Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

We propose a method for deterministic implantation of single atoms into solids which relies on a linear ion trap as an ion source. Our approach allows a deterministic control of the number of implanted atoms and a spatial resolution of less than 1 nm. Furthermore, the method is expected to work for almost all chemical elements. The deterministic implantation of single phosphor or nitrogen atoms is interesting for the fabrication of scalable solid state quantum computers, in particular for silicon and diamond based schemes. A wide range of further applications is expected for the fabrication of nano and sub-nano electronic devices.

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The quadrupole moment of the 8+ yrast state in 84Kr was measured using the level-mixing spectroscopy technique to be Q = 36(4) efm2. The result is compared with predictions of the shell model using common sets of effective charges. The comparison of experimental quadrupole moments with calculated values for 8+ states in Kr, Sr and Zr isotopes with N = 48, 50 and for 9/2+ states in isotopes with N = 47, 49 suggests a modification of the effective charges used in this region.

We investigated the local atomic and electronic structures of two related systematic sets of ternary uranium oxides, NaUO3 - KUO3 - RbUO3 and BaUO3 - Ba2U2O7 - BaUO4, by measuring the X-ray absorption near edge structure (XANES). The results are compared with calculations based on self-consistent real space full multiple scattering analysis. We found a very good agreement between measured and calculated spectra, which indicates that the uranium ions are in pure U5+ oxidation state in these compounds. The low energy shoulder observed in the U L3 edge XANES is an intrinsic feature of the uranium unoccupied 3d electronic state of the U5+ ions. Specific double shoulder features in the higher energy range of the U L3 edge XANES can be interpreted as indicative of the pure cubic perovskite structure.

The preferred orientation and thermal stability of MAX phase (M: early transition metal; A: A group element; X: C and/or N) Ti2AlN thin films sputtered onto MgO(111) and Al2O3(0001) substrates have been investigated by in situ x-ray diffraction and Rutherford backscattering (RBS). Low-temperature deposition results in epitaxial growth with inclined basal planes. An increased substrate temperature changes the preferred orientation to a parallel basal plane growth. In contrast to high bulk thermal stability, thin films decompose in vacuum already at 800°C by outward Al diffusion, followed by de-twinning of the remaining Ti2N slabs and concurrent TiN and Ti4AlN3 formation.

In the environment uranium is one of the ubiquitous elements. One million tons of uranium metal have been produced so far. Between 1945 and 1990 about one quarter of the total world production of uranium have been mined and milled in Eastern Germany. Especially these former intense uranium mining and milling activities have caused contaminations within urban areas.

In the hexavalent oxidation state uranium is soluble in aqueous media. The determination of different species with non-invasive methods can be performed by Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLIFS) under natural conditions. This is due to the different fluorescence emission and fluorescence lifetime of the varying uranium species.
The concentration of uranium ranges from the upper ng/L scale for table waters to the mg/L scale for mining related seepage and surface waters of the tailing ponds. From speciation calculations uranium in the most of these waters should be found as carbonate species. Unfortunately the carbonate species show poor fluorescence properties under ambient temperatures. By cooling of the samples to temperatures below 220 K an increase in the fluorescence intensity and lifetime can be observed. The dynamic quench effect of the carbonate ion decreases with decreasing temperature. Therefore the direct determination of the uranium speciation also in drinking waters can be performed. Examples of uranium carbonate speciation in a wide variety of natural waters will be given.

As an example, the dependence of the uranium speciation on pH of a tailing pond water was detected in detail by TRLIFS measurements. This water contains complexing anions like carbonate, phosphate, arsenate and sulphate. The uranium speciation in this surface water is strongly changed with pH. At pH 5 uranium arsenate species are dominating. The fluorescence emission of this species is in good agreement with data obtained from the uranium mineral troegerite.

Dynamics of the Molten Salt Reactor, one of the 'Generation IV International Forum' concepts, was studied in this paper. The graphite-moderated channel type MSR was selected for the numerical simulation. The MSR represents a liquid fueled reactor and its dynamics is very specific because of two physical peculiarities: the delayed neutrons precursors are drifted by the fuel flow and the fission energy is immediately released directly into the coolant. Presently, there are not many accessible numerical codes appropriate for the MSR simulation, therefore the DYN3D-MSR code was developed based on the Light Water Reactor dynamics code DYN3D. It allows calculating of full 3D transient neutronics in combination with parallel channel type thermal-hydraulics. The code was validated on experimental results of Molten Salt Reactor Experiment (from Oak Ridge National Laboratory) and applied to several transients typical for the liquid fuel system. Those transients were initiated by reactivity insertion, by overcooled or overfueled fuel slug, by the fuel pump start-up or coast-down, or by the blockage of single fuel channels. In these considered transients, the response of the MSR is characterized by the immediate change of the fuel temperature according to the power level. This causes fast feedback reactivity insertion, which is negative in the case of power increase. On the other hand, the graphite response is slower and its feedback coefficient is in some cases positive. The addition of erbium to the graphite can ensure the negative feedback and inherent safety features. The DYN3D-MSR code has been shown to be an effective tool for MSR dynamics studies.

The HADES (High Acceptance Di-Electron Spectrometer) is a tool designed for lepton pair (e+e-) spectroscopy in pion, proton and heavy ion induced reactions in the 1-2 A GeV energy range. One of the goals of the HADES experiment is to study in-medium modifications of hadron properties like effective masses, decay widths, electromagnetic form factors etc. Such effects can be probed with vector mesons (rho , omega , phi ) decaying into e+e- channel. The identification of vector mesons by means of a HADES spectrometer is based on invariant mass reconstruction of e+e- pairs. The combined information from all spectrometer sub-detectors is used to reconstruct the di-lepton signal. The recent results from 2.2 GeV p+p, 1 A GeV and 2 A GeV C+C experiments are presented.

The sorption characteristics of U and Cs on bentonite and montmorillonite were investigated in batch experiments to understand the near-field behaviour in geological nuclear repositories. The pH dependence and the retention of Cs are small and only caused by cation exchange. For the U sorption, maxima were found between pH 5.5 to 6.0 and at pH > 10.5. The linear sorption isotherm indicates only one binding type.
The retention of Cs in the concentration range between 3·E-9 and 1·E-4 mol/L on bentonite and montmorillonite (KD between 30 and 189 mL/g, retention from 7 to 31 %) is low compared to U (KD between 430 and 31500 mL/g, retention from 50 to 100 %)

This work is supported by the European Commission (NF-PRO C2-ST-C-01)

We studied the U(VI) sorption onto kaolinite in batch experiments in the absence and presence of humic acid (HA) under different experimental conditions: [U]0 = 1×10-6M or 1×10-5 M, [HA]0 = 10 or 50 mg/L, I = 0.1 M or 0.01 M NaClO4, pH = 3 – 10, CO2 or N2 atmosphere. The study showed that the U(VI) sorption onto kaolinite is influenced by pH, CO2 and HA presence. In the absence of CO2, the U(VI) uptake increases with increasing pH value up to pH 6. Above pH 6 it remains unchanged. Because of the formation of negatively charged uranyl carbonate complexes, the decrease in the U(VI) sorption onto the negative surface of kaolinite was observed above pH 8 in the presence of CO2. In presence of HA, the adsorption of U(VI) closely follows the adsorption of HA. In the acidic pH range the U(VI) uptake is enhanced compared to the system without HA due to the formation of additional binding sites for U(VI) coming from HA adsorbed onto kaolinite. The formation of aqueous uranyl humate complexes reduces the U(VI) sorption in the near neutral pH range. The enhancement of the U(VI) concentration from 1×10-6 M to 1×10-5 M results in the shift of the sorption pH edge by one pH unit to higher pH values. The ionic strength has only a slight influence on the U(VI) sorption onto kaolinite, whereas the HA sorption shows a dependence on the ionic strength.

