Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The formation of nanocrystals after implantation of silver
alone as well as together with the halogen ions Cl, Br, and I into a SiO2
layer of about 100 nm was studied by X-ray diffraction and transmission
electron microscopy. The co-implantation of Ag and Cl/Br results in the
formation of cubic AgX-crystals which are stable in size under annealing.
The co-implantation of Ag and I as well as single Ag implantation result
in Ag crystallites, which grow under annealing. The annealing procedure
causes a redistribution of the particles within the layer.

In recent years, single crystal SiC has become an important electronic material due to its excellent physical and chemical properties. The present paper reports a study of the defect reduction and recrystallization during annealing of Ge+-implanted 6H-SiC. Implants have been performed at 200 keV with doses of 1x1014 and 1x1015 cm-2. Furnace annealing has been carried out at temperatures of 500 °C, 950 °C and 1500 °C. Three analytical techniques including Rutherford backscattering spectrometry in conjunction with channelling (RBS/C), positron annihilation spectroscopy (PAS) and cross-sectional transmission electron microscopy (XTEM) have been employed for sample characterization. It has been shown that damage removal is more complicated than in ion-implanted Si. The recrystallization of amorphized SiC layers has been found to be unsatisfactory for temperatures up to 1500 °C. The use of ionbeam-induced epitaxial crystallization (IBIEC) has been more successfull as lattice regrowth, altough still imperfect, has been observed to occur at a temperature as low as 500 °C.

Thermal SiO2 films have been implanted with Si+ ions using double-energy implants (200 + 100 keV) at a substrate temperature of about -20 °C to total doses in the range 1.6 x 1016 - 1.6 x 1017 cm-2 followed by short-time thermal processing, in order to form a Si nanostructure capable of yielding blue photoluminescence (PL). The intensity and the peak position of the PL band have been investigated as a function of ion dose, manner of heat treatment, anneal time and anneal temperature. For the formation
of blue PL emitting centres, optimum processing conditions in terms of excess Si concentration and overall thermal budget are mandatory. The nature of the observed blue emission is discussed.

The present paper reports a study of the defect reduction
and recrystallization during annealing of Ge+-implanted 6H-SiC. Implants
have been performed at 200 keV with doses of 1x1014 and 1x1015 cm-2. Furnace
annealing has been carried out at temperatures of 500°, 900° and
1500 °C. Several analytical techniques including Rutherford backscattering
spectrometry in conjunction with channeling (RBS/C), cross-sectional transmission
electron microscopy (XTEM), positron annihilation spectroscopy (PAS), Raman
scattering (RS), and electron paramagnetic resonance (EPR) measurements
have been employed for sample characterization. It has been shown that
damage removal is more complicated than in ion-implanted Si. Using PAS
deeply diffusion vacancy-like defects were found far below the region defined
by TRIM as containing the maximum amount of nuclear energy deposition.
The amorphized SiC is characterized by carbon dangling bonds as revealed
by EPR. The recrystallization of amorphized SiC layers has been found to
be unsatisfactory for temperatures up to 1500 °C. The use of ion-beam-induced
epitaxial crystallization (IBIEC) has been more successful as lattice regrowth,
altough still imperfect, has been observed to occur at a temperature
as low as 500 °C.

Ion beam synthesis of ß-SiC in the top-Si-layer of a SIMOX (Separation by Implantation of OXygen) substrate is reported.
All the implanted C is captured inside the 200 nm top-Si-layer and is accumulated preferentially at the Si/SiO2-interface during annealing. A Si/SiC/SiO2-structure is obtained by C implantation at high temperature (T>500 °C) and subsequent annealing treatment (T=1250 °C). It consists of a crystalline Si overlayer, a Si layer with a high density of perfectly aligned ß-SiC grains and a buried oxide layer. Implantation at elevated temperatures is crucial for the quality of the SiC layer. However, the use of such implantation temperatures is limited by the dissolution of the buried oxide layer.

Point defects and defect agglomerates are created by high
energy ion implantation into Si and subsequent annealing. These defects
are investigated by gettering of metal atoms and analysing the metal concentration
distribution. Beside a gettering layer at the projected ion range (Rp)
where the implant is deposited also a gettering layer is also found at
the Rp/2-position. This Rp/2-layer is related to implantation defects -
not to the implant atoms. It is thermally stable up to a temperature of
more than 1000 °C.

