Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

At the Forschungszentrum Rossendorf (FZR) the new facility ELBE for research with various kinds of radiation is presently under construction. ELBE is centered around a superconductiong Electron Linac which will produce quasi-continuous beams of high Brillance and low Emittance. Preliminary results of the first stage accelerator tests will be shown. The 40 MeV / 1 mA electron beam will be used to drive free-electron lasers for the production of infrared light in the 5-510µm wavelength range. X-ray beams in the 5-50 keV energy range will be generated using channeling radiation. Additionally, the ELBE facility will provide bremsstrahlung and photoneutron beams for investigations in nuclear physics and technology.

The new radiation source "Strahlungsquelle ELBE" at the Forschungszentrum Rossendorf (FZR) will us the high brilliance electron beam from a linac with superconducting rf-cavities to porduce various secondary beams for experiments in nuclear science, solid state physics, materials research, environmental chemisty and in the life sciences. During the year 2001 all components belonging to the first stage of ELBE (20 MeV) were successfully put into operation. The main beam properties were measured and it was shown that all major design parameters could be reached with the ELBE accelerator. In the low bunch charge mode of operation intended for radiation physics applications a transversal emittance of 3 mm mrad was measured. At the 77 pC maximum bunch charge the emittance is still better than 10 mm mrad, well sufficient for driving the ELBE free-electron lasers. The longitudinal phase space properties are mainly determined by the capture process of the electron bunches into the first accelerator cavity and could, thus, be characterized by measuring the energy spectrum of the beam while scanning the rf-phase of the second cavity. These measurements at 3 pC bunch charge vielded an emittance of 57 keV ps and a bunchlength of 2 ps, in good agreement with theoretical predictions and with bunch length measurements using an autocorrelation technique.

The homogeneous dynamo effect is at the root of cosmic magnetic field generation. With only a very few exceptions, the numerical treatment of homogeneous dynamos is carried out in the framework of the differential equation approach. The present paper tries to facilitate the use of integral equations in dynamo research. Apart from the pedagogical value to illustrate dynamo action within the well-known picture of Biot-Savart's law, the integral equation approach has a number of practical advantages. The first advantage is its proven numerical robustness and stability. The second and perhaps most important advantage is its applicability to dynamos in arbitrary geometries. The third advantage is its intimate connection to inverse problems relevant not only for dynamos but also for technical applications of magnetohydrodynamics. The paper provides the first general formulation and application of the integral equation approach to time-dependent kinematic dynamos in finite domains. For the spherically symmetric alpha^2 dynamo model it is shown how the general formulation is reduced to a coupled system of two radial integral equations for the defining scalars of the poloidal and the toroidal field components. The integral equation formulation for spherical dynamos with general velocity fields is also derived. Two numerical examples, the alpha^2 dynamo model with radially varying alpha, and the Bullard-Gellman model illustrate the equivalence of the approach with the usual differential equation method.

ohne Abstract

A detailed investigation of the emission instabilities and their frequency spectra for Ga, AuGeSi and AuGe Liquid Metal Alloy Ion Sources (LMAIS) was carried out. The ion sources with an extracting electrode (triode scheme) as well as without an extractor (diode scheme) were applied. The ion current coming from the collector electrode was amplified and converted into a voltage. The amplifier has a -3 dB edge at 40 MHz and converting factor of 200 mV/µA. The frequency spectra from 0 to 50 MHz were measured using a HP8591 spectrum analyzer with a resolution of 300 kHz. Simultaneously the corresponding oscillograms of the emission current were obtained using a digital oscilloscope. The measured spectra are correlated with these oscillograms. The current for which pulses appear superimposed on the d.c. level of current is well predicted by existing theory. The pulses are believed to be the result of droplet emission, and their terminal frequency appears to coincide with the frequency of vibrations of the sides of the liquid cone at high currents. The emitted droplets screen the source tip. This leads to decreasing of the effective electric field and fluctuation of the emission process. The following disintegration of the droplets into smaller ones aggravate the noise. The fluctuation depends nonlinearly on the emission current. This dependence shows an S shape, at low emission current the noise is negligible, at 40-60 µA the fluctuation goes up rapidly and higher than 70 µA it comes to a saturation. The onset of the pulses (at about 30 µA) is also predictable from a criterion developed by Mair for the critical current. At 100 µA emission current the typical fluctuation is about 5 µA(rms). The alloy sources were investigated also at temperatures from 400 to 1100 °C and the dependence of the fluctuations of the emission on the temperature were obtained. With increasing temperature a stronger fluctuation was registered. Better understanding of the instabilities that develop on the liquid anode can be useful for deposition purposes.

In Umweltproben sind verschiedene langlebige Actinide von Bedeutung. In Wässern oder Salzlösungen mobilisierte Actinide sind mittels LS auch in Multielementspektren ohne chemische Trennung zuverlässig und schnell nachzuweisen. Die Energien müssen allerdings mindestens eine Halbwertsbreite auseinander liegen, um eine Trennung der Einzelnuklide mittels Spektrenentfaltung [1] zu erreichen. Für die Nuklide ²²⁶Ra (4,78 MeV), ²³⁷Np (4,79 MeV) und ²³³U (4,82 MeV) liegen die Energielinien zur einfachen Bestimmung mittels Peakfitting zu dicht beieinander. Bei der Bestimmung dieser Nuklide werden die Gleichgewichte von ²²⁶Ra und ²³⁷Np mit ihren radioaktiven Zerfallsprodukten ausgenutzt. Das ²²⁶Ra ist Glied der Uran-Radium-Familie. Es entsteht aus ²³⁸U durch Zerfall über Th und Pa. ²²⁶Ra zerfällt in der Reihe: 1600 a ²²⁶Ra (alpha: 4,78 MeV) 3,82 d ²²²Rn (alpha: 5,49 MeV) 3,05 min²¹⁸Po (alpha: 6,00 MeV) 26,8 min ²¹⁴Pb (beta) 19,8 min ²¹⁴Bi (beta) 164 µs ²¹⁴Po (alpha: 7,69 MeV) 22,3 a ²¹⁰Pb (beta) 5,02 d ²¹⁰Bi (beta) 138 d ²¹⁰Po (alpha: 5,31 MeV) st. ²⁰⁶Pb. Im alpha-LS-Spektrum treten folglich mit aufsteigender Energie die Peaks des ²²⁶Ra, des ²²²Rn, des ²¹⁸Po und des ²¹⁴Po auf. Mit zunehmenden Alter des ²²⁶Ra-Präparates bildet sich eine Spur des beta-Stahlers ²¹⁰Pb, aus der dann etwas ²¹⁰Po (5,31 MeV) hervorgeht. Die beta-Strahler erscheinen im LS-Spektrum, werden aber durch beta-Diskriminierung eliminiert, so dass ein reines alpha-Spektrum resultiert. Da das Tochterprodukt ²²²Rn gasförmig ist, kann ein Teil bei der Probenahme entweichen, so dass das radioaktive Gleichgewicht innerhalb der Zerfallskette gestört ist. Nach 8-10 Halbwertszeiten des ²²²Rn, also 5-6 Wochen, ist das Gleichgewicht wieder hergestellt. Die Probe wird also nach etwa 6 Wochen Wartezeit gemessen, wobei die Gesamtimpulsrate der alpha-Aktivität des ²²⁶Ra im Gleichgewicht mit seinen Folgeprodukten entspricht. Der ²²⁶Ra-Anteil wird aus den Peaks der ²²⁶Ra-Folgeprodukte bestimmt, die im Spektrum auf den ²³⁷Np/²³⁴U/²²⁶Ra-Summenpeak, der bei 4,8 MeV liegt, folgen und mittels Peakfitting zugänglich sind. Diese machen im radioaktiven Gleichgewicht mit dem ²²⁶Ra genau 74,9% der alpha-Counts aus, so dass 25,1% auf das ²²⁶Ra entfallen [2,3]. Das ²³⁷Np (2,144·106 a) ist im Gleichgewicht mit seiner Tochter ²³³Pa (26,976 d), das durch alpha-beta-Trennung gut von den alpha-strahlenden Nukliden abgetrennt werden kann. Auf Grund von Einzelmessungen bzw. Berechnungen ist das Aktivitätsverhältnis von ²³⁷Np zu ²³³Pa bekannt. So kann aus dem beta-Peak des ²³³Pa die ²³⁷Np-alpha-Aktivität berechnet werden. Durch Subtraktion des ²²⁶Ra und ²³⁷Np-Anteils des Summenpeaks bei 4,8MeV erhält man den Anteil des ²³³U.

