Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The core neutronic and thermal-hydraulic simulator code DYN3D, which has been developed for three-dimensional analyses of steady states and transients in thermal reactors with quadratic or hexagonal fuel assemblies, is based on nodal methods for the solution of the two group neutron diffusion equation. Loading cores with higher content of MOX fuel, the increase of the fuel cycle length, and the consideration of new reactor types require improvements of these standard methods. Therefore, DYN3D has been extended to use more than two energy groups. Furthermore, a nodal expansion method for solving the equations of the simplified P3 approximation (SP3) of the multigroup transport equation was developed for quadratic fuel assembly geometry. The new extensions of DYN3D are verified with calculations of the OECD/NEA and U.S. NRC PWR MOX/UO2 Core Transient Benchmark. The nodal SP3 method in DYN3D is outlined and results for a steady state are shown, which are compared with the solutions of the diffusion and transport codes used by the participants of the benchmark. Improved DYN3D-results are obtained with the new extensions.

The morphology and bonding structure of nanocomposite C:Ni (~30 at.%) thin films grown by ion beam sputtering at different temperatures (RT-500°C) is investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray absorption near edge structure (XANES). XRD and XANES of the LII–LIII absorption edges of Ni reveal that at lower temperatures nickel is in the carbidic state whose signatures become more pronounced when the growth temperature increases up to ~300°C. Further increase in the growth temperature results in XRD and XANES features resembling to those of metallic nickel. According to the TEM observations, nickel or nickel carbide nanoparticles embedded in a carbon matrix have a columnar structure with the diameter of ~2-5 nm, which is typical for the films grown under surface diffusion dominated conditions for the structuring processes. At lower temperatures carbon is mainly in an amorphous state, while the transition into a fullerene-like structure, which consists of nickel nanoparticles wrapped in curved graphite sheets, can be clearly identified at ~200°C. The present results show that the nickel nanoparticles act as nucleation centres for the formation of the fullerene-like structures in both carbidic and metallic states, provided that the temperature for surface diffusion during the deposition process is sufficiently high.

Scanning electron microscopy (SEM) and cathodoluminescence (CL) in combination with scanning transmission electron microscopy (STEM) have been used to investigate thermally grown amorphous silicon dioxide layers implanted isoelectronically with group IV ions (C+, Si+, Ge+, Sn+, Pb+) as well as with group VI ions (O+, S+, Se+). Besides the main luminescent centers in a-SiO2 layers: the red R luminescence (650 nm; 1.9 eV) of the non-bridging oxygen hole centers (NBOHC), the blue B (460 nm; 2.7 eV) and the UV (290 nm; 4.3 eV) band of the oxygen deficient centers (ODC), in ion implanted silica additional emission bands are observed. E.g. in Ge+ implanted layers a huge violet band appears at 410 nm (3.1 eV) increasing with the thermal annealing process due to formation of Ge dimers, trimers or higher aggregates, finally leading to destruction of the luminescence centers by further growing to Ge nanoclusters. The Ge cluster size is shown by STEM cross section micrographs.
Generally, group IV element implantation and partial substitution of silicon increases the luminescence in the blue/violet region whereas group VI elements and additional oxygen increase the intensity in the red region, confirming the association of the blue and the red luminescence with oxygen deficient centers and oxygen excess centers, respectively. Thus, the cathodoluminescence spectra of sulfur and oxygen implanted SiO2 layers under special conditions show besides the characteristic luminescence bands a multimodal structure beginning in the green region at 500 nm and extending up to the near infrared region at 820 nm. The energy step differences of the sublevels amount in the average 120 meV and indicate vibronic-electronic transitions, probably of O − interstitial molecules.

While the Master Curve (MC) method is gradually entering brittle fracture safety assessment procedures world-wide, knowledge is still lacking about its limits of applicability to highly neutron irradiated material. In this paper two reactor pressure vessel (RPV) steels A533B Cl. 1 (IAEA reference material code JRQ) and A508 Cl.3 (code JFL) were scrutinized for possible deviations of the postulated invariant MC shape and the MC validity for macroscopically inhomogeneous microstructure. Besides tensile and Charpy-V tests, MC tests were performed on Charpy size three-point bend specimens in the unirradiated, neutron irradiated with fluences up to nearly 10²⁰ n/cm² (E>1MeV) and recovery heat treated condition. Evaluation procedures include Master Curve reference temperature T₀ determination according to ASTM E1921-05 as well as additional analysis methods such as SINTAP, multimodal MC method (MML) and the Unified Curve (UC). Integrity assessment according to ASME Code Cases N-629 and N-631has been applied. It is shown that the standard MC concept provides a precise description of the fracture toughness for all conditions, even exceptionally well for the highly irradiated state. No MC shape change could be observed, whereas the UC concept indicates a significant influence of irradiation on the fracture toughness curves for the highly irradiated JRQ.

High resolution transmission electron microscopy, scanning transmission electron microscopy and cathodoluminescence have been used to investigate Si and Ge cluster formation in amorphous silicon dioxide layers. Commonly, cathodoluminescence emission spectra of pure SiO2 are identified with particular defect centers within the atomic network of silica including the nonbridging oxygen hole center associated with the red luminescence at 650nm (1.9 eV) and the oxygen deficient centers with the blue (460 nm; 2.7 eV) and ultraviolet band (295 nm; 4.2 eV). In Ge+ ion implanted SiO2 an additional violet emission band appears at 410 nm (3.1 eV). The strong increase of this violet luminescence after thermal annealing is associated with formation of low-dimension Ge aggregates like dimers, trimers and higher formations, further growing to Ge nanoclusters. On the other hand, pure silica layers were modified by heavy electron beam irradiation (5 keV; 2.7 A/cm2) leading to electronic as well as thermal dissociation of oxygen and appearance of under-stoichiometric SiOx. This SiOx will undergo a phase separation and we observe Si cluster formation with a most probable cluster diameter of 4 nm. Such largely extended Si clusters will diminish the SiO2 related luminescence and Si crystal related luminescence in the near IR appears.

Currently, several countries already have adopted or are in the process of adopting the Master Curve (MC) approach into their reactor pressure vessel (RPV) integrity assessment. Yet, for highly irradiated material some open technical issues like the postulated invariance of the MC shape and the behaviour of inhomogeneous material need resolving to allow the MC approach to be incorporated into the German Nuclear Regulatory Guidelines (KTA). Within the framework of the reactor safety research of BMWi specimens from 3 different RPV steels were scrutinized (IAEA reference material 3JRQ57, 1JFL11 comparable to German RPV steel 22NiMoCr3-7 and Russian WWER-440 type base metal KAB-B), which were neutron irradiated up to 10²⁰ n/cm² (E>1MeV). Additionally, the effects of a recovery heat treatment at 475°C/100h are investigated. Besides microstructure analysis, hardness, tensile and Charpy-V tests, the experimental part focuses on fracture mechanical testing, i.e. J_R curves according to ASTM E1820-06 and MC reference temperature T₀ determination according to ASTM E1921-05 as well as additional analysis methods like SINTAP, multimodal MC method and the Unified Curve. Resistance against ductile crack initiation (J_R curves) remains relatively unaffected by irradiation (J_Q/J_0,2BL according to ASTM E1820 and J_0,2 according to GKSS). The MC provides a precise description of the fracture toughness vs. temperature of unirradiated, irradiated, and recovery annealed material. Especially for the highly irradiated states, the MC describes the K_Jc(1T) values exceptionally well, including data points outside the temperature limit T₀±50K according to Standard ASTM E1921 and even for highly irradiation sensitive and inhomogeneous RPV steel 3JRQ57. Some scatter occurs for low and medium irradiated states, where more points than expected lie below the MC for 5% failure probability. The Unified Curve, which allows for MC shape changes at high irradiation levels, overpredicts the influence of irradiation with 3JRQ57. For none of the 3 investigated RPV steels a shape change of the Master Curve due to irradiation could be observed. Embrittlement prediction formulae according to Reg. Guide 1.99, Rev. 2 and WWER specific VERLIFE procedure tend to underpredict the irradiation-induced changes in material properties.

SIMOX (Separation-by-Implantation-of-Oxygen) is an established techniques to fabricate silicon-on-insulator (SOI) structures by oxygen ion implantation into silicon. The main problem of SIMOX is the very high oxygen ion fluence and the related defects. It is demonstrated that vacancy defects promote and localize the oxide growth. The crucial point is to control the distribution of vacancies. Oxygen implantation generates excess vacancies around RP/2 which act as trapping sites for oxide growth outside the region at the maximum concentration of oxygen at RP. The introduction of a narrow cavity layer by He implantation and subsequent annealing is shown to be a promising technique of defect engineering. The additional He implant does not initiate oxide growth in the top-Si layer of SOI.

Tin-doped-indium oxide In2O3:Sn (ITO) is a degenerate n-type semiconductor with high transparency and nearly metallic conductivity. Thin films of ITO find applications in optoelectronics, photovoltaics, and in the liquid crystal display and OLED industry as transparent electrodes. The interaction of the plasma with the growing film surface may be employed to manipulate adatom mobilities and nucleation rates [1, 2], and thus contribute to the definition of the film structure and the evolution of its properties during deposition. The aim of the present work is to investigate the influence of the intentionally inhomogeneous plasma flow on the distribution of the ITO film thickness and optical properties along the substrate. For large scale inspection spectra of Delta/Psi were recorded scanning across the sample. For high resolution (small scale inspection) a spectrum of Delta micromaps with wavelength from 420 nm to 830 nm with 4 µm lateral resolution was recorded using Nanofilm’s imaging ellipsometer EP3 within 3 min. From fitting of the spectrum one obtains maps for each of the fit parameters. It is observed, that the damping of the oscillator is rather independent of location. A negative oscillator frequency shift with thickness increasing from the low towards the high plasma flow area is observed. Such a correlation can be explained by the change of the film microsctructure and stoichiometry [3] which, in turn, can affect electronic structure of the film.
References
1. I. Petrov, P.B. Barna, L. Hultman, and J.E. Greene, J. Vac. Sci. Technol. A 21, 1 (2003).
2. W. Ensinger, Nucl. Instr. Meth. B 127/128, 796 (1997).
3. A. Rogozin, M. Vinnichenko, N. Shevchenko, A. Kolitsch, and W. Moeller, Thin Solid Films 496, 197 (2006) .