The development of irradiation-induced precipitates in VVER-440 type reactor pressure vessel steels 15Kh2MFA and weld metals SV-10KhMFT during irradiation and post-irradiation annealing is studied by small angle neutron and X-ray scattering. The kinetic conditions for the precipitation of particles, which already exist in the unirradiated state, seem to be improved at temperatures of about 270 °C due to the irradiation. The size distribution of the irradiation-induced precipitates depends on the copper content and differs between weld and base metal. A strong correlation between the formation of irradiation-induced precipitates and the irradiation hardening is found. The hardness nearly linearly depends on the number of these precipitates.

Comparing with the exact solutions of the model system of one and two particles coupled to an axial rotor, the quality of the semi classical tilted axis cranking approximation is investigated. Extensive comparisons of the energies and M1 and E2 transition probabilities are carried out for the lowest bands. Very good agreement is found, except near band crossings. Various recipes to take into account finite K within the frame of the usual principal axis cranking are included into the comparison. A set of rules is suggested that permits to construct the excited bands from the cranking configurations, avoiding spurious states.

Iron implanted and subsequently annealed n-type Si(111) was
studied by conversion electron Mössbauer spectroscopy and Auger electron
spectroskopy. Phase analysis performed with Mössbauer spectroscopy indicates
the formation of a mixture of ß- and a-FeSi2 due to the implantation
(200 keV, 3 x 1017 cm-2, 350 °C) and a complete transition of the mixture
to the ß-and the a-phase after annealing at 900 °C for 18 h and
1150 °C for 1 h, respectively. The iron concentration profiles were
determined with Auger electron sputter depth profiling. The profile of the
as-implanted sample is Gaussian shaped and boradened after annealing at 900
°C to a pleateau-like profile. For the sample annealed at 1150 °C
only a slight broadening occurs and depth depending fluctuations of the iron
concentration can be observed. Investigations with scanning Auger electron
spectroscopy show in the case of the as-implanted sample only slight fluctuations
of the iron distribution in the center of the profile. After annealing at
900 °C large separated ß-FeSi2 precipitates grow due to the process
of Ostwald ripening. At 1150 °C Ostwald ripening and coalescence of
the precipitates occur and a wide extended penetration a-FeSi2 network structure
is formed. It is the first time that such large iron disilicide structures
could be proved by scanning Auger electron spectroscopy.

A short review is presented concerning problems of ion beam processing of single crystalline silicon carbide. Emphasis is given
to recent results on point defect, extended defects, amorphization and recrystallization, electrical activation of dopant atoms, and metallization.

Plasma source ion implantation (PSII) has been developed
as an alternative technique to circumvent the limitations of conventional
ion implantation, like the requirements of a complicated target handling
and beam raster system for a uniform ion implantation of 3-dimensional
samples. In this method, a plasma sheath conformably surrounds the target.
By applying negative high voltage pulses, positively charged ions are accelerated
from the plasma trough the sheath and implanted into the target. Critical
parameters for the further development of this implantation process are
the ion implantation current and the sheath expansion characteristics.

This paper presents a detailed study of boron nitrides
at hydrostatic pressures. Cohesive properties of zincblende (c-BN), wurtzite
(w-BN), hexagonal (h-BN), rhombohedral (r-BN) and rocksalt structure are
calculated by systematic optimization of unit cell parameters using total-energy
DFT techniques. With focus on the very rarely discussed layered modifications
the p - V equation of states are derived. It is confirmed taht the isothermal
bulk modulus of the sp2 bonded phases is more than ten times smaller in
comparison to the dense phases. Additionally the equilibrium line of c-BN
and h-BN in the phase p, T diagram is estimated. According to recent experimental
reports c-BN is predicted as stable modification at standard conditions.