[1] Nebelung, C., Nitsche, H.: Stillegung und Rückbau Direktmessung alpha-aktiver Nuklide in Bauschutt zur Freigabeentscheidung Schlussbericht, BMBF Förder-vorhaben 02S7655, 1999
[3] Baraniak, L., Nebelung, C., Thieme, M.: Report FZR-343 (2002) 48
[4] Nebelung, C.: Report FZR-373 (2003) 53

Ziel dieser Arbeit war die experimentelle und numerische Beschreibung der Kühlmittelvermischung in Druckwasserreaktoren (DWR). Die Kühlmittelvermischung ist für zwei Klassen von Störfällen relevant: Borverdünnungsstörfälle und Kaltwassertransienten. Ausgehend von den Vermischungsphänomenen, die für diese Szenarien bedeutsam sind, wurde die Versuchs-anlage ROCOM (Rossendorf Coolant Mixing Facility) konzipiert und aufgebaut. Die Untersuchung der Vermischung erfolgt anhand eines Tracers (verdünnte Salzlösung) und der Messung der Leitfähigkeit des Fluids. Die numerische Simulation der Kühlmittelvermischung wurde mit dem CFD-Code CFX-4 durchgeführt. Vergleichende CFD-Rechnungen für das Modell und die Geometrie des Originalreaktors vom Konvoi-Typ belegten, dass eine Skalierung im Maßstab 1:5 die Übertragbarkeit der Ergebnisse gewährleistet. An ROCOM wurden umfangreiche Geschwindigkeitsmessungen im Ringspalt des Reaktordruckbehälters (RDB) für verschiedene Schleifendurchsätze und bei Teilschleifenbetrieb durchgeführt und mit CFX-4 Rechnungen verglichen. Bei Vollschleifenbetrieb konnte im Modell eine Unabhängigkeit der Geschwindigkeitsprofile von der Reynoldszahl im Bereich des Nominaldurchsatzes nachgewiesen werden. Besonders aufschlussreich waren, in Übereinstimmung mit CFX-4 Rechnungen, die Messungen transienter Geschwindigkeitsfelder mittels Hochleistungs-LDA-Technik. Damit konnte die Phänomenologie der beschleunigten Strömung aufgeklärt werden, wobei zunächst das Fluid als Ganzes beschleunigt wird, sich später aber große Wirbel im Ringraum herausbilden. Die Experimente an der ROCOM-Anlage zur stationären Kühlmittel-zirkulation bei Betrieb aller Schleifen zeigen übereinstimmend mit CFX-4 Berechnungen eine Beschränkung der Verteilung des eingespeisten Tracers am Kerneintritt überwiegend auf den Quadranten der mit dem Tracer beaufschlagten Schleife. Bei der Simulation von transienten Strömungsvorgängen, wie z. B. dem Anlaufen der Kühlmittelzirkulation, ergibt sich dagegen im Experiment und der korrespondierenden Rechnung ein vollständig anderes Vermischungsbild. Die Strömung teilt sich in eine linke und rechte Komponente um den Kernbehälter auf, so dass die Störung zuerst den Sektor des Reaktorkerns erfasst, der der Schleife gegenüberliegt, über die der Tracer eingespeist wird. Ergänzend wurden Berechnungen zum WWER-440 bei stationärem Durchsatz durchgeführt, die die Anwendbarkeit eines analytischen Ver-mischungsmodells für WWER-440 Reaktoren bestätigen. Für turbulente Strömungen konnte CFX-4 somit anhand von Experimenten validiert werden und steht für weitere Untersuchungen in der Reaktorsicherheitsforschung zur Verfügung. Weiterhin konnte gezeigt werden, dass die Vermischungsmittelwerte im DWR mit Hilfe einer Lösung der eindimensionalen konvektiv-diffusiven Transportgleichung längs des Strömungsweges abbildbar sind. Die Ergebnisse der Experimente, CFD-Rechnungen und des analytischen Modells fließen in ein Modul ein, welches in Computercodes für Störfallanalysen integriert wird. Über eine verbesserte Beschreibung der Vermischung wird dadurch eine realistische Störfallsimulation erreicht.

Zur hochempfindlichen in situ Detektion aquatischer Kolloide mit Partikeldurchmessern < 50 nm wurde im Institut für Radiochemie ein modulares Gerätesystem zur Laser-induzierten Breakdown-Detektion (LIBD) aufgebaut, orientiert an den im Institut für Nukleare Entsorgung des FZK entwickelten LIBD-Systemen. [1-2]
Als gepulste 532 nm-Lichtquelle wird ein diodengepumpter Nd:YAG-Laser eingesetzt. Die Laserpulsenergie wird mit einer Kombination aus Fresnelrhomben und Calcitpolarisator variiert und einem pyroelektrischen Detektor gemessen. Zur Strahlcharakterisierung wird ein kommerzielles Laserstrahlanalysesystem verwendet, das eine 14 Bit-CCD-Kamera, Abschwächungsoptik, PCI-Einschubkarte und Software umfasst. Durchfluss-Küvetten aus Quarzglas werden als Probengefäße benutzt. Die Breakdown-Ereignisse werden akustisch mit einem piezoelektrischen Sensor detektiert und gleichzeitig mit einem Kamerasystem ortsaufgelöst registriert. Systemsteuerung, Datenerfassung und –verarbeitung erfolgen über ein digitales Oszilloskop sowie zwei PC.
Test- und Kalibriermessungen erfolgten an wässrigen Dispersionen von Polystyrolpartikeln. Erste Anwendungen befassten sich mit der Charakterisierung von realen Wässern.
Das Gerätesystem wird ausführlich vorgestellt, die Ergebnisse der Kalibriermessungen, der Messungen an realen Wässern sowie erster Untersuchungen zur Kolloidbildung von U(IV) werden mitgeteilt und diskutiert.

Literatur:

[1] T. Bundschuh, W. Hauser, J.I. Kim, R. Knopp, F.J. Scherbaum, Colloid. Surf. A 180, 285, (2001)
[2] C. Walther, C. Bitea, W. Hauser, J.I. Kim, F.J. Scherbaum, Nucl. Instr. and Meth. B 195, 374, (2002)

Die Parameter des Elektronenstrahls an der Strahlungsquelle ELBE des Forschungszentrums Rossendorf sind u. a. eine variable Elektronenenergie zwischen 12 und 40 MeV, 1 mA maximaler Strom, eine maximale Frequenz von 13 MHz, eine Pulslänge kleiner als 10 ps und eine geringe Emittanz. Dies führte zu der Idee, die kurzen und intensiven Elektronenpulse in kurze Neutronenpulse umzuwandeln, um an einem Flugzeitspektrometer energieabhängige Neutronenwirkungsquerschnitte zu messen bei einem kurzen Flugweg von nur einigen Metern. Hauptziel ist dabei die Messung solcher Querschnitte für Konstruktionsmaterialien von Fusions- und Spaltreaktoren und für Abfälle aus solchen Reaktoren. Aufgrund der sehr hohen Energiedeposition durch die Elektronen im Neutronenradiator wurde flüssiges Metall (Blei) als Neutronenradiator gewählt. Für das Wandmaterial ist Molybdän der Hauptkandidat, für den thermomechanische und neutronenphysikalische Rechnungen vollständig gemacht wurden. Mit den Monte Carlo Codes MCNP und FLUKA wurden die Teilchentransportrechnungen ausgeführt. U.a. wurden aus Berechnungen des Neutronenstrahls und des Untergrunds am geplanten Messplatz Schlussfolgerungen für die Konstruktion und Materialauswahl der Photoneutronenquelle gezogen. Die Berechnung des Neutronenpulses am Messplatz zeigt, dass die Neutronen von den Photonen separiert werden können, sich die Neutronenpulse jedoch stark überlappen. Wird z. B. nur jeder vierte Elektronenpuls verwendet, so beträgt der Neutronenfluss am Messplatz zirka 2.5×10⁶ n/cm²/s und der nutzbare Energiebereich 660 keV bis 6 MeV bei den Parametern 3.6 m Flugweg, 11.2×11.2 mm² Querschnitt des Radiators, 30 MeV Elektronenenergie und 1 mA Elektronenstrom. Das FZR arbeitet an einer Elektronenquelle mit kleinerer Pulsfrequenz bei gleichem mittleren Strom von 1 mA. Die Energieauflösung am Messplatz liegt unter 1%.

The irradiation induced microstructure for two reactor pressure vessel (RPV) steel types after irradiation at different neutron exposure levels and annealing treatments were investigated by means of small-angle neutron scattering (SANS).
The radiation susceptibility changes with the type of steel. Therefore the role of particular material parameters in defect evolution was worked out. The characteristics of the irradiation-induced defects such as number density, size distribution, and volume content were determined and potential defect compositions were estimated. The type of RPV steel influences the volume content but hardly the size distribution. The evolution of the defect content for a particular steel is not linear in response to the irradiation damage described in displacements per atom. The defect type is not uniform. The average composition of defects is partly changed with increasing dpa-doses. Additionally, the redissolving behaviour of irradiation induced defect clusters was studied after gradual thermal treatments.
The analysis bases on the hypothesis that the radiation defects are not-ferromagnetic. The consistency of the results supports the correctness of the hypothesis.

There is a hypothesis which postulates the microstructural radiation defects as effective traps for hydrogen and this deposited hydrogen as a stabilizer of these features. Thus, the higher the content of hydrogen the stronger is the embrittlement. The H-trap hypothesis should be examined by a SANS experiment.

The measurement has shown that we can detect H-deposition (> 1x10-4 wt.-%) by SANS.

New oxorhenium complexes with 2-(diphenylphosphanyl)-N-(2-thioethyl)benzamide (H₂PNS) and trimethyl-, triethyl- and triphenyl-hydroxyl silylated monodentate thiols are reported. These new complexes have been prepared by reacting [NⁿBu₄][R-e(O)Cl₄] with the tridentate H₂PNS and the corresponding silylated thiol at room temperature. The characterization of the complexes involved elemental analysis, ³¹P and ¹H NMR spectroscopies and X-ray crystallographic analysis for the triethyl-silylated Re complex.