Overview of electroproduction experiments on light nuclei

Report on RPC test at the FZ Rossendorf ELBE linac

The speciation of curium(III) with L-Threonine and O-Phospho-L-Threonine was determined by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at trace Cm(III) concentrations (3x10-7 M).
Curium species of the type MpHqLr were identified in the L-Threonine- and O-Phospho-L-Threonine system. These complexes are characterized by their individual luminescence spectra and luminescence lifetimes. The following formation constants were determined a) for L-Threonine: log β101 = 6.72 ± 0.07, log β102 = 10.22 ± 0.09, and log β1-22 = -(7.22 ± 0.19) at ionic strength I = 0.5 M and b) for O-Phospho-L-Threonine: log β121 = 18.03 ± 0.13 and log β111 = 14.17 ± 0.09 at ionic strength I = 0.154 M. Possible structures of the identified curium species will be discussed on the basis of the luminescence lifetime measurements and the magnitude of the formation constants.

The ion implantation induced interfacial mixing and subsequent amorphization of ultrathin Ni81Fe19/Ta bilayers has been investigated by TRIDYN simulations and cross-sectional transmission electron microscopy. Due to the 30 keV Ni implantation the initially sharp interface between Ni81Fe19 and Ta broadens and an intermixing between both layers is observed. Consequently the Ni81Fe19 layer is increasingly doped with Ta in the interface near region. For the region exhibiting a Ta concentration above a threshold value of about 20 atom-% irradiation induced amorphization takes place. For an increasing Ni implantation fluence this region is succeedingly enlarged. A quantitative agreement between simulations and experimental data is found.

Der Washout der Positronenemitter bei in-beam PET

In-beam PET wird am experimentellen Schwerionentherapieplatz an der Gesellschaft für Schwerionenforschung (GSI) Darmstadt bereits erfolgreich eingesetzt. Durch Vergleich der simulierten Positronen-emitterverteilung mit der gemessenen sind Aussagen zur Reichweite des Kohlenstoffstrahls möglich, es können anatomische Veränderungen detektiert werden und die Feldlage kann beurteilt werden. In der Simulation werden alle Prozesse von der Erzeugung der Positronenemitter bis zum Nachweis der Photonen im Detektor behandelt. Im lebenden Gewebe jedoch nehmen die Positronenemitter an metabolischen Vorgängen teil. Es kommt zum Transport und der Annihilationsort kann vom Erzeugungsort verschieden sein.
Durch Auswertung von Messungen an Patienten kann eine effektive Halbwertszeit bestimmt werden, die sich aus der physikalischen und der biologischen Halbwertszeit zusammensetzt. Die physikalische Halbwertszeit wird nur durch die Stöchiometrie des Gewebes und die Bestrahlungsdauer determiniert. Die biologische Halbwertszeit unterliegt dagegen vielfältigen Einflüssen, die in der vorliegenden Arbeit untersucht wurden. Die Berücksichtigung der effektiven Halbwertszeiten verbessert die Simulation.

Bei der Radiotherapie von Tumoren erlauben Ionenstrahlen auf Grund ihrer physikalischen und biologischen Eigenschaften ein höheres Maß an Präzision und Wirksamkeit als die konventionellen Strahlenmodalitäten, Photonen und Elektronen. Ein in-situ Monitoring der Dosisapplikation ist deshalb für die Qualitätssicherung der Ionentherapie in hohem Maße wünschenswert. Das einzige gegenwärtig bekannte, dafür geeignete Verfahren ist die in-beam Positronen-Emissionstomographie (PET). Dabei werden während der Tumorbestrahlungen Annihilationsereignisse registriert, die in Folge des Zerfalls von Positronenemittern auftreten, welche durch nuklare Wechselwirkungen zwischen den Projektilen des Therapiestrahles und den Atomkernen des Gewebes erzeugt werden. In die experimentelle Ionentherapie-Anlage an der Gesellschaft für Schwerionenforschung Darmstadt wurde ein in-beam PET-Scanner integriert und die Methode erstmals klinisch angewendet. An dieser Anlage wurden seit 1997 etwa 300 Patienten, vornehmlich mit stahlenresistenten Tumoren in komplizierten anatomischen Positionen im Kopf-Hals-Bereich mit Kohlenstoff-Ionenstrahlen behandelt. Alle Bestrahlungen wurden mit PET kontrolliert.Die nunmehr vorliegenden Erfahrungen erlauben gesicherte Aussagen zum Potential der in-beam PET für die Qualitätssicherung der Ionentherapie.

Abstract not available

The paper presents the method of determination of mass transfer coefficient in electromagnetic stirred liquid phase based on Lamont, Scott and Kolmogorov turbulent models. For this purpose the numerical model allowing the determination of electromagnetic force and turbulent filed was utilized. As result the values of Lorentz forces and metal surface velocity have been determined. Calculations were made for IS5/III Leybold Heraeus induction furnace.

The Institute of Radiochemistry (IRC), one of the six institutes of the Forschungszentrum Rossendorf (FZR), performs application-oriented research in the fields of radiochemistry and radioecology. Motivation and background of our research are environmental processes relevant for the installation of nuclear waste repositories, for remediation of uranium mining and milling sites, and for radioactive contaminations caused by nuclear accidents and fallout. Due to their high radiotoxicity and long half-life the actinides are of special interest. Hence our research focuses on the chemical behavior of actinides at the molecular level in order to predict the relevant macroscopic processes in the environment.
Within this framework, special emphasis is on the interface between geological and biological systems.
In the last year our research topics were as follows:
– Aquatic chemistry of actinides
– Actinides in bio-systems
– Interaction of actinides with solid phases
– Reactive transport of actinides
About 60 scientists, technicians and PhD students are employed in the Institute of Radiochemistry.
We have achieved a wide range of new scientific results in the past year, which are presented in this Annual Report. Among them only a few can be highlighted here in this preface. For the first time it was possible to determine uranium speciation in situ in drinking and mineral waters e.g. by a dedicated fluorescence spectrometer at lowest μg/L concentrations. This methodical progress is an important prerequisite to study the uranium toxicity and its dependence on chemical speciation. We were very successful in the determination of formation pathways and structure of various actinide complexes, e.g., the surface complexes of uranium (VI) onto mica and iron hydroxides over a wide range of pH and carbonate concentration. These results contribute to a better understanding of actinide speciation in geo- and bio-systems, especially with respect to the chemical processes on the interfaces.
Studies to the interaction of uranium with biofilms, green algae and bacteria coming from extreme habitats extended our research on the field of bio-systems.
Major progress in the structural analysis of multiple uranium species has been achieved by applying Monte Carlo simulations and iterative transformation factor analysis to EXAFS spectroscopy.
Furthermore, our new radiochemical experimental station at the Free Electron Laser of the Rossendorf accelerator ELBE is now in full operation. We have started first experiments on the uranium and neptunium complexation on selected mineral surfaces.