Abstract
The time dependent Schrödinger equation of an open quantum mechanical system is solved by using the stationary bi-orthogonal eigenfunctions of the non-Hermitean time independent Hamilton operator. We calculate the decay rates at low and high level density in two different formalism. The rates are, generally, time dependent and oscillate around an average value due to the non-orthogonality of the wavefunctions. The decay law is studied disregarding the oscillations. In the one-channel case, it is proportional to t^-b with b ≈ 3/2 in all cases considered, including the critical region of overlapping where the non-orthogonality of the wavefunctions is large. Starting from the shell model, we get b ≈ 2 for 2 and 4 Open decay channels and all coupling strengths to the continuum. When the closed system is described by a random matrix, b ≈ 1 + K/2 for K = 2 and 4 channels. This law holds in a limited time interval. The distribution of the widths is different in the two models when more than one channel are open. This leads to the different exponents b in the power law. Our calculations are performed with 190 and 130 states, respectively, most of them in the critical region. The theoretical results should be proven experimentally by measuring the time behaviour of de-excitation of a realistic quantum system.

The wind energy ressources of a selected saxon area ( 64 km x 48 km ) has been investigated in detail. The region can be characterized by a mean height of 200 m a.s.l. with complex surface profil. Based on measuring data of three stations WASP calculations with a grid step of 0.25 km has been performed for the whole region. The data are compared with calculation results of the mesoscale KAMM model. These calculations based on the geostrophic wind data of the Czech station Prague and used a 1.0 km and a 2.5 km grid ( coresponding 60 and 120 wind clusters ). The ressource maps ( mean wind velocity and power density ) calculated by both methods show corresponding high wind potential in the investigated area. Differences in the absolute values are explained by the features of the used programmes.

In ein mit Erdgas betriebenes konventionelles Fernwärmesystem (Anschlußleistung 2,1 MW) wurde eine Solarkollektoranlage mit einer Bruttofläche von 100 m² integriert. Die Kollektoranlage diente vorrangig der Erzeugung von Brauchwarmwasser für eine Schule, solare Überschüsse wurden in den Rücklauf des Fernwärmesystems eingespeist. Ein umfangreiches Meßprogramm ermöglichte Effizienzuntersuchungen des Systems.
Der jährliche Nettoertrag des Solarsystems lag bei 280 kWh/m² bei einem Systemnutzungsgrad von 26%. Beide Kennziffern wurden durch hohe Rücklauftemperaturen (58°C) im Nahwärmenetz negativ beeinflußt. Weitere Untersuchungen galten dem Betrieb der eingesetzten Schichtspeicher (Reihenschaltung). Bei Ausschöpfung der in dem Vorhaben deutlich gewordenen Optimierungspotentiale können in künftigen Anlagen Nettoerträge von 350 kWh/m² und solare Wärmegestehungskosten unter 30 Pf./kWh erreicht werden.

Vom Zentrallager der Plus-Warenhandelsgesellschaft in Ottendorf-Okrilla (Sachsen) wird seit 1992 eine hybride Wind-Photovoltaik-Anlage zur Erzeugung von Elektroenergie betrieben. Die Anlage besteht aus einer 60-kW-Windkraftanlage des Typs TW 60 und einer Phtovoltaikanlage mit einer Spitzenleistung von 3,18 kW. Letztere zeichnet sich durch einige Besonderheiten (Orientierung des Generators, Master-Slave-Betieb der Wechselrichter) aus. Das Betriebsverhalten der Anlagen wurde über einen längeren Zeitraum untersucht.

Im Jahr 1995 erreichte die Photovoltaikanlage mit 689 Vollastbetriebsstunden etwa die Ergebnisse anderer Photovoltaikanlagen in Sachsen. Mögliche höhere Erträge werden durch die gewählte Orientierung des Generators verhindert. Die Master-Slave-Schaltung der Wechselrichter arbeitete ohne Probleme. Ihre Effektivität wurde durch die nicht optimale Abstimmung von Generator- und Wechselrichterleistung beeinträchtigt.

A neural-network approach has been developed for localizing leakages and estimating the leak rate in pressurized plants with complicated three-dimensional structures. Results are presented from experiments with simulated leaks at a VVER-440 reactor vessel head. As features for characterizing the occurrence and the location of a leak, RMS values of acoustic emission sensors and coherence values and power spectra of microphone signals were used. Three-layer perceptron networks were found to be best suited for leak localization and for estimation of leak rates. However, the estimation of leak rates required an additional neural network because a different normalization procedure was necessary for extracting features from the RMS values of the acoustic emission sensors. Perceptron networks with continuously valued outputs corresponding to the coordinates of the leak positions were useful for classifying even positions which had not been offered during training.