Understanding of the free electron generation mechanisms in tin-doped indium oxide (ITO) films grown at elevated temperatures is important to decrease their electrical resistivity and keep high optical transmittance. Therefore, this study is focused on a real-time determination of the free electron optical absorption of ITO layers by in situ spectroscopic ellipsometry (SE). The experiments were carried out using rotating compensator ellipsometer M-2000 (J.A. Woollam Inc., U.S.A.) during: (i) growth by reactive magnetron sputtering at elevated temperatures (Ts=RT-500 °C); and (ii) post deposition annealing (Ta=200-320 °C) of the amorphous films. In situ four-point probe resistivity measurements during annealing and the data of separate in situ X-ray diffraction (XRD) experiments at the European Synchrotron Radiation Facility were used to contrast the SE results.
Free electron density, Ne, and mobility, µe, are estimated for the growing film from parameterization of the complex dielectric function in Drude-Lorentz approach. The Drude term accounts for the free electron optical absorption. The Ne values range from ~3x10^20 to 10^21 cm^-3 depending on the growth temperature. It is in good agreement with results of ex situ Hall effect measurements, while µe values are usually overestimated by SE. The SE applicability as a non-contact and in situ tool for monitoring of the film resistivity during growth is demonstrated. The method indicates decrease of the resistivity with increasing film thickness at Ts<270 °C mainly due to enhancement of the Ne while there is no such variation of resistivity observed at Ts>400 °C. This result is discussed in terms of thickness-dependent film morphology.
It is shown that postdeposition annealing modifies the film properties in two stages. During the first stage film remains amorphous according to in situ XRD; SE shows slight enhancement of Ne, while film resistivity strongly decreases. It is attributed to relaxation of In-O bonds in the amorphous phase and subsequent rearrangement of defect structure mainly improving the electron mobility. During the second stage, time dependence of the resistivity changes slope and Ne increases by a factor of 2. It coincides in time with the film crystallization outset, which enables Sn-donor activation by removal of interstitial oxygen. Therefore, this study provides experimental evidence of Sn donor activation during crystallization of the films and shows its kinetics depending on the annealing conditions.

New and precise astronomical observations call for nuclear data of equal precision for their interpretation. This will permit a better understanding of big-bang nucleosynthesis and of asymptotic giant branch stars.

Based on the recent experimental study of the 3He(alpha,gamma)7Be nuclear reaction directly in the energy range of big-bang nucleosynthesis, a new experiment to study the 2H(alpha,gamma)6Li reaction at big-bang energies is proposed. The new data will allow to test a possible nuclear solution to the big-bang Li-6 posed by new observations of Li-6 in very old stars.

The 14N(p,gamma)15O reaction determines the rate of the CNO cycle. A new study of this reaction is currently underway at the LUNA accelerator deep underground in Gran Sasso, Italy. The new data can be coupled with observations of low-energy solar neutrinos (for example in the Borexino and SNO+ detectors) in order to determine the solar metallicity.

Satellite-based observations of the decay of radioactive Al-26 allow to determine the rate of supernovae in our galaxy, provided the production process is sufficiently well known. The 25Mg(p,gamma)26Al reaction influences Al-26 production; its rate is under experimental study at LUNA.

The cross section of the 14N(p,gamma)15O reaction directly influences the rate of the CNO cycle of hydrogen burning. In order to reliably extrapolate the cross section to the solar Gamow peak, in a previous LUNA experiment capture to the ground state and several excited states in O-15 has been measured and used in an R-matrix fit [1,2].
The data on the ground state capture had been affected by the true coincidence summing effect in a large volume HPGe detector placed in close geometry [1], limiting the precision of the extrapolation. A new measurement of the cross section for capture to the ground state in O-15 is running at LUNA in Gran Sasso (Italy). A clover HPGe detector is used to reduce the summing correction and its consequent uncertainty. We concentrate on energies above the 259 keV resonance, where the R-matrix fit can be constrained by precision data.

[1] A. Formicola et al., Phys. Lett B 591, 61 (2004)
[2] G. Imbriani et al., Eur. Phys. J. A 25, 455 (2005)

The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of Be-7 and B-8 neutrinos from the Sun predicted by solar models and in the Li-7 abundance obtained in big-bang nucleosynthesis calculations.
Here we report on a precision cross section measurement performed by the LUNA collaboration at Gran Sasso (Italy). At energies directly relevant to big-bang nucleosynthesis, the cross section has been studied by both the activation [1] and the prompt-gamma technique.
Using a windowless gas target, high beam intensity, a low background in beam gamma-detector and low background gamma-counting facilities, the 3He(alpha,gamma)7Bee cross section has been determined at 90-170 keV center-of-mass energy with a total uncertainty as low as 4%.

The new data can be used in big-bang nucleosynthesis
calculations and to constrain the extrapolation of the
3He(alpha,gamma)7Be astrophysical S-factor to solar
energies.

[1] D. Bemmerer et al., Phys. Rev. Lett. 97, 122502 (2006)

New and precise astronomical observations call for nuclear data of equal precision for their interpretation. This will permit a better understanding of big-bang nucleosynthesis and of asymptotic giant branch stars.

Based on the recent experimental study of the 3He(alpha,gamma)7Be nuclear reaction directly in the energy range of big-bang nucleosynthesis, a new experiment to study the 2H(alpha,gamma)6Li reaction at big-bang energies is proposed. The new data will allow to test a possible nuclear solution to the big-bang Li-6 posed by new observations of Li-6 in very old stars.

The 15N(p,alpha)12C reaction is an important source of uncertainty in the prediction of the F-19 yield observed in asymptotic giant branch stars. Based on angular distribution measurements performed at TU Berlin, the potential of a new precision cross section measurement is evaluated.

The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis calculations. In the seminar I will report on a new precision experiment on this reaction performed by the LUNA collaboration [1].

Using a windowless gas target, the high beam intensity of the LUNA2 accelerator, and the Gran Sasso low background gamma-counting facilities, the 3He(alpha,gamma)7Be cross section has been determined by the activation method at 90 -- 170 keV center-of-mass energy with a total uncertainty as low as 4%. The present low energies are directly relevant to big-bang nucleosynthesis and had previously been reached experimentally only by the prompt-gamma technique and with inferior precision.

The new LUNA data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3He(alpha,gamma)7Be astrophysical S-factor to solar energies.

[1] D. Bemmerer et al. (LUNA Collaboration), Phys. Rev. Lett. 97, 122502 (2006)

We demonstrate the imaging of ferroelectric domains in BaTiO3, using an infrared-emitting free-electron laser as a tunable optical source for scattering scanning near-field optical microscopy and spectroscopy. When the laser is tuned into the spectral vicinity of a phonon resonance, ferroelectric domains can be resolved due to the anisotropy of the dielectric properties of the material. Slight detuning of the wavelength gives rise to a contrast reversal clearly evidencing the resonant character of the excitation. The near-field domain contrast shows that the orientation of the dielectric tensor with respect to the sample surface has a clear influence on the near-field signal.

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Modern technologies allow the amplification of short laser pulses to energies of some tens of kJ. Additionally, ultrashort pulses containing only some optical cycles can be generated. By merging these techniques nowadays focused laser beams can reach unprecedented intensities in the range of 1021 W/cm2 and will reach even higher ones in the near future. At these intensities the electric and magnetic field strength is many orders of magnitude higher than those that will ever possible in a static generation scheme. By applying these fields on a target it becomes possible to get access to a new interaction regime of light and matter: relativistic optics. This opens a new wide area in experimental science where classical optics meets plasma dynamics, relativistic quantum mechanics, and high energy physics.

Soils, sediments, and waters heavily polluted with radionuclides and other toxic metals, are a reservoir of unusual bacteria well adapted to these toxic environments. These bacteria possess fascinating mechanisms for interaction with and bio-transformation of radionuclides and other heavy metals, thus regulating the mobility of the metals in the environment. This paper will give an overview on the different mechanisms of interaction between radionuclides/metals and bacterial strains isolated from different extreme habitats including uranium mining waste piles as well as groundwater of a radioactive repository. For this purpose, a combination of spectroscopic (EXAFS, XANES, TRLFS), microscopic (TEM), microbiological and wet chemistry techniques is used. Elucidating the interaction mechanisms microbe/metals is helpful for understanding the role which bacteria play in the transport and mobility of toxic metals in the environment as well as their biotechnological application in the bioremediation of heavy metal contaminated waters. Another application of the isolated bacterial cells and their biocomponents is in the field of nanotechnology. Thus, the surface layer (S-layer) protein of Bacillus sphaericus JG-A12, a bacterium isolated from a uranium mining waste pile near the town of Johanngeorgenstadt in Germany, is used as template for the formation of noble metal (Pd, Pt, Au, etc.) nanoparticles for industrial application (e.g. catalysis). The structure and the size of these metallic nanoparticles were characterized using synchrotron radiation-based methods such as X-ray absorption spectroscopy.

Was ist Physik für eine Wissenschaft? Eine populärwissenschaftliche Betrachtung der Wissenschaft Physik, erläutert anhand einer Liste ungelöster physikalischer Aufgaben.