A superconducting electron linear accelerator of high brilliance and low emittance (ELBE) is under operation at Forschungszentrum Rossendorf since January 2003. The first stage of ELBE is based on an electron energy of 20 MeV, whereas in the future a 40 MeV beam will be provided. The relativistic electron beam is used to drive various kinds of secondary radiation sources. Among all, X-rays in a wide energy range can be obtained. One method for production of intensive, quasi-monochromatic Xrays in the energy range 10 - 100 keV, tunable in photon energy, is by channeling of relativistic electrons in a perfect crystal. This unconventional photon source with variable time structure will be optimised and used for radiobiological studies. Its first test operation was in October 2003. This thesis is part of the first radiobiological project – the determination of relative biological effectiveness (RBE) of the X-rays in this energy range.
The most important aspects of medical application of low-energy X-rays are imaging and radiation therapy, but they can also be helpful in the study of radiation effects in living matter. However, the RBE depends on the photon energy, dose range, cell line and biological endpoint. Up to now no definitive conclusions can be made about their biological effectiveness due to the large spread of the published data. Therefore, in order to precisely determine the RBE, studies have to be performed at an intensive, tunable photon source, for several practically relevant cell lines and biological endpoints. The possibility of using channeling radiation (CR) for medical applications has been widely discussed in the literature, but building and optimisation of a dedicated source is for the first time performed at the ELBE accelerator.

One of the most interesting features of Earth's magnetic field reversals is their asymmetric shape including a rather slow decay of a given axial dipole and a very fast recreation of the dipole with opposite polarity. With focus on this asymmetry, we consider a simple mean-field dynamo model with a spherically symmetric isotropic helical turbulence parameter alpha that is quenched by the magnetic field energy and subjected to some noise. With an appropriate radial dependence of alpha(r) (including at least one sign change), this dynamo model exhibits typical features of reversals. The asymmetric shape and the very fast recreation are attributed to the dynamical behaviour in the vicinity of a branching point of square root type (exceptional point) of the spectrum of the non-selfadjoint dynamo operator. Other features, like the possible correlation of magnetic field amplitude and reversal rate, the bimodal field distribution, and the inhibition time are also addressed within our simple model. We discuss a tendency of highly supercritical dynamos to self-tune into reversal prone states, and hypothesize that reversing dynamos might be much more common in nature than what could be expected from a purely kinematic perspective.

Differential production cross sections of K± mesons have been measured in p+C and p+Au collisions at 1.6, 2.5, and 3.5 GeV proton beam energy. At beam energies close to the production threshold, the K- multiplicity is strongly enhanced with respect to proton-proton collisions. According to microscopic transport calculations, this enhancement is caused by two effects: the strangeness exchange reaction NY-->K-NN and an attractive in-medium K-N potential at saturation density.

The timing performance of various multigap resistive plate chambers (MRPCs) has been tested at the Rossendorf electron linac ELBE. Electrons with energies of 20-40 MeV have been used to mimic minimum ionizing hadrons to be detected in large-scale time-of-flight (TOF) detectors for future heavy-ion collision experiments. Referencing the TOF measurement to the accelerator radio frequency, no fast start detector was necessary. A typical time resolution (including contributions from front-end electronics) of about 70 ps has been achieved for individual strips of the multistrip-anode MRPCs. Rate dependencies and detection efficiencies have been investigated.

Strangeness production by both hadronic and leptonic projectiles with beam energies of up to a few GeV is reviewed. The focus is on the production of strangeness using proton and ion beams, as well as the photo- and electroproduction of strangeness, as observed at modern facilities. The elementary production of K+- and phi mesons as well as Lambda, Sigma hyperons on the nucleon is described. Based on these results, the production of strange mesons and strange baryons on nuclear targets, as well as the creation of light hypernuclei is discussed, emphasizing the influence of the nuclear medium.

Results from coincidence Doppler broadening (CDB) measurements on various Si samples and Brazilian quartz having low quartz structure are presented with the aim to give further strong indication of the existence of a low quartz structure, but not of Si divacancies as frequently considered, at the SiO2/Si interface.

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For the validation of Computational Fluid Dynamics (CFD) codes experimental data with high resolution in time and space are needed.
ROCOM (Rossendorf Coolant Mixing Model) is a test facility for the investigation of coolant mixing in the primary circuit of pressurized water reactors. This facility describes the primary circuit of a German KONVOI type reactor. All important details of the reactor pressure vessel are modelled in a linear scale of 1:5. The facility is characterized by flexible possibilities of operation in a wide variety of flow regimes and boundary conditions. The flow path of the coolant from the cold legs through the downcomer until the inlet into the core is equipped with high resolution measurement technique. Especially, wire mesh sensors in the downcomer of the vessel with a mesh of 64 x 32 measurement positions and in the core inlet plane with one measurement position for the entry into each fuel assembly allow to carry out high-level CFD code validation. The measured data can contribute significantly to the validation of the CFD-codes.

Multiply twinned FePt nanoparticles with icosahedral structures were prepared by dc magnetron sputtering in argon. The icosahedral structure of these particles is known to be very stable against structural transformations into both the face-centered cubic phase (fcc, A1) and the chemically ordered tetragonal L10 phase upon in-flight or post-deposition thermal annealing. Irradiation of these multiply twinned FePt particles with 5 keV He ions, however, resulted in a transformation into predominantly single crystalline fcc particles at high ion fluences of f>1017 ions/cm2. Adjacent particles were observed to coalesce under the effect of He irradiation, and the size of individual particles was found to be slightly reduced, which indicates a high atomic mobility owing to temporarily enhanced defect concentrations caused by the ion bombardment. Strikingly, there was no indication for the occurrence of L10 ordered FePt nanoparticles upon ion irradiation in these samples.

A new multipurpose thermalhydraulic test facility TOPFLOW (TwO Phase FLOW) was built and put into operation at Forschungszentrum Rossendorf in 2002 and 2003. Since then, it has been manly used for the investigation of generic and applied steady state and transient two phase flow phenomena and the development and validation of models of Computational Fluid Dynamic (CFD) codes in the frame of the German CFD initiative. The advantage of TOPFLOW consists in the combination of a large scale of the test channels with a wide operational range both of the flow velocities as well as of the system pressures and temperatures plus finally the availability of an special instrumentation that is capable in high spatial and temporal resolving two phase flow phenomena, for example the wire-mesh sensors.