Im Rahmen eines Programms des Bundesministeriums für Umwelt, Naturschutz und Reaktorsicherheit zur Zusammenarbeit mit den MOE-Staaten und der GUS auf dem Gebiet der kerntechnischen Sicherheit hat das Forschungszentrum Rossendorf, Institut für Sicherheitsforschung, im Verbund mit dem technischen Überwachungsverein TÜV Rheinland, Institut für Kerntechnik und Strahlenschutz, die einvernehmlich spezifizierten "technischen Mittel für ein System zur verbesserten betrieblichen Überwachung" an das Kernkraftwerk Saporoshje, Ukraine, geliefert und in Betrieb genommen. Im vorliegenden Bericht (Textteil) werden die Lieferungen und Leistungen nach Art und Umfang (Beschaffenheit, Abnahmeprüfung, Übergabe und Inbetriebnahme der technischen Mittel und die für das ukrainische Fachpersonal durchgeführten Schulungsmaßnahmen) bis zur Aufnahme des Probebetriebes am 30. November 1995 beschrieben. Dem Bericht ist ein Anhang A mit Vorschlägen zur Gestaltung der automatisch arbeitenden Überwachungssoftware und ein Anhang B mit wissenschaftsorganisatorischen Details beigefügt.

Abstract
Extended calculations of the deuteron's static properties, based on the numerical solution of the Bethe-Salpeter equation,.are presented. A formalism is developed, which provides a comparative analysis of the covariant amplitudes in various representations and nonrelativistic wave functions. The magnetic and quadrupole moments of the deuteron are calculated in the Bethe-Salpeter formalism and tlie role of relativistic corrections is discussed.

Abstract
We consider the mass of the one-loop hedgehog soliton of the bosonized SU(2) Nambu & Jona-Lasinio model embedded in hot nuclear matter mimiced by a gas of constituent quarks. We prove that the proper-time regularized and self-consistently determined soliton in a heat bath obeys Poincare's invariance up order V². At ifinite temperature and chemical potential, we show that the inertial mass obtained in the perturbative pushing approach coincides with the total internal energy of the soliton.

By measuring the fast particles emerging from a proton-nucleus interaction the number of participants can be determined. This might be the key quantity for understanding subthreshold particle production.

The results on epitaxial crystallization of Si by mega-electron-volt heavy-ion-beam irradiation recently published by Nakata can be understood in the framework of the "point defect diffusion model". It is not necessary to consider inelastic electronic scattering effects in order to explain the decrease in the normalized crystallization rate with increasing nuclear deposited energy.

A neutron double-crystal diffractometer (DCD) with a position sensitive detector for a simultaneous measurement of the whole scattering range was realized. In this arrangement the angular dependence of the intensity distribution is transformed into a positional one by an elastically bent perfect analyzer (second) crystal set in a completely asymmetric diffraction geometry. The DCD may be used for the study of neutron beams scattered, diffracted, refracted or reflected in small angles. By tuning the curvature of the analyzer in the momentum transfer a range from 2x10-1 nm-1 is available. At present this DCD is used for small-angle neutron scattering investigations as well as for experiments with neutron optical components.

The report gives an overview on the scientific work of the Institute of Safety Research in 1994.

The temperature dependence of the ion beam induced interfacial amorphization process (IBIIA) in silicon has been investigated at temperatures above 80 K using Rutherford Backscattering Spectroscopy/Channeling (RBS/C) and Cross-Sectional Transmission Electron Microscopy (XTEM). Three regimes are observed. Above temperatures of about 320 K there is strong temperature dependence of the IBIIA rate (thermal regime). At lower temperatures the rate moves towards a saturation value (transition regime). Below approximately 150 K IBIIA is nearly temperature independent (ballistic regime). This low temperataure regime can be explained by an athermal transport of point defects like in ballistic mixing processes.

Surface Light Scattering Spectroscopy of the Gallium Liquid-Vapor Interface
Vladimir Kolevzon
Abstract
High frequency capillary waves at liquid Ga surface have been studied by means of quasielastic light scattering spectroscopy. The observed frequencies and damping constants of waves differed greatly from classical theoretical treatment of liquid Ga surface as that of a simple liquid. This effect was explained due to the presence of the surface layer, possessing visco-elastic properties extracted, for the first time, from the fit of experimental spectra with an appropriate theoretical form. Negative value of the surface dilational viscosity has been derived for all wavenumbers studied. At high wavenumbers q the wave dispersion behavior is best likely linked to the weak coupling between dilational and capillary modes, while at low and moderate q the experimental data resemble to the mixed oscillations. This behavior is consistent with an influence of diffusion and adsorption on the waves propagation.