Pulsed laser deposited epitaxial PrxCo100−x (x=8.7–27.6 at. %) thin films were systematically studied as a function of Pr content. Structural and magnetic measurements reveal different phases for specific composition ranges. In some cases, the phases observed are in contrast to their bulk counterparts. Uniaxial anisotropy at room temperature is observed in all the films enabling excellent hard magnetic properties. Polarization decreases monotonically with the increase of Pr content, whereas coercivity exhibits a broad maximum near the highly anisotropic PrCo5 composition. For the optimum combination of coercivity and polarization, the measured (BH)max reaches values of 310 kJ/m3, which exceeds the highest-energy product value reported for RE-Co (RE=rare-earth) systems. Temperature-dependent ac susceptibility measurements reveal that films with x=8.7–20.4 undergo a spin reorientation from easy axis to easy cone, but films with x=22.9–27.6 maintain their uniaxial anisotropy throughout the temperature range of investigation. From the comparison of the structural investigations and the spin reorientation temperature measurements, it is concluded that the spin reorientation temperature is insensitive to the change in the lattice parameter of the PrCo5±δ phase.

Intersubband transitions in semiconductor quantum wells (QW) are crucial for mid-infrared lasers, detectors, and modulators. New compound materials such as lattice matched InGaAs/AlAsSb and strain compensated InGaAs/AlAs, both grown on InP, feature large conduction band discontinuities (>1eV) and allow the extension of the available wavelength range into the near infrared. Such short wavelengths require narrow QWs (<3 nm) where the first excited state inside the QW may be raised above indirect (X or L) valleys within the Brillouin zone. Quantum cascade lasers involving subbands above the indirect minimum have recently been reported [1].
We have studied intersubband relaxation dynamics in In0.53Ga0.47As/AlAs0.56Sb0.44 multiple QWs with thicknesses from 2.9 to 4 nm (corresponding to absorption wavelengths of 2.4 to 3.2 µm) by femtosecond pump-probe experiments [2]. At early delay times, all samples show an exponential decay of the transient transmission occurring with time constants of 0.8 to 1.5 ps. The relaxation dynamics at later delay times strongly depends on the QW thickness and doping. For very narrow QWs the observed bi-exponential decay indicates several competing relaxation channels. Here transfer of electrons to X- and L-states in the barriers, which exist in the case of n-type modulation doping, or in the wells is energetically possible. The data are analyzed in terms of an effective three-level configuration. Our results indicate that intervalley scattering in QWs is in the ps regime, much slower than in a bulk semiconductor. This observation suggests that intersubband lasing involving states above indirect minima of the well material should be possible, as also confirmed by the results of [1].
[ ] D. G. Revin et al., Appl. Phys. Lett. 90, 021108 (2007).
[2] C. V.-B. Tribuzy et al., Appl. Phys. Lett. 89, 171104 (2006).

We describe a fast optical tomography sensor which has been designed for the investigation of single and two phase flows in pipes and bubble columns. It enables image acquisition at frame rates of up to 4.5 kHz at roughly 2 mm image resolution. The sensor works similar to a conventional CT with 256 light emitters and 32 light receivers arranged about the object’s cross-section. The light emitters are sequentially flashed while the light receiver intensities are recorded synchronously. Primary area of application is single phase flows with dye tracers. Another potential application is the investigation of bubbly two phase flows at low gas fractions. Principle tests have been made for both problems.

For each accelerator the choice of the injector is a crucial topic and the best gun concept for the different demands must be determined. This paper presents simulated data of beam parameters from the Rossendorfer SRF gun and other RF photo injectors. The differences between normal and superconducting guns lead to different possibilities in the operation modes. Due to the high RF power losses in normal conducting guns the duty cycle is low and cw-mode operation impossible. For the SRF gun an exterior magnetic field for emittance compensation close to the photo cathode is prohibited due to the Meißner-Ochsenfeld effect. Therefore, other mechanisms for emittance compensation must be used. In the simulations RF focusing with a proper cathode visor and a solenoid focusing after the gun are considered.

It has been shown that the performance of GaAs/AlGaAs quantum cascade lasers (QCL) can be severely hampered by the influence of indirect states [1]. Yet very recent work has demonstrated that lasing can be achieved in InGaAs/AlAsSb [2], InGaAs/AlAs [3] and InAs/AlSb [4] at around 3 um, where the upper laser level lies above some indirect minima.
We have performed a pump-probe investigation of intersubband relaxation in doped narrow In0.53Ga0.47As/AlAs0.56Sb0.44 multi quantum wells (QW) with different thicknesses from 2.9 to 4 nm, grown by MBE latticed matched to an InP substrate. The measurements were performed with a high-repetition-rate (78 MHz) optical parametric oscillator tunable between 1.1 and 3.3 um with a pulse length of 280 fs [5]. The extremely high signal-to-noise ratio allows us to analyze the decay dynamics in detail.
In a well-doped 3 nm QW, where the second subband lies above the InGaAs X-minimum, we observe a non-exponential decay, which can be very well reproduced with two exponentials of 1.5 and 6.2 ps (Fig. 1). Analyzing this behavior with three-level rate equations, the first time constant represents the combined decay rate from the upper subband to the lower one and to the X-state. The second time constant correspond to the return time from the X-level to the ground state. This means that the intervalley transfer time (≥ 2 ps) is much longer than known from bulk systems and implies that population inversion in a QCL can persist, thus explaining the functioning of QCLs at wavelength as short as 3 um.
As a cross check, we also investigated wider-QW samples, where the second subband lies below the X-level. As expected, these were found to exhibit a simple mono-exponential behavior. We are presently extending this investigation to coupled QWs, which bear closer similarity to actual QCL structures.

References
[1] L. R. Wilson et al., Appl. Phys. Lett. 81, 1378 (2002).
[2] D. G. Revin et al., Appl. Phys. Lett. 90, 021108 (2007).
[3] M. P. Semtsiv et al., Appl. Phys. Lett. 90, 051111 (2007).
[4] K J. Devenson et al., Appl. Phys. Lett. 90, 111118 (2007).
[5] C. V.-B. Tribuzy et al., Appl. Phys. Lett. 89, 171104 (2006).

Amorphous silicon (a-Si) layers for the passivation of p-type silicon wafer surfaces are investigated. The first main topic is the thermal stability of a-Si passivation. Here, an improved thermal stability in annealing (400 °C) and firing processes (wafer temp. 550 °C) could be achieved by stacking a-Si layers with PECVD SiOx layers of different thickness. Hydrogen depth profiling using nuclear reaction analysis shows a hydrogen concentration of 11 at% in the bulk of the a-Si. After firing of single layer a-Si samples a hydrogen concentration peak at the a-Si / c-Si interface could be observed. The second major topic of this paper is the deposition of a-Si layers using an industrial-type inline PECVD reactor. Excellent surface passivation (>1 ms on 1 Ohm cm FZ wafers) can be reported. These a-Si layers are further characterised using FTIR and spectroscopic ellipsometry.

A status report of the superconducting RF photo electron injector development at Forschungszentrum Dresden-Rossendorf (FZD) will be given. The SRF gun project is a collaboration of BESSY, DESY, MBI and FZD and aims at the installation of a high average current CW photo injector at the ELBE superconducting electron linac. Main design parameters of the SRF gun are an electron energy of 9.5 MeV, a maximum average current of 1 mA, transverse normalized emittances (rms) of 1 mm mrad at 77 pC and 2.5 mm mrad at 1 nC bunch charge. The 1.3 GHz niobium cavity consists of three full cells with TESLA geometry, a specially designed half-cell in which the photo cathode is placed, and a choke filter in order to prevent rf losses at the cathode side of the cavity. Presently, the helium tank welding and cavity treatment have been finished. The cavity is now in the FZD and the cryomodule assembly has been started. Various subsystems like cathode cooler, cavity tuners, cryostat components, and the niobium cavity are still being tested and measured. A photo cathode preparation system was developed and installed. The equipment is now in operation and the first series of Cs2Te photo cathodes have been produced. The development of the 262 nm driver laser system for the high charge mode (500 kHz, 1 nC) is finished. A diagnostic beamline, which is especially designed for the SRF gun parameter measurement, is being build up.

Almost eight years ago the first magnetic field was created by an intense flow of molten sodium in the Riga dynamo experiment. Since then a lot of data has been collected. In our geometry dominates the vertical field component which is remarkably strong with a maximum value exceeding 0.1 T. The principal symmetry of field with respect to the vertical axis is m=1. As such a strong field considerably deforms the flow some additional modes with m=3, 5 and even 7 are observed. The field pattern rotates around the axis with a frequency of 1.2 to 1.8 Hz. This rotation is not completely constant as turbulence in sodium flow creates some turbulence in magnetic field, too. Nevertheless the field turbulence is low and causes no considerable secondary electromagnetic effects. Hence the field generation itself is based on the mean-flow only. For the last two years the experiment was in repair. Among other things a worn out sodium seal was replaced by a magnetic coupler. This will widen the measuring possibilities particularly in the vicinity of the threshold. We plan to restart experiments this summer and hope to report new results at the meeting.

