron scattering; Aluminium alloys; Plasticity

  • Nuclear Instruments and Methods in Physics Research B 23(2005), 92-97
  • Poster
    4th Conference on Synchrotron Radiation in Materials Science (SRMS4), 23.-25.08.2004, Grenoble, France

Publ.-Id: 6577

Centrality Dependence of Thermal Parameters Deduced from Hadron Multiplicities in Au + Au Collisions at sqrt{s_{NN}} = 130\ GeV

Cleymans, J.; Kämpfer, B.; Kaneta, M.; Xu, N.

We analyse the centrality dependence of thermal parameters deduced from hadron multiplicities in Au + Au collisions at sqrt{s_{NN}} = 130\ GeV. While the chemical freeze-out temperature and chemical potentials are found to be roughly centrality-independent, the strangeness saturation factor gamma_S increases with participant number towards unity, supporting the assumption of equilibrium freeze-out conditions in central collisions.

Keywords: strangeness; heavy ion collisions; statistical model

  • Physical Review C 71(2005), 054901

Publ.-Id: 6576

Above room temperature operation of short wavelength (lambda=3.8 µm) strain-compensated In0.73Ga0.27As–AlAs quantum-cascade lasers

Semtsiv, M. P.; Ziegler, M.; Dressler, S.; Masselink, W. T.; Georgiev, N.; Dekorsy, T.; Helm, M.

We demonstrate the design and implementation of a broad-gain and low-threshold (Jth = 860 A/cm2 at 8 K) quantum-cascade laser emitting between 3.7 and 4.2 µm. The active region design is based on strain-compensated In0.73Ga0.27As–AlAs on InP. Laser operation in pulsed mode is achieved up to a temperature of 330 K with maximum single-facet output peak powers of 6 W at 8 K and 240 mW at 296 K. The temperature coefficient T0 is 119 K.

Keywords: QCL; quantum cascade laser; room temperature; low-threshold; strain-compensated; InGaAs; AlAs

Publ.-Id: 6575

Syntheses of 18F-Labeled Acyclic Purine and Pyrimidine Nucleosides Intended for Monitoring Gene Expression

Grote, M.; Noll, S.; Noll, B.

  • Lecture (Conference)
    6th International Conference on Nuclear and Radiochemistry, 29.08.-03.09.2004, Aachen, Germany
  • Lecture (Conference)
    European Symposium on Radiopharmacy and Radiopharmaceuticals, 09.-12.09.2004, Danzig, Poland

Publ.-Id: 6574

Mikromilieu und Hypoxie im Bild

Baumann, M.; Krause, M.; Eicheler, W.; Petersen, C.; Zips, D.; Beuthien-Baumann, B.; Kotzerke, J.; Laniadao, M.; Herrmann, T.

Der prätherapeutischen Identifikation prognosebestimmender Parameter des Tumormikromilieus, z. B. dem Ausmaß der Hypoxie kommt eine besondere Bedeutung für die Individualisierung der Tumortherapie zu. Entsprechende Informationen können heute grundsätzlich durch die biologisch-funtionelle Bildgebung zur Verfügung gestellt werden. Ein wesentliches Problem sämtlicher bildgebender Verfahren ist jedoch, dass nicht zwischen klonogenen, also rezidivfähigen, und nicht -klonogenen Zellen unterschieden werden kann. Klonogene Zellen machen oft nur einen verschwindend kleinen Anteil aller Zellen im Tumor aus und können ausschließlich durch funktionell-tumorbiologische Methoden erfasst werden. Voraussetzung für die Entwicklung und Validierung funktioneller Bildgebungsmethoden ist daher die Integration von Methoden der Strahlen- und Tumorbiologie mit bildgebenden Untersuchungensverfahren. Diese Integration setzt den Aufbau und die gezielte Förderung entsprechender fachübergreifender Gruppen voraus. Bisherige Ergebnisse zur funktionellen Bildgebung des Tumormikromilieus, die anhand eines Beispiels illustriert werden, zeigen eine hohe Relevanz für unser Verständnis der Biologie von Krebserkrankungen. Es besteht wenig Zweifel daran, dass in der Zukunft die Integration der funktionellen Bildgebung in die Krebsbehandlung, z. B. in die computergeschützte Strahlentherapieplanung, wesentliche Impulse für die Verbesserung der Therpaie krebskranker Patienten bieten wird.

  • Nova Acta Leopoldina NF 89, Nr. 337 (2004) 119-125

Publ.-Id: 6573

SANS-Untersuchungen zur Charakterisierung von Strahlendefekten in RDB-Stählen nach Bestrahlung, Ausheilung und Wiederbestrahlung

Ulbricht, A.; Bergner, F.

Bekanntlich bewirkt das Strahlungsfeld schneller Neutronen in der Umgebung des Reaktorkerns eine Verschlechterung der mechanischen Eigenschaften des Druckbehälters. Zur Vertiefung des Verständnisses und zur quantitativen Beschreibung der Zusammenhänge zwi-schen Gefüge, mechanischen Eigenschaften und Strahlenbelastung wurden an Reaktor-druck-behälter-(RDB)-Stählen Strukturuntersuchungen mit Neutronen-Kleinwinkelstreuung (SANS) durchgeführt. Dazu wurden Proben aus verschiedenen Chargen von west- und ost-europäischen RDB-Stählen verwendet, die im Rahmen des Bestrahlungsprogramms am ehemaligen Proto-typ-Reaktor WWER-2 in Rheinsberg oder der Versprödungsüberwachungsprogramme laufender WWER-Reaktoren (u. a. in Loviisa, Finnland) bestrahlt worden sind.
Die bestrahlungsbedingten Strukturveränderungen sind abhängig vom Stahltyp. Charakteristische Parameter der Strahlendefekte, wie Anzahldichte, Größenverteilung und Volumen-gehalt wurden bestimmt und mögliche Defektzusammensetzungen abgeschätzt. Es zeigte sich, dass mit zunehmender Strahlenbelastung für einen bestimmten Stahltyp der Gehalt an Defekten nicht proportional ansteigt, wobei die Größenverteilung nahezu unverändert bleibt. Defektradien bis 3 nm treten auf. Das Maximum der Größenverteilung liegt bei 1 nm. Zudem kann sich mit der Bestrahlungszeit auch die mittlere Zusammen-setzung der Punktdefekt-agglomerate aus Leer-stellen und Fremdatomen ändern. Kupferverunreinigungen im Stahl beeinflussen die Clusterbildung am stärksten. Reine Kupfer-ausschei-dungen wurden nicht beobachtet.
Eine Wärmebehandlung über 400 °C reduziert den Anteil an Defekten, aber stabilisiert kupferreiche Cluster.
Erstmals wurden SANS-Untersuchungen an nach einer Ausheilwärmebehandlung (475 °C / 100 h) wiederbestrahltem RDB-Stahl durchgeführt. Die Defekterzeugung war im zweiten Bestrahlungszyklus wesentlich geringer. Auch die Zusammensetzung der neuen Strahlendefekte wich von der des ersten Bestrahlungszustandes ab.

  • Poster
    Deutsche Neutronenstreutagung, 1.-3. September 2004, Dresden

Publ.-Id: 6572

Heating effect on steady and unsteady horizontal laminar flow of air past a circular cylinder

Shi, J.-M.; Gerlach, D.; Breuer, M.; Biswas, G.; Durst, F.

Extensive numerical experiments were carried out to study the effect of cylinder heating on the characteristics of the flow and heat transfer in a two-dimensional horizontal laminar flow of air past a heated circular cylinder for the range of Reynolds numbers $0.001 \le Re \le 170$. The fluid was treated as incompressible (density is independent of the pressure) while the variation of the fluid properties with temperature was taken into account. By including the transient density term of the continutiy equation, which was neglected in a previous study by Lange et al.(IJHMT, 41(22), 1998), we were able to predict correctly the vortex shedding frequency at various overheat ratios using an incompressible flow solver. The effect of dynamic viscosity and density variations on the flow dynamics occurring with the cylinder heating was analyzed separately. Another emphasis of the work was to investigate the physical mechanism behind the ``effective Reynolds number'' concept widely applied in engineering correlations. Similarity was discovered for the distribution of the local dimensionless viscous force, the vorticity and the Nusselt number at the cylinder surface and the pressure force in the rear part of the cylinder. Two characteristic temperatures, T_{ef\!f} = T_\infty + 0.28(T_W-T_\infty) for the flow dynamics and T_{f} = T_\infty + 0.5(T_W-T_\infty) for the heat transfer, were identified.

Keywords: heat transfer; forced convection; vortex shedding; local similarity; characteristic temperature

Publ.-Id: 6571

In-situ conformational analysis of fibrinogen adsorbed on Si surfaces

Tunc, S.; Maitz, M. F.; Steiner, G.; Vázquez, L.; Pham, M. T.; Salzer, R.

Fibrinogen is a major plasma protein. Previous investigations of structural changes of fibrinogen due to adsorption are mostly based on indirect evidence after its desorption, whereas our measurements were performed on fibrinogen in its adsorbed state. Specific enzyme linked immunosorption experiments showed that the amount of adsorbed fibrinogen increased as the surface became more hydrophobic. AFM investigations revealed the trinodular shape of fibrinogen molecules adsorbed on hydrophilic surfaces, whereas all of the molecules appeared globular on hydrophobic surfaces. The distribution of secondary structures in adsorbed fibrinogen was quantified by in-situ FTIR analysis. Substrates of identical chemical bulk composition but different surface hydrophobicity permit direct comparison among them. Adsorption properties of fibrinogen are different for each degree of hydrophobicity. Although there is some increase of turn structure and decrease of β-sheet structure, the secondary structure of adsorbed fibrinogen on hydrophilic surface turned out to be rather similar to that of the protein in solution phase with a major α-helix content. Hydrophilic surfaces exhibit superior blood compatibility as required for medical applications.

Keywords: Silizium; Protein; Fibrinogen; Adsorption; Konformation; AFM; FTIR

  • Colloids and Surfaces B: Biointerfaces, 42(2005)3-4, 219-225

Publ.-Id: 6569

Experimental Evidence of Si Nanocluster delta-Layer Formation in the Vicinity of Ion-Irradiated SiO2-Si Interfaces

Röntzsch, L.; Heinig, K. H.; Schmidt, B.; Mücklich, A.