Heavy metals like actinides in the aquatic environment are normally transported as a complexed species. Knowledge about the complex formation is therefore essential for prediction of the migration of these elements.
Spectroscopic methods have the advantage to be non-invasive and non-destructive. The high intensity of laser light sources allows the excitation of all species in the illuminated volume. Therefore laser-based methods reach low detection limits.
Mainly Laser-induced Photoacoustic Spectroscopy (LIPAS) and Time-resolved Laser-induced Fluorescence Spectroscopy (TRLFS) were used for such speciation studies. The introduction of lasers with pulse durations in the pico- and femtosecond range in combination with short gated CCD-cameras enables studies of fluorescing species with fluorescence lifetimes of some nanoseconds.
As uranium, americium and curium show fluorescence properties time-resolved laser-induced fluorescence spectroscopy enables the determination of species up to detection limits of 10-8 M and less. Besides the study of the complex formation of uranium (sulfate, phosphate, arsenate) we determined the formed species in several mining related waters as function of pH. The change in the speciation of uranium(VI) is shown to be in agreement with calculations.
Absorption spectroscopy can be used as tool for direct determination of actinide species. However the concentration of actinides in the natural environment is much lower than the detection limit of conventional UV-VIS spectroscopy. Using laser-induced photoacoustic spectroscopy the detection limit can be decreased by about three orders of magnitude. Studies of the complex formation of uranium(IV) with phosphate and arsenate will be presented, demonstrating the advantage of direct speciation methods.
Polyelectrolytic organic macromolecules, like humic substances, are important complexing agents towards actinide metal ions. The investigation of simple model ligands would give a more detailed description of the binding behavior of humic substances. Especially the different bonding of carboxylic and phenolic group and the discrimination between them is of interest to get more detailed information about the complex formation. The various hydroxy benzoic acids allow the investigation of these differences.
To study the complex formation of actinides with such organic ligands we used the new, recently developed femtosecond laser-induced time-resolved fluorescence spectroscopy. The method uses the fluorescence properties of organic ligands with p-electron systems and their change by interaction with actinide ions to study the complex formation of these systems.

Commisioning and operating the ELBE accelerator requires exact knowledge and easy, reproduceable control of various parameters. Important parameters like beam position and beam current are measured with a system consisting of a stripline BPM (beam position monitor), conversion electronics, data acquisition and monitoring software. The design of the electronics is described in this paper

The ELBE accelerator needs a non-destructive system for measurement of the electron beam position at about 30 location. To obtain the required resolution about 100mm a system of stripline beam position monitors (BPM) is under design. The stripline BPM can work in full power mode almost without influence on the beam. To study the basic operational principle of the stripline BPM and to get information about BPM signal behavior we did a set of measurements on the ELBE injector. The resolution of the BPM was measured as a function of the electron beam current; at an electron beam current of 1mA the resolution is about 30mm. One of the BPMs will be installed upstream the cryomodule. Since the power spectrum of the beam depends on the bunch length, measurements in the frequency domain give us information about the length of the bunch entering the cryomodule with RF cavities. The BPM will serve for setting correct phase and power of both bunchers of the injector and for measurements of the electron beam position at the entrance of the cryomodule. There are two alternative schemes of the BPM electronics under development. The first one is based on the logarithmic amplifier AD8313, another one on the an rms-responding true power detector AD8361. Both of them can process RF signals with frequencies up to 2.5GHz. The BPM electronics will operate at the fundamental frequency of the accelerator which is 1.3GHz.

1 FZR, , Germany
2 TJNAF, Newport News, Virginia, USA

The radiation source ELBE is based on a superconducting LINAC. Initially it was designed to operate in CW mode with repetition rates either 13 MHz either 260 MHz. Later it was decided to operate the accelerator with reduced repetition rates for diagnostic reasons and for certain users. Now it is possible to operate with bunch frequency 13/n MHz, where n can be 2,4,8,16,32,64 and 128. It is required that the BPM system supports any of these operation modes. A core element of the BPM electronics is a logarithmic detector AD8313 made by Analog Devices Inc. The logarithmic detector is a direct RF to DC converter rated up to 2.5 GHz. Initial design of the BPM electronic was sophisticated only for CW operation with repetition rate more than 10 MHz, since bandwidth of the AD8313 is about of 10 MHz. Additionally a sample and hold amplifier is built in to provide enough time for an ADC to make measurements. The sample and hold amplifier is synchronized with a bunch frequency. In the paper we present results of the modified BPM electronics test.

The radiation source ELBE at Forschungszentrum Rossendorf is based on a superconducting linear accelerator that produces a CW electron beam up to 40 MeV and 1 mA. In 2001 the first stage of the ELBE LINAC was put into operation. The main electron beam parameters like energy, transverse emittance, bunch length and energy spread were determined. Moreover, optimal machine parameters were specified and several online diagnostics were tested, such as beam position monitors and beam-loss detectors.

n the research center Rossendorf, the radiation source ELBE based on a super conducting LINAC is under construction. In the year 2001 the first accelerating module was commissioned. The electron beam parameters like emittance, bunch length, energy spread weremeasured. Here we present results of the measurements as well as the methodics used to make the measurements. In the ELBE injector, where electron beam energy is 250 keV, the emittance was measured with the help of multislit device. Emittance of the accelerated beam was measured by means of quadrupole scan method and is 8 mm×mrad at 77 pC bunch charge. Electron bunch length was measured using coherent transition radiation technique. At the maximum design
bunch charge of 77 pC the RMS bunch length was measured to be 2 ps. A set of online diagnostic systems is also under development. One of them a system of stripline beam position monitors is also described here. A BPM resolution of about 10 µm was achieved using logarithmic amplifier as the core element of the BPM electronics. A system of beam loss monitors based on the RF Heliax cable working as an ionization chamber is intended to be another online diagnostic system.

At the Forschungszentrum Rossendorf (FZR), the superconducting electron linear accelerator ELBE is under construction. It will deliver an electron beam with an energy of up to 40 MeV at an average beam current of up to 1mA. The accelerator uses standing wave DESY type RF cavities operating at 1.3 GHz. A non-destructive system for the measurement of the beam position at about 30 locations is needed. To obtain the required resolution of 100 mm, a system of stripline beam position monitors (BPM) is under design.

A comparison between ultra-fast X-ray CT and a wire-mesh sensor is presented. The measurements were carried out in a vertical pipe of 42 mm inner diameter, which was supplied with an air-water mixture. Both gas and liquid superficial velocities were varied. The X-ray CT delivered 263 frames per second, while the wire-mesh sensor was operated at a frequency four times higher. Two different gas injectors were used: 4 orifices of 5 mm diameter for creating large bubbles and gas plugs and a sintered plate with a pore size of 100 µm for generating a bubbly flow. It was found that the wire-mesh sensor has a significantly higher resolution than the X-ray CT. Small bubbles, which are clearly shown by the wire-mesh sensor, cannot be found in the CT images, because they cross the measuring plane before a complete scan can be performed. This causes artifacts in the reconstructed images, instead. Furthermore, there are large deviations between the quantitative information contained in the reconstructed tomographic 2D distributions and the gas fractions measured by the sensor, while the agreement is very good, when the gas fraction is obtained by a direct evaluation of the X-ray attenuation along the available through-transmission chords of the tomography setup. This shows that there are still potentials for an improvement of the image reconstruction method. Concerning the wire-mesh sensor it was found, that the gas fraction inside large bubbles is slightly underestimated. Furthermore, a significant distortion of large Taylor bubbles by the sensor was found for small liquid velocities up to 0.24 m/s. This effect vanished with growing superficial water velocity.

Positron emission tomography (PET) is currently the only technically feasible method for the non-invasive and in situ control of the precision of the irradiation in highly conformal heavy ion tumour therapy. ...

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) is a useful tool to study the speciation of uranyl in solution and at mineral surfaces. Sorption experiments of U(VI) adsorbed on ferrihydrite samples were carried out under ambient atmosphere for pH 5.8 and 7.8 in 0.1 N NaClO₄ solution. A ferrihydrite suspension with an iron concentration of 1 mM was used. The total U(VI) concentration used in these experiments were 5·10^-5, 1·10^-5, and 1·10^-6 M. The contact time of U(VI) with the mineral was two to three days.
TRLFS measurements in the ferrihydrite system were carried for the supernatant. Characteristic fluorescence spectra of the remaining U(VI) in solution were found for the samples with 5·10^-5 and 1·10^-5 M at pH 5.8. No uranium fluorescence signal could be obtained for all samples at pH 7.8 indicating that the sorption at this pH was stronger than at pH 5.8. The emission bands for the detected fluorescence signal of the two pH 5.8 samples were found at 486, 502, 522, and 546 nm. These emission bands and the determined respective lifetime of 28500 ± 1000 ns and 4600 ± 100 ns are indicative for UO₂OH^-5 and UO₂(OH)₂.
Batch sorption experiments of U(VI) adsorbed on gibbsite samples were carried out under ambient atmosphere for pH 3.5 to 9.5 in 0.1 N and 0.01 N NaClO₄ solution. The solid concentration for the gibbsite samples was 12.5 g/L. The total U(VI) concentration used in these experiments was 1·10^-6 M. The gibbsite was separated from solution by centrifugation at 3500 rpm for 15 minutes. The residue was then taken up with a solution of the same ionic strength and pH and subsequently measured the suspension by TRLFS. This procedure guarantees that the detected signal was exclusively attributed to adsorbed U(VI) on the gibbsite surface.
The detected emission bands for the adsorbed U(VI) surface species on gibbsite were observed at 495, 518, 543, and 568 nm. The fluorescence spectrum was best described with one lifetime of 2400 ± 660 ns indicating that one U(VI) surface species on gibbsite has formed.

We report a X-ray diffraction and Fourier transform infrared spectroscopy study of highly strain compensated InxGa1-xAs/AlAs/ InyAl1-yAs multiple quantum well structures grown pseudomorphically grown on InP substrate by gas-source molecular beam epitaxy. X-ray diffraction patterns show compositional grading of the interfaces that markedly influence the absorption spectra. The intersubband transitions at wavelengths shorter than 2.0 mm for QW structures with different InGaAs well thickness are presented.

Porous silicon (PS) has a great potential in optical applications due to its tuneable refractive index. In particular, multilayer structures consisting on PS layers with different refractive indexes can be used as interference filters. Due to the characteristics of PS and its production process, many types of interence filters can be produced: Bragg reflectors, monochromatic filters for light emitting devices (which can also be based on PS), microcavities for biosensing applications, Fabry-Pérot resonators, photonic crystals, etc.