The study of crystal damage by ion implantation with high current densities is important to obtain better insights in the processes taking place during ion implantation (under ex-treme conditions). The combination of a Focussed Ion Beam (FIB) and an ion microprobe with channeling capability within the same institute facilitates this type of research by enabling small implanted areas to be analysed. In this study Ga was implanted by FIB into ZnO at different ion fluxes and fluences. The fluences were varied from 10¹³ to 10¹⁷ at/cm² and fluxes of 5•10¹³ at. cm-²s^-1 and 5•10¹⁸ at. cm-²s^-1 were used. The implanted areas were analysed with a 1 MeV He+ ion beam focussed to well below the size of the implanted area (about 100 x 100 µm²). The damage of the sample caused by the ion microprobe was also studied and the fluence for the analysis has been chosen so low that no significant damage occurs.
The results of the channeling measurements are presented and the effects of flux and flu-ence on the crystal damage are discussed. It can be noted that the effects of the flux are relatively minor and crystal damage in ZnO occurs only at high fluences

Surface passivation stack systems, all deposited using PECVD, are investigated. Stacks of SiOx, SiNx and SiOx (PECVD-ONO) are shown to be a suitable passivation layer system for the rear of silicon solar cells (p type bulk). The thermal stability during annealing at 425 °C and firing of screen printed front contacts could be shown with surface recombination velocities below 60 cm/s after firing. Solar cell precursors without metallisation showing implied Voc values above 680 mV are presented. Hydrogen depth profiling using nuclear reaction analysis (NRA) shows the hydrogen distribution after deposition and different thermal treatments. Finally, solar cells using the new stack system as rear passivation and laser-fired rear contacts are presented with a peak efficiency of 19.4 %.

We have investigated the dephasing time associated with intersubband transitions of photocarriers in an undoped GaAs/AlGaAs multi quantum-well heterostructure. Our measurements were performed directly in the time-domain. After optical generation of electron-hole pairs across the band-gap, a resonant THz-pulse excited the electrons to the second subband and generated a coherent polarization involving the ground and the first excited subbands. The re-radiation from this polarization was detected by a cross-correlation technique with a second THz-pulse. The polarization was observed to decay with short decay-times between 50 fs and approx. 200 fs. They depend on the carrier concentration which was adjusted by the optical excitation power. These time constants determine directly the linewidth of this intersubband transition. At low temperatures, the dephasing signals show a pronounced beating at all optical excitation powers which we attribute to excitonic effects such that more than two energy levels are involved in the interaction with the THz-pulse.
By varying the excitation power, we also found a strong depolarization shift of the absorption line.

The superconducting linear accelerator ELBE (Electron Linac for beams with high Brilliance and low Emittance) at the Forschungszentrum Rossendorf has a wide variety of experimental applications ranging form nuclear spectroscopy over x-ray to infrared FEL experiments. The two superconducting accelerator modules of ELBE and in the future a superconducting photo injector are connected to a 200 W at 1.8 K cryogenics plant. The presentation will give an overview of the ELBE cryogenics system and will focus on operation experience with the superconducting cryogenics modules and of the cryogenic plants. Specific problems of fill level stability, helium pressure stability and cool down procedures of the cryomodules will be presented. Specific problems of the ELBE helium plant concerning purity, maintenance and reliability will be discussed, too.

Ultrafast midinfrared spectroscopy is used to probe dynamics in the intermediate bandwidth manganite Nd_0.5Sr_0.5MnO₃. In the majority paramagnetic and ferromagnetic phases, the early time dynamics are consistent with the excitation and subsequent redressing of uncorrelated lattice polarons, with longer time dynamics related to spin-lattice thermalization. These polaron excitations reveal the intrinsically inhomogeneous nature of these phases. At lower temperatures we observe ultrafast melting of charge-orbital order, liberating quasiparticles that subsequently relax into bound polaronic states on a subpicosecond time scale. The temperaturedependent amplitude of the polaron excitations scales with the volume fraction of the CE phase. Thus, polaron dynamics, as measured using ultrafast spectroscopy, serve as a sensitive probe of phase inhomogeneity.

Elastomer materials filled with magnetically and/or electrically susceptible particles promise to have different functionality than conventional elastomers, and therefore, could likely be applied in state-of-the-art control technologies [1]. Of particular interest are elastomers, which are filled with metal coated carbon black (MCCB). These filler fulfils its reinforcing function to the rubber and also chang the elastomer´s electro-magnetic properties. Thus, the rheological and viscoelastic properties of rubber can be changed and controlled by subjecting the compound to a magnetic field.

If the easy magnetization direction of a 3d-4f ferrimagnet is perpendicular to a high-symmetry axis, a magnetic field applied in the easy direction may induce a number of first-order transitions, the first (lowestfield ) one of which carries the information sufficient for an unambiguous determination of the intersublattice molecular field. This idea has been used to find the molecular field in Er2Fe17. To this end, magnetization curves have been measured in pulsed magnetic fields of up to 50 T applied along [100] or [001]. In order to obtain a reference value of the molecular field by a conventional method, high-field measurements have also been performed on crystals free to rotate. The molecular fields determined by both techniques are in good agreement with each other as well as with the values deduced from literature data.

We present a novel non-resonant photoconductive THz detection antenna. Compared to electro-optic sampling, photoconductive antennas can be integrated more easily into compact THz setups, e.g. by using substrate lenses and by coupling to optical fibers. However, since the antenna gap of typical THz detection antennas is usually only a few µm wide, the alignment of photoconductive antennas is not simple and the possibility to move the antenna is limited. Here we report on a THz system consisting of a scalable THz emitter based on an interdigitated electrode structure [1] and a detection antenna with similar electrode geometry. While the THz emitter is fabricated on SI-GaAs substrate, various materials with short carrier lifetimes are used for the detection antenna. Detection antennas based on LT-GaAs and GaAs implanted with As+ (dual-energy implants, 1MeV and 2.4 MeV, doses in the range from 10e13 cm-2 to 10e16 cm-2) and N+ (dual-energy implants, 0.4 MeV and 0.9 MeV, doses in the range from 10e12 cm-2 to 10e14 cm-2) are compared. The strongest detected signals are found for As+ implantations with a dose in the 10e14 cm-2 range. Antenna currents of up to 7 nA are observed, the corresponding spectrum extends up to 4 THz. The THz radiation was modulated with frequencies up to 100 kHz. In this range the detected signal was independent of the modulation frequency. From the RC time constant of the detector antenna a cut-off frequency of 500 kHz is expected. Furthermore the dependence of the detected signal on the gating laser power and the spot size of the gating beam were studied and optimum conditions are discussed. In conclusion, the scalable antennas constitute an efficient, easy-to-use, symmetric emitter-detector pair.
[1] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114 (2005).

We present a terahertz transceiver consisting of a photoconductive emitter and detection antenna with similar electrode geometry. Here, we focus on the detection antenna. Compared to electro-optic sampling, photoconductive antennas can be integrated more easily into compact THz setups, e.g. by using substrate lenses and by coupling to optical fibers. However, since the antenna gap of typical THz detection antennas is usually only a few µm wide, the alignment of photoconductive antennas is not simple and the possibility to move the antenna is limited. The transceiver consists of a scalable THz emitter based on an interdigitated electrode structure [1] and a detection antenna with similar electrode geometry. While the THz emitter is fabricated on SI-GaAs substrate, various materials with short carrier lifetimes are used for the detection antenna. Detection antennas based on LT-GaAs and GaAs implanted with As+ (dual-energy implants, 1 MeV and 2.4 MeV, doses in the range from 1013 cm-2 to 1016 cm-2) and N+ (dual-energy implants, 0.4 MeV and 0.9 MeV, doses in the range from 1012 cm-2 to 1014 cm-2) are compared. The strongest detected signals are found for As+ implantations with a dose in the 1014 cm-2 range. In Fig. 1 THz transients detected with an LT-GaAs antenna are shown together with the corresponding power spectra. The spectra extend up to 4 THz. The dependence of the detected signal on the gating laser power and the spot size of the gating beam were studied. For 70 mW power of the gating beam, a spot size in the range from 200 to 700 µm yields the strongest signals. The lower limit is determined by a nonlinear dependence of the signal on the excitation density. The upper limit for the spot size is simply given by the size of the antenna, which was 1 mm x 1 mm. Furthermore the detector was used to study the spatial profile of the THz beam. In this experiment, the detector was placed 27 mm behind the emitter and scanned across the THz beam. The beam had Gaussian shape with a full width at half maximum of 8 mm and 18 mm for frequency components of 0.7 THz and 0.2 THz, respectively. This experiment demonstrates the potential of the detector to map out unfocussed THz fields with a good signal-to-noise ratio. In conclusion, the scalable antennas constitute an efficient, easy-to-use, symmetric emitter-detector pair.
[1] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114 (2005).

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The development of quantum cascade lasers (QCL) for frequencies as low as 3 µm has gained a lot of attention recently. While the performance of GaAs/AlGaAs quantum cascade lasers (QCL) can be severely hampered by the influence of indirect states [1], very recent work has demonstrated that lasing can be achieved in InGaAs/AlAsSb [2], InGaAs/AlAs [3] and InAs/AlSb [4] at around 3 m, where the upper laser level lies above some indirect minima.
We have performed a pump-probe investigation of intersubband relaxation in doped narrow In0.53Ga0.47As/AlAs0.56Sb0.44 multi quantum wells (QW) with different thicknesses from 2.9 to 4 nm, grown by MBE latticed matched to an InP substrate. The measurements were performed with a high-repetition-rate (78 MHz) optical parametric oscillator tunable between 1.1 and 3.3 µm with a pulse length of 280 fs [5]. The extremely high signal-to-noise ratio allows us to analyze the decay dynamics in detail.
The relaxation dynamics of a well-doped 3 nm QW was studied with 130 pJ pulses at a wavelength of 2.4 µm. In this sample, where the second subband lies above the InGaAs X-minimum, we observe a non-exponential decay, which can be very well reproduced with two exponentials of 1.5 and 6.2 ps (Fig. 1). Analyzing this behavior with three-level rate equations, tau =1.5 ps represents the combined decay rate from the upper subband to the lower one and to the X-state. 6.2 ps correspond to the return time from the X-level to the ground state (see inset). This means that the intervalley transfer time (tauX2 ≥ 2 ps) is much longer than known from bulk systems and implies that population inversion in a QCL can persist, thus explaining the functioning of QCLs at wavelength as short as 3 µm.
As a cross check, we also investigated wider-QW samples, where the second subband lies below the X-level. As expected, these were found to exhibit a simple mono-exponential behavior. We are presently extending this investigation to coupled QWs, which bear closer similarity to actual QCL structures.