Predictions on formation and self-alignment of Si nanocrystals (NCs) at ion-irradiated SiO2-Si interfaces have been proven by cross-section transmission electron microscopy (TEM). The model is based on ion-mixing of SiO2-Si interfaces resulting in a region of SiOx (x<2). During annealing, the interface restores and in the tail of the mixing profile Si excess precipitates and forms a self-aligned layer
of Si NCs in SiO2. This Si NC delta-layer structure can hardly be observed by conventional TEM techniques due to the very low mass contrast of tiny Si NCs embedded in SiO2. Here, a contrast enhancing method is presented based on alloying of these Si NCs with Ge. For this purpose, a thin Ge layer is embedded into the oxide sufficiently far from the interface mixing range, thus, preventing interference with the Si precipitate formation. During annealing, diffusing Ge monomers attach to the Si NCs, mainly due to the energetically favored Si-Ge bond, resulting in Si1-xGex NCs with an enhanced mass contrast in TEM.

Keywords: SiO2-Si interface; ion irradiation; phase separation; silicon; nanocrystal; self-alignment

  • Poster
    14th International Conference on Ion Beam Modification of Materials, 05.-10.09.2004, Monterey, USA
  • Nuclear Instruments and Methods in Physics Research B 242(2006), 149-151

Publ.-Id: 6565

Renormalization of the Two-Photon Vacuum Polarization and the Self Energy Vacuum Polarization for a Tightly Bound Electron

Zschocke, S.; Plunien, G.; Soff, G.

The renormalization method of Bogoljubov-Parasuik-Hepp-Zimmermann (BPHZ) is used in order to derive the renormalized energy shift due to the gauge invariant Källén-Sabry diagram of the two-photon vacuum polarization (VPVP) as well as the self energy vacuum polarization S(VP)E beyond the Uehling approximation. It is outlined, that no outer renormalization is required for the two-photon vacuum polarization and that only the inner renormalization has to be accomplished. It is shown that the so-called nongauge invariant sourious term is absent for a wide class of vacuum polarization (VP) diagrams if one applies the widely used sherical expansion of bound and free-electron propagator. This simplifies significantly calculations in bound state quantum electrodynamis. As one result of our paper the use of the BPHZ-approach in bound state QED is established.

Publ.-Id: 6564

An Electromagnetic Undulator for the Far Infrared at ELBE

Dekorsy, T.; Fahmy, K.; Grosse, E.; Michel, P.; Seidel, W.; Wolf, A.; Wünsch, R.

The parameters of an envisaged undulator at the radiation source ELBE for the production of IR radiation up to 150 microns are presented.

  • Poster
    Poster, 26th Int. Free-Electron Laser Conf. and 11th FEL Users Workshop, Trieste, 29.08.-3.09.2004
  • Open Access Logo Contribution to proceedings
    26th International Free-Electron Laser Conference and 11th FEL Users Workshop, 29.08.-03.09.2004, Trieste, Italy
    Proceedings of the 26th International Free-Electron Laser Conference and 11th FEL Users Workshop, 389-391

Publ.-Id: 6563

Ion-induced stress relaxation during the growth of cubic boron nitride thin films

Abendroth, B.

  • Open Access Logo Wissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-406
    ISSN: 1437-322X


Publ.-Id: 6560

Modification of magnetic thin films and nanostructures by ion irradiation and implantation

Fassbender, J.

He ion irradiation in an applied magnetic field allows to tailor the magnetic properties of thin films and nanostructures after preparation due to structural phase transformations, interfacial mixing, chemical ordering and magnetic ordering. Some examples for the case of exchange bias films, soft magnetic films and FePt nanoparticles will be demonstrated.
In addition ion implantation can be used for the doping of magnetic materials to modify the Curie temperature, the saturation magnetization or the magnetic damping behavior of ferromagnetic films. First results of these ion implantation studies will be presented.

Keywords: magnetism; magnetic films; nanoparticles; ion irradiation; ion implantation

  • Lecture (others)
    Seminar talk: Hitachi Almaden, San Jose, USA, 08.10.2004

Publ.-Id: 6557

Maßgeschneiderte magnetische Materialien – neue Wege durch Ionenbestrahlung

Fassbender, J.

Aufgrund der reduzierten Dimensionen in ultradünnen magnetischen Schichten und Nanopartikeln hängen die magnetischen Eigenschaf-ten, wie z. B. magnetische Anisotropien und Austauschkopplung, empfindlich von der Grenzflächen- bzw. Oberflächenmorphologie ab. Zusätzlich werden chemisch geordnete intermetallische Verbindungen zunehmend anwendungsrelevant. In beiden vorgenannten Bereichen bietet die Ionenbestrahlung eine exzellente Möglichkeit nach der Prä-paration die magnetischen Eigenschaften gezielt auf einer lokalen Skala zu beeinflussen. Somit lässt sich eine rein magnetische Na-nostrukturierung ohne Veränderung der Oberflächentopographie reali-sieren.
Der Vortrag gibt eine Übersicht über die neuen Möglichkeiten mit Hilfe der Ionenbestrahlung künstliche magnetische Strukturen herzustellen und ordnet diese im Bezug auf deren Anwendungsrelevanz ein.

Keywords: magnetism; ion irradiation

  • Lecture (others)
    Physikalisches Kolloquium TU Dresden

Publ.-Id: 6556

Reflectivity characterization of ion irradiated exchange bias FeMn-FeNi films

Solina, D.; Liedke, M. O.; Tietze, U.; Fassbender, J.; Schreyer, A.

X-ray reflectivity, neutron reflectivity studies and Magneto-Optic Kerr Effect measurements (MOKE) have been carried out on thermally evaporated FeMn-FeNi exchange biased films before and after ion irradiation by helium ions. MOKE shows that ion irradiation reduces the exchange bias characteristics of the samples with increased dose. Modelling of the reflectivity data infers that atoms from the buffer layer are displaced by the ions and imbedded into the substrate material. The correlation between these strong structural modifications by irradiation with the reduction of the exchange bias will be discussed.

Keywords: magnetism; magnetic films; exchange bias; x-ray diffraction; neutron scattering

  • Journal of Magnetism and Magnetics Materials 286(2005), 225

Publ.-Id: 6555

Analysis of theNi81Fe19/Fe50Mn50 exchange bias system with a varying Cu spacer thickness and position for partial decoupling

Liedke, M. O.; Nembach, H.; Hillebrands, B.; Fassbender, J.

In order to study the role of the exchange interaction at and near the interface, Ni81Fe19/Fe50Mn50 bilayers have been studied, which have an intervening layer of varying thickness and position in the antiferromagnetic Fe50Mn50 layer. The role of the intervening layer is to generate partial exchange decoupling. As a result, samples were obtained, where in one in-plane direction the position of the intervening layer varies from the interface to the top surface of the Fe50Mn50 layer at a constant intervening layer thickness, and in the other in-plane direction the intervening layer thickness varies. Two-dimensional maps of the resulting exchange bias field and the coercive field were obtained from magneto-optic Kerr effect magnetometry measurements. The role of the position and strength of the partial decoupling within the antiferromagnetic layer on the exchange bias effect and the coercive field is discussed.

Keywords: magnetism; magnetic films; exchange bias; antiferromagnets; exchange coupling

  • Journal of Magnetism and Magnetic Materials 290-291(2005)1, 588-590

Publ.-Id: 6554

Modified Gilbert Damping due to Exchange Bias in NiFe/FeMn Bilayers

Weber, M.; Nembach, H.; Hillebrands, B.; Fassbender, J.

A picosecond all-optical pump-probe technique is used to investigate magnetization dynamics and magnetic damping behaviour in the Ni81Fe19/Fe50Mn50 exchange bias system. Within the laser excitation the spin temperature is elevated resulting in a collapse of the exchange bias field. Thus, a fast anisotropy field pulse is launched which triggers the magnetization precession. The extracted Gilbert damping increases linearly with the exchange bias field magnitude which can be understood taking local fluctuations of the interfacial exchange coupling as an additional dissipation mechanism into account.

Keywords: magnetism; magnetic films; exhange-bias; magnetic damping; pump-probe; time-resolved

  • Journal of Applied Physics 97(2005), 10A701

Publ.-Id: 6552

Real time temperature dynamics in exchange biased bilayers upon laser excitation

Weber, M.; Nembach, H.; Hillebrands, B.; Fassbender, J.

A hot spin and phonon gas in exchange biased metallic bilayers is induced by an 8.5 ps laser excitation. The spin-lattice temperature dynamics is sensed in real time by the time evolution of the exchange bias field on the picosecond timescale. A calibration with temperature dependent quasistatic Kerr measurements explains a pump pulse induced temperature increase of about 100°C at the interface. Upon photoexcitation the exchange coupling across the interface between the ferromagnetic and antiferromagnetic layer is reduced within the first 10 ps, leading to a reduction of the bias field to about 50% of its initial value. The fast thermal unpinning of the exchange coupling is followed by a heat diffusion dominated recovery with a relaxation time on the order of 160 ps. A heat transport analysis reveals the diffusivity of the studied bilayer system.

Keywords: magnetism; magnetic films; time-resolved MOKE; exchange-bias; pump-probe

  • IEEE Transactions on Magnetics 41(2005)3, 1089-1092

Publ.-Id: 6551

Grazing incidence X-ray diffraction of SS304 steel surfaces modified by high- and low- pressure ion nitriding processes

Ueda, M.; Gomes, G.; Abramof, E.; Reuther, H.

Ion nitriding is an important surface modification technique used routinely in the advanced technology industries aiming at producing high added value components with increased hardness and wear resistance. To further improve the performance of critical-part components, new methods for surface strengthening are being developed with success, like plasma immersion ion implantation (PIII) and hybrid surface treatments combining PIII and ion nitriding. In particular, a combination of high-pressure (40 Pa), high-temperature (up to 450 degreesC) glow discharge nitriding with low-pressure (8 x 10(-2) Pa) ion nitriding was implemented. Analysis of the SS304 samples treated by this hybrid method was carried out using the technique of grazing incidence X-ray diffraction (GIXRD) and the results were supported by both normal XRD and AES (Auger electron spectroscopy) measurements. Depending on the particular conditions of the treatment and the depth probed, mixed phases Of gamma(N), and gamma were measured. The improvement in hardness of the obtained surfaces (up to a factor of 2.7) was also in accordance with the results of other analysis.

Publ.-Id: 6550

Surface modification of metal alloys by plasma immersion ion implantation and subsequent plasma nitriding

Kostov, K. G.; Ueda, M.; Lepiensky, A.; Soares, P. C.; Gomes, G. F.; Silva, M. M.; Reuther, H.