In the present work the optical properties of PS single layers and multilayer structures were studied. Since the refractive index of PS varies depending on the air content of the porous matrix, the PS structures were modelled as an homogeneous mixture of silicon and air (and, eventually, silicon dioxide), according to the effective medium theories (EMTs). By adjusting the refractive index and thickness of each individual layer, we can obtain a stack of PS layers with the desired optical properties, resulting in interference filters of predetermined band width.

The optical characterization has been carried out by spectrophotometric and spectroscopic ellipsommetry measurements. In addition, compositional analysis has been performed by means of Ion Beam Analysis (IBA) techniques, with special interest devoted to oxygen and hydrogen, in order to correlate the optical parameters and the chemical composition. Finally, multilayers structures have been produced and their operation has been checked.

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Cubic boron nitride (c-BN) films with low stress have been produced by simultaneous 35 keV N+ ion implantation during growth by Ion Assisted Sputtering (IAS). The stress release is achieved at the lost of a decrease in the c-BN content. Despite this fact, films with high c-BN content and relatively large thickness (~0.4 mm) have been successfully produced. The decrease on the c-BN content is discussed in terms of the damage induced by the medium energy ion implantation.

Ion beam assisted deposition (IBAD)techniques have been employed to produce thin films composed of boron, carbon, and nitrogen atoms, including amorphous carbon (a-C), carbon nitride (CNx) and ternary compounds (BCxNy). The films were deposited by evaporating either graphite or boron carbide (B4C) targets, with simultaneous ion bombardment from a precursor N2 +CH4+Ar gas mixture. The composition and bonding structure of the films have been carefully studied, including the analysis with time of flight elastic recoil detection analysis (TOF-ERDA) and X-ray absorption near-edge spectroscopy (XANES). Mechanical characterisation of the films has been also performed, including measurements of hardness, elastic modulus, and friction coefficients. The optimal values encountered are hardness of B35 GPa, and friction coeffivients of B0.05. Also, the thermal stability of the films has been examined by annealing under vacuum conditions. The applicability of the coatings is discussed in terms of all these parameters.

We report on the production of nanoscale patterning on Si substrates by low-energy ion-beam sputtering. The surface morphology and structure of the irradiated surface were studied by atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM). Under ion irradiation at off-normal incidence angle (~50◦), AFM images show the formation of both nanoripple and sawtooth-like structures for sputtering times longer than 20 min. The latter feature coarsens appreciably after 60 min of sputtering, inducing a large increase in the surface roughness. This behaviour is attributed to the preferential direction determined on the substrate by the ion beam for this incidence angle, leading to shadowing effects among surface features in the sputtering process. Under irradiation at normal incidence, the formation of an hexagonal array of nanodots is induced for irradiation times longer than 2 min. The shape and crystallinity of the nanodots were determined by HRTEM. At this incidence angle, the surface roughness is very low and remains largely unchanged even after 16 h of sputtering. For the two angle conditions studied, the formation of the corresponding surface structures can be understood as the interplay between an instability due to the sputtering yield dependence on the local surface curvature and surface smoothing processes such as surface diffusion.

The isopeptide N^Epsilon-(Gamma-glutamyl)-L-lysine 4 was labelled with ¹⁸F via N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB). A modified approach for the convenient synthesis of [¹⁸F]SFB was used, and [¹⁸F]SFB could be obtained in decay-corrected radiochemical yields of 44-53% (n=20) and radiochemical purity >95% within 40 min after EOB. For labelling N^Epsilon-(Gamma-glutamyl)-L-lysine with [¹⁸F]SFB the effects of isopeptide concentration, temperature, and pH were studied to determine the optimum reaction conditions. The coupling reaction was shown to be temperature and pH independent while being strongly affected by the isopeptide concentration. Using the optimized labelling conditions, in a typical experiment 1.3 GBq of [¹⁸F]SFB could be converted into 447 MBq (46%, decay-corrected) of [¹⁸F]fluorobenzoylated isopeptide within 45 min, including HPLC purification.

Im kernnahen Bereich verändert der Reaktordruckbehälter unter dem Einfluss der Neutronenbestrahlung seine Eigenschaften. Das Phänomen ist als Neutronenversprödung bekannt und insbesondere für die Reaktoren vom Typ WWER von hoher sicherheitstechnischer Relevanz. Zur Vertiefung des Verständnisses und zur quantitativen Beschreibung der Zusammenhänge zwischen Gefüge, mechanischen Eigenschaften und der Strahlenbelastung wurden an WWER-Reaktordruckbehälterwerkstoffen Mikrostrukturuntersuchungen durchgeführt.

RF photo cathode electron guns are the source of choice for most high-performance accelerator systems. Their advantages are the ability to produce very bright beams of electrons with high bunch charges and small transverse and longitudinal emittance. At present, their drawback is the limited average current since the high RF fields in the cavities require a pulsed operation. On the other hand, advanced light sources uses linear accelerators with superconducting RF cavities which can operate in the cw mode and produce high average current electron beams. A preferred injector for these accelerators would be a photo electron gun with superconducting RF cavities.
In a research project [1], a superconducting RF photo electron gun (SRF gun) has been developed during the last years at the Forschungszentrum Rossendorf. The SRF gun was put successfully into operation and the efficiency of the concept could be demonstrated. A helium bath cryostat was used to cool down the cavity to 4.2 K. In a daily cycle the cryostat was filled up with liquid helium in the morning followed by a measuring time in the afternoon. The resonator is a niobium half cell with TESLA geometry operating at 1.3 GHz. The photo cathode had a Cs2Te layer deposited in a preparation chamber connected with the gun. In contrast to the cavity the cathode is not superconducting. It is thermally isolated from the cavity and its heat load goes in a special nitrogen cooling system. The cathode was illuminated by an UV laser of 263 nm with a micropulse frequency of 26 MHz and a maximum energy per pulse of 40 nJ. For evaluation of the produced electron beam a beam line is installed with two solenoids, insertable mask and view screens, spectrometer magnet, kicker cavity and Faraday-cup. Measurements of beam parameters and operation experience are reported.

1] E. Barthels et al., Nucl. Instr. and Meth. A 445 (2000) 408

The ELBE accelerator, an electron linac with superconducting cavities, is designed for a beam energy of 40 MeV, a cw current up to 1 mA and a maximum bunch charge of 77 pC. After acceleration, the electron beam will be used to generate a variety of electromagnetic radiation with different wavelength. Two FELs will deliver coherent infrared radiation, monochromatic x-rays will be produced by means of a channeling target and MeV photons with a bremsstrahlung target. Furthermore, converter targets for neutron and positron production will be installed. For the first commissioning tests, the injector with a thermionic gun, one cryomodule with two nine-cell cavities and a diagnostic beam-line were in operation. Energy and energy width were determined with a magnetic spectrometer. Transverse emittance measurements were carried out with a pepper-pot mask in the injector and with the quadrupole scan method for the accelerated beam. Bunch length was determined from the autocorrelation function of the coherent transition radiation with a Martin-Puplett interferometer. The results of these beam parameter measurements will be presented and evaluated

In the last year at the Forschungszentrum Rossendorf, in cooperation with the Budker Institute Novosibirsk, DESY, the Max-Born-Institut Berlin, and ACCEL, a superconducting rf photoelectron gun (SRF gun) has been developed [1]. On March 5, 2002 this gun is going into successful operation. A half-cell niobium cavity has been cooled down to 4 K. In this cavity a CsTe2 photocathode is installed, isolated electrically and thermally by a vacuum gap. The Q value and the accelerating field strength in the cavity have been measured. For 4 K, Q=2x108 and an accelerating field of 14 MV/m have been obtained. This value did not change during four weeks of operation time. The energy of the electron beam is 0.62 MeV and the bunch charge is 26 pC. The laser works with a duty factor of 25% and with a repetition rate of 26 MHz inside the macropulse. The average laser power in the macropulse is 1.5 W (l=260 nm), and the length of the micropulse is 5 ps FWHM. This corresponds to an average electron current of 170 µA and 680 µA in the macropulse. The isolated cathode allows the separate measurement of the cathode current and the current inside the beam dump as a function of the laser phase. The next measurements are planned for late May.

[1] E. Barthels et al., Nucl. Instr. and Meth. A 445 (2000) 408

Last year the first ELBE accelerating module was commissioned. During the commissioning the electron beam parameters such as transverse emittance, energy spread and bunch length were measured. Each of them was studied at different bunch charges as a function of RF field phase in the first accelerating cavity. Especially for an accelerator like ELBE, which is intended to be a driver for free electron laser (FEL), bunch length measurement in picosecond range becomes very important and impose some challenge. Coherent transition radiation (CTR) technique was used to measure bunch length. This technique uses the Martin-Puplett interferometer to measure the autocorrelation of the CTR pulse yielding a minimum 2 ps RMS bunch length at 77 pC bunch charge. Short description of the method, experimental setup, data evaluation procedure and results of the measurements will be presented.

By December 2001 we concluded the first operational period of the ELBE accelerator. The achieved results and measured parameters of the electron beam are presented here. Although the full beam energy of 20 MeV for the first accelerator stage has been reached, most measurements were performed at 12 MeV beam ernergy with varying beam currents and bunch charges. The nominal bunch charge of 77 pC was reched and the measured beam parameters (energy spread, bunchlength and transverse emittance) meet the previous predictions.