Nanocluster of Pr compounds were formed in a silicon-on-insulator (SOI) structure by ion beam synthesis. The processes of ion-induced atomic mixing, phase segregation, texturing, and nanocluster formation were investigated by transmission electron microscopy, Auger electron spectroscopy and X-ray diffraction. After annealing at 900°C an array of Si precipitates is formed in SiO2. The size distribution of Si precipitates corresponds to the concentration profile of excess Si atoms in SiO2. Implanted Pr atoms are trapped at the Si/SiO2 interface around the Si precipitates and decorate them. During annealing at 1100°C crystalline nanocluster of Pr9.33Si6O26 or Pr2Si2O7 silicate are formed. In the top-Si-layer a narrow layer of Pr compound nanocluster is formed by self-organization rather equidistant of the Si/SiO2 interface. Pr oxide (Pr2O3), the most desired high-k material, was not definitely verified. The predominant process in solid-state synthesis of Pr compounds is Pr silicate formation. It provides both, a high atomic package density and a low reordering energy.

We investigated the imaging capability of a fast linearly scanned electron-beam X-ray tomography approach with respect to the phase structure recovery for two-phase flows in a cylindrical pipe. As a consequence of the suggested linear electron beam deflection pattern we need to solve an inverse problem of the limited-angle type which introduces some artefacts in the reconstructed images. To reduce these artefacts we have devised a modified iterative image reconstruction algorithm denoted as binary ART including a level-set based image smoothing operation. To assess the achievable quality of spatial phase structure recovery from the limited-angle data we performed a simulation study on three-dimensional flow data sets obtained with a fast and high-resolution conductivity wire-mesh sensor under real two-phase flow conditions. The simulations revealed that the reconstruction error remains below 2% for up to 1% of Gaussian noise in the projection data and even for up to 5% noise in the case of the bubble diameters below 3 mm.

EXAFS gives average values of all neighbor distances as a radial distribution function. The determination of the coordination number has a relative high error (10-25%), therefore the structure interpretation is strongly focused onto distance determination (delta R ± 0.01Å). EXAFS combined with Factor analysis allows separation of different species from solutions. EXAFS allows to korrelate structural features to thermodynamically proposed solution species.

In a Pressurized Water Reactor, negative reactivity is present in the core by means of Boric acid as a soluble neutron absorber in the coolant water. The main functions of the boric acid are to compensate for the fuel burn up and Xenon poisoning during normal operation and to provide the required sub-criticality of reactor shutdown during refueling and maintenance. During a so called Boron Dilution Transient (BDT), the borated coolant water is diluted by mixing with unborated water. The resulting decrease in the boron concentration leads to an insertion of positive reactivity in the core, which may lead to a reactivity excursion. The associated power peak may damage the fuel rods.

The most severe BDT involves scenarios in which a slug of unborated water has been formed in a cold leg in a stationary (Main Coolant Pumps are down) primary circuit. An inadvertent start-up of the MCP of the affected loop causes the transport of the unborated slug into the Reactor Pressure Vessel (RPV). The resulting positive reactivity insertion in the core is governed by the degree of mixing of the unborated slug and the borated water in the cold leg, the downcomer and the lower plenum. This mixing of borated and unborated water is an important process, because it mitigates and determines the degree of reactivity insertion.

The objective of the present study is to develop a validated Computational Fluid Dynamics (CFD) model for the prediction of the boron concentration distribution in the RPV as function of time during a BDT. This CFD model has been validated using the measurement data from the Rossendorf coolant mixing model (ROCOM) experiment. The ROCOM test facility represents the primary cooling system of a KONVOI type of PWR (1300 MWel). The linear scale of the ROCOM experimental facility is 1:5. The RPV is connected with four circulation loops. Various experiments on boron dilution scenarios have been performed. For the pump start-up experiments the following boundary conditions were varied: the length of the pump ramp, the final mass flow rate of the loop with the start–up pump, the volume of the deborated slug and initial position in the cold leg and the status of the unaffected loops.

A detailed CFD model of the RPV including the inlet nozzles, the downcomer, and the lower plenum has been developed to predict the mixing of deborated and borated water in this RPV. The validation of the model has been achieved by comparison of the calculated and measured relative boron concentration at the core inlet plane as function of time. For code validation a slug mixing experiment with 14 s ramp length, 185 m³/h final flow rate, 4 m³ slug volume and 10 m initial slug position was taken. The unaffected loops were open. In spite of the complicated spatial, temporal, and geometrical aspects of the flow in the RPV, the agreement between the calculated and the experimental data is good. The minimal relative boron concentration measured at the core inlet is 64% and the calculated minimum value is 60%.

Structural and magnetic properties in Mn-implanted, p-type Si were investigated. High resolution structural analysis techniques such as synchrotron x-ray diffraction revealed the formation of MnSi1.7 nanoparticles already in the as-implanted samples. Depending on the Mn fluence, the size increases from 5 nm to 20 nm upon rapid thermal annealing. No significant evidence is found for Mn substituting Si sites either in the as-implanted or annealed samples. The observed ferromagnetism yields a saturation moment of 0.21µB per implanted Mn at 10 K, which could be assigned to MnSi1.7 nanoparticles as revealed by a temperature-dependent magnetization measurement.

Mit dem CFX-Code wurden Nachrechnungen von Durchsatzparametern eines Leser Sicherheitsventils mit einem Gitter von 1 Million Hexaeder Zellen vorgestellt. Sehr gute Übereinstimmung wurde bei den Durchsatzkennlinien der vom Hersteller angegebenen Charakteristiken für Luft und Wasser erzielt, es existierte jedoch eine systematische Abweichung bei Luft im Falle hoher Ansprechdrücke durch Realgasverhalten und das Auftreten hohe Ma-Zahlen. Eine CFX-Rechnung mit einem Zweiphasengemisch bei 2 bar Überdruck, 1 mm Blasendurchmesser und 4% Gasanteil ergab eine realistische Erhöhung des Volumenanteils der Gasphase in Abhängigkeit von der Druckabsenkung. Weitere Untersuchungen, insbesondere das Verhalten bei Sattdampf und bei Stoffgemischen im Vergleich mit Experimenten sind geplant.

Auftriebsgetriebene Strömungen in Druckwasserreaktoren stellen sich in einer Vielzahl von sicherheitsrelevanten Szenarien, insbesondere unter Naturumlaufbedingungen z. B. nach einem postulierten Leckstörfall bzw. bei “pressurized thermal shocks (PTS)” Szenarien, wie der Einspeisung von kaltem Notkühlwasser in die noch heissen Loops in, ein. Bei der Untersuchung dieser Klasse von Strömungen mit numerischen Tools zeigte sich, dass vor allem die vorhandenen Turbulenz- und Zweiphasenmodelle noch weiterzuentwickeln sind. Die umfangreichen Messdaten der Versuchsanlage ROCOM bieten eine Grundlage für die Validierung von CFD-Codes. Zur Bewertung der Güte der Modelle wurden quantitative Vergleiche zwischen Rechnung und Messung durchgeführt.

Im ersten Teil des Vortrages erfolgt eine Vorstellung von Nachrechnungen dichtegetriebener Experimente an der ROCOM-Versuchsanlage. Im zweiten Teil wird näher auf Simulationen eines Auffüllvorganges eines Notkühlbehälters mit deborierten Kühlmittel eingegangen.

Hierbei werden Fortschritte in der CFD-Modellierung der letzten Jahre, der Einbezug von sogenannten Best Practice Guidelines in der CFD-Modellierung in der Reaktorsicherheitsforschung aber auch bestehender Forschungsbedarf und Grenzen der Modellierung dargestellt.