Thermal or radiation enhanced diffusion of nitrogen are extensively utilized for the surface hardening of metallic components. Plasma-immersion ion implantation (PIII) is a newly developed technology, which provides ion implantation at moderate energy (10-50 keV), and thereby allowing penetration depths deeper than the surface oxide barrier. The damage caused by ion implantation together with the surface sputtering may create favorable boundary conditions for an efficient subsequent diffusive treatment such as nittiding. Surface modification of aluminum alloy 5052, Ti6A14V alloy and steels (AISI 304 and H13) by a combination of PIII and plasma nitriding (PN) has been investigated. Nitrogen ions were implanted into specimens at 15 W and then ion nitrided at low pressure with bias of - 800 V. Compared to the untreated samples the hardness of Ti6Al4V alloy and AISI 304 steel could be improved significantly. The hardness of H13 steel can be increased by 20+ACU- using a duplex process with 4-h nitriding time. X-ray diffraction (XRD) results have shown some structural modification of the metallic samples and formation of a double-layer structure in AISI 304, treated by PIII and PN. Nitrogen depth profile of the same stainless steel sample, obtained by Auger electron spectroscopy (AES), shows two rather well-defined nitrogen enriched regions with different N contents: high (25-30 at.+ACU-) in a surface layer and medium (similar to 10+ACU-) in a subsurface layer.

Publ.-Id: 6549

Use of low energy and high frequency PBII during thin film diposition to achieve relief of intrinsic stress and microstructural changes

Bilek, M. M. M.; Mckenzie, D. R.; Möller, W.

In this paper, we describe the use of plasma based ion implantation (PBII) together with physical vapour deposition (PVD) to achieve low intrinsic stress and influence the microstructure of coatings. A model is proposed to explain the physical mechanisms responsible for observed changes in stress. The model is compared with other available models and all are tested against data obtained using a cathodic vacuum arc in background gas to deposit aluminium nitride, titanium nitride and carbon in the presence of high voltage pulsed bias on the substrate. In all three systems, application of PBII resulted in large reductions in intrinsic stress as the voltage and frequency of the pulses was increased. We observed a trade off between applied bias voltage and pulsing frequency. The crystalline systems, aluminium nitride and titanium nitride showed changes in preferred orientation in PBII treated films. An important conclusion of significance to industry is that even relatively low energy (0.5¿5 keV) ion implantation can be effectively employed to achieve stress relief.

Keywords: Intrinsic stress; Plasma immersion ion implantation; Deposition

Publ.-Id: 6548

Stability of the N=50 shell gap in the neutron-rich Rb, Br, Se, and Ge isotones

Zhang, Y. H.; Podolyàk, Z.; de Angelis, G.; Gadea, A.; Ur, C.; Lunardi, S.; Marginean, N.; Rusu, C.; Schwengner, R.; Kröll, T.; Napoli, D. R.; Menegazzo, R.; Bazzacco, D.; Farnea, E.; Lenzi, S.; Martinez, T.; Axiotis, M.; Tonev, D.; Gelletly, W.; Langdown, S.; Regan, P. H.; Valiente Dobon, J. J.; von Oertzen, W.; Rubio, B.; Quintana, B.; Medina, N.; Broda, R.; Bucurescu, D.; Ionescu-Bujor, M.; Iordachescu, A.

The low- and medium-spin states of the N = 50 neutron-rich, 37 87Rb, 35 85Br, 34 84Se and 32 82Ge isotones have been populated in deep-inelastic processes produced by the interaction of 460 MeV 82Se ions with a 192Os target. The subsequent gamma decay has been investigated at the Laboratori Nazionali di Legnaro using the GASP gamma-ray detector array. The comparison of the experimentally observed excited states with shell-model calculations performed with and without neutron degrees of freedom has allowed the investigation of the role of the neutron-core breaking excitations and therefore of the N = 50 shell gap. The inclusion of neutron configurations in the shell-model calculations results in an improved agreement between the experimental and calculated level energies at medium and high spin. These results highlight the considerable contribution of neutron-core particle-hole excitations across the N = 50 shell gap to the level configurations. The overall agreement of the measured excited states with the shell-model predictions is indicative of the persistence of the N = 50 shell gap down to Z = 32.

Keywords: nuclear shell model; gamma-ray spectra; deep inelastic heavy ion reactions; nuclei with mass number 190 to 219; nuclear spin; nuclear energy level transitions; high spin states; nuclei with mass number 59 to 89

Publ.-Id: 6547

Ion Irradiation through SiO2/Si-interfaces: Non-conventional Fabrication of Si Nanocrystals for memory applications

Schmidt, B.; Heinig, K.-H.; Röntzsch, L.; Müller, T.; Stegemann, K.-H.; Votintseva, E.

Si nanocrystals for multi-dot floating-gate memories have been produced by non-conventional ion beam synthesis (IBS). Due to ion beam mixing irradiation with 1015-1016 Si+ cm-2 at 50-100 keV through 50 nm poly-Si and 15 nm SiO2 on (001)Si results in a considerable Si excess within the oxide. At the upper and lower interfaces of the gate oxide, this ion irradiation forms a metastable SiOx (x < 2) composition. Post-irradiation RTA thermal treatment leads to phase separation into Si and SiO2. Adjacent to the recovering interfaces, narrow SiO2 zones become denuded of excess Si. More distant excess Si precipitates as Si NCs in the gate oxide. MOSFET characteristics in terms of write/erase voltage, duration of the programming time, endurance and retention have been evaluated.

Keywords: nanocluster; ion irradiation; phase separation; non-volatile multi-dot floating-gate memory

  • Lecture (Conference)
    14th Int. Conf. on Ion Beam Modification of Materials (IBMM 2004), September 5-11, 2004, Monterey, California, USA
  • Nuclear Instruments and Methods in Physics Research B 242(2006)1-2, 146-148

Publ.-Id: 6546

Nitrogen Transport in the Primary Circuit of a VVER during a LOCA occurring under Plant Cool-down Conditions

Prasser, H.-M.; Baranyai, G.; Böttger, A.; Ezsöl, G.; Guba, A.; Perneczky, L.; Toth, I.; Zschau, J.

The paper presents the results of a thermal hydraulic experiment conducted at the PMK-2 test facility in the frame of the IMPAM-VVER project. The test was dedicated to a scenario assuming a loss-of-coolant event dur-ing the cool-down phase of the plant, when neither core flooding tanks nor safety injections systems are available by an automatic action. This may lead to core heat-up in case of a larger LOCA. The assumed break size in the test was 30%. The experiment has shown that an overheat of the fuel rod cladding occurs. Finally, one of four lines of the low pressure ECC injection system is sufficient to handle the situation and to cool down the reactor safely. A speciality of the scenario is the fact that the primary circuit is pressurised with nitrogen. In the course of the transient, non-condensable gas can enter the primary circuit from the pressurizer during the blow-down phase. In order to study the transport of the nitrogen along the primary circuit, the test facility PMK-2 was equipped with a novel type of probe: a local void probe equipped with an integrated micro-thermocouple. The new probes delivered a global picture of the process. Nitrogen coming from the pressurizer after its depletion enters the circuit via the surge line and turns first in the direction of the main coolant flow towards the steam generator and finally towards the break in the cold leg. However, nitrogen is also moving in the opposite direc-tion and reaches the reactor outlet and enters even the upper plenum of the reactor closely after the main circulation pump has stopped. RELAP5 was found to be able to predict the behaviour of the non-condensable gas in the primary circuit. Despite of some delay in the prediction of the time of the appearance of the gaseous phase in the begin of the process, most details of the transient were reproduced by the calculation with a good accuracy.

Keywords: pressurized water reactors; loss-of-coolant transients; VVER; nitrogen transport; void probes; RELAP

  • Kerntechnik 69(2004)4, 167

Publ.-Id: 6545

Long-time operation of a modified Cs sputter ion source 860-C

Friedrich, M.; Tyrroff, H.

The modified Rossendorf Cs sputter ion source 860-C presented at the SNEAP 2002 [1] was opened in July 2004 for maintenance after 4100 operation hours. The main reason for this first source opening after its installation in July 2002 was a short circuit of the sputter voltage due to material deposition on the cathode insulator. Beside the long lifetime the modification of the source has resulted in a reduced erosion of the spherical ioniser and in a concentrated deposition of the sputtered material on the front side of the additional shielding electrode and on the cathode-shielding electrode. After cleaning all inner source parts can be reused in the ion source.

[1] M. Friedrich and H. Tyrroff, Nucl. Instr. and Meth. B 201 (2003) 645.

Keywords: Ion sources; Negative ions; Electrostatic accelerators

  • Lecture (Conference)
    SNEAP-2004, Hamilton/Ontario, Canada
  • Contribution to external collection
    Proceedings of SNEAP-2004, Hamilton/Ontario, Canada, 2005 (auf CD)

Publ.-Id: 6544

RBS with high depth resolution using small magnetic spectrometers

Grötzschel, R.; Klein, C.; Mäder, M.

The increasing importance of ultra-thin layers for novel technologies demands quantitative analysis techniques with a depth resolution of atomic monolayers, which can be obtained for RBS and ERDA by magnetic spectrometers.

We operate at the 3 MV Tandetron accelerator a magnetic spectrometer consisting of an UHV scattering chamber and a simple dipole magnet with circular field boundaries (Browne¿Buechner spectrometer). Since in many cases of high resolution ion beam analysis the samples must be prepared in situ in UHV, the chamber with a base vacuum of 4 × 10-10 mbar is equipped with an ion sputter gun and two low rate e-beam evaporators for in situ layer deposition. A RHEED system is used to check the surface reconstruction and monitor the layer growth. Samples are transferred, together with a BN heater, to the precision 5-axes channelling goniometer. The magnet with a mean radius of 0.65 m is mounted vertically and can be positioned either at 35.5° or 144.5°. The backward position offers the advantage of a high mass resolution, but the Rutherford cross sections are a factor of about 100 lower than at the forward angle, which is the preferred position if kinematically possible.

At the 5 MV tandem accelerator a QQDS magnetic spectrometer is being installed. The facilities for in situ sample preparation in UHV are similar. These spectrometers are described in detail and recent applications are discussed.

Publ.-Id: 6542

Electronic structure and mechanical properties of Cr7C3

Music, D.; Kreissig, U.; Mertens, R.; Schneider, J. M.

We have studied the Dr7C3 phase by means of ab initio calculations and found covalentionic Cr-C-Cr chains in a metallic matrix. Furthermore the structure of thin films, synthesized by Rf magnetron sputtering, is schown to be in good agreement with our theoretical prediction.

  • Physics Letters A 326(2004)5-6, 473-476

Publ.-Id: 6540

Cross-Section Measurement of Charged-Pion Photoproduction from Hydrogen and Deuterium

Zhu, L. Y.; Arrington, J.; Averett, T.; Et, A.

We have measured the differential cross section for the b3;n࢐c0;-p and b3;n࢐c0;+ reactions at c.m. = 90° in the photon energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at E3.3 GeV exhibit a global scaling behavior for both c0;- and c0;+ photoproduction, consistent with the constituent counting rule and the existing c0;+ photoproduction data. Possible oscillations around the scaling value are suggested by these new data. The data show enhancement in the scaled cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio of exclusive c0;- to c0;+ photoproduction at high energy is consistent with the prediction based on one-hard-gluon-exchange diagrams.