References
[1] L. R. Wilson et al., Appl. Phys. Lett. 81, 1378 (2002).
[2] D. G. Revin et al., Appl. Phys. Lett. 90, 021108 (2007).
[3] M. P. Semtsiv et al., Appl. Phys. Lett. 90, 051111 (2007).
[4] K J. Devenson et al., Appl. Phys. Lett. 90, 111118 (2007).
[5] C. V.-B. Tribuzy et al., Appl. Phys. Lett. 89, 171104 (2006).

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Terahertz radiation from a large-area Photoconductive device

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We present studies of the radiation properties of a photoconductive terahertz (THz) structure [1]. It consists of an interdigitated electrode structure fabricated on GaAs. Illuminating this structure by a femtosecond laser pulses yields accelerated photocarriers, which are the source of THz radiation. For avoiding destructive interference of radiation generated in regions of opposite field direction a second metallization isolated from the first one covers every second electrode spacing. Intense THz radiation with fields of the order of 1 kV/cm is observed. We use a photoconductive detection antenna for measuring the spatial profile. The detection antenna is placed in a distance of 13 mm from the emitter. The beam profile is resolved for spectral components in the range from 0.5 to 1.5 THz. All beam profiles have Gaussian shape. The divergence increases with decreasing frequency. For wavelengths significantly smaller than the excitation spot size, the results can be well described by Gaussian optics. However, at longer wavelength, where the paraxial approximation fails, diffraction has to be considered in a more general way.
[1] A. Dreyhaupt, S.Winnerl, M.Helm, T. Dekorsy, Opt. Lett. 31, 1546 (2006)

We present a novel scalable photoconductive THz detection antenna. Compared to electro-optic sampling, photoconductive antennas can be integrated more easily into compact THz setups, e.g. by using substrate lenses and by coupling to optical fibers. However, since the antenna gap of typical THz detection antennas is only a few µm wide, the alignment of photoconductive antennas is not simple and the possibility to move the antenna is limited. Here we report on a THz system consisting of a scalable THz emitter based on an interdigitated electrode structure [1] and a detection antenna with similar electrode geometry. While the THz emitter is fabricated on SI-GaAs substrate, various materials with short carrier lifetimes are used for the detection antenna. Detection antennas based on LT-GaAs and GaAs implanted with As+ (dual-energy implants, 1 MeV and 2.4 MeV, doses in the range from 1013 cm-2 to 1016 cm-2) and N+ (dual-energy implants, 0.4 MeV and 0.9 MeV, doses in the range from 1012 cm-2 to 1014 cm-2) are compared. The strongest detected signals are found for As+ implantations with a dose in the 1014 cm-2 range. A comparison with electro-optic sampling indicates that the carrier lifetime in this material is of the order of 0.4 ps. Furthermore the dependence of the detected signal on the gating laser power and the spot size of the gating beam were studied and optimum conditions are discussed. The antenna is suitable for detection of THz radiation which is not focussed to a spot of the order of the THz wavelength. We demonstrate this by showing the frequency resolved beam profile of an unfocussed THz beam. In conclusion, the scalable antennas constitute an efficient, easy-to-use, symmetric emitter-detector pair.
[1] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114 (2005).

Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot is studied using far-infrared, time-resolved pump-probe measurements for transitions between the s-like ground and p-like first excited conduction band states. Spin-flip transitions between singlet and triplet states are observed experimentally in the decay of the absorption bleaching, which shows a clear biexponential dependence. The initial fast decay (∼ 30ps) is associated with the singlet polaron decay, while the decay component with the longer time constant (∼ 5 ns) corresponds to the excited state triplet lifetime. The results are explained by considering the intrinsic Dresselhaus spin-orbit interaction, which induces spin-flip transitions by acoustic phonon emission or phonon anharmonicity. We have calculated the spin-flip decay times, and good agreement is obtained between the experiment and the simulation of the pump-probe signal. Our results demonstrate the importance of spin-mixing effects for intraband energy relaxation in InAs/GaAs quantum dots.

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In the present work we applied the Optically read out PArticle track Chamber, OPAC, for the measurement of radial dose distributions, d(r), around tracks of heavy ions passing through the gas-filled sensitive volume of the chamber. The measured data were compared with d(r) functions derived from data calculated with the Monte Carlo particle transport code, TRAX – which is used for the heavy ion therapy planning at GSI.
To measure this quantity we have used here an optically read out time projection chamber (OPAC) with a parallel-drift field and one or several electron and light amplification stages. The two dimensional projection of the three dimensional ionization pattern caused by the ionizing particle passing through the chamber is captured by an image intensified CCD camera.
The work is motivated by the role the radial dose distribution plays in the estimation of the relative biological effectiveness (RBE) of heavy ions, e.g. in radiation therapy and in radiation protection. The most successful model for high-dose irradiation with ions (applicable e.g. for heavy ion therapy) is found to be the local effect model (LEM). The present work intends to deliver measured data for one of the basic physical parameters which serve as input for the application of the local effect model: the radial dose distribution, d(r).
The first goal of our measurement program was the measurement of d(r) distributions around carbon ions of different energies from 400 MeV/u down to the Bragg peak regions. We found an excellent agreement between the measured and simulated distributions at all carbon energies for the r–range in which the measurements deliver useful results. The lower limit of this range is about 100 nm and the upper limit is 6000 nm at a resolution of down to 33 nm ­ if scaled to water density.
Despite the simplifications in the TRAX code (e.g. binary encounter theory for the emission ionization electrons), the discrepancies between the simulated and measured d(r) distributions are found to be lower than the measurement uncertainties at most measured carbon ion energies in almost the whole observed r-range. Hence, within the limitations of our measurements we can conclude that the precision of TRAX is sufficient to simulate the d(r) distributions around carbon ions to serve as input parameter for therapy planning. However, this conclusion is only valid for larger radial distances (r >100 nm). For smaller radial distances the measured data are dominated by the diffusion.
Apart from carbon ion tracks, tracks of very heavy ions (40Ar, 84Kr and 238U) were also measured with OPAC. The simulated d(r) values were typically slightly or significantly higher than the measured data in the 100 nm < r < 5000 nm region.
The energy values of the very heavy ions were selected with the aim of comparing the track structures - and namely the d(r) distributions - of ions with largely different atomic mass but similar LET values. From the Z­dependency of the stopping power we know that for heavier ions a higher specific ion energy (expressed in MeV/u) is required to provide the same LET. For example the common LET of 315 keV/μm was achieved at largely different specific energy levels of 4,4 MeV/u for 12C, 65 MeV/u for 40Ar and 650 MeV/u for 84Kr ions. The difference in the track structures was expected mainly due to the different ion velocities and thus e.g. different ranges of δ-electrons. This expectation could be confirmed by the measurements. The reason why ­in line with the simulations ­ no strong differences could be observed in the d(r) distributions of the argon and krypton ions is the relatively small difference in the velocities of the both ion types in conjunction with the limited range in r, where the data can be compared. In contrary, the d(r) function of the carbon ion shows a qualitatively different behavior than the heavier ions inside the observable radius-range ­ in agreement with the simulations.

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A 2-dimensional massively parallel self-actuated piezoresistive cantilever arrays are a possible candidate for use in high speed AFM based surface imaging systems. By the utilization of cantilever arrays consisting of several hundred of cantilevers the AFM inspection speed of the next generations IC can be significantly increased. In the frame of the European Project PRONANO we are developing such arrays based on self-actuated piezoresistive cantilever. For design optimization and error detection in MEMS structures, which include a lot of CMOS processing, a device and process simulations are useful and essential. Besides the description of the electro-mechanic behaviour of the MEMS-part a parasitic effects (electrical crosstalk, noise, and temperature influence) of the CMOS have to be included. SPICE is well suitable tool for these investigations.
The investigated cantilever system consists of a p-doped piezoresistive sensor and a p-doped heater is represented qualitatively in Figure 2. The meander structure is the ion implanted heater to control the thermal bending of the silicon beam. An AC power supply is applied to the heater to bring the beam in resonant oscillation and to steer the position of the free end of the beam. The beam bending is determined with the piezoresistor. Experimentally a current crosstalk was detected. At the configuration a voltage peak occurs during the positive half wave of the sensor signal. The magnitude of this voltage peak depends on the dc-voltage of the heater signal. To simulate this effect a suitable equivalent network model was developed. The heater and the piezoresistor were modeled by resistor chains, which are interconnected to the n-doped silicon body (resistor network) by diodes including the junction capacitance. Additionally pnp-transistors were inserted in the equivalent circuit, where heater and piezoresistor are located near each other.
With this circuit model the experimentally determined behavior could be simulated and the effect could be explained as a current crosstalk across the parasitic transistors. We will show that this effect can be significantly suppressed by applying a certain substrate bias and corresponding design optimisation.

We present a family of equations of state described within a quasiparticle model adjusted to first-principles lattice QCD calculations and study the impact on azimuthal flow anisotropies and transverse momentum spectra within hydrodynamic simulations for heavy-ion collisions at energies relevant for LHC.