Clay minerals and organic substances are common components of soils, sediments, and rocks. Little is known, however, on the migration behavior of humic substances in clay formations. Here, we studied the diffusion of humic acid (HA) in clay at different porosities to understand the migration of organic colloids in narrow pore systems.
The migration behavior of HA in compacted clay has to be understood as the diffusion of a size distributed matter, which is affected by filtration effects, size exclusion, the conformity of HA as a function of pH, and probably the distribution of functional groups in the different molecular fractions.

In diesem Vortrag sollte dargestellt werden, mit welchen Methoden sich die Astrophysik Information über den Kosmos beschafft und welche Vor- und Nachteile erdgebundene gegenüber weltraumgestützten Beobachtungen haben. Dabei wurde insbesondere auf den Nachweis elektromagnetischer Strahlung und kosmischer Teilchen eingegangen.
Ein Teil des elektromagnetischen Spektrums wird durch die Erdatmosphäre absorbiert und geladene Teilchen werden bis zu einer bestimmten Energie durch das Erdmagnetfeld abgelenkt. Hochenergetische Teilchen können jedoch in der Atmosphäre Teilchenschauer auslösen, die mit verschiedenen Detektortypen am Erdboden nachgewiesen werden können, wobei unterschiedliche physikalische Prozesse ausgenutzt werden. Als Beispiele wurde die Funktionsweise von Radioantennen, CCD-Kameras (Charge-Coupled Device), des Chandra-Röntgenteleskops und EGRET (Energetic Gamma Ray Experiment Telescope) für den Nachweis elektromagnetischer Strahlung, sowie des AMS-Experimentes (Alpha Magnetic Spectrometer) mit seinen vielfältigen Detektortypen zum Nachweis kosmischer Teilchen vorgestellt.
Ohne die Resultate dieser Messungen wäre die Entwicklung kosmolgischer Modelle nicht möglich. Gleichzeitg werden dadurch neue Fragen aufgeworfen, die Anstoß zur Verbesserung und Entwicklung neuer Detektortypen liefern.

The two commonly used approaches to describe the VVER radial reflectors in diffusion codes, by core-reflector albedos and by a ring of diffusion assembly size nodes, are discussed. The advantages and disadvantages of the first approach are presented first, then the Koebke's equivalence theory is outlined and its implementation for the VVER radial reflectors is discussed. Results for the VVER-1000 reactor are presented.

In-situ x-ray diffraction (XRD) during the growth of Ni-Ti thin films was chosen in order to investigate their texture development using a deposition chamber installed at a synchrotron radiation beamline. Near-equiatomic films were co-sputtered from Ni-Ti and Ti targets. The texture evolution during deposition is clearly affected by the substrate type and the ion bombardment of the growing film. On naturally oxidized Si(100) substrates the NiTi B2 phase starts by stacking onto (h00) planes, and as the thickness increases evolves into a (110) fiber texture. For the deposition on thermally oxidized Si(100) substrates, this pronounced cross-over is only observed when a substrate bias voltage (-45 V) is applied. The oxide layer plays an important role on the development of the (100) orientation of the B2 phase during deposition on heated substrates (≈ 470ºC). If this layer is not thick enough (naturally oxidized Si substrate) or if a bias voltage is applied, a cross-over and further development of the (110) fiber texture is observed,which is considered as an orientation that minimizes surface energies. Electrical resistivity measurements showed different behaviour during phase transformation for the NiTi film deposited on thermally oxidized Si without bias and those on thermally oxidized Si(100) with bias and on naturally oxidized Si(100) without bias. This is related to stresses resultant from the fact that the NiTi films are attached to the substrates as well as with the existence of distinct textures.

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Ein kleiner Leckstörfall kann im Druckwasserreaktor unter bestimmten Bedingungen zum Abriss der einphasigen Naturzirkulation in einer oder mehrerer Schleifen führen. Bei einem bestimmten Druckniveau ist der Massenstrom der Noteinspeisung groß genug, um die Leckverluste wieder zu kompensieren. Der Primärkreislauf wird aufgefüllt und der einphasige Naturumlauf springt wieder an. Minderborierte Kondensatpfropfen, die sich in der vorhergehenden Reflux-Condenser-Phase gebildet haben, werden in Richtung Reaktorkern transportiert. Basierend auf aus PKL-Experimenten abgeleiteten Randbedingungen wurden an ROCOM Experimente zur Quantifizierung der Vermischung innerhalb des Druckbehälters durchgeführt.

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EPOS, the acronym of ELBE Positron Source, describes a running project to build an intense pulsed beam of mono-energetic positrons (0.2-40 keV) for materials research. Positrons will be created via pair production at a tungsten target using the pulsed 40 MeV electron beam of the superconducting electron linac with high brilliance and low emittance (ELBE) at Forschungszentrum Rossendorf (near Dresden, Germany). The chosen design of the system under construction is described and results of calculations simulating the interaction of the electron beam with the target are presented, and positron beam formations and transportation are also discussed.

Our studies on bacterial diversity in waters, sediments and soils of different uranium mining wastes and repositories in Germany and in the USA demonstrated that these extreme environments are populated by dense groups of various bacteria, most of which are site-specific [14, 16, 17]. Recently we have found that these bacterial populations respond very vigorously to the addition of uranyl or sodium nitrate to their natural habitats [5, 6]. The predominant bacterial groups in the original, untreated samples, which contained about 26 mg U/kg, were rapidly replaced after the supplementations by bacteria, which were underrepresented in the untreated samples. The shifting in the structure of the natural bacterial community was dependent on the salt solution added, on the aeration conditions, and on the duration of the experiments...

Peptide YY (PYY) is a 36 amino acid peptide amide that belongs to the PP-hormone family. It selectively binds to at least two G-protein coupled receptors named Y2- and Y5-receptor. These receptors are overexpressed in various tumors like neuroblastomas and nephroblastomas. With regard to clinical applications, NPY2 and NPY5 receptors may act as in vivo targets for receptor-directed therapy and diagnosis of the tumors, however there are specific and in vivo stabile ligands still required. The aim of the work was to synthesize receptor ligands, which could be radioloabeled with the positron emitting nuclides Ga-68 for small animal positron emission tomography (PET).
PYY derivatives were synthesized by solid-phase peptide synthesis, characterized by HPLC/MALDI techniques, purified by preparative HPLC and linked to DOTA (1,4,7,10-tetraazacyclotetradecane-N, N´,N´´,N´´´tetraacetic acid) as chelator for the ⁶⁸Ga(III). The in vitro binding affinity of the peptides was studied in SMS-KAN cells expressing NPY2 receptors by competitive receptor binding assays. Two Ga-68 labeled derivatives were studied in vivo by small animal PET in rats and mice. The activity was fast eliminated into the urine. One hour after injection was only in the kidneys remaining activity detected.
Optimized Y2-selective and in vivo stabile peptides will be developed and tested for tumor targeting.