Publ.-Id: 6539

Multiphonon vibrations at high angular momentum in 182Os

Pattison, L. K.; Cullen, D. M.; Smith, J. F.; Et, A.

Evidence is presented for multiphonon excitations based on a high-spin (25) intrinsic state in the deformed nucleus 182Os. Angular momentum generation by this mode competes with collective rotation. The experimental data are compared with tilted-axis cranking calculations, supporting the vibrational interpretation. However, the lower experimental energies provide evidence that more complex interactions of states are playing a role.

Publ.-Id: 6538

Formation and evolution of sp2 clusters in amorphous carbon networks as predicted by molecular dynamics annealing simulations

Belov, A. Y.; Jäger, H. U.

A comparative study of clustering of sp2 -bonded atoms in the as-deposited and annealed tetrahedral amorphous carbon (ta-C) films is presented. The as-deposited ta-C with a grown-in compressive stress of ~10 GPa is modelled using amorphous networks generated by ion-beam film deposition simulations with a modified hydrocarbon potential of Brenner. The ta-C networks were annealed in the temperature range of 600-1200 K, using molecular-dynamics with the same interatomic potential. The size and type of the sp2 clusters were analysed as a function of the annealing temperature as well as the simulation parameters for the stress and temperature control. An essential finding of this study is that at the density less than 3.0 g/cm3 the structure of ta-C can become unstable with respect to formation of large sp2 clusters. The influence of the sp2 clustering on the film stress is also discussed.

Keywords: tetrahedral amorphous carbon; simulation; thermal stability; microstructure

  • Diamond and Related Materials 14(2005)1014-1018

Publ.-Id: 6537

Precise measurement of the 7Be (p, gamma) 8B S factor

Junghans, A. R.; Mohrmann, E. C.; Snover, K. A.; Et, A.

We present new measurements of the 7Be(p,gamma)8B cross section from pc.m. = 116 to 2460 keV (where c.m. means center-of-mass), which incorporate several improvements over our previously published experiment, also discussed here. Our new measurements lead to S17(0) = 22.1±0.6(expt)±0.6(theor) eV b based on data from pc.m. = 116 to 362 keV, where the central value is based on the theory of Descouvemont and Baye. The theoretical error estimate is based on the fit of 12 different theories to our low-energy data. We compare our results to other S17(0) values extracted from both direct [7Be(p,gamma)8B] and indirect (Coulomb-dissociation and heavy-ion reaction) measurements, and show that the results of these three types of experiments are not mutually compatible. We recommend a "best" value, S17(0) = 21.4±0.5(expt)±0.6(theor) eV b, based on the mean of all modern direct measurements below the 1+ resonance. We also present S factors at 20 keV which is near the center of the Gamow window: the result of our measurements is S17(20) = 21.4±0.6(expt)±0.6(theor) eV b, and the recommended value is S17(20) = 20.6±0.5(expt)±0.6(theor) eV b.

Publ.-Id: 6536

Electromagnetic-induced fission of 238U projectile fragments, a test case for the production of spherical super-heavy nuclei

Heinz, A.; Schmidt, K.-H.; Junghans, A. R.; Et, A.

Isotopic series of 58 neutron-deficient secondary projectiles (205,206At, 205,209Rn, 208,212,217,218Fr, 211,223Ra, 215,226Ac, 221,229Th, 226,k231Pa, 231,234U) were produced by projectile fragmentation using a 1 A GeV 238U beam. Cross sections of fission induced by nuclear and electromagnetic interactions in a secondary lead target were measured. They were found to vary smoothly as a function of proton and neutron number of the fissioning system, also for nuclei with large ground-state shell effects near the 126-neutron shell. No stabilization against fission was observed for these nuclei at low excitation energies. Consequences for the expectations on the production cross sections of super-heavy nuclei are discussed.

Publ.-Id: 6535

Towards strangeness saturation in central heavy ion collisions at high-energies

Cleymans, J.; Kämpfer, B.; Wheaton, S.

Analyses of the centrality binned identified hadron multiplicities at SPS (Ýa;s = 17 AGeV) and RHIC (Ýa;s = 130 AGeV) within the statistical-thermal model point to strangeness saturation with increasing centrality and energy

Publ.-Id: 6534

Annual Report 2003 - Institute of Nuclear and Hadron Physics

Enghardt, W.; Fahmy, K.; Grosse, E.; Kämpfer, B.; Schneidereit, C.; Wagner, A.; (Editors)

  • Open Access Logo Wissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-401 2004
    ISSN: 1437-322X


Publ.-Id: 6533

Dipole and quadrupole excitations in 88Sr up to 6.8 MeV

Käubler, L.; Schnare, H.; Schwengner, R.; Prade, H.; Dönau, F.; von Brentano, P.; Eberth, J.; Enders, J.; Fitzler, A.; Fransen, C.; Grinberg, M.; Herzberg, R.-D.; Kaiser, H.; von Neumann-Cosel, P.; Pietralla, N.; Richter, A.; Rusev, G.; Stoyanov, C.; Wiedenhöver, I.

Dipole and quadrupole excitations in the semimagic N=50 Nucleus 88Sr were investigated at the superconducting Darmsatdt linear accelerator S-DALINAC with bremsstrahlung of an endpoint energy of 6.8 MeV. Many new dipole excitations could be identified, and their reduced transition probabilities were determined. The experimental findings are discussed in the context of quasiparticle-phonon-model and shell-model calculations. A breaking of the N=50 core is essential to describe the structure of the observed excitations. The two-phonon quadrupole-octupole Jpi=1- state exhibits unusual features which are presently not understood.

Keywords: Nuclear resonance fluorescence; quasiparticle-phonon model; shell model

Publ.-Id: 6531

[Re6Q7O(3,5-Me2PzH)6]Br2*3,5-Me2PzH (Q = S, Se) - new octahedral rhenium cluster complexes with organic ligands: original synthetic approach and unexpected ligand exchange in cluster core

Mironov, Y. V.; Shestopalov, M. A.; Brylev, K. A.; Yarovoi, S. S.; Romanenko, G.; Fedorov, V. E.; Spies, H.; Pietzsch, H.-J.; Stephan, H.; Geipel, G.; Bernhard, G.; Kraus, W.

Two new octahedral rhenium cluster complexes, [Re6S7O(3,5-Me2PzH)6]Br2*3,5-Me2PzH and [Re6Se7O(3,5-Me2PzH)6]Br2*3,5-Me2PzH, with the organic ligand 3,5-dimethylpyrazole (3,5-Me2PzH) have been synthesized by reaction of rhenium chalcobromides Cs3[Re63-Q7Br)Br6] (Q = S, Se) with molten dimethylpyrazole. During the reaction all six apical bromine ligands of the cluster complexes are substituted by the organic ligand which is coordinated via the aromatic nitrogen N2. Additionally, the inner ligand μ3-Br in the cluster core [Re63-Q7Br)]3+ is substituted by oxygen given cluster cores, [Re63-Q7O)]2+, with mixed chalcogene/oxygen ligands.

Keywords: Rhenium; Cluster compounds; Ligand exchange; Organic ligands; Structure elucidation; Laser Fluorescence Spectroscopy

  • European Journal of Inorganic Chemistry (2005)4, 657-661

Publ.-Id: 6530

Pegylation of 1,4,8,11-Tetraazacyclotetradecane (Cyclam) and its Copper(II) Complexation

Stephan, H.; Geipel, G.; Bernhard, G.; Appelhans, D.; Tabuani, D.; Komber, H.; Voit, B.

Two novel star-like cyclam derivatives with appended four amino groups and four PEG-arms have been synthesized. The complex formation of cyclam and the cyclam derivatives and with Cu(II) has been studied by UV-Vis and time-resolved laser-induced fluorescence (TRLFS) measurements. The cyclam ligands investigated form 1:1 complexes with Cu(II). Rapid complex formation was found for cyclam and the pegylated derivative. By way of contrast, complete attainment of complexation between Cu(II) and cyclam ligand having external primary amino groups needs approx. 1 hour. By TRLFS measurements a quench effect of the Cu(II) on the fluorescence of the ligand was found. This behaviour was used to determine the complex formation between Cu(II) and the ligands.

  • Tetrahedron Letters 46(2005), 3209-3212

Publ.-Id: 6529

Fluid mixing and flow distribution in the reactor circuit - Part 1: Measurement data base

Rohde, U.; Kliem, S.; Höhne, T.; Karlsson, R.; Hemström, B.; Lillington, J.; Toppila, T.; Elter, J.; Bezrukov, Y.

Experimental investigations and Computational Fluid Dynamics (CFD) calculations on coolant mixing in Pressurised Water Reactors (PWR) have been performed within the EC project FLOMIX-R. The project aims at describing the mixing phenomena relevant for both safety analysis, particularly in steam line break and boron dilution scenarios, and mixing phenomena of interest for economical operation and the structural integrity. Measurement data from a set of mixing experiments have been gained by using advanced measurement techniques with enhanced resolution in time and space. Slug mixing tests simulating the start-up of the first main circulation pump are performed with two 1:5 scaled facilities: the Rossendorf Coolant Mixing model ROCOM and the Vattenfall test facility. Additional data on slug mixing in a VVER-1000 type reactor have been gained at a 1:5 scaled metal mock-up at EDO Gidropress. Experimental results on buoyancy driven mixing of fluids with density differences have been obtained at ROCOM and the Fortum PTS test facility.
Concerning mixing phenomena of interest for operational issues and thermal fatigue, flow distribution data available from commissioning tests at PWRs and VVER are used together with the data from the ROCOM facility as a basis for the flow distribution studies.
In part 1 of the paper, the experiments performed are described, results of the mixing experiments are shown and discussed. In part 2, efforts on computational fluid dynamics codes validation on selected mixing tests under steady-state flow conditions will be reported about. The application of Best Practice Guidelines in code validation will be demonstrated.

Keywords: nuclear reactor; boron dilution; turbulent mixing; pre-stressed thermal shock; measurement data; wire mesh sensor; test facility

  • Nuclear Engineering and Design, 235(2005), 421-443

Publ.-Id: 6528

PWR Vessel Failure Experiments & Analysis

Sehgal, B. R.; Willschuetz, H.-G.