The ELBE project is intended to produce a wide spectrum of different radiation and neutrons, especially coherent light in the medium and far infrared by means of a free electron laser. The driver of the radiation source is a super-conducting electron linear accelerator designed for a wide range of beam parameters in order to satisfy the various requirements.
Recently, the forst cryostat containing two nine-cell niobium standing wave rf cavities for an acceleration up to 20 MeV has been put into operation and has been cooled down. For the first time, an accelerated electron beam has been produced successfully.
The helium cooling for the cryostat working at 1.8 K, the electron beam injector, the radio frequency system, beam control and beam diagnostics operate very reliably. In the paper the results of the beam parameter measurements and the experiences gained in the first operation period will be presented.

Als Bestandteil der Nebengesteine von Uranlagerstätten und potentiellen Endlagern für radioaktive Abfälle stellen Feldspäte eine mögliche Mineraloberfläche für die Sorption von mobilisierten Uran(VI) dar. Durch die Wechselwirkung zwischen Feldspat und der wässrigen Phase kann es neben der Adsorption von Uran(VI) zu Mineralauflösung und -neubildung kommen. Ziel der Arbeit ist es, mittels EXAFS-Spektroskopie (Extended X-ray Absorption Fine Structure) und TRLFS (Time-Resolved Laser-induced Fluorescence Spectroscopy) die Sorptionsmechanismen von Uran(VI) an Albit in Abhängigkeit von Mineralauflösungs-prozessen aufzuklären.

Die U(VI)-Adsorption an Albit, Na_0,86K_0,09Ca_0,05[Al_1,05Si_2,95O₈] /1/, der Korngröße 2-6,3 µm (BET: 2,2 m²/g) wurde in 0,01 M NaClO₄- Lösung im batch- und Durchflussversuch untersucht. Im Batchversuch erfolgte die U(VI)-Adsorption bei pH 6,4 und 1·10^-5 M U innerhalb einer Sorptionsdauer von 24 Stunden. Die Durchflussversuche erfolgten ohne vorherige Equilibrierung des pH-Wertes mit U(VI)-Konzentrationen von 5·10^-6 und 2·10^-5 M bei pH 5,8 über einen Zeitraum von 170 Tagen. Zusätzlich wurde Albit bei 40°C über mehrere Monate in 0,01 M HClO₄ sauer aktiviert. Die entstehende siliziumreiche Oberflächenschicht (leached layer) wurde duch Ultraschall abgetrennt und angereichert. An diesem sauer aktivierten Albit <2 µm (BET: 21,6 m²/g) wurde U(VI) bei pH 5,8 (2·10^-5 M U) und 5,0 (1·10^-4 M U) adsorbiert. Die Sorptionsproben wurden als feuchte Paste für die Messung der zeitaufgelösten Fluoreszenzspektren (TRLFS) und Röntgenabsorptionsspektren (EXAFS: ESRF, ROBL) präpariert.

Die Anpassung der EXAFS-Spektren ergeben für die U(VI)-Sorption an Albit im Batch und Durchflussversuch einen kurzen Abstand der äquatorialen Sauerstoffe O_eq von ca. 2,35 Å. Der kurze Abstand und der hohe Debye-Waller Faktor weisen auf eine inner-sphere Oberflächenkomplexierung hin. Bei Uran(VI) adsorbiert an sauer aktivierten Albit ist die äquatoriale Sauerstoffschale in zwei Schalen bei 2,23 Å und 2,44 Å gesplittet, was als die Präsenz einer inner-sphere Oberflächenspezies interpretiert werden kann. Bidentat koordiniertes Silizium wurde mit der EXAFS-Spektroskopie nicht beobachtet, die Detektion monodentater Uran(VI)-Silikat Koordinationen ist jedoch wegen destruktiv interferierender Mehrfachstreupfade stark eingeschränkt. Uran-Uran Rückstreuungen wurden nicht beobachtet, eine Präzipitation von Uran(VI)phasen ist demnach kein dominierender Sorptionsmechanismus. Die TRLFS zeigt für U(VI) sorbiert an Albit (Durchflussversuch) zwei Fluoreszenzspezies mit t₁ = 1 µs und t₂ = 10-20 µs, während bei U(VI) adsorbiert an sauer aktivierten Albit nur eine Fluoreszenzspezies beobachtet wurde, deren Spektrum und Lebensdauer t₂ entspricht. Das Spektrum von Fluoreszenzspezies t₂ ist dem von Uran(VI) adsorbiert an Silica ähnlich /2/. Sowohl die EXAFS- als auch die TRLFS-Ergebnisse zeigen, dass die Sorption von Uran(VI) an Albit bevorzugt als inner-sphere Oberflächenkomplexierung am Silikattetraeder stattfindet.

/1/ Zorn T. (2000) Dissertation TU Dresden.
/2/ Gabriel et al. (2001) J. Colloid Interface Sci. 239, 358-368.

Implantation-induced excess vacancies and the related Cu gettering was studied in Si and in the solid solution Si0.93Ge0.07. The excess vacancy and interstitial generation during normal and inclined ion implantation can be simulated by a simple binary collision process. The excess defects are shown to control the Cu gettering observed in Si after rapid thermal annealing at a temperature of 900oC. The vacancy concentration in the SiGe layer is higher than in Si. An additional vacancy concentration of about 3.15´1018cm-3 was determined in the SiGe layer beside the implantation-generated excess vacancies. These vacancies are presumably incorporated by the SiGe layer deposition on Si substrate. Vacancies agglomerate and form cavities during annealing. The cavities in SiGe were found to be significantly larger than in Si.

The neutron-deficient nucleus ⁶⁶Ge was populated at high spin in two experiments using the reaction ⁴⁰Ca(³²S, α2p) at beam energies of 105 and 95 MeV. In the first experiment, a self supporting ⁴⁰Ca target was used, while a gold-backed target of similar thickness was used in the second experiment. αγ rays were detected with the EUROBALL array, combined with the charged-particle detector array EUCLIDES and the Neutron Wall. The level scheme of ⁶⁶Ge was extended up to E=18MeV and I=(23^- ). Above angular momentum 10⁺, we found two sequences, connected by energetically staggered I=1 M1 transitions. The total Routhian surface calculations describe ⁶⁶Ge at lower spins as a αγ-soft nucleus a having moderate deformation of β₂=0.23, while a triaxial deformation is predicted for the band structures above I^π=10⁺. To our knowledge, this is the first observation of staggered M1 transitions in a deformed four-quasiparticle π(g² _9/2)νg² _9/2 ) structure.

precipitation formation in Al4Cu by ion irradiation

For a thermionic electron gun working at 250 kV and producing bunches up to 85 pC, the measurement of the transverse emittance shows an increase with bunch charge. In order to understand this effect numerical simulations have been performed. The influences of the electrode geometry, space charge and of the control mesh in front of the cathode are included in the simulation. The results show that the growth of the transverse emittance for higher bunch charges is mainly caused by the control mesh. The strong acceleration field penetrates through the mesh holes and for the electrons each hole acts as a small lens. The portion of electrons which are deflected increases with electron current. In the numerical simulation the field and electron trajector calculation was performed with the EGUN code [1]. For the electron transmission throught the potential barrier of the mesh the free electron gas approximation with a local penetration probability, a Maxwell energy distribution and an angular distribution according to Lambert´s law was applied.

1. W.B. Herrmannsfeldt, Report SLAC-331-UC-28, Stanford University, 1988

The β decay of ⁹⁶Ag (Z=47,N=49) was investigated by measuring positrons, X rays as well as β-delayed protons and γ rays. The γ radiation was studied by means of germanium detectors and a NaI total-absorption spectrometer. Two β-decaying isomers in ⁹⁶Ag were established with half-lives of 4.40(6) and 6.9(6) s and tentative spin–parity assignments of (8⁺) and (2⁺), respectively. For both isomers, the intensities of β transitions to low-lying levels of ⁹⁶Pd (Z=46,N=50) and β-delayed proton decays to levels in ⁹⁵Rh (Z=45,N=50) were measured. Several new ⁹⁶Pd levels were firmly established. The level energies, their γ decays and the Gamow–Teller decay of ⁹⁶Ag are compared to shell-model predictions. A new low-lying level in ⁹⁵Rh was found at 680 keV excitation energy. Through a comparison with low-lying states of N=50 isotones, this level is interpreted as the first excited 7/2⁺ state built on the proton 9/2⁺ ground state. The assignments of further excited states in ⁹⁵Rh are discussed.

kein Abstract

Microorganisms and their biocomponents affect the mobility and toxicity of radionuclides through solubility and speciation changes, biosorption, bioaccumulation or other bio-transformations. In this paper we report structural information on the uranium complexes formed by different bacterial isolates from uranium mining waste piles and by their purified biocomponents. EXAFS fits indicate that for B. sphaericus JG-A12 U(VI) is coordinated to carboxyl groups in a bidentate arrangement (RU-C= 2.91 ± 0.02 Å), and to phosphates in a mondentate fashion (RU-P = 3.59 ± 0.02 Å). For the other bacteria strains belonging to A. ferrooxidans, S. maltophilia, P. stutzeri, and P. migulae species U (VI) was bound only to phosphate (RU-P = 3.63 ± 0.02 Å). Because of the ability of the strain B. sphaericus JG A12 to bind selectively and reversibly U and other heavy metals, it was chosen for construction of so called biological ceramics (biorers) for bioremediation of metal-polluted waters. The sorption of U(VI) by the biocers occurs mainly via silicate and phosphate groups. The SiO2-matrix alone binds U (VI) via hydroxyl (water molecules) and silicate groups.