Mit Hilfe moderner Scannertechnologien und hohen Magnetfeldstärken erreicht die Magnetresonanzbildgebung (MRT) eine nahezu zelluläre Auflösung. Damit bietet sie eine zerstörungsfreie, nicht-invasive und wiederholbare Möglichkeit zur Verfolgung einzelner Zellen oder kleiner Gruppen von Zellen nach deren Transplantation oder Injektion in lebende Organismen. Dafür müssen die Zellen mit einem geeigneten MRI-Kontrastmittel (KM) markiert sein, um sie in vivo vom umgebenden Gewebe unterscheiden zu können.
In der vorliegenden Studie haben wir MRT-Messungen an KM-markierten Zellen des nicht-kleinzelligen Lungenkarzinoms (NSCLC) bei Magnetfeldstärken von 1,5 T und 7 T durchgeführt. Systematisch bestimmten wir die Zellmarkierungseffizienz von gadolinium- und Mangan-basierten KM in Abwesenheit und gegenwart verschiedener Transfektionsmittel (TM). Das Ziel der Studie war, diejenige Kombination von KM und TM zu finden, welche den besten Kontrast in der MRT liefert.

With the currently available scanner technology and high field strengths, magnetic resonance imaging (MRI) allows a near cellular resolution and, thus, provides a non-invasive and repetitive means of tracking single cells or small groups of cells after their transplantation or injection in living organisms. Therefore, cells need to be labeled with a suitable MRI contrast agent (CA) in order to differntiate them from surrounding tissue in vivo. Using CA labeled tumor cells it would be possible to follow tumor progression over time with MRI after transplantation or injection.

The phosphorylation of p-tert-butylcalix[8]arene (1) with phosphorus pentachloride and hydrolysis gives intramolecular bridging tert-butylcalix[8]arene triphosphates. The reactivity (esterification, dehydratisation, complexation), the structure (nmr and x-ray), and stereochemical behaviour of the phosphates will be discussed.

With QCD sum rule evaluations, spectral changes of hadrons inside nuclear matter are considered, which shed light on QCD condensates and thus on the non-perturbative structure of the QCD ground state. For some light quark configurations, omega meson and nucleon, the relevance of four-quark condensates is compared; the D meson as a representative of heavy-light quark systems is also briefly discussed.

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The present paper reports on our efforts in modelling the pressure drop buildup across beds of fibrous materials at strainers. Special attention is drawn to the compressibility of the fibrous filter cake. The influence of penetration depth of sludge particles onto the overall pressure drop in conjunction with the thin-bed effect has been investigated. Numerical results are compared with pressure drop measurements.

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We investigate numerically the transition to turbulence in a flat-plate boundary layer controlled by electromagnetic forces. The fluid considered is incompressible, Newtonian and low conductive. Similar to boundary layer suction, when applying a steady, wall-parallel, and streamwise orientated Lorentz force as suggested by Gailitis and Lielausis [1] in the early 1960s, the Blasius velocity profile is transformed to an exponential one gaining a critical Reynolds number which is increased by two orders of magnitude.

Two and three dimensional direct numerical simulation (DNS) of both linear and nonlinear stages of the transition process were performed, as well as a linear stability analysis (LSA) of the intermediate velocity profiles. DNS and also LSA results confirm the expected increased stability of the controlled flow. Depending on Lorentz force strength transition to turbulence is delayed or even stopped by either damping primary instability, or, in the nonlinear case, by suppressing the emerge of Omega-vortices which usually preceeds the breakdown to turbulence. Surprisingly, both DNS and LSA results suggest interesting linear stability characteristics of the intermediate velocity profiles.

In DNS (Re based on inflow displacement thickness is 360), to initiate transition, small amplitude disturbances are introduced near the inflow boundary, forming Tollmien-Schlichting waves (TSW) which grow and decay in uncontrolled case corresponging to linear stability theory. When applying a Lorentz force, all investigated TSW are damped. The decay rate is maximum in a region near the onset of control and decreases as the velocity profile evolves towards the exponential shape. This observation could suggest that in the intermediate region there are profiles more stable than the exponential one, although we are aware that from these decay rates one cannot conclude directly the stability of a velocity profile, notably its critical Reynolds number. However, our assumption is confirmed by LSA results where critical Reynolds numbers of intermediate profiles are found to be larger than for the exponential profile.

References:

1. A. Gailitis, O. Lielausis , „On a possibility to reduce the hydrodynamic resistance of a plate in an electrolyte,“ Applied Magnetohydrodynamics, Reports of the Physics Institute Riga, v. 12, p. 143-146, 1961

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Lorentz forces originating from surface-mounted actuators of permanent magnets and electrodes in weakly conducting fluids like seawater can be used to control flow separation at hydrofoils [1].
As induction effects are typically weak, external currents have to be applied to achieve control. An alternating arrangement of electrodes and magnets is considered which creates a mainly streamwise Lorentz force that is exponentionally decaying in wall-normal direction. As spanwise influences are supposed to be small, the problem is treated as two-dimensional in the following.
We investigate control by steady forcing at the suction side and by oscillatory forcing near the leading edge of symmetric foils, mostly in the post-stall regime. Similarities to the onventional method of oscillatory blowing for separation control do exist [2].
The numerical results presented cover direct numerical simulations based on a highly accurate spectral element method in the laminar flow regime in order reveal basic control phenomena as well as simulations using turbulence modelling at higher Reynolds numbers which are closer to possible naval application.
Although strong-enough steady control is able to suppress separation completely, appropriate time-periodic control turns out to be more effective for finite lift-enhancement when comparing the energetical effort. Optimum control frequencies are found, typical flow structures are analyzed, and "lock-in" phenomena with the natural shedding process are discussed [3]. The scaling behaviour of the energetical effort is compared with recent experimental results [4].

References:

1. T.Weier, G.Gerbeth, G.Mutschke, O.Lielausis, G.Lammers, "Control of Flow Separation using electromagnetic Forces",
Flow, Turbulence and Combustion 71 (2003) 5-17.
2. D.Greenblatt, I.J.Wygnanski, "The control of flow separation by periodic excitation",
Prog. Aero. Sci. 36 (2000) 487-545.
3. G.Mutschke, G.Gerbeth, T.Albrecht, R.Grundmann, "Separation Control at Hydrofoils using Lorentz
Forces", European Journal of Mechanics/B - Fluids, 2006, in press.
4. T.Weier, G.Gerbeth, "Control of separated Flows by time-periodic Lorentz Forces",
European Journal of Mechanics/B - Fluids 23(2004) 835-849.

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Multilayered materials have been shown to enhance mechanical properties compared to single-phase materials. In this paper, we describe the deposition of Ti/C multilayered systems using a novel high current pulsed cathodic arc system with dual cathodes. We have chosen this system because titanium forms a ductile crystalline layer (which can be hardened by alloying with carbon) and carbon forms an amorphous layer (which can be tailored from soft sp2 to hard sp3 material). The mechanical properties of both layers can thus be varied over a wide range. The multilayer deposition was monitored with in-situ spectroscopic ellipsometry to accurately determine the thickness and optical constants of the individual layers. Important information regarding film nucleation and growth in the interfacial regions is deduced from the ellipsomteric data. The film thicknesses determined from the ellipsometric analysis are compared with cross-sectional transmission electron microscopy (TEM) images.