Six different types of boron nitride films were investigated by polarized infrared reflection spectroscopy. Films with a highly cubic, mixed cubic and non-cubic, and exclusively non-cubic phase composition were synthesized using ion beam assisted deposition. Additionally, post-deposition argon ion irradiated cubic and non-cubic boron nitride films as well as nitrogen implanted boron sample were analyzed. Using this technique, besides the cubic phase, two different non-cubic modifications, layered anisotropic and amorphous, could be distinguished. A preferentional orientation of the normal axis of the sp2-bonded basal planes parallel to the substrate surface was observed.

Die 40-kWp-Photovoltaikanlage ist die größte netzgekoppelte PV-Anlage in Sachsen. Trotz nichtoptimaler Lage (Abschattungen im Winter) erreichte die Anlage im Jahr 1995 einen Ertrag von fast 700 kWh/kWp und liegt damit im Durchschnitt der Erträge anderer sächsischer Anlagen. Bewährt hat sich die Master-Slave-Schaltung der Wechselrichter. Der Strombedarf der Kirntzschtalbahn wird durch die Anlage zu knapp 30% gedeckt.

Durch das FZR werden an 50 PV-Anlagen in Sachsen mehrjährige Untersuchungen zum Betriebsverhalten durchgeführt. Das Untersuchungsprogramm umfaßt die technische Überprüfung des Anlagenzustandes und die Messung der Kennlinien des PV-Generators und der eingesetzten Wechselrichter. Ferner erfolgt eine monatliche Analyse der erreichten Energieerträge. Die ersten Ergebnisse dieser Untersuchungen sind im Beitrag zusammengefaßt.

In order to improve the operational surveillance of a VVER-1000 unit of the Ukrainian nuclear power plant Zaporosh´ye a technical monitoring system has been specified. The system will enable the state regulatory and supervisory bodies to survey the unit operation independently of operators to assess its safety status, and to impose appropriate conditions. Due to its up-to-date configuration the system provides early indication of any operational incident and emission of radioactive materials connected. Based on the system an immediate warning in mergency situations is possible as well as an effective emergency management. For this purpose 49 different operational parameters of the unit, 18 radiological parameters of the unit and the plant site and 6 meteorological parameters are monitored.
The monitoring concept and its technical realization are described.

Results of a small break loss of coolant accident experiment, conducted on the PMK-2 integral type test facility are presented. The experiment simulated a 1% break in the cold leg of a VVER-440-type reactor. The main phenomena of the experiment are discussed and in case of selected events a more detailed interpretation with the help of measured void fraction, obtained by a special measurement device is given. Two thermohydraulic computer codes, RELAP5 and ATHLET, are used for post test calculations. The aim of the presented calculations is to investigate the code capability for modeling natural circulation phenomena in VVER-440-type reactors. Therefore the results of the experiment and both calculations are compared. Both codes predict most of the transient events well, with the exception that RELAP5 fails to predict the dry-out-period in the core. In the experiment the hot and cold leg loop seal clearing is accompanied by natural circulation instabilities, which can be explained by means of the ATHLET calculation.

Abstract
The guiding centre appraximation for the motion of charged particles in axially symmetric magnetic and electric fields is formulated and stopping as well as small angle scattering on a multicompenent plasma are included. The developed code has been applied to the design of a 14-MeV neutron source according to the concept of the Budker Institute Novosibirsk. It is demonstrated that the self-interaction of injected D and T ions by stopping, scattering and induced internal Fields has to be taken into account, if the injected power exceeds a few megawatt.

The dependence of the mechanical properties on the depth position in the unirradiated state and after irradiation up to neutron fluences of approximately 5x1018 and 70x1018 cm-2 (E > 0.5 MeV) is tested on a forging made out of VVER 440 reactor pressure vessel (RPV) steel 15CrMoV. The near-surface position shows a higher strength and a lower transition temperature than the positions greater than 1/4 wall thickness. Irradiation does not change these differences in a significant manner. The testing of specimens from the 1/4 depth position withi the surveillance programme, as normally laid down in the legal rules relating to nuclear power plants, results in a conservative safety assessment against brittle failure up to the EOL fluence. On taking into account fluence attenuation, the flux effect, etc., the toughness gradually increases from the inside to the outside of the wall after longer RPV operating times.