For calculations of Lower Head Failure experiments like FOREVER it is necessary to model the melt pool convection and the temperature field within the vessel as well as creep and plasticity processes. Therefore a 2D Finite Element Model is developed based on a commercial Finite-Element-code. The CFD module is used to calculate the thermodynamics. The resulting temperature field of the vessel wall is applied to the mechanical model. To describe the visco-plastic deformation a numerical creep data base (CDB) is developed where the creep strain rate is evaluated in dependence on the current total strain, temperature and equivalent stress. In this way the use of a single creep law, which employs constants derived from the data for a limited stress and temperature range, is avoided. For an evaluation of the failure times a damage model is applied.
After pre- and post-test calculations of the ealier FOREVER-experiments, pre-test calculations for the forthcoming experiments were performed. Taking into account both - experimental and numerical results - gives a good opportunity to improve the simulation and understanding of real accident scenarios.
After analyzing the results of the calculations, it seems to be advantageous to provide a vessel support, which can unburden the vessel from a part of the mechanical load and, therefore, avoid the vessel failure or at least prolong the time to failure. This can be a possible accident mitigation strategy. Additionally, it may be advantageous to install a passive automatic control device to initiate the flooding of the reactor pit to ensure external vessel cooling in the event of a core melt down.

Keywords: Core Melt Down Accident; Temperature Field and Creep Simulation; Accident Mitigation Strategy

  • Lecture (Conference)
    PHARE-Seminar, 13.-17.09.2004, Bulgaria, BG 01.10.01

Publ.-Id: 6527

Lead Corrosion in Soils: A Literature Review

Scheinost, A.

wird nachgereicht

  • Invited lecture (Conferences)
    International Symposium: Sport Shooting and the Environment - Sustainable Use of Lead Ammunition, 09.-10.09.2004, Rome, Italy

Publ.-Id: 6526

Reduction of the annealing temperature of radiation-induced defects in ion-implanted MOS structures

Kaschieva, S.; Dmitriev, S. N.; Skorupa, W.

Si-SiO2 structures irradiated with 11-MeV electrons for 10 s and then implanted with B+ ions with an energy of 10 keV at a dose of 1.0×1012 cm-2 through the oxide were annealed at different temperatures. MOS capacitors including such oxide layers were studied by quasi-static C/V and thermally stimulated current (TSC) methods. A comparison of the radiation defect annealing of double-treated (electron-irradiated and ion-implanted) samples and of implanted-only samples was carried out. It is shown that a preceding low-dose high-energy electron irradiation of the samples leads to a lowering of the annealing temperature of radiation defects introduced by ion implantation. After annealing at 500 °C for 15 min, no TSC spectra for the double-treated samples were observed. The spectra of the other samples (which were not previously irradiated) showed that after the same thermal treatment only some of the radiation defects introduced by ion implantation are annealed. The difference between the annealed interface state density of previously electron-irradiated and current MOS structures is also demonstrated. A possible explanation of the results is proposed .

Publ.-Id: 6524

Acoustic and magnetic anomalies near the saturation field of the S=1/2 antiferromagnetic Heisenberg chain studied on a Cu(II) coordination polymer - art. no. 092403

Wolf, B.; Zherlitsyn, S.; Luthi, B.; Et, A.

By means of magnetic measurements as a function of temperature and magnetic field, the coordination polymer Cu(II)-2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene has been identified as a model system for a homogeneous antiferromagnetic S=1/2 Heisenberg spin chain with a moderate exchange-coupling constant |J| = 21.5 K/kB. Measurements of the longitudinal elastic constant in pulsed fields up to 50 T, i.e., across the saturation field gmu(B)B(S)=2|J|, reveal a pronounced acoustic anomaly near BS at T<|J|kB. We show that this feature is directly related to the shape of the magnetization curve and thus represents a generic property of a one-dimensional S=1/2 Heisenberg antiferromagnet with finite spin-lattice interaction.

Publ.-Id: 6522

Low-dimensional quantum spin systems in pulsed magnetic fields

Wolf, B.; Zherlitsyn, S.; Low, U.; Luthi, B.; Pashchenko, V.; Lang, M.

Low-dimensional spin systems in high magnetic fields reveal new and unexpected physical phenomena. Typical examples are the distinct plateaus in the magnetization found in quasi- 1d and 2d compounds. Even for the well known case of the quasi-1d homogeneous Heisenberg spin chain or its dimerized variant, unexpected elastic anomalies can be detected in high magnetic fields. Since the exchange constants of these low-dimensional spin systems are of the order of the dominant energy scale, pulse-field experiments are particularly well suited for investigations of the magnetic properties. In this contribution we present and discuss our results for selected low-dimensional compounds such as the homogeneous antiferromagnetic Heisenberg spin chain Cu(II)-bispyrazoldihydroxybenzene, which is a new quasi-1d Cu(II)-coordination polymer, the dimerized spin chain (VO)2P2O7, and the plateau systems SrCU2(BO3)2 and NH4CUCl3.

  • Physica B 346(2004), 19-26

Publ.-Id: 6521

K+- and K--production in heavy-ion collisions at SIS energies

Forster, A.; Böttcher, I.; Grosse, E.

The production and the propagation of K+- and of K--mesons in heavyion collisions at beam energies of 1 to 2 A GeV have systematically been investigated with the Kaon Spectrometer (KaoS) at the SIS at the GSI. The ratio of the K+-production excitation function for Au+Au and for C+C reactions increases with decreasing beam energy, which is expected for a soft nuclear equation-of-state. At 1.5 A GeV a comprehensive study of the K+- and of the K--emission as a function of the size of the collision system, of the collision centrality, of the kaon energy, and of the polar emission angle has been performed. The K-/K+ ratio is found to be nearly constant as a function of the collision centrality. The spectral slopes and the polar emission patterns are different for K- and K+. These observations indicate that K+-mesons decouple earlier from the reaction zone than K--mesons.

  • Journal of Physics G 30(2004)1, S393-S400

Publ.-Id: 6520

System-size dependence of strangeness saturation

Cleymans, J.; Kämpfer, B.; Steinberg, P.; Wheaton, S.

The final state in heavy-ion collisions has a higher degree of strangeness saturation than the one produced in collisions between elementary particles such as p-p or p-(p) over bar. A systematic analysis of this phenomenon is made for C-C, Si-Si and Pb-Pb collisions at the CERN SPS collider and for Au-Au collisions at RHIC and at AGS energies. Strangeness saturation is shown to increase smoothly with the number of participants at AGS, CERN and RHIC energies.

  • Journal of Physics G 30(2004)1, S595-S598

Publ.-Id: 6519

Pinhole interferometry with coherent hard X-rays

Leitenberger, W.; Wendrock, H.; Bischoff, L.; Weitkamp, T.

This paper discusses the experimental realisation of two types of X-ray interferometer based on pinhole diffraction. In both interferometers the beam splitter was a thin metal foil containing micrometer pinholes to divide the incident X-ray wave into two coherent waves. The interference pattern was studied using an energy-dispersive detector to simultaneously investigate in a large spectral range the diffraction properties of the white synchrotron radiation. For a highly absorbing pinhole mask the interference fringes from the classical Young's double-pinhole experiment were recorded and the degree of coherence of X-rays could be determined. In the case of low absorption of the metal foil at higher X-ray energies (> 15 keV) the interference pattern of a point diffraction interferometer was observed using the same set-up. The spectral refraction index of the metal foil was determined.


Publ.-Id: 6518

Slow highly charged ions for nanoscale surface modifications

Kentsch, U.; Landgraf, S.; Schmidt, M.; Et, A.

With the Dresden EBIT (electron beam ion trap) a room temperature, compact and long-term stable source of slow highly charged ions (HCIs) has been developed and prepared for small series production. A wide spectrum of ions such as Ar18+, Fe24+ and Xe44+ have been produced and extracted with energies less than q x 10 keV (q - ion charge). At ion-surface interactions the high neutralization energy of the ionic projectiles leads to high power densities of 10(12)-10(13) W/cm(2) at the surface. Thus, these ions can produce nanoscale material modifications and can be used for surface analysis techniques as well. Examples of extracted and magnetically analyzed ion beams are given. Fields of applications of slow highly charged ions are summarized.

Publ.-Id: 6517

Microstructure of beta-FeSi2 buried layers synthesis by ion implantation

Ayache, R.; Richter, E.; Bouabellou, A.

A buried layer of iron disilicide was synthesized by ion implantation in (111) Si p-type maintained at 500 degreesC using 195 keV Fe ions with a dose of 2 x 10(17) at./cm(2), followed by annealing in a N-2 atmosphere at 850 degreesC for 90 min. The investigation of the phase composition is carried out by Rutherford backscattering spectrometry (RBS), whereas the structural characterization is obtained by means of both X-ray diffraction (XRD) pole figure and cross-sectional transmission electron microscopy (XTEM). The precipitates favor epitaxial growth with respect to (111) Si planes with epitaxial relationships (220) beta-FeSi(2)parallel to(111) Si and/or (202) beta-FeSi(2)parallel to(111) Si. A mixture of beta-FeSi2 and alpha-FeSi2 Silicides is observed in the as-implanted state. After annealing of the samples at 1000 degreesC, the XRD pole figures show the transition from beta-phase to alpha-phase.

  • Nuclear Instruments and Methods in Physics Research B 216(2004), 137-142

Publ.-Id: 6516

Multidimensional cosmological models: Cosmological and astrophysical implications and constraints

Günther, U.; Starobinsky, A.; Zhuk, A.

  • Physical Review D 69(2004), 044003

Publ.-Id: 6515

Is C-11-choline the most appropriate tracer for prostate cancer?

Zophel, K.; Kotzerke, J.

Publ.-Id: 6511

Intranuclear cascade plus percolation plus evaporation model applied to the C-12+Au-197 system at 1 GeV/nucleon

Volant, C.; Turzo, K.; Trautmann, W.; Et, A.

The nucleus-nucleus Liege intranuclear-cascade+percolation+evaporation model has been applied to the C-12+Au-197 data measured by the INDRA-ALADIN collaboration at GSI. After the intranuclear cascade stage, the data are better reproduced when using the Statistical Multiframentation Model as afterburner. Further checks of the model are done on data front the EOS and KAOS collaborations.

  • Nuclear Physics A 734(2004), 545-548

Publ.-Id: 6510

Chemical and nuclear studies of hassium and element 112

Gaggeler, H. W.; Bruchle, W.; Dullmann, C. E.; Et, A.

  • Nuclear Physics A 734(2004), 208-212

Publ.-Id: 6509

Chemical identification and properties of element 112

Yakushev, A. B.; Zvara, I.; Oganessian, Y. T.; Dmitriev, S. N.; Eichler, B.; Hübener, S.; Sokol, E. A.; Turler, A.; Yeremin, A. V.; Et, A.

  • Nuclear Physics A 734(2004), 204-207
  • Radiochimica Acta 91(2003)8, 433-439

Publ.-Id: 6508

Magnetic dipole and electric quadrupole rotational structures and chirality in Rh-105

Oliveira, J. R. B.; Cybulska, E. W.; Medina, N. H.; Et, A.