CFD code validation requires experimental data that characterize distributions of parameters within large flow domains. On the other hand, the development of geometry-independent closure relations for CFD codes have to rely on instrumentation and experimental techniques appropriate for the phenomena that are to be modelled, which usually requires high spatial and time resolution. The presentation reports about the use of wire-mesh sensors to study turbulent mixing processes in the single-phase flow as well as to characterize the dynamics of the gas-liquid interface in a vertical pipe flow. Experiments at a pipe of a nominal diameter of 200 mm are taken as the basis for the development and test of closure relations describing bubble coalescence and break-up, interfacial momentum transfer and turbulence modulation for a multi-bubble-class model. This is done by measuring the evolution of the flow structure along the pipe. The transferability of the extended CFD code to more complicated 3D flow situations is assessed against measured data from tests involving two-phase flow around an asymmetric obstacle placed in a vertical pipe. The obstacle, a half-moonshaped diaphragm, is movable in the direction of the pipe axis; this allows the 3D gas fraction field to be recorded without changing the sensor position. In the outlook, the pressure chamber of TOPFLOW is presented, which will be used as the containment for a test facility, in which experiments can be conducted in pressure equilibrium with the inner atmosphere of the tank. In this way, flow structures can be observed by optical means through large-scale windows even at pressures of up to 5 MPa. The so-called “Diving Chamber” technology will be used for Pressurized Thermal Shock (PTS) tests. Finally, some important trends in instrumentation for multi-phase flows will be given. This includes the state-of-art of X-ray and gamma tomography, new multi-component wire-mesh sensors, and a discussion of the potential of other non-intrusive techniques, such as neutron radiography and Magnetic Resonance Imaging (MRI).

Für das unwahrscheinliche Szenario eines Kernschmelzunfalls in einem Leichtwasserreaktor mit Bildung eines Schmelzesees in der Bodenkalotte des Reaktordruckbehälters (RDB) ist es notwendig, mögliche Versagensformen des RDB sowie Versagenszeiträume zu ermitteln, um die daraus resultierende mögliche Belastung des Sicherheitsbehälters bestimmen zu können. In dieser Arbeit wird ein integrales Modell entwickelt, das die Vorgänge im unteren Plenum beschreibt. Dabei sind zwei prinzipielle Modellbereiche zu unterscheiden: Das Temperaturfeld in der Schmelze und im RDB wird mit einem thermodynamischen Modell berechnet, während für die Strukturanalyse des RDB ein mechanisches Modell verwendet wird.
Das thermodynamische und das mechanische Modell können rekursiv gekoppelt werden, wodurch die wechselseitige Beeinflussung berücksichtigt werden kann. Insbesondere werden damit neben der Temperaturabhängigkeit der Materialparameter und den thermisch induzierten Spannungen im mechanischen Modell auch die Rückwirkungen der Behälterverformung auf das Temperaturfeld selber erfasst.
Für die Kriech- und Schädigungssimulation werden in dieser Arbeit neue Verfahren angewendet. Durch die Entwicklung und den Einsatz einer Kriechdatenbasis konnte die bei sehr unterschiedlichen Temperaturen, Spannungen und Dehnungen ungeeignete Verwendung einzelner Kriechgesetze umgangen werden. Aufbauend auf experimentellen Untersuchungen wurde eine Kriechdatenbasis für einen RDB-Stahl entwickelt und an Hand von Kriechversuchen verschiedener Geometrie und Dimension validiert.
Die wesentlichen Ergebnisse dieser Arbeit lassen sich wie folgt zusammenfassen: Aufgrund des thermodynamischen Verhaltens eines großen Schmelzesees mit inneren Wärmequellen erfolgt die höchste thermomechanische Belastung des RDB im oberen Drittel der Bodenkalotte. Dieser Bereich wird als heißer Fokus bezeichnet. Der untere Bereich der Kalotte weist hingegen eine höhere Festigkeit auf und verlagert sich deswegen bei entsprechender Belastung des RDB im wesentlichen senkrecht nach unten. Bei einer externen Flutung besteht auch bei hohen Innendrücken für einen Reaktor großer Leistung die Möglichkeit, die Schmelze im RDB zurückzuhalten. Ohne interne oder externe Flutung besteht für das betrachtete Szenario keine Aussicht für eine Schmelzerückhaltung im RDB.
Aus den gewonnenen Erkenntnissen wurden zwei Patente abgeleitet. Dabei handelt es sich um passiv wirkende Einrichtungen zur Schadensbegrenzung: Die erste reduziert durch Abstützen des unteren Kalottenzentrums die Maximalspannungen im hochbeanspruchten Bereich des heißen Fokus und kann damit ein Versagen verhindern oder zumindest verzögern. Die zweite Einrichtung ermöglicht die passive Auslösung einer Flutung, indem die Abwärtsbewegung der Kalotte zur Steuerung genutzt wird. Hierdurch kann beispielsweise ein Ventil geöffnet werden, um Wasser aus im Gebäude höher angeordneten Reservoirs in die Reaktorgrube zu leiten.
Abweichend von bisherigen Annahmen kann festgehalten werden, dass eine Kernschmelzerückhaltung im Reaktordruckbehälter auch für Reaktoren größerer Leistung möglich ist.

In explosive nucleosynthesis temperatures are high enough for photodissociation reactions to occur, e.g. leading to the production of p-process nuclei. In order to understand the reaction rates of element production and element disruption we started an experimental program at the new bremsstrahlung facility of the superconducting electron accelerator ELBE of FZ-Rossendorf, Dresden. The bremsstrahlung facility and the detector setup are designed such that the scattering of photons from nuclei and the photodissociation of nuclei around the particle separation energies can be studied under optimized background conditions. In activation measurements with bremsstrahlung at end-point energy from 10.0 to 16.5 MeV (gamma,p), (gamma,n) and (gamma,alpha) reactions of 92,100 Mo. have been studied. Our activation yields can be described within a factor 2-3 or better with calculations using the cross sections from recent Hauser-Feshbach models.

In our experiments we investigated the consequence of an application of a DC magnetic field on both the bubble and the liquid velocity. The motion of single argon bubbles rising in GaInSn were analyzed in terms of the terminal velocity, the drag coefficient, the oscillation frequency of the bubble velocity and the Strouhal number. Because the gas bubble is electrically non-conducting, it does not experience the effect of the electromagnetic force directly. However, the bubble behaviour is influenced by the magnetically induced modifications in the liquid flow structure around the bubble. The measurements reveal a distinct effect of the magnetic field on the bubble velocity as well as the bubble wake. The magnetic field application leads to a mitigation of the horizontal components of the bubble velocity resulting in a more rectilinear bubble path. A restructuring of the entire flow field can be observed if a bubble plume is exposed to a DC magnetic field. As a result of the interaction between magnetic field and liquid flow electric currents were induced inside the liquid causing a damping of the flow by Joule dissipation. However, a characteristic feature of the electromagnetic dissipation is the anisotropy. Thus, the application of a transverse field leads not only to a general damping of the flow, but also favours the occurrence of vortices aligned parallel to the magnetic field direction.

A superconducting radio frequency (RF) photoelectron injector (SRF gun) is under development at the Research Center Dresden–Rossendorf. This project aims mainly at replacing the present thermionic gun of the superconducting electron linac ELBE. Thereby the beam quality is greatly improved. Especially, the normalized transverse emittance can be reduced by up to one order of magnitude depending on the operating conditions. The length of the electron bunches will be shortened by about two orders of magnitude making the present bunchers in the injection beam line dispensable. The maximum obtainable bunch charge of the present thermionic gun amounts to 80 pC. The SRF gun is designed to deliver also higher bunch charge values up to 2.5 nC. Therefore, this gun can be used also for advanced facilities such as energy recovery linacs (ERLs) and soft X-ray FELs. The SRF gun is designed as a Click to view the MathML source cell cavity structure with three cells basically TESLA cells supplemented by a newly developed gun cell and a choke filter. The exit energy is projected to be 9.5 MeV. In this paper, we present a description of the design of the SRF gun with special emphasis on the physical and technical problems arising from the necessity of integrating a photocathode into the superconducting cavity structure. Preparation, transfer, cooling and alignment of the photocathode are discussed. In designing the SRF gun cryostat for most components wherever possible the technical solutions were adapted from the ELBE cryostat in some cases with major modifications. As concerns the status of the project the design is finished, most parts are manufactured and the gun is being assembled. Some of the key components are tested in special test arrangements such as cavity warm tuning, cathode cooling, the mechanical behavior of the tuners and the effectiveness of the magnetic screening of the cavity.

Water-soluble clips are possible molecules to study the weak, non-covalent interactions, responsible for many biological processes. The complexation behaviour of naphthalene clips, substituted with phosphonate or phosphate groups, has been studied by time-resolved laser-induced fluorescence spectroscopy with ultrafast pulses (fs-TRLFS). As guest molecules we used nucleic bases, nucleosides and nucleotides. The determined association constants for the 1:1 complexes range from log Ka = 3.9 – 4.2 and are around one order of magnitude higher than published in literature. The measured spectra assumes the appearance of excited state species or dimerization of the clip.

Ripple formation with a spatial periodicity in the sub-micrometer range on obliquely ion-bombarded solid surfaces has become a topic of intense research in the context of fabrication of nanoscale textured materials [1]. Ion-beam induced ripples are produced by the interplay between a roughening process caused by ion beam erosion (sputtering) of the surface and smoothening processes caused by thermally or ion-induced surface diffusion. Recently we have shown that Ar- ion irradiation of Si (001) surfaces under an angle of about 60° with respect to surface normal and using ion energies of about 60keV results in periodic crystalline ripple formation where the crystalline ripples are covered by a partially amorphous surface layer [2, 3]. In this contribution we report on investigations of patterned Si (001) surfaces after irradiation with Xe-ions using ion-energies between 5 and 40keV. Besides AFM measurements, the structure of the amorphous layer and the amorphous-crystalline interface were studied by means of grazing-incidence small-angle scattering (GISAXS) and grazing-incidence diffraction (GID) using synchrotron-radiation. The data reveal that both surface and crystalline ripples appear for all ion energies used. These ripples show asymmetric side-facets where the degree of asymmetry decreases for increasing ion-energy. They show short-range ordering; the ripple wavelength and thickness of the amorphous layer increase as a function of the ion-energy.