Uranium mining waste pile environments are rich in uranium and heavy metals. In addition, they are a reservoir of a large variety of metal-resistant microorganisms. These natural microbial populations are able to influence dissolution and/or precipitation of actinides. The overall goals of this work are to identify the key interactions between uranium and the strains JG-2 and JG-7B isolated from a uranium mining waste pile situated near the town of Johanngeorgenstadt in the Southeast of Germany. Phylogenetic analysis of these strains revealed that they are related to Stenotrophomonas maltophilia and Bacillus sphaericus. As estimated by using ICP-MS these natural isolates possess high ability to accumulate uranium and other heavy metals such as Cd, Pb, Ni and Ag. The Extended X-ray Absorption Fine Structure (EXAFS) analysis of the uranium complexes formed by these isolates demonstrated that the U(VI) is coordinated to phosphate groups in a monodentate fashion with an average distance between the U atom and the P atom of 3.59 ± 0.02 Å. Transmission Electron Microscope (TEM) and Enery-Dispersive X-ray (EDX) analysis demonstrated that the uranium accumulated by S. maltophilia JG-2 is located at the cell wall and in the cytoplasm. In the case of B. sphaericus JG-7B the uranium was located only in the cell wall. In addition, we applied TRLFS in order to determine the energy and the shape of the emission bands and the fluorescence lifetime of the uranium complexes formed by the two different bacterial strains studied in this work. The results may ultimately help to develop effective methods for remediation of heavy-metal-polluted environments.
This work was supported by grant No.03I4004B from the Bundesministerium für Bildung und Forschung, Germany.

An important factor to influence the microstructure of a metallic alloy is the convection in the melt during the solidification process. The application of time varying magnetic fields can be considered as an effective tool to organise a well-defined flow structure in the liquid phase affecting the nucleation and growth process. The aim of our work is to improve the understanding of the basic mechanisms allowing us to find a strategy to refine the microstructure of castings by an optimal combination of magnetic field intensity, field frequency and cooling rate.
Directional solidification experiments were carried out with a Pb85wt%Sn alloy contained inside a cylindrical crucible with a diameter of 50 mm. A melt height of 60 mm was chosen. The container bottom is positioned on a water cooled copper chill allowing a directed solidification by a vertical heat flux. A rotating magnetic field was generated by an inductor system with 6 coils. A maximum field amplitude up to 25 mT can be applied. The frequency of the field can be varied between 10 and 400 Hz.
Local temperatures were determined during the solidification process using a set of thermocouples. Cooling curves measured at different locations inside the sample reveal the significant influence of the electromagnetic convection on the local heat transfer. Profiles of the melt velocity were obtained applying the ultrasonic Doppler method..
The Pb-85wt%Sn alloy shows a microstructure with primary tin-crystallites and eutectic. Specimens solidified without a rotating magnetic field showed a columnar dendritic microstructure which is orientated in heat flux direction. The tin crystallites and the eutectic are homogeneously distributed over the whole sample. If the alloy solidifies in a rotating magnetic field the microstructure changes. The shape of the dendrites changes from columnar to equiaxed in direction from the bottom to the top of the specimen and in the same direction the volume content of the eutectic increases.
Numerical calculations were performed using a continuum two-phase model for the directional solidification of a binary alloy. The model includes mass, momentum, energy and species mass conservation equations written in compressible form in order to be able to model shrinkage flow. The Lorentz force term has been included into the momentum conservation equation in order to take into account the effect of the applied magnetic field. The temperature-solute coupling has been described by conduction dominated solidification rule. The two-phase mushy zone has been treated by means of a porous medium approach.
The system of equations has been solved by means of a Finite Volume method. SIMPLE algorithm was used. The results are compared with numerical solutions for diffusion dominated solidification of PbSn and with the data obtained from the experiment.
It was demonstrated by the numerical simulations that a rotating liquid phase forms a mushy zone front showing a convex shape with the maximum on the axis of rotation. This phenomena is confirmed by the experiment.

Various types of AC magnetic fields are already used as stirrers in liquid metal processing. We distinguish between rotating, travelling and pulsating magnetic fields depending on the coil arrangement and the number of phases of the coil currents. Considering the melt motion driven by each of these AC fields in a cylindrical cavity, the basic flow structures are well-known. As a common feature, they typically consist of a rather rigid flow configuration providing only a limited mixing of the melt. Instead a three-dimensional mixing is often needed in metallurgical applications, for instance, in order to dissolve alloying agents or to submerge lighter parts into the melt.

We report on model experiments for the electromagnetic stirring of liquid metals in a cylindrical cavity using a rotating and a travelling magnetic field. The eutectic melt InGaSn in a cylindrical cavity of aspect ratio 1.5 has been used. Velocity measurements are performed using both the ultrasonic Doppler velocimetry (UDV) as well as local potential probes. UDV delivers profiles of the vertical velocity component. In the case of the rotating magnetic field this component represents only a weak secondary flow compared to the primary azimuthal rotation. In case of a separate action of the two field types, the expected flow fields have been found. An efficient mixing can be obtained by a combination of the two field types. Velocity measurements will be presented demonstrating this behaviour.

During last decades magnetohydrodynamic (MHD) effects have attracted growing interest because of its potential impact on numerous industrial technologies. In processes involving electrically conducting liquids, the application of an external magnetic field offers efficient opportunities for a contactless flow control and fluid handling. However, for a well-aimed optimisation of the flow structure local information about flow quantities like velocity, pressure, temperature, concentration or void fraction is necessary. In case of liquid metals the choice of a suitable measuring technique is a crucial problem, because the user is confronted with the problems of opaque fluids, high temperatures as well as a strong chemical reactivity of the fluid against many materials. Therefore, measuring techniques well-known from ordinary hydrodynamics generally fail for liquid metal applications. As a consequence, no commercial measuring systems are available for liquid metal applications.
We present a summary of our R&D activities on velocity measuring techniques for liquid metal flows. Various model experiments with various liquid melts at different temperatures have been performed to test and evaluate local sensors as well as integral methods.
A mechano-optical probe was developed which has to be positioned directly inside the liquid metal flow. The measuring principle is based on the separation of a direct mechanical interaction between flow and sensor tip and the optical acquisition and processing of the signal. The insensitivity of the system to electrical noise and external magnetic fields can be considered as an important advantage. Until now, the sensor has been tested in metallic melts up to temperatures of about 450 °C, but an extension up to 800°C should be straightforward.
The ultrasound Doppler velocimetry (UDV) represents a powerful tool since it delivers a full velocity profile along the ultrasonic beam. It can operate with a direct contact to the melt, but also through a channel wall. To overcome the thermal restriction of the ultrasonic transducers an acoustic wave guide has been used. The acoustic wave guide and the piezoelectric element are combined in form of an integrated sensor. This approach allowed the first successful application of the ultrasound Doppler technique in liquid metals at temperatures above 200°C. The feasibility of this integrated sensor concept was demonstrated in experiments in metallic melts as sodium, PbBi, CuSn or aluminium.
In some applications even a coarse knowledge of the flow topology and the direction of the main eddies would be of high value. We study the possibility of velocity reconstruction in electrically conducting fluids from external measurements of induced magnetic fields. The method is based on the fact that an external magnetic field is deformed by the motion of the fluid. Applying the primary magnetic field in two different directions and measuring the related induced magnetic fields at an array of Hall probes, the three-dimensional velocity field can be reconstructed. Besides some theoretical basics, we present experimental results of a demonstration experiment.

Induction heating is widely used in metals processing. In many cases, additional wishes exist for the flow structure in the melt and the heat and mass transport processes connected with it. The induction heating itself causes a flow in the melt which in a cylindrical configuration typically consists of a double vortex driving the flow on the melt surface radially outwards.
We present the solution of a two-phase stirrer which allows a flexible control of the melt motion in addition to the induction heater. In order to demonstrate this approach, we consider the case of a float-zone arrangement working with an RF induction heater. The two-phase stirrer basically consists in an additional coil superimposed to the primary induction coil. The additional coil is not connected to any power supply, but it is part of a secondary circuit with adjustable capacitor and resistance. The current in the secondary coil is solely induced by the primary coil. In that way, an electromagnetic pump is created. Its intensity and resulting flow direction can easily be adjusted to the process needs. The flexible system parameter are the location and distance of the secondary coil with respect to the primary one, and the capacitor and resistance of the secondary circuit.
We present numerical and experimental results for two applications: the float-zone crystal growth of Ni-based single crystals, and the solidification of the magnetic material NdFeB under varying flow conditions at the solidification front. In the first case, the main interest is directed to a change of the phase boundary geometry in order to obtain a single crystal at all. In the second case, the interest is focused on the microstructure of the solidified material resulting from strongly differing flow conditions in the molten phase. Compared to the usual induction heater, the two-phase stirrer allows to provide a much stronger flow in both directions, i.e. radially inwards or outwards at the solidification front, or an almost stagnant melt. The theoretical part contains the solutions of the electromagnetic problem and the heat & fluid flow equations with a free form of the solid-liquid interface. Experimental results will be given for the resulting phase boundary in single crystals of Ni97Si3, and the convection influenced microstructure of NdFeB alloys.

We present results on the design and application of a DC magnetic field in order to control the filling process of the investment casting of aluminium alloys. The configuration basically consists in a U-bend, and the main request is to reduce the high flow velocities during the starting phase of the filling process. They are considered as the main source of problems like bubble or inclusion entrapment.
At first, the process was simulated using a plexiglas model and the eutectic melt InGaSn which is liquid at room-temperature. Local velocity measurements have been performed using the ultrasonic Doppler velocimetry, whereas flow rates have been determined independently using the contact-less transmitter technique. Measurements and video visualizations clearly show effects like flow rate oscillations or gas bubble entrapment. Depending on geometric and process parameters, the time-scale to get rid of these entrapments may become longer than the process itself. In order to decrease the maximum values of the velocity at the beginning of the process, an external steady magnetic field has been applied. The measurements show that it is capable of reducing the velocity peaks significantly.
Numerical simulations have been performed for the filling process with DC magnetic field control. The free surface problem which occurs in the riser of the casting process is taken into account by a Volume of Fluids Method. The simulations have been compared to the model experiments, and afterwards scaled up to the realistic aluminium casting process. For the influence of the DC magnetic field, a good agreement is obtained between numerics and measurements.
Real tests have been performed with molten aluminium at an industrial investment caster. They clearly show the primary action of the magnetic field, i.e. to reduce the velocity peaks at the beginning of the process. In a second set of experiments the amplitude of the DC field has been adapted in such a way that at the beginning of the process the full breaking action takes place, whereas the field strength has been reduced during the course of the filling process. In this regime, a clear reduction of the peak velocities is obtained with an only slight increase of the overall filling period.
For the flow rate measurement in a pipe, a contact-less solution based on a pair of alternating magnetic field transmitter and receiver has been developed. Test results from a laboratory model and the industrial Al casting process will be presented.