The Rh-105 nucleus has been studied with the Mo-100(B-11, alpha2ngamma) reaction at 43 MeV incident energy. A rich variety of structures was observed at high and low spins, using gamma-gamma-t and gamma-gamma-particle coincidences and directional correlation ratios. A gamma-vibrational band was observed for the first time in this nucleus. A new structure based on the intruder 1/2(+)[431] proton orbital was identified. Four magnetic dipole bands have also been observed at high spin, three of which have negative parity and have similar characteristics: they are very regular in energy spacing, presenting nearly constant alignments and large values of B(M1)/B(E2) ratios. Two of them are nearly degenerate in excitation energy and could be chiral partners. The pig(9/2)xnuh(11/2)(g(7/2),d(5/2)) configuration is tentatively assigned for these bands, with the angular momenta of the proton and neutron intruders and the collective angular momentum aligning along the three perpendicular axes of the triaxial core, as predicted by tilted axis cranking calculations.

  • Physical Review C 6902(2004)2, 4317

Publ.-Id: 6507

Recent investigations and applications of thin diamond-like carbon (DLC) foils

Liechtenstein, V. K.; Ivkova, T. M.; Olshanski, E. D.; Golser, R.; Kutschera, W.; Steier, P.; Vockenhuber, C.; Repnow, R.; Hahn, R.; Friedrich, M.; Kreissig, U.

In previous experiments, diamond-like carbon (DLC) foils which are being produced at Kurchatov Institute, have proven well suited as long-lived stripper foils for high-energy heavy-ion tandem accelerators and ultra-thin secondary-electron (SE) emitting foils for TOF spectrometers. This paper presents the recent advances in DLC targetry with main emphasis given to development and applications of ultra thin foils (maximum diameter up to 70 mm) with high uniformity and minimal possible effective thickness, the latter has been evaluated by measurements of both energy loss and angular scattering of heavy ions transmitting through the foils. Comparative lifetimes of DLC stripper foils irradiated with high intensity (150 muA) He+ beams at 2 MeV are reported, which demonstrate the favorable use of DLC foils as supplemental strippers at RFQ linacs. In conclusion, the attractive results of application of ultra-thin DLC foils to TOF detectors for heavy ion AMS as well as to compact tandem accelerators are presented.

Publ.-Id: 6506

Ion beam studies of TiNxOy thin films deposited by reactive magnetron sputtering

Alves, E.; Ramos, A. R.; Barradas, N. P.; Et, A.

Titanium oxynitride compounds exhibit interesting properties for applications in fields ranging from protective/decorative coatings to solar panels. The properties of TiNxOy are related to the oxide/nitride ratio and can be tailored playing with this ratio. In this work we studied the influence of substrate bias voltage and flow rate of reactive gases (a mixture of N-2 and O-2) on the properties of TiNxOy films. The films were deposited on steel substrates at a constant temperature of 300 degreesC by r.f. reactive magnetron sputtering. The depositions were carried out from a pure Ti target. The composition throughout the entire thickness was determined by Rutherford backscattering spectrometry. To obtain information on the profile of light elements (0, N) and detect the presence of hydrogen on the films, heavy ion elastic recoil detection analysis was performed. The results indicate a nearly constant stoichiometry through the entire analysed depth. The colouration varied from the shiny golden for low oxygen contents (characteristic of TiN films) to dark blue for higher oxygen contents. The electrical resistivity of the samples was obtained at room temperature and the values varied from approximately 120 muOmega cm for a sample with very low oxygen content to values up to 350 muOmega cm, for the highest oxygen contents.

  • Surface & Coatings Technology 180-181(2004), 372-376

Publ.-Id: 6499

Spectroscopic quadrupole moments of high-spin isomers in Pb-193

Balabanski, D.; Ionescu-Bujor, M.; Iordachescu, A.; Et, A.

The quadrupole interaction of high-spin isomers in Pb-193 implanted into solid Hg cooled at a temperature T=170 K has been investigated by the time-differential perturbed gamma-ray angular-distribution method. Spectroscopic quadrupole moment values of \Q\=0.22(2) eb and 0.45(4) eb have been deduced for the 21/2(-) and 33/2(+) three-neutron states, respectively. A much higher value \Q(s)\=2.84(26) eb has been determined for the 29/2(-) isomer, the band head of a magnetic rotational band.

Publ.-Id: 6495

High-spin states in the vibrational nucleus Cd-114

Jungclaus, A.; Algora, A.; Axiotis, M.; Borge, M. J. G.; Fernandez, M. A.; Gadea, A.; Galindo, E.; Hausmann, M.; Lenzi, S.; Martinez, T.; Napoli, D. R.; Piqueras, I.; Schwengner, R.; Ur, C.

High-spin states of the neutron-rich vibrational nucleus Cd-114 have been studied using the incomplete fusion reaction Pd-110(Li-7, p2n) and the GASP spectrometer in conjunction with the ISIS Si ball. About 50 new states with excitation energies up to 7 MeV and angular momentum I less than or equal to (18(+)) were observed and for many of them, spin and parity could be firmly assigned. The band-like stuctures in Cd-114 are compared to the corresponding ones in the even-even neighbour Cd-112.

Publ.-Id: 6494

Dissipative continuum model for self-organized pattern formation during ion-beam erosion

Facsko, S.; Bobek, T.; Stahl, A.; Kurz, H.; Dekorsy, T.

A continuum model, based on the damped Kuramoto-Sivashinsky equation, is shown to reproduce the morphology evolution during ion sputtering quite successfully. In a very narrow range of the damping parameter alpha, the alignment of the structures into hexagonal domains is obtained under normal incidence of ions with striking resemblance to the experimentally observed dot patterns. The origin of this damping factor is discussed.

  • Physical Review B 6915(2004)15, 3412-3412

Publ.-Id: 6493

Gross shell structure at high spin in heavy nuclei

Deleplanque, M. A.; Frauendorf, S.; Pashkevich, V. V.; Chu, S. Y.; Unzhakova, A.

Experimental nuclear moments of inertia at high spins along the yrast line have been determined for a large number of nuclei. They are found to systematically differ from the rigid-body values. The difference is attributed to shell effects which are well described by a rotating Woods-Saxon potential. The data and quantal calculations are interpreted by means of the semiclassical periodic orbit theory. From this new perspective, features in the moments of inertia as a function of neutron number and spin as well as their relation to the shell energies can be understood. Gross shell effects persist up to the highest angular momenta observed.

  • Physical Review C 6904(2004)4, 4309-4309

Publ.-Id: 6492

Effect of N-2 to C2H2 ratio on r.f. plasma surface treatment of austenitic stainless steel

El-Rahman, A. M.; El-Hossary, F. M.; Fitz, T.; Negm, N. Z.; Prokert, F.; Pham, M. T.; Richter, E.; Möller, W.

Adding C2H2 gas to nitrogen gas during r.f. plasma processing has significant influence on the properties of 304 austenitic stainless steel. The elemental depth profiles of the compound layer were investigated by glow discharge optical spectroscopy. Different treatment gas compositions of N-2/C2H2 mixtures (0-100% in steps of 10 or 5% in some cases) have been investigated. The total gas pressure during the plasma processing was 8.4 x 10(-2) mbar. The sample temperature was approximately 550 degreesC with an insignificant variation for different gas compositions. A high rate of diffusion of carbonitriding has been obtained without initial sputter removal of the surface oxide layer, which demonstrates that the surface oxide layer does not play a crucial role during r.f. plasma carburizing and carbonitriding of stainless steel. The nitrogen concentration and the compound layer thickness increase, when 10% of C2H2 is added. With further increase in the C2H2 ratio, they decrease again gradually. The carbon depth profiles for the same samples show lower concentration and opposite tendency with respect to the nitrogen content. X-ray diffraction was used to characterize the microstructure of the compound layers. Fe2N, Fe3N, CrN, nitrogen-expanded austenite (gamma(n)) and carbon-expanded austenite (gamma(c)) phases are detected. The relative proportions of the phases are critically dependent on the N-2/C2H2 ratio. Depending on the N-2/C2H2 ratio, the microhardness value increases by a factor between 2 and 6. The carbonitrid layer exhibits a corrosion resistance better than pure nitrided and carburized layers.

  • Surface & Coatings Technology 183(2004)2-3, 268-274

Publ.-Id: 6491

Structure and phase transformations of AISI M2 high-speed tool steel treated by PIII and subsequent compression plasma flows of nitrogen

Uglov, V. V.; Anishchik, V.; Vetushka, A. M.; Rumyanceva, I. N.; Richter, E.

The effect of 'double' treatment (plasma immersion ion implantation and subsequent compression plasma flows) on microstructure, element and phase composition, as well as on microhardness properties of AISI M2 high-speed tool steel has been studied. Such methods as Auger electron spectrometry (AES), X-ray diffraction (XRD), energy-dispersive X-ray (EDX) microanalysis and scanning electron microscopy (SEM) allowed us to obtain the data suggesting that under such treatment a martensite-austenite conversion and the formation of a deep (approx. 35 mum) three-layered structure take place. It has been established that transformations of near-surface regions are associated with the complete dissolution of the initial (Fe, Cr)(3)(W, Mo)(3)C carbide (less than or equal to 5-10 mum) and the formation of gamma(N,M) expanded austenite doped with atoms of nitrogen, carbon and alloying metals M=W, Mo, Cr.

Publ.-Id: 6490

Correlation between magnetic properties of CoFe single and CoFe/SiO2multi-layer thin films and their microstructure, texture and internal stress state

Dieter, S.; Pyzalla, A.; Bauer, A.; Schell, N.; McCord, J.; Seemann, K.; Wanderka, N.; Reimers, W.

CoFe single and multi-layer systems are diposited by a radio-frequency sputter process. Thickness, roughness, morphology, texture and internal stress state of the layers are determined by X-ray reflectometry, transmission electron microscopy, and diffraction methods. The texture and the internal stress of the layers depend strongly on the parameters of the sputter process. The magnetic properties of the layers are determined from hysteresis curve measurements and magneto-optical Kerr microscopy. A strong correlation between the texture, the internal stress and the magnetic properties of the CoFe layers os observed.

Keywords: Reflectometry Microstructure Residual stresses Texture Coercivity Kerr microscopy

  • Zeitschrift für Metallkunde 95/2004 164-175

Publ.-Id: 6489

Exclusive eta production in proton-proton reactions

Balestra, F.; Dressler, R.; u. a.