The mechanism behind energetic ion impact induced stress reduction in highly stressed tetrahedral amorphous carbon and cubic boron nitride thin films is investigated by real time in situ spectroscopic ellipsometry and ex situ electron microscopy. Highly stressed carbon and boron nitride films were grown by filtered cathodic vacuum arc and RF magnetron sputtering, respectively. The films were then implanted by 5–10 keV argon ions and the film optical properties and thickness monitored in situ by spectroscopic ellipsometry. In both cases the films were observed to expand due to a reduction in the density of the ion-modified layer. Cross-sectional transmission electron microscopy and electron energy loss spectroscopy of the carbon films showed that this reduction in density is associated with a conversion of diamond-like bonding to graphite-like bonding. In situ stress measurements performed on the boron nitride films revealed a simultaneous reduction in stress with expansion of the material.

Ultra-thin metal films are widely utilised for surface-enhanced Raman scattering and surface adsorption spectroscopy. We present in situ spectroscopic ellipsometry investigations of the growth of ultra-thin silver films, from island growth through percolation and continuous film growth. Silver films are deposited using a pulsed filtered cathodic vacuum arc, which provides precise control and reproducibility of the film growth conditions. Plasmon polariton resonances are determined for the growing islands below the percolation threshold. As the surface coverage increases a second oscillator, attributed to bulk plasma resonances, is required to accurately model the ellipsometric data. Post-deposition optical and electronic changes are observed for island films and the origins of these changes are investigated using the ellipsometric data.

A coating of polyurethaneurea was made from a solution on the surface of metal stents. The influence of cleaning, etching, chemical and ion beam modification (plasma immersion ion implantation) of the metal surface on the adhesion strength of the polyurethaneurea was analysed. Polyurethaneurea films imbedded with tantalum particles as a radiopaque filler maintained their strength and elasticity and produced clear X-ray contrast images of vascular stents

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The L10 transition temperatures for chemical ordering in FePd and FePt intermetallic alloys may be substantially reduced by ion irradiation [1]. Alignment of the strong magnetic axis normal to the surface layer was achieved. Recently, we showed via kinetic Monte Carlo simulations [2] that (i) ion-beam-induced reduction of the L10 transition temperature may be understood in terms of vacancy-assisted atomic ordering and that (ii) superstructure alignment results from a small initial directional short range order (DSRO).
In this contribution (i) we present systematic studies of the ion-irradiation-induced L10 ordering in thin layers, (ii) we predict the evolution of chemical ordering in layers with well-designed initial DSRO, (iii) we study in non-stoichiometric alloys (e.g. Fe1-xPdx) the competition of L10 ordering (FePd) with L12 ordering (Fe3Pd), (iv) we extend our atomistic simulations of ion-irradiation-induced ordering to more complex systems like Heusler alloys and (v) we evaluate the influence of interfaces in nanostructures on the ordering process.
[1] D. Ravelosona, C. Chappert, V. Mathet and H. Bernas, Appl. Phys. Lett. 76 (2000) 236.
[2] H. Bernas, J.-Ph. Attane, K.-H. Heinig, D. Halley, D. Ravelosona, A. Marty, P. Auric, C. Chappert, Y. Samson, Phys. Rev. Lett. 91 (2003) 77203.

Scalability and performance of current flash memories can be improved substantially by novel devices based on Multi-Dot Floating Gate MOSFETs. The multi-dot layer in the very thin gate oxide can be fabricated CMOS-compatibly by ion beam synthesis (IBS). Here, we present both experimental and theoretical studies on IBS of multi-dot layers consisting of Si nanocrystals (NCs). The NCs are produced by ultra low energy Si+ ion implantation, which causes a high Si supersaturation in the shallow implantation region. During post-implantation annealing, this su-persaturation leads to phase separation of the excess Si from the SiO2. Till now, the study of this phase separation suffered from the weak z contrast between Si and SiO2 phases in Transmission Electron Microscopy (TEM). Here, this imaging problem is solved by Energy Filtered Scanning Transmission Electron Microscopy (EFSTEM). Additionally, kinetic lattice Monte Carlo simula-tions of Si phase separation have been performed and compared with EFSTEM images. It has been predicted theoretically that the morphology of the multi-dot Si floating gate changes with increasing ion fluence from isolated, spherical NCs to percolated spinodal Si pattern. These pattern agree remarkably with EFSTEM images. However, the predicted fluence for spinodal pattern is lower than the experimental one. Because oxidants of the ambient atmosphere penetrate into the as-implanted SiO2, a substantial fraction of the implanted Si is lost due to oxidation.

The magnetic domain configuration and the magnetization reversal behavior of micropatterned exchange bias elements were investigated by means of magnetic force microscopy. In addition to the unidirectional anisotropy the shape anisotropy determines the overall magnetization reversal behavior. In order to modify the ratio between both anisotropy contributions the exchange bias field strength was reduced by means of 5 keV He+ ion irradiation. For the as-prepared samples a mono-domain magnetization state with the magnetization direction aligned along the exchange bias field direction was found regardless of the element shape. After irradiation the unidirectional anisotropy contribution is reduced and hence the previously homogeneous magnetization state brakes up into small domains with 360° domain walls in between. The appearance of these domain walls, which was mainly observed for the descending branch of the magnetization reversal, is found to depend strongly on the structure width and orientation.

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Kinetic Lattice Monte Carlo (KLMC) simulations can predict reaction pathways for the ion-beam-assisted fabrication of functional nanostructures. Three examples will be presented, which are based on different kinds of ion-solid-interaction: (i) High-dose ion implantation of species which are immiscible within the surface layer produces a far-from-equilibrium state. A post-implantation thermal treatment activates phase separation. According to predictive simulations the location and size of the nanoclusters can be controlled by boundary conditions (i.e. interfaces) and annealing parameters. (ii)Ion irradiation through an interface between immiscible phases leads to a thin interface film of an unstable alloy which is formed by collisional mixing. KLMC simulations of phase separation in this film predicts a self-organised/self-aligned nanocluster formation . (iii) Ion irradiation can produce chemical order in intermetallic alloys at low temperature. KLMC simulations show that chemical ordering requires mobile vacancies. Without irradiation the formation of vacancies requires high temperatures at which the ordered phase might be unstable. Thus, thermally activated ordering of some intermetallics like Heusler alloys is difficult, whereas ion irradiation orders them.
Process simulations of phase separation in Si implanted gate oxides and at ion irradiated Si/SiO2 interfaces have been performed in the framework of an European GROWTH, where a novel non-volatile memory transistor was developed. Irradiation-induced ordering studies are focussed on future magnetic recording materials (FePt) and materials for spintronics.