Acknowledgement: One of the authors (A.B.) would like to thank the ESRF for financial support and the ID01 beamline-staff for providing beamtime and valuable help.

[1] M.A. Makeev, R Cuerno and A.-L. Barabasi, Nuc. Inst. and Meth. in Phys. Res. B 197, 185 (2002).
[2] S.Hazra, T.K. Chini, M.S. Sanyal, J. Grenzer U. Pietsch , Phys. Rev. B 70, 121307(R) (2004).
[3] S. Grigorian, J. Grenzer, D. P. Datta, S. Hazra, T.K. Chini, M. K. Sanyal and U. Pietsch, Appl. Phys. Lett 89, 231915 (2006).

Model experiments with low melting point liquid metals become an important tool to investigate the flow structure and related transport processes in melt flows being relevant for metallurgical applications. This approach is encouraged by recent developments of appropriate flow measuring techniques in the temperature range until about 300°C. The ultrasound Doppler velocimetry (UDV) which delivers instantaneous profiles of the local velocity along the ultrasonic beam is a very attractive technique to attain experimental data from flows of opaque liquids.
AC magnetic fields are used in industrial practice for melt stirring. The requirements are manifold for miscellaneous metallurgical operations or casting technologies, mainly the magnetic field application should provide an efficient mixing of the melt in order to achieve homogeneous distributions of solute and/or temperature. Here the standard case of electromagnetic stirring by means of a rotating magnetic field (RMF) is considered. Especially, we study the melt flow excited by an application of an intermittently or alternately applied RMF.

Melt convection significantly affects the solidification of metallic alloys. Therefore, the knowledge of the flow field in the melt is an important issue. Velocity measurements in liquid metals are complicated by the specific material properties. Recently, X-ray radioscopic methods became an important diagnostic tool for real-time and in-situ observation of the solidification front with a spatial resolution of a few microns.
Solidification experiments using Ga-30wt%In alloy (melting point 38°C) have been conducted. The metal alloy was poured in a flat solidification cell (150 µm gap thickness) made from two fused quartz glasses, and was heated above its melting point and was subsequently solidified by cooling of the solidification cell’s bottom. A microfocus X-ray radioscopy setup was used for qualitative real-time visualisation of concentration fluctuations and structure formation within the solidifying melt at a spatial resolution of 10 µm. Image integration times of 440 ms were found to be sufficient to ensure sufficient temporal sampling rate.
The optical flow method has been adopted to determine the velocity of the liquid and the dendritic growth rate from translocations of concentration contour lines appearing during a temporal image sequence. Therefore, Gaussian spatio-temporal low-pass filtering was applied to smooth spurious image brightness fluctuations caused by noise. Then any local image brightness variation was related to a certain amount of change in the local alloy composition. Image pattern deformations due to diffusion processes in the melt are considered to be negligible. Thus, the continuity equation was used as a first constraint to compute local translocation velocities in image regions providing sufficient brightness gradients. The physically justified assumption of a locally smooth velocity field was used as a second constraint.

In conjunction with flow measurements in nontransparent fluids like liquid metals, there is an increasing demand for the measurement of non-stationary flows. Ultrasound Doppler Velocimeters (UDV) belong to the standard equipment of research. UDV emit short ultrasound pulses into the fluid which have a typical duration equivalent to 2-8 wavelengths. The ultrasound is partly reflected by small tracer particles inside the fluid and scattered back to the transducer. The tracer particles are assumed to move without slip. The echo signals are recorded and can be used to estimate the fluid velocity. The measurement principle of Ultrasound Doppler Velocimetry, however, suffers from inherent constraints: One problem is the limitation in time resolution. For non-stationary flows a high temporal resolution is essential, because the relevant flow information is partly contained in the high frequency part of the velocity signal. In practice, the problem of time resolution occurs because an average over several send pulses is required to get accurate velocity estimates. Another problem is the maximum velocity that can be detected unambiguously with Doppler systems and related narrowband methods.
The actual problem with time resolution is the fact that a sending pulse only contains little signal energy, which leads to a low signal to noise ratio (SNR). The method under examination uses sending signals with a length of ~20µs instead of typically 0.5µs to 2µs in UDV systems. Long sending signals contain more signal energy and result in a better signal quality in terms of SNR. With higher SNR, however, less averaging is needed, and thus time resolution can be increased.
We propose the use of a short (~20µs) sending signal with linear chirp. The coding is similar to that of Frequency Modulated Continuous Wave Radar (FMCW Radar). The chirp is needed to obtain spatial resolution. Similar to the crosscorrelation approach in Pulse Doppler Ultrasound, it finally leads to a time-of-flight evaluation. The main difference, however, is that more ultrasound energy can be sent into the fluid. Also, the maximum velocity limit of Doppler systems does not apply for this method, so that velocities above this limit can be detected unambiguously.

The Ultrasound Doppler Velocimetry (UDV) is a non-intrusive technique to measure velocities of liquid flows. Because of the ability to work in opaque fluids and to deliver complete velocity profiles in real time it becomes very attractive for liquid metal applications. But, in case of hot metallic melts the user is confronted with a number of specific problems: First of all the application of the ultrasonic transducers is usually restricted to maximum temperatures of about 150°C. The transmission of a sufficient amount of ultrasonic energy from the transducer to the fluid has to be guaranteed. Here, the acoustic coupling and the wetting conditions have to be considered as important issues. Moreover, the flow has to be seeded with reflecting particles to obtain Doppler signals from the fluid.
The feasibility of velocity profile measurements by UDV has already been demonstrated for low temperature liquid metals as mercury and gallium. Complications arise for UDV applications in liquid metal flows at high temperatures. Recently, an integrated ultrasonic sensor with acoustic wave guide has been developed. The feasibility of this sensor concept has been demonstrated in diverse experiments using miscellaneous metallic melts until temperatures of about 700°C.
In this presentation we show various applications of UDV in liquid metal flows to demonstrate the capabilities and current restrictions of this technique. For instance, we consider single- and multi-transducer arrangements for flow mapping or present velocity measurements obtained during the solidification of a metallic melt. Besides the determination of velocity profiles in the liquid phase the UDV data allow for an assessment of the current position of the solidification front too. Specific problems arising in the context of UDV measurements in liquid metal experiments will be discussed.

Bubble driven flows have found wide applications in industrial technologies. In metallurgical processes gas bubbles are injected into a bulk liquid metal to drive the liquid into motion, to homogenize the physical and chemical properties of the melt or to refine the melt. For such gas-liquid metal two-phase flows, external magnetic fields provide a possibility to control the bubble motion in a contactless way.
Compared to the numerous experimental studies on the movement of bubbles in transparent liquids , especially in water, the number of publications dealing with gas bubbles rising in liquid metals is comparatively small. The shortage of suitable measuring techniques can be considered as one reason for the slow progress in the investigations of gas-liquid metal flows. Powerful optical methods are obviously not available for measurements in liquid metals. The majority of measurements in liquid metal two-phase flows published until now was obtained using local conductivity probes, hot wire anemometer or optical fiber probes to determine quantities such as void fraction, bubble and liquid velocity or the bubble size. However, measurements with any local probe disturb the flow in a significant way, especially if the structures to be investigated reach dimensions comparable to the probes. In the case of opaque liquids the application of acoustic or ultrasonic sensors offers a possibility to get information about the flow structure and bubble quantities. We applied the Ultrasound Doppler Velocimetry (UDV) for measurements of the velocity structure in liquid metal bubbly flows. Because of the ability to work non-invasively in opaque fluids and to deliver complete velocity profiles in real time it is very attractive for liquid metal applications.

It is well known, that Lorentz forces can be used to enhance the convection in electrochemical cells. Usually this enhancement leads to an increased limiting current [1], i.e. an increased deposition rate in the case of metal deposition. Currently, the influence of magnetic fields on the morphology of the deposited layers and the action of concentration gradient forces on deposition are intensively discussed. [2,3]
In our presentation, we will discuss the effect of magnetic forces during the electrodeposition of copper for different orientations of the external magnetic field. Besides new analytical findings, by numerical methods, we will present results on the velocity and the concentration field inside the cell from which interesting conclusions can be drawn. Three-dimensional effects are found to play an important role. Finally, a comparison with recent experimental results [4] will be performed.

[1] T.Z. Fahidy, J. Appl. Electrochem. 13 (1983) 553.
[2] A. Bund, H.H. Kuehnlein, J. Phys. Chem. B 109 (2005) 19845.
[3] J. M. D. Coey, F. M. F. Rhen, P. Dunne, S. McMurry, J. Solid State Electrochem. DOI: 10.1007/s10008-006-0254-4.
[4] A. Bund, S. Koehler, H.H. Kühnlein, W. Plieth, Electrochimica Acta 49 (2003) 147.