Differential cross sections for the exclusive reaction pp-->ppeta observed via the eta-->pi(+)pi(-)pi(0) decay channel have been measured at T-beam=2.15 GeV, 2.50 GeV, and 2.85 GeV (excess energies 324 MeV, 412 MeV, and 554 MeV). The influence of the N(1535)S-11 resonance is clearly seen in the invariant mass and momentum dependent differential cross sections. The extracted resonance parameters are compatible with existing data. No significant evidence for further resonance contributions has been found. In addition, angular distributions of the ppeta final state have been measured. The polar angle distribution of the eta shows an anisotropy with respect to the beam axis for the lowest beam energy, which vanishes for the higher energies. The sign of this
anisotropy is negative and expected to be sensitive to the dominant production mechanism. In contrast, the proton polar angle in the pp rest frame tends to be more strongly aligned along the beam axis with increasing beam energy. The analyzing power A(y) is compatible with zero for all beam energies.

Keywords: meson production

Publ.-Id: 6488

Verlässlichere Prognosen zur Schadstoffausbreitung mit Hilfe einer mineral-spezifischen Sorptionsdatenbank des Forschungszentrums Rossendorf (FZR)

Brendler, V.; Richter, A.; Bernhard, G.

Anwendungsbereiche der hier vorgestellten thermodynamischen Sorptionsdatenbank RES³T (Rossendorf Expert System for Surface and Sorption Thermodynamics) sind Deponien unterschiedlichster Art und ihre Sickerwässer, aber auch die Sicherung von kontaminierten Militärstandorten und Industrieanlagen. Neben chemo-toxischen Abfällen wird zudem die Lagerung radioaktiver Abfälle adressiert, wobei sowohl das Wirtsgestein als auch das Deckgebirge erfasst wird, und damit nicht nur für ein einziges Endlagerumfeld (z. B. Salzstock) Relevanz besitzt. Und schließlich stellt die Datenbank auch einen Beitrag dar zur Problematik der Stilllegung, Verwahrung oder Rekultivierung von Bergwerken (Uranerz, Kupferschiefer, Steinkohle) und offenen Tagebauen (Steinkohle, Braunkohle). Die avisierten Anwender umfassen Behörden, Gutachter, die WISMUT GmbH, Ingenieurbüros sowie universitäre und außeruniversitäre Forschungseinrichtungen.

Keywords: Sorption; Datenbank; Oberflächenkomplexierung; Schadstoffausbreitung; reaktiver Transport; Sanierung; Endlager; Deponie; Prognose; Actiniden

  • Umwelt-Report (2004), 10-11(Ausgabe Dresden), 56-57(Ausgabe Chemnitz)

Publ.-Id: 6487

Measurement Techniques for Multiphase Flows, Multiphase Flows in Industrial Applications CFX-Germany

Prasser, H.-M.


  • Invited lecture (Conferences)
    ANSYS-CFX/FZR Workshop, FZ-Rossendorf 28.-30.04.2003, Invited lecture at Short Course on Multiphase Flows

Publ.-Id: 6484

Excitation function of elliptic flow in Au + Au collisions and the nuclear matter equation of state

Andronic, A.; Barret, V.; Basrak, Z.; Bastid, N.; Benabderrahmane, L.; Berek, G.; Caplar, R.; Crochet, P.; Dupieux, P.; Dzelalija, M.; Fodor, Z.; Gasparic, I.; Grishkin, Y.; Hartmann, O. N.; Herrmann, N.; Hildenbrand, K. D.; Hong, B.; Kecskemeti, J.; Kim, Y. J.; Kirejczyk, M.; Koczon, P.; Korolija, M.; Kotte, R.; Kress, T.; Lebedev, A.; Leifels, Y.; Lopez, X.; Merschmeyer, M.; Neubert, W.; Pelte, D.; Petrovici, M.; Rami, F.; Reisdorf, W.; de Schauenburg, B.; Schüttauf, A.; Seres, Z.; Sikora, B.; Sim, K. S.; Simion, V.; Siwek-Wilczynska, K.; Smolyankin, V.; Stockmeier, M. R.; Stoicea, G.; Tyminski, Z.; Wagner, P.; Wisniewski, K.; Wohlfarth, D.; Xiao, Z.-G.; Yushmanov, I.; Zhilin, A.

We present measurements of the excitation function of elliptic flow at midrapidity in Au + Au collisions at beam energies from 0.09 to 1.49 GeV per nucleon. For the integral flow, we discuss the interplay between collective expansion and spectator shadowing for three centrality classes. A complete excitation function of transverse momentum dependence of elliptiv flow is presented for the first time in this energy range, revealing a rapid change with incident energy below 0.4 AGeV, followed by an almost perfect scaling at the higher energies. The equation of state of compressed nuclear matter is addressed through comparisons to microscopic transport model calculations.

  • Physics Letters B 612(2005), 173


Publ.-Id: 6481

Spectroscopic characterization of the uranium carbonate andersonite Na2Ca[UO2(CO3)3]*6H2O

Amayri, S.; Arnold, T.; Reich, T.; Foerstendorf, H.; Geipel, G.; Bernhard, G.; Massanek, A.

The uranium carbonate andersonite Na2Ca[UO2(CO3)3]*6H2O was synthesized and identified with classical analytical and spectroscopic methods. The classical methods applied were X-ray powder diffraction (XRD), nitric acid digestion, and Scanning Electron Microcopy combined with Energy-Dispersive Spectroscopy (SEM/EDS). To spectroscopically characterize andersonite, Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS), X-ray Photoelectron Spectroscopy (XPS), and Fourier-Transformed InfraRed spectroscopy (FT-IR) were used. Natural and synthetically prepared andersonite samples were characterized with the non-destructive TRLFS by six fluorescence emission bands at 470.6, 486.1, 505.4, 526.7, 549.6, and 573.9 nm. In addition, andersonite was characterized by FT-IR measurements by the appearance of the asymmetric stretching vibration of the uranyl cation (v3 UO2 2+) at 902 cm-1 with a shoulder at 913 cm-1 and by XPS measurements. The intensity of the measured XPS lines was used to verify the composition of the synthetic andersonite sample. The measured intensity ratios and the calculated atomic ratios agree with the stoichiometry of Na2Ca[UO2(CO3)3]*6H2O. The results of both the qualitative and quantitative XPS analysis confirmed the formation of andersonite. The spectral features of the U 4f electrons and the distinct splitting of the U 6p3/2 electrons can be used to identify the [UO2(CO3)3]4- structural moiety.

These spectroscopic methods proved to be suitable to identify in a fingerprinting procedure secondary U(VI) phases in mixtures with other phases or as thin coatings on mineral and rock surfaces. The obtained spectroscopic information are of fundamental importance for uranyl(VI) adsorption studies, since such information are useful to identify the coordination environment and structure of unknown adsorbed uranium(VI) surface species on minerals surfaces.

Keywords: uranium; carbonate; andersonite; synthesis; TRLFS; XPS; FT-IR; spectroscopy

Publ.-Id: 6480

Compressive stress, preferred orientation and film composition in Ti-based coatings developed by plasma immersion ion implantation-assisted deposition

Mukherjee, S.; Prokert, F.; Richter, E.; Moeller, W.

Plasma immersion ion implantation-assisted deposition has been employed to develop Ti-based coatings in the presence of reactive nitrogen environment. When TiN coatings have been developed on Al alloys, it has been observed that a large compressive stress develops in the coating, which can be reduced when the substrate bias is increased. The formation of stress is attributed to atomic peening effect that densifies a coating, whereas thermal spikes generated at high substrate bias reduce the stress. For TiAl coatings delivered from an alloy plasma source, the Al content decreases with increasing negative substrate bias. This is in agreement with the results of TRIDYN ballistic computer simulations, which confirm that the film composition is significantly influenced by preferential sputtering. The results indicate that time-averaged substrate bias is an ill-defined parameter to describe coating properties deposited by the above technique.

Keywords: Ion implantation; TiN; stress; TRIDYN

  • Surface & Coatings Technology 186(2004), 99-103

Publ.-Id: 6478

Contactless inductive flow tomography

Stefani, F.; Gundrum, T.; Gerbeth, G.

The three-dimensional velocity field of a propeller driven liquid metal flow is reconstructed by a contactless inductive flow tomography (CIFT). The underlying theory is presented within the framework of an integral equation system that governs the magnetic field distribution in a moving electrically conducting fluid. For small magnetic Reynolds numbers this integral equation system can be cast into a linear inverse problem for the determination of the velocity field from externally measured magnetic fields. A robust reconstruction of the large scale velocity field is already achieved by applying the external magnetic field alternately in two orthogonal directions and measuring the corresponding sets of induced magnetic fields. Kelvin's theorem is exploited to regularize the resulting velocity field by using the kinetic energy of the flow as a regularizing functional. The results of the new technique are shown to be in satisfactory agreement with ultrasonic measurements.

Publ.-Id: 6477

Correlation between bonding structure and microstructure in fullerene-like carbon nitride thin films

Gago, R.; Jiménez, I.; Neidhardt, J.; Abendroth, B.; Caretti, I.; Hultman, L.; Möller, W.

The bonding structure of highly ordered fullerene-like (FL) carbon nitride (CNx) thin films have been assessed by x-ray absorption near edge spectroscopy (XANES). In particular, samples with different degree of FL character (in terms of graphene plane alignment, extension, and cross-linking) have been analyzed to discern spectral signatures of the microstructure formation. The XANES spectra of FL-CNx films resemble that of graphitic CNx, indicating a sp2 character for C and N atoms. However, the evolution of the FL structure is promoted with the dominance of three-fold N over pyridine-like and cyanide-like bonding environments. In addition, the p*/s* ratio at the C(1s) edge is constant, independent of the change in FL character, while it decreases ~40% at the N(1s) edge with the formation of FL arrangements. This result indicates that there is no appreciable introduction of C-sp3 hybrids with the development of FL structures and, additionally, that a different spatial localization of p electrons at C and N sites takes place in curved graphitic structures. The latter has implications for the elastic properties of graphene sheets and could as such explain the outstanding elastic properties of FL-CNx.

  • Physical Review B, 71(2005)12, 5414

Publ.-Id: 6476

Breakdown of Burton-Prime-Slichter approach and lateral solute segregation in radially converging flows

Priede, J.; Gerbeth, G.

A theoretical study is presented of the effect of a radially converging melt flow on the radial solute segregation in simple solidification models. We show that the classical Burton-Prim-Slichter (BPS) theory describing the effect of a diverging flow on the solute incorporation into the solidifying material breaks down for converging flows. The breakdown is caused by a divergence of the integral defining the effective boundary layer thickness which is the basic concept of the BPS theory. The divergence can formally be avoided by restricting the axial extension of the melt to a layer of finite height. This allows us to obtain radially uniform solutions for the solute distributions which, however, are valid only for weak melt flows with an axial velocity away from the solidification front comparable to the growth rate. There is a critical melt velocity for each growth rate at which the solution passes through a singularity and becomes physically inconsistent for stronger melt flows. Thus, the radially uniform solute distribution becomes incompatible with a converging flow exceeding the growth rate, and a radial segregation sets in. The solute distribution is analysed in detail for a solidification front presented by a disk of finite radius R0 subject to a converging melt flow. We obtain similarity and matched asymptotic solutions showing that the radial solute concentration depends on the radius r as ln1/3(R0/r) and ln(R0/r) close to the rim and at large distances from it, respectively. The converging flow causes a solute pile-up forming a logarithmic concentration peak at the symmetry axis which might be an undesirable feature for crystal growth processes.