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For the generation of nanostructures for novel applications such as in electronics and optics, self-organisation under large-area processing appears to be the most promising way in terms of cost-efficiency. However, so far it has turned out to be extremely difficult to fulfil the requirements of macroscopic-range ordering, narrow particle size distribution, and, in the case of buried nanostructures, in-depth location control. The introduction of ion beams, which are widely established in industrial production, into this area is challenging but hampered by the statistical processes of ion slowing down and the physico-chemistry of precipitation and ripening during ion beam synthesis. These processes will, in general, result in disorded rather than ordered structures.
In the present paper it is demonstrated that, nevertheless, ion beam techniques may contribute to ordering. Different mechanism of ion-driven self-organisation are available for this purpose: (i) For ensembles of nanoclusters in a matrix material, ion-beam mixing may cause an inversion of the Ostwald ripening effect and thus drive the system towards a narrow particle size distribution. (ii) Processes of ion mixing, diffusion and precipitation at interfaces may result in a well-defined location of a nanocluster sheet close to the interface. (iii) Ordering of intermetallic nanostructures may be achieved at low temperature by ion-induced generation of mobile point defects.
For these phenomena, both theoretical predictions and experimental evidence will be presented. In addition, the results will be related to promising applications, such as for new non-volatile memories and novel media for magnetic recording.

In silicon nanocrystal based metal–oxide–semiconductor memory structures, tuning of the electron tunneling distance between the Si substrate and Si nanocrystals located in the gate oxide is a crucial requirement for the pinpointing of optimal device architectures. In this work it is demonstrated that this tuning of the ‘‘injection distance’’ can be achieved by varying the Si1 ion energy or the oxide thickness during the fabrication of Si nanocrystals by ultralow-energy silicon implantation. Using an accurate cross-section transmission electron microscopy ~XTEM! method, it is demonstrated that two-dimensional arrays of Si nanocrystals cannot be positioned closer than 5 nm to the channel by increasing the implantation energy. It is shown that injection distances down to much smaller values ~2 nm! can be achieved only by decreasing the nominal thickness of the gate oxide. Depth profiles of excess silicon measured by time-of-flight secondary ion mass spectroscopy and Si nanocrystal locations determined by XTEM are compared with Monte-Carlo simulations of the implanted Si profiles taking into account dynamic target changes due to ion implantation, ion erosion, and ion beam mixing. This combination of experimental and theoretical studies gives a safe explanation regarding the unique technological route of obtaining Si nanocrystals at distances smaller than 5 nm from the channel: the formation of nanocrystals requires that the interface mixing due to collisional damage does not overlap with the range profile to the extent that there is no more a local maximum of Si excess buried in the SiO2 layer.

Nanocluster ensembles and nanowires can be synthesised in surface layers of various substrates by high dose ion implantation. A detailed understanding of the complex processes like ion deposition and subsequent phase separation has been achieved by atomistic computer simulations. This understanding helps to tailor nanostructures for applications. Examples are the synthesis of very narrow Si nanocluster layers in SiO2 for nanodot memories and of ion-beam-shaped metallic nanorods for photonics. Recently it has been demonstrated that ion beams can be also used to change properties of nanostructures drastically. Thus, at elevated temperatures, irradiation of nanocluster ensembles results in a narrowing of the size distribution, which can be described as “inverse Ostwald ripening”. Irradiation of single-crystalline but chemically disordered nanostructures assists chemical ordering of alloys like FePt, which has a very high magnetic anisotropy and is, therefore, a favorite material for future magnetic recording.

An overview of our recent research on ion beam synthesis and processing of nanostructures will be presented. Predictions of atomistic simulations on reaction pathways of nanostructure formation and processing will be compared with experimental results. (i) delta-layers of nanoclusters were formed by ion beam mixing and phase separation at interfaces. In an industrial environment, these delta-layers have been proven to be good candidates as distributed charge storage centers in future FLASH memories. (ii) Nanowires formed by focussed ion implantation and other techniques can be processed into nanocluster chains and other functional structures for electronics and photonics. (iii) Metallic nanospheres can be shaped by swift heavy ions into rods and wires, which may have interesting applications in photonics.

We present a novel type of ion-beam-induced deformation of metal nano-objects. Under heavy-ion irradiation Au nanospheres in a silica matrix first elongate, and at higher dodes combine into nanowires that continue to grow under the ion beam. Such anisotropically shaped metal nanoparticles may have great potential in a wide range of fields. For example, nanorods exhibit a split plasmon resonance, with one of the bands shifting as far as the infrared. Arrays of such nanoparticles have great potential as nanophotonic guides in the (infra)red, an important telecom wavelength regime, but outside the range of plasmon resonances of spherical particles. Our samples consist of Au spheres (15 nm) in a single plane 150 nm below the surface of the silica matrix. At low dose (2x10^14 cm^-2) the nanospheres elongate into nanorods, with their long axis oriented in the beam direction (also verified by changing the ion incidence angle). At high doses, nanowire form, still parallel with the ion path. This intriguing effect (the wires must have formed from many primary particles) will be discussed in detail, along with the elongation mechanism, based on kinetic Monte Carlo computer experiments. We also observe a clear threshold in the electronic energy loss. This threshold can be explained, assuming that the ion track have to be continuous for elongation to occur.

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Recently it has been demonstrated that ion irradiation of nanostructures, interfaces and ultrathin magnetic films can modify substantially the nanocluster size distribution [1], the spatial nanocluster alignment [2], the nanocluster shape [3] and the chemical order of metal alloys [4]. Furthermore, low-energy ion-erosion of semiconductor [5] and metal [6] surfaces can result in the formation and self-organization of nanostructures.
For all phenomena listed above (disregarding chemical ordering), ion-irradiation-activated interface/surface processes have been identified as the driving force. Thus, a fundamental understanding of the basic mechanisms of the interface/surface evolution under ion-irradiation might allow a controlled growth and a taming of properties of nanostructures.
This contribution will review theoretical studies and atomistic computer simulations which demonstrate that the above listed phenomena have the same origin: a competition of surface erosion or interface mixing on the one hand and diffusional processes on the other. The far-from-equilibrium processing of nanostructures can lead to exotic properties like “negative interface energy” and “inverse Ostwald ripening”.
[ 1] K.-H. Heinig, T. Müller, B.Schmidt, M. Strobel, W. Möller, Appl. Phys. A 77, 17–25 (2003).
[ 2] L. Röntzsch, K.-H. Heinig, and B. Schmidt, Mater. Sci. Semicond. Proc. 7, 357 (2004).
T. Müller, K.H. Heinig, and W. Möller, Appl. Phys. Lett. 81, 2373 (2002).
T. Müller, K.H. Heinig, W. Möller, C. Bonafos, H. Coffin, N. Cherkashin, G. Assayag,
S. Schamm, G. Zanchi, A. Claverie, M. Tencé, C. Colliex, Appl. Phys. Lett. 85, 2373 (2004).
[ 3] K.-H. Heinig, in Proc. Workshop „Ion Beam Shaping of Metal Nanoparticles“, ed. A. Polman, Amsterdam (Netherlands), Dec17 (2004).
[ 4] H. Bernas, J.-Ph. Attane, K.-H. Heinig, D. Halley, D. Ravelosona, A. Marty, P. Auric, C. Chappert, Y. Samson, Phys. Rev. Lett. 91, 077203 (2003).
[ 5] S. Facsko et al., Science 285, 1551 (1999).
[ 6] M. Strobel, K.H. Heinig, T. Michely, Surf. Sci. 486, 136 (2001).
T. Michely, M. Kalff, G. Comsa, M. Strobel, K.H. Heinig, Phys. Rev. Lett. 86, 2589 (2001).

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