Preface:

For more than a decade, Prof. Frank Pobell has worked as an editor for the Journal of Low Temperature Physics. He has, along with Horst Meyer (Duke University, USA) and recently Neil Sullivan (University of Florida, USA), advanced the Journal of Low Temperature Physics to a highly regarded magazine and the worldwide leading publication medium in low temperature and high magnetic field physics. Due to his imminent retirement, Frank Pobell has recently handed on the baton to Mikko Paalanen (Helsinki University of Technology, Finland) who has taken over as an editor as of September 2005.
During the 24th International Conference on Low Temperature Physics (LT24) in Orlando, the undersigned and Horst Meyer have discussed the idea to thank Frank Pobell for his dedication to the JLTP with a special issue. Publishing special issues has emerged as an effective concept for JLTP to honour outstanding members of the community. It has been applied at various occasions. We have therefore invited colleagues and companions of his scientific locations at Munich, Cornell, Jülich, Bayreuth and Dresden to contribute articles to the "Special Issue to honour Frank Pobell for his dedication for the Journal of Low Temperature Physics as an editor".
Frank Pobell dedicated his scientific live to the physics of matter under extreme conditions. Most of us got to know him because of his quest for ultra-low temperatures. With his groups in Jülich and Bayreuth, he achieved several records of the lowest equilibrium temperature. The last record, T = 1.5 µK, which has been reached through the adiabatic demagnetization of Platinum is valid since a decade. He built up several nuclear demagnetization refrigerators which enabled outstanding investigations of solids and quantum fluids down to lowest temperatures. He attended to nuclear-spin ordering phenomena, superconductors, magnetic superconductors, magnets, spin glasses, liquid and solid 3He and 4He. Frank Pobell shared his cooling machines and experimental techniques with the international community. His institute in Bayreuth had the status of an EU user facility during the 3rd and 4th research framework programme of the European Community.
Frank Pobell was acting as an editor for JLTP even in the recent years, when he was the director of Forschungszentrum Rossendorf, a large research centre in Dresden, as well as the president of the Leibniz Society, one of the four large science communities of Germany. During that time, Frank Pobell also emphasized the importance of physics in high magnetic fields, in particular in a field range, B > 50 T, not available to the wide science community, so far. In consequence, he accompanied by four other research institutes in Dresden, successfully applied for the German high magnetic field project, the Hochfeld-Magnetlabor Dresden (HLD), a user facility for experiments in very high pulsed magnetic fields up to 100 T which will go into operation in mid 2007. Until 2005, Frank Pobell also headed the build-up team of the HLD.
One of us (TH) had the opportunity both to be a member of his µK laboratory in Bayreuth and to participate in the construction of pulsed field installations at HLD. "As one of his former students, I am very grateful to Frank Pobell who has taught us much more than only low-temperature physics." Acting as guest editors for this issue, we got an impression on the amount of work which is necessary to keep that journal a vivid publication medium for the international low-temperature science community.
Thomas Herrmannsdoerfer, Hochfeld-Magnetlabor Dresden (HLD), Germany
Eckhard Krotscheck, Johannes Kepler Univ. Linz, Austria

Suboxic soils and sediments contain the reactive Fe-bearing mineral phases mackinawite (FeS), siderite (FeCO3) and the Fe(II/III) oxide magnetite (Fe3O4), which should be able to reduce Se to oxidation states 0, -I and –II, forming elemental Se and iron selenides of low solubility. While the reduction of selenate or selenite to Se(0) by green rust, pyrite and by Fe2+ sorbed to montmorillonite in a slow (weeks), kinetically limited redox reaction has been demonstrated earlier, we show here that selenite is rapidly (within one day) reduced by nanoparticulate mackinawite and magnetite, while only 1/3 of selenite is reduced by the larger siderite crystals within one day. Depending on Fe(II)-bearing phase and pH, we observed four different reaction products, α-monoclinic and trigonal (hexagonal) elemental Se, and two iron selenides with structures similar to Fe7Se8 and FeSe. The thermodynamically most stable iron selenide, ferroselite (FeSe2) was not observed. The local structure suggests formation of nanoscale clusters, which may be much more soluble than the corresponding macrocrystalline solids, and mobile in case the physicochemical parameters of groundwater favor formation of stable colloidal suspensions (e.g. low IS and pH close to PZC).

Turbulent fluid has often been conceptualized as transient thermodynamic phase. Here, a finite-time thermodynamics (FTT) formalism [1] is proposed to compute mean flow and fluctuation levels of unsteady incompressible flows. That formalism builds upon a traditional Galerkin model which simplifies a continuum 3D fluid motion into a finite-dimensional phase-space dynamics and subsequently, into a thermodynamics energy problem. This model consists of a velocity field expansion in terms of flow configuration dependent eigenmodes and of a dynamical system describing the temporal evolution of the mode coefficients. Each mode may be considered as a wave, parameterized by a wave number and frequency. In our FTT framework, the mode is treated as one thermodynamic degree of freedom, characterized by an energy level. The dynamical system approaches local thermal equilibrium where each mode has the same energy if it is governed only by internal (triadic) mode interactions. However, the full system approaches only partial thermal equilibrium by strongly mode-dependent external interactions. In these interactions, large-scale modes typically gain energy from the mean flow while small-scale modes loose energy to the heat bath. The energy flow cascade from large to small scales is thus a finite-time transition phenomenon. The FTT model has been successfully applied to predict the cascade for flows with simple to complex dynamics. Examples include laminar vortex shedding which is dominated by 2 eigenmodes and homogeneous shear turbulence which has been modeled with 1459 modes. In addition, the onset of vortex shedding is described in the Galerkin and vortex picture as a finite time scale phenomenon.

[1] B. Andresen, P. Salamon & R.S. Berry, J. Chem. Phys. 66 (1977) 1571-1577.

The radiation emitted by a relativistic electron/positron planar channeled in a single crystal and simultaneously swinging transversely due to longitudinal hypersonic (HS) vibrations excited in the crystal has been investigated. In the present case, the continuum potential of crystallographic planes has been obtained by averaging the corresponding atomic potentials over an area, the size of which was chosen to be much smaller than the wavelength of the quasi-stationary HS field. The spatially periodic HS superlattice involves a zone structure of the channeling states. Probabilities of radiant transitions as well as spectral-angular distributions of the intensity of the corresponding channeling radiation have been calculated assuming parametric resonance. Numerical results are given for several crystallographic planes of -quartz.

Uranium is a widespread radioactive toxic heavy metal, released into the biosphere mostly by military purposes and nuclear industry. It is taken up by plant root systems and its chemical toxicity is much more dangerous than the radiological. Thus cell suspensions of rape (Brassica napus) revealed similar defence mechanisms after uranium exposure as described for other heavy metals (Clemens 2001). Some of them will be presented in detail, e.g. the pH-shift of the outer medium, the uptake and cellular sequestration revealed by ICP-MS and confocal microscopy. The speciation of uranium in aqueous solutions depends strongly on the pH-value, ionic composition and strength and thus plays an important role in bioavailability and cytotoxicity respectively. To investigate the speciation of uranium time-resolved laser-induced fluorescence spectroscopy (TRLFS) was performed. Further investigations are under way to clarify the molecular interactions between possible cellular ligands and uranium.

Clemens, S. (2001). "Molecular mechanisms of plant metal tolerance and homeostasis." Planta V212 (4): 475-486.

Recently we have developed a high resolution gamma ray CT system for process diagnostics. Typical application areas are nuclear fuel element bundles, chemical reactors, and hydrodynamic machines. In order to use gamma ray CT for non-destructive testing application, such as crack detection in metal parts, an increase in spatial resolution is required. Therefore, we tested two different approaches to increase resolution. The first technique is spatial collimation, where scanning is performed with an additional tungsten collimator in front of the detector arc. The collimator comprises slits of 0.33 mm width to narrow the sampling radiation beams. Thereby, the collimator also provides additional protection against false detection of scattered gamma photons. For a full sampling of the projection space the collimated detector needs to be laterally moved in six steps in addition to the rotary scanning motion of the tomography system. This increases measurement time proportionally and further requires an additional precise translation stage. As an alternative we tested a so called wobbling scheme that has frequently been used with earlier emission and transmission type CT scanners to increase resolution. With the wobbling scheme the detector has to be displaced laterally as with the collimation technique but the collimator is no longer needed. Since the collimator is a heavy and expensive part this approach is attractive. However, to produce the same sharp projection information as with the collimated detector it is necessary to deconvolve the measurement data with the detector’s response function. In our article and presentation we will discuss the methods in details and compare results that have been obtained from practical measurements on phantoms and industrial objects.

This paper presents methods to compute j-integral values for cracks in two- and three-dimensional thermo-mechanical loaded structures using the finite element code ANSYS. The developed methods are used to evaluate the behavior of a crack on the outside of an emergency cooled reactor pressure vessel (RPV) during a severe core melt down accident. It will be shown, that water cooling of the outer surface of a RPV during a core melt down accident can prevent vessel failure due to creep and ductile rupture. Further on, we present j-integral values for an assumed crack at the outside of the lower plenum of the RPV, at its most stressed location for an emergency cooling (thermal shock) scenario.

no abstract available

Stability in mean electron beam energy is of highest interest for a number of experiments performed at the ELBE accelerator. Energy drifts affect parameters of the generated Bremsstrahlung spectra, X-rays or infrared light, as well as the beam trajectory at the production targets or through the FEL waveguide, respectively.
In practise, we observe a slow drifting of the effective accelerating field during the first hours after a machine power-up or after switching to different nominal beam energies. Initially, this effect was compensated manually. A first order automation solution has been developed that corrects the resulting energy drift continuously, using a non-intrusive beam position monitor placed in a transversely dispersive part of the beam guide.
This paper describes the beam line setup and the simplified dynamic model of the control loop derived from it. Calculation of controller parameters using standard a standard method is shown. The user interface of the control system and working conditions for the loop are explained. Operational performance and conclusions towards improvements close this contribution.

Survey on recent investigations on the superconducting and magnetic properties of micro- and nanogranular materials at very low temperatures and high magnetic fields

Der Vortrag gibt einen Überblick über den Einsatz von Röntgen- und Gamma-Tomographieverfahren in der Technik.

The presentation is a short-course lecture that introduced the current state of the art in multiphase flow measurement techniques. Sensors and measurement techniques, which are intoduced are: electrical and optical needle probes, wire-mesh sensors and tomography techniques. Beside the physical measuring and sensor construction principles the presentation addresses the topic of data processing for multiphase flow measurements.