  • Journal of Crystal Growth 285(2005), 261-269

Publ.-Id: 6475

Erneuerbare Energien: Das Beispiel Windenergie

Rindelhardt, U.

Die Nutzung der Windenergie zur Stromerzeugung hat in den letzten Jahren ein beachtliches Niveau erreicht. Im Vortrag wird der gegenwärtige technische Entwicklungsstand vorgestellt. Ferner werden die mit der Aufnahme von großen fluktuierenden Mengen Windstromes im Höchstspannungsnetz auftretenden Probleme und Konsequenzen diskutiert.

Keywords: Windenergie

  • Lecture (others)
    Antrittsvorlesung, Technische Universität Chemnitz, 04.05.2004, Chemnitz, Germany

Publ.-Id: 6474

Grid connected PV systems in Saxony: A ten years review

Hiller, W.; Rindelhardt, U.; Voigtländer, I.

Even in former East Germany, photovoltaic has being used increasingly for generating electricity. Through weaker buying power the absolute numbers of pv plants are far lower than in the western federal states, but a series of remarkable results was achieved nonetheless. This paper discusses the developments in the federal state Saxony.
It is based on long term results on selected photo-voltaic plants as well as data recorded by the regional electricity supplier Energieversorgung Ostsachsen (ESAG), Dresden, in 2002 and 2003 [1].

Keywords: photovoltaic plants

  • Contribution to proceedings
    EuroSun 2004, 20.-23.06.2004, Freiburg, Germany
    Procedings Vol. 3, 401-407
  • Lecture (Conference)
    EuroSun 2004, 20.03.-23.06.2004, Freiburg, Germany

Publ.-Id: 6472

Regional differences in cerebral blood flow and cerebral ammonia metabolism in patients with cirrhosis

Ahl, B.; Weissenborn, K.; van den Hoff, J.; Fischer-Wasels, D.; Köstler, H.; Hecker, H.; Burchert, W.

Clinical and histopathological findings hint at regional differences in the brain's sensitivity to metabolic changes in cirrhosis. The aim of the present study was to examine regional differences in cerebral ammonia metabolism in patients with cirrhosis and grade 0-to-I hepatic encephalopathy (HE). 13N-ammonia, 15O-water positron emission tomography (PET) and magnetic resonance imaging (MRI) were performed. Quantitative values of cerebral blood flow (CBF) and the initial cerebral ammonia uptake rate (K1) were derived for several regions of interest from images of the desired parameters after interactive coregistration with the patients' MRI-studies. CBF (mL/mL/min), K1 (mL/mL/min), and the ammonia extraction fraction (K1/CBF) showed marked regional variance with the highest levels in the thalamus, the lenticular nucleus, and the cerebellum. In conclusion, the regional differences in cerebral ammonia uptake correspond to the distribution of histopathological changes in the brain of patients with cirrhosis as well as clinical features of HE, characterized by signs of basal ganglia and cerebellar dysfunction with corresponding signs of functional impairment, especially of the frontal cortex and cingulate gyrus.

Publ.-Id: 6471

UV Raman Spectroscopy of Boron Nitrides

Ferrari, A. C.; Reich, S.; Casiraghi, C.; Arenal De La Concha, R.; Loiseau, A.; Gago, R.; Abendroth, B.; Zhang, W. J.; Chan, C. Y.; Bello., I.; Robertson, J.

Boron nitride is a remarkable material both for its fundamental properties and for applications [1]. It exists in different polytypes, the sp2 bonded hexagonal and rhombohedral structure and the sp3 cubic and wurzite phases. All phases are high band-gap semiconductors (>5 eV). Cubic boron nitride, c-BN, is of particular interest for its properties similar to diamond [1,2]. Physical or chemical vapor deposition of boron nitride films can often result in the growth of the less dense hexagonal BN (h-BN). BN nanotubes are also very interesting [3]. Their properties, as for carbon nanotubes, can be derived by studying the wrapping of a single layer of h-BN [3]. Boron Carbon Nitrides (BCN) thin films are also useful for hard coating applications and as semiconductors with variable gap [4]. Raman and Infrared spectroscopy are quick and non-destructive tools to assess the properties of materials. Infrared spectroscopy is the most widely used for boron nitrides. It is very sensitive to polar BN bonds. The spectra for c-BN and h-BN show distinct IR features [1]. Raman spectroscopy is rarely used because of its inferior sensitivity, especially in the case of thin films. This is opposite to diamond-like carbons, where Raman spectroscopy is a standard characterization tool [5]. However, Infrared spectroscopy has limitations. The substrate has to be either transparent or not highly reflective. Raman spectroscopy has no requirement on the substrate properties and can simultaneously give informantion on phase composition, crystallinity, size and stress. In order to successfully apply Raman scattering in boron nitrides its low sensitivity has to be overcome. This can be done by measuring the samples with an excitation energy closer to their band gap. Here we measure BN samples by UV Raman spectroscopy at 5.1 and 5.4 eV. This enhances the BN Raman cross-section and paves the way to making UV Raman scattering a standard tool for boron nitrides characterization. We studied the UV Raman spectra of a variety of boron nitride samples ranging from h-BN (bulk and thin film), c-BN (single crystals, thin and thick films), mixed h-BN and c-BN phases and BCN films with varying B, C and N content. The UV Raman spectra of c-BN and h-BN are so intense that a clear measurement of the second order Raman signal is possible. This allows us to probe for the first time non-zone center phonons and validate the predictions of first-principles calculations of BN properties [3,6]. For h-BN, whilst the high-energy optical phonons are in excellent agreement with theory, the ionic interaction was strongly underestimated for lower energies [6]. The first order UV Raman spectra of thin and nano-crystalline films also show the activation of non-zone center phonons and IR modes, such as the 800 cm-1 mode of h-BN. In BCN films the direct signature of BN bonds can be seen, being no more shadowed by the carbon phase.

1. P. B. Mirkarimi, K. F. McCarty, D. L. Mellin, Mat. Sci. Eng. R21, 47 (1997)
2. W. Zhang, S. Matsumoto, Q. Li, I. Bello, S. T. Lee, Adv. Funct. Mat. 12, 250 (2002)
3. L. Wirtz et al. Phys. Rev. B 68, 045425 (2003)
4. R. Gago et al. J. Appl. Phys. 92, 5177 (2002)
5. A.C. Ferrari, J. Robertson, Phys. Rev. B 61, 14095 (2000); 64 075451 (2001)
6. G. Kern, G. Kresse, J. Hafner, Phys. Rev. B 59, 8551 (1999)

  • Lecture (Conference)
    15th European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes, Nitrides & Silicon Carbide (DIAMOND 2004), 12-17 September 2004, Riva del Garda, Trentino, Italy

Publ.-Id: 6470

Efecto del Ar en películas CNxHy depositadas mediante ECR-CVD

Camero, M.; Gago, R.; Gómez-Aleixandre, C.; Albella, J. M.

Carbon nitride films have been deposited by ECR-CVD, from Ar/CH4/N2 gas mixtures with different methane concentrations. Infrared Spectroscopy and Elastic Recoil Detection Analysis have been used for films characterisation and Optical Emission Spectroscopy for plasma analysis. Argon concentration in the gas mixture controls the growth rate as well as the composition of the film. In the proposed model, argon plays a key role in the activation of methane molecules. Also, during the growth of the film, two processes may be considered: i) Film formation and ii) Etching of the growing surface. Changing the gas mixture composition affects both processes, which results in films with different composition and structure as well as different deposition rates.

Keywords: Thin films; hydrogenated carbon nitride; ECR-CVD; Argon

  • Boletín de la Sociedad Española de Cerámica y Vidrio 43(2004), 491-493

Publ.-Id: 6469

X-ray diffraction study of stress relaxation in cubic boron nitride films grown with simultaneous medium-energy ion bombardment

Abendroth, B.; Gago, R.; Eichhorn, F.; Möller, W.

Relaxation of the intrinsic stress of cubic boron nitride (cBN) thin films has been studied by x-ray diffraction (XRD) using synchrotron light. The stress relaxation has been attained by simultaneous medium-energy ion bombardment (2-10 keV) during magnetron sputter deposition, and was confirmed macroscopically by substrate curvature measurements. In order to investigate the stress-release mechanisms, XRD measurements were performed in in-plane and out-of-plane geometry. The analysis shows a pronounced biaxial state of compressive stress in the cBN films grown without medium-energy ion bombardment. This stress is partially released during the medium-energy ion bombardment. It is suggested that the main path for stress relaxation is the elimination of strain within the cBN grains due to annealing of interstitials.

  • Applied Physics Letters 85(2004)24, 5905-5907

Publ.-Id: 6467

Interaction of nitrogen atoms in expanded austenite formed in pure iron by intense nitrogen plasma pulses

Piekoszewski, J.; Sartowska, B.; Walis, L.; Werner, Z.; Kopcewicz, M.; Prokert, F.; Stanislawski, J.; Kalinowska, J.; Szymczyk, W.

The paper presents the results of experiments on modification of pure iron by high intense nitrogen pulsed-plasma treatment. The duration of nitrogen plasma pulses is approximately 1 µs, and the energy density amounts to about 5 J/cm2. Such pulses are capable to melt the surface layer of the substrate (1-2 µm) and to introduce a significant concentration of nitrogen into the molten layer. Nuclear reaction analysis (NRA), X-ray diffraction (XRD) and conversion electron Mössbauer spectroscopy (CEMS) were used for characterisation of the treated samples. The main results of the data analysis are as follows:it has been stated that such treatment leads to gradual transformation of initial alpha-phase into austenitic gamma structure in which expanded austenite gammaN is present. Treatment with 20 pulses results in almost complete transformation and introduces a retained dose of nitrogen estimated as 5.5 x 1017 N/cm2. The susceptibility for expansion of the lattice transformed to austenite in this way is smaller than in the case when the steel subjected to conventional nitriding is orginally of austenite type. The analysis of the ratio of gamma to gammaN as a function of the nitrogen content provides a firm evidence that strong repulsion forces act between the first and the second nearst-neighbour nitrogen atoms in the fcc austenitic structure as a result of nitriding of pure iron by intense nitrogen plasma pulses.

Keywords: alpha-to-gamma phase transformation in iron; expanded austenite; intense nitrogen plasma pulses

  • Nukleonika 49(2004)2, 57-60

Publ.-Id: 6466

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