The influence of melt convection on the microstructure formation of Nd-Fe-B alloys, mainly the volume fraction and grain size of the a-Fe phase, has been investigated experimentally using the electromagnetic levitation technique and a specially designed floating-zone arrangement. Samples were subjected to a strong rotation during levitation and compared to fixed samples without additional sample rotation in the levitation facility. A controlled influence on the melt flow is possible via tailored magnetic fields which enable enhancement or suppression of the melt convection using the floating-zone arrangement. A specially designed magnetic two-phase stirrer offers a strong influence on the melt flow in the floating-zone facility. The microstructure pattern, mainly the volume fraction and grain size of the a-Fe phase, vary strongly with the strength of the internal flow motion in both types of experiments. The melt flow in a levitated droplet was studied numerically under the additional effect of a global sample rotation giving rise to the assumption of strong suppression of internal motions. The electromagnetically driven flow in the inductively heated melt zone of the floating-zone arrangement turned out to be of strong influence on the resulting microstructure. Numerical simulations are essential in order to understand and design the various flow structures.

The performance and in-beam imaging capabilities of two Hamamatsu avalanche photodiode arrays (S8550) individually coupled to crystals of cerium-doped lutetium oxyorthosilicate (LSO) are presented. The two detectors were operated in coincidence at the GSI medical beam line of the Gesellschaft für Schwerionenforschung, in Darmstadt, Germany. In a first set of experiments their imaging performance was tested before, during and after the irradiation of phantoms of polymethylmethacrylate with carbon ion beams with fluences equivalent to 1000 typical daily therapeutic fractions. Only minor energy, time and spatial resolution deterioration was observed, with the initial values being recovered after stopping the irradiation. A second set of experiments successfully imaged the depth distribution of positron emitter radionuclides created in a phantom that stopped the high energy carbon ion beam. The particular details for the in-beam PET acquisition are shortly outlined. The obtained results show that LSO is a suitable material for in-beam PET and that its coupling with avalanche photodiode arrays is feasible for a PET system dedicated to in-beam monitoring of ion therapy.

The response of pig lungs to irradiation with ¹²C-ions was assessed in two experiments to validate the procedures for heavy ion therapy planning at the Gesellschaft für Schwerionenforschung (GSI) and to explore their range of applicability. In both experiments, the target volume (spread-out Bragg peak, SOBP) was planned to be a 4 cm long cylinder with a diameter of 4 cm. Doses in the SOBP were prescribed to be equivalent to 5x4 Gy, 5x5.5 Gy and 5x7 Gy of x-rays in the first experiment, and to 5 fractions of 7 Gy and 9 Gy in the second experiment. The lung response in the first experiment was less than exepted on the basis of earlier experiments with photons. Pneumonitis reaction and chronic fibrotic changes were observed outside the prescribed high-dose region. In the second experiment, the effects were more pronounced than had been expected on the basis of the first experiment. Changes were most intense in the high-dose region, but were also seen troughout the lung along the beam channel. Moreover, significant skin reactions were observed at the beam entrance site in all animals and - less pronounced - at the beam exit site in 3 of the 6 animals. In conclusion, the complex irradiation geometry of the pig lung, the changes of body weight between the two experiments, and in sufficient accounting for a change in the relative biological effectivness (RBE) computation led to substantial deviations of the observed reactions from expectations, the reasons for which could be identified in a subsequent analysis. The less pronounced lung reaction in the first experiment was due to an overestimation of RBE in a preliminary version of the algorithm for its determination. The extension of the fibrotic reaction resulted from the smear-out of the high-dose region due to density variations in tissue structures, respiratory movement, and limited positioning accuracy. The skin reactions at the entrance port reflect the different treatment geometry in the two experiments. The one unexplained observation is the mild skin reaction that was observed in the second experiment at the beam exit site.

Purpose: Compared to photon irradiation, carbon ions provide physical and biologic advantages that may be exploited in chordomas and chondrosarcomas.
Methods and Materials: Between August 1998 and December 2000, 37 patients with chordomas (n = 24) and chondrosarcomas (n = 13) were treated with carbon ion radiotherapy within a Phase I/II trial. Tumor conformal application of carbon ion beams was realized by intensity-controlled raster scanning with pulse-to-pulse energy variation. Three-dimensional treatment planning included biologic plan optimization. The median tumor dose was 60 GyE (GyE =Gy x relative biologic effectiveness).
Results: The mean follow-up was 13 months. The local control rate after 1 and 2 years was 96% and 90%, respectively. We observed 2 recurrences outside the gross tumor volume in patients with chordomas. Progression-free survival was 100% for chondrosarcomas and 83% for chordomas at 2 years. Partial remission after carbon ion radiotherapy was observed in 6 patients. Treatment toxicity was mild.
Conclusion: These are the first data demonstrating the clinical feasibility, safety, and effectiveness of scanning beam delivery of ion beams in patients with skull base tumors. The preliminary results in patients with skull base chordomas and low-grade chondrosarcomas are encouraging, although the follow-up was too short to draw definite conclusions concerning outcome. In the absence of major toxicity, dose escalation might be considered.

Für eine genauere Einordnung der Sicherheitsreserven des Reaktordruckbehälters im Falle einer Kernschmelze ist es notwendig, mögliche Versagensformen des Reaktordruckbehälters sowie Versagenszeiträume zu untersuchen. Dazu wurden und werden derzeit weltweit Versuche zum Versagen der Bodenkalotte durchgeführt, wie die amerikanischen OLHF- (OECD Lower Head Failure) oder die schwedischen FOREVER-Experimente (Failure of Reactor Vessel Retention). Am Institut für Sicherheitsforschung des Forschungszentrums Rossendorf bei Dresden wurde ein Finite-Elemente-Modell entwickelt, das sowohl die Thermofluiddynamik der Schmelze als auch die elastoplastische Mechanik des Reaktordruckbehälters modelliert. Nach der Validierung und der Anwendung dieses numerischen Modells bei der Simulation der FOREVER-Experimente führte die Analyse der Ergebnisse zur Einführung einer "Kriechstütze" und einer passiven Steuereinrichtung zur Flutung der Reaktorgrube im Falle einer Kernschmelze. Die Funktion der Kriechstütze besteht darin, die am schwersten beanspruchten Bereiche des Reaktordruckbehälters im Falle einer Kernschmelze zu entlasten und somit ein Versagen zu verhindern oder zumindest deutlich zu verzögern. Dazu werden unterhalb der Bodenkalotte des Reaktordruckbehälters sehr tragfähige Stützen in geeigneter Form angeordnet. Beide Ideen wurden erfolgreich als Patent angemeldet (vgl. "Abstützvorrichtung für Stahlbehälter" Patent DE 100 41 778 C2"Steuereinrichtung für einen Kühlkreislauf" Patent DE 100 41 779 C1). In ersten Rechnungen zur FOREVER-Anordnung wurde gezeigt, dass selbst bei einem Szenario ohne Flutung der Bodenkalotte von außen die Zeit bis zum Versagen durch die Wirkung der Kriechstütze bei gleichbleibenden Randbedingungen verdoppelt werden kann. Eine weitere Verfolgung dieses Konzepts bei bestehenden und zukünftigen Kernkraftwerken erscheint daher vielversprechend.

In vivo dose localization in light ion tumour therapy can be performed by measuring the range distributions of beta⁺ active ions in tissue employing positron emission tomographic techniques. For this purpose a multiplicative iteration scheme for reconstructing three-dimensional images from shift-variant, limited-angle data is presented. In the iterative correction steps the algorithm uses the geometric means of quotients calculated from the three-dimensional Radon transforms of the backprojected measured and approximated source distributions. When sources measured with poor statistics are reconstructed, an effective noise suppression is achieved.

A multiplicative iteration scheme for reconstructing three-dimensional images from shift variant, limited angle data taken by a large area positron camera is proposed. Using simulated data sets the algorithm has been proven to be capable of reconstructing source distributions from a very small number of annihilation events. This suggests the applicability of the technique to the measurement of range distributions of beta⁺-active ions implanted into thick targets and especially to in-vivo dose localization in light ion tumor therapy by means of positron emission tomography.

Kein Abstract.

In-line nitrogen PIII/ion nitriding processing of metallic materials

Improvements of tribological properties of Ti6Al4V by nitrogen plasma
immersion ion implantation

Dynamic in situ analysis of stress and film thickness provide fast and more physical information on growth and stress evolution in cBN layers than integrating (ex situ) methods. Especially features of the layered structure of boron nitride films, like the evolution of instantaneous stress and growth rates during deposition can be resolved by in situ methods. This work is concerned with dynamic in-situ stress measurement by means of cantilever bending during magnetron sputter deposition of cBN thin films. Laser deflection in combination with in situ ellipsometry is used to determine the instantaneous stress of the films. The results show, in agreement with results that were obtained previously from ion beam assisted deposition (IBAD), that the hBN and cBN layers exhibit different levels of stress under constant deposition conditions. The stress increases from less than -4 GPa to very high values (-10 GPa) after the coalescence of the cBN nuclei. It is therefore possible to establish the point of cBN nucleation instantly. A simultaneous medium energy ion bombardment is used for stress relaxation during film deposition. A modified substrate bias voltage, combining negative high and low voltage pulses, is used to enable an ion bombardment of the growing film with energies up to 8 keV. In this way, cBN films with a stress as low as –1.7 GPa could be produced without destroying the sp3-bonds significantly.

Backround: Radiobiological and physical examinations suggest clinical advantages of heavy ion irradiation. We report the results of 23 women and 22 men (median age 48 years) with skull base tumors irradiated with carbon ion beams at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, from December 1997 until September 1999.
Patients and Methods: The study included patients with chordomas (17), chondrosarcomas (10) and other skull base tumors. It is the first time that the intensity-controlled rasterscan-technique and the application of positron emission tomography (PET) for quality assurance was used. All patients had computed tomography for three-dimensional treatment planning. Patients with chordomas and chondrosarcomas underwent fractionated carbon ion irradiation in 20 consecutive days (median total dose 60 GyE). Other histologies were treated with a carbon ion boost of 15 to 18 GyE delivered to the macroscopic tumor after fractionated stereotactic radiotherapy (median total dose 63 GyE).
Results: Mean follow-up was 9 months. Irradiation was well tolerated by all patients. Partial tumor remission was seen in 7 patients (15.5%). One-year local control rate was 94%. One patient (2.2%) deceased. No severe toxicity and no local recurrence within the treated volume were observed.
Conclusion: Clinical effectiveness and technical feasibility of this therapy modality could clearly be demostrated in our study. To evaluate the clinical relevance of the different beam modalities studies with larger patient numbers are necessary. To continue our project a new heavy ion accelator exclusively for clinical use is planned to be constructed in Heidelberg.

Beams of heavy charged particles like protons or carbon ions represent the optimum tool for the treatment of deep seated, inoperable and radioresistant tumors growing in close vicinity to organs at risk. In contrast to photon therapy, the dose deposited by heavy charged particles increases with the penetration depth, culminating in a sharp maximum at the end of the particle range - the Bragg peak. This peak can be shifted in depth by energy variation and distributed laterally through magnetic deflection of the particle beam, thus allowing a precise and conform irradiation of the selected target volume. Furthermore, carbon ions offer the most conform irradiation due to their lower lateral scattering when compared to protons. In addition to this excellent physical selectivity, the biological efficiency concerning cell killing increases towards the end of carbon ion range. Therefore, the high dose at the Bragg peak is further enhanced by an increase in biological efficiency. Finally, by applying PET techniques, an in-situ dose localization control can be performed by tracing the small amount of beta⁺ emitters (¹¹C, ¹⁵O, ¹⁰C) which are produced in nuclear fragmentation reactions between the ¹²C projectiles and atomic nuclei in the target volume. A pilot tumor therapy unit that fully exploits the advantages of carbon ions is under operation at GSI since December 1997 and has now treadet more than 70 patients. Its mayor goal is to demonstrate the safe and routine application of charged particle beams for radiotherapy.

The performance of Hamamatsu S8550 avalanche photodiode (APD) arrays for scintillator matrices readout has been evaluated. The S8550 device is a monolithic 8 X 4 pixels structure with an active area of 2.56 mm² for each pixel. The device allows stable operation at gains up to 74, with a detection efficiency of about 60% for photons of 420 nm wavelength. It is characterized by a low noise equal to 27 electrons equivalent noise charge at room temperature. The energy resolution of 14.6%, for the 511 keV peak from ²2;Na source has been recorded with a 2 x 2 x 10 mm³ LSO crystal coupled to one pixel. The number of electron-hole pairs produced by the 511 keV photopeak absorbed in LSO is equal to 4830 ± 240 e-h/MeV. Coupling LSO scintillator crystals to individual pixels of the APD array a coincidence timing resolution of 3.0 ± 0.2ns FWHM has been measured for the 511 keV b3;-rays from a ²2;Na source. Finally, we compared the characteristics and readout performance of the Hamamatsu array with the results measured earlier under the same conditions for the quadrant large area avalanche photodiodes of Advanced Photonics Inc.

In this paper, we evaluate the performance of recently released avalanche photodiode arrays from Advanced Photonix, Inc. (API) for scintillator matrices readout. The Large Area Avalanche Photodiode (LAAPD) quadrant device is a monolithic 2x2 pixels structure with an active area of 5.3 mm² per pixel. The device allows stable operation with high gains up to 200, and detection efficiency of 73+-10 % for 420 nm photons. It is furthermore characterized by a high linear fill factor of 90 % and low noise equal to16 electrons ENC measured at room temperature. We have measured pixel-to-pixel gain non-uniformity smaller than 1.5 % and inter-pixel crosstalk of 1.7 % in the pulse mode operation with the device gain of 50. The energy resolution of 12.3+-0.5 % was achieved for the 511 keV photopeak from a ²²Na source placed on top of a 2x2x10 mm³ LSO crystal coupled to one pixel of quadrant LAAPD. An array of LSO crystals was later coupled to the device allowing individual crystal identification. Coincidence timing resolution of 1.9+-0.1 ns FWHM has been obtained for quadrant pixel with LSO scintillator for the 511 keV peak from ²²Na source. Finally, we compared the characteristics and readout performance of LAAPD array with published results of some commerciallly available APD arrays.

The Master Curve (MC) approach and the associated reference temperature, T0, as defined in the test standard ASTM E1921, is rapidly moving from the research laboratory to application in integrity assessment of components and structures. T0 is the index temperature for the universal MC, which considers the behaviour of a specific material. “The Structural Integrity Assessment Procedures for European Industry” (SINTAP) contain a MC extension for analysing the fracture behaviour shown by inhomogeneous ferritic steels statistically. This paper presents the application of the MC approach to the T0 determination of different types of Russian WWER-type reactor pressure vessel (RPV) steels. In addition the SINTAP-MC approach was applied to determine an alternative reference temperature, TR.
The influence of different microstructures and compositions within one type of RPV steel and the effect of irradiation with fast neutrons on T0 are experimentally evaluated. In general the MC based T0 is about 72 K below the Charpy V-notch transition temperature related to an impact energy of 48J. The paper demonstrates the application of MC based T0 and TR as an alternative reference temperature for neutron embrittled RPV steels used in the RPV integrity assessment.

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The in-beam dual head positron camera BASTEI (Beta+ Activity meaSurements at the Therapy with Energetic Ions) is used to monitor and control the applied dose distributions simultaneously to tumor irradiations with carbon ion beams at the experimental heavy ion therapy facility at GSI Darmstadt. Therefore, the PET system has been mounted directly at the treatment site. A fully 3D reconstruction algorithm based on the Maximum Likelihood Expectation Maximization algorithm has been developed and adapted to a spatially varying imaging situation. The scatter and attenuation correction is applied to the measured list mode data before each iterative step. This requires an attenuation map containing the information on the tissue composition and densities. This information is derived from the X-ray computed tomograms (CT) of the patient and the patient fixation system including the head rest. The scatter correction is included into the forward projection step of the Maximum Likelihood image reconstruction. The normalization of scattered events relative to the unscattered events is done by a global scatter fraction factor. The results are presented.

The in beam dual head positron camera BASTEI (Beta+ Activity meaSurements at the Therapy with Energetic Ions) is used to monitor and control the applied dose distributions simultaneously to the irradiation of the therapeutic carbon ion beam at the experimental heavy ion therapy facility at GSI Darmstadt. Therefore, the PET system is mounted directly at the treatment site. A fully 3D reconstruction algorithm based on the Maximum Likelihood Estimator algorithm has been developed and adapted to a strongly spatially-varying imaging situation. A scatter and attenuation correction is applied to the measured listmode data prior to the image reconstruction. The scatter correction method uses the subtraction of scattered data from measured data.
In this particular PET-application the fluence of registered events at the detector surface is in the order of 10 to 102 cm-2 compared with about 102 to 103 cm-2 in conventional PET in nuclear medicine. The emission data are acquired in a 3D list mode. Since less than 100,000 out of approximately 4 millions coincidence channels typically contribute to the reconstruction all detected events have to be utilized. In addition, they cannot be sorted into sinograms because the necessary interpolation procedures would generate additional noise. Therefore the reconstruction algorithm takes the original emission data as input. The non-zero elements of the 3D transition matrix are calculated at run-time using a Monte-Carlo code that models the individual crystal responses along each coincidence channel. Results of the performance studies as well as examples of use are presented.

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Bei der Gesellschaft für Schwerionenforschung in Darmstadt ist in den vergangenen Jahren eine experimentelle Anlage für die Krebstherapie mit Strahlen schwerer Ionen errichtet worden. Im Herbst 1997 sollen dort die ersten Tumorpatienten mit ¹²C Strahlen behandelt werden. Bei der Installation der Anlage und ihrem künftigen Betrieb kooperieren die Gesellschaft für Schwerionenforschung Darmstadt (GSI), das Deutsche Krebsforschungszentrum in Heidelberg (DKFZ), die Radiologischen Kliniken der Universitäten Heidelberg und Magdeburg sowie das Forschungszentrum Rossendorf. Der Rossendorfer Beitrag zur Therapieanlage besteht in der Entwicklung und Realisierung von Verfahren, welche die Positronen-Emissions-Tomographie (PET) für die Kontrolle der Schwerionen-Tumortherapie anwenden. Mit der Installation des in Rossendorf gebauten Positronen-Emissions-Tomographen am Therapieplatz in Darmstadt im Januar 1996 konnten die methodischen Vorarbeiten zum Einsatz von PET für die Tumortherapie mit schweren Ionen beginnen.

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Abstract wir von Dr. Geipel nachgereicht