With the aim of obtaining a general insight into the technetium/S-unprotected MAG3 system, the reactions of technetium with the unprotected MAG3 ligand, both at c.a.- and n.c.a.-level, were investigated. Based on a profound knowledge of the reaction route and the Tc-MAG3 complexes, sources of by-products were identified and eliminated as far as possible. Only a small percentage (< 5%) of intermediate Tc-MAG3 complexes occurs, but this may increase by improper handling of the kit, such as insufficient reaction time. In conclusion, two kinds of impurity may occur in the Tc/MAG3 kit: One is derived from impurities in the ligand (MAG3 disulphide), which can be excluded by careful preparation and storage. The other is caused by insufficient reaction time according to the scheme (Fig. 3) during the labelling procedure. Its content can be minimized by optimization of the reaction time. A reaction time of at least 10 minutes gives about 98% of the renal agent.

Zur Verbesserung der betrieblichen Überwachung eines WWER-1000-Reaktors im ukrainischen Kernkraftwerk Saporoshje wurde ein technisches Informationssystem entworfen. Dieses System soll die Staatliche Aufsichtsbehörde in die Lage versetzen, den Reaktorbetrieb unabhängig vom Betreiber zu beobachten, den Sicherheitszustand der Anlage zu erfassen und nötigenfalls Auflagen zu erteilen. Durch eine moderne Ausrüstung soll das System eine Früherkennung von betrieblichen Störungen und Unfällen sowie der damit verbundenen Emission radioaktiver Substanzen ermöglichen. Auf dieser Grundlage soll sowohl eine rechtzeitige Warnung als auch ein effektives Störfall-Management möglich werden. Zu diesem Zweck werden 49 betriebliche Parameter, 18 radiologische Parameter und 6 meteorologische Parameter kontinuierlich überwacht. Das Überwachungskonzept und die technische Realisierung werden vorgestellt.

Boron doped crystalline superlattices grown by low-temperature molecular beam epitaxy have been used by Stolk et al. to study the diffusion of self-interstitials I in Si. After 40 keV Si+ ion implantation and subsequent annealing, stationary interstitial profiles are found at 670 °C for periods t ~1 h, demonstrating that the penetration depth of interstitials is limited by trapping at hitherto unidentified impurities. In addition, the high
interstitial supersaturation causes B de-activation in the near-surface region. We show that both these effects, the trap-limited interstitial diffusion and the enhanced B clustering, can be explained, if the transient de-activation of boron is modeled by the first-order reaction I + B_s ->B_i. Boron atomic and electrical profiles as well as self-interstitial profiles are computed which are in good agreement with the experimental results.

Abstract
The trapping effect is investigated close to the elastic threshold. The nucleus is described as an Open quantum mechanical many-body system embedded in the continuum of decay channels. An ensemble of compound nucleus states with both discrete and resonance states is investigated in an energy-dependent formalism. It is shown that the discrete states can trap the resonance ones and also that the discrete states can directly influence the scattering cross section.

Abstract
Within an effective one- boson exchange parametrization of the T matrix of NN interactions we calculate cross sections for the reactions pp → ppγ and pn → pnγ for proton incidence energies in the order of 1 GeV. Besides bremsstrahlung processes we consider photons from ∆ decays and contributions from the η → γγ process, where the η is excited via the N₁₅₃₅ resonance. At beam energies above 700 MeV the ∆ decay channiel dsminates for large photon energies, while above the η threshold the η decay photons ähsw up only in a narrow window. The low energy photons stem frorn pure bremsstrahlunig processes.

Abstract

The role of the massive photon decay via intermediate states of electron-electron-holes and proton-proton-holes into neutrino-anti-neutrino pairs in the course of neutron star cooling is investigated. These reactions may ba operative in hot neutron stars in the region of proton pairing. The corresponding contribution to the neutrino emissivity is calculated. It varies with the temperature as T^3/2e^−m_γ/T for T < m _γ, where m_γ is an effective photon mass in superconducting matter. Estimates show that this process appears as strong cooling channel of neutron stars at temperatures T ≈ ( 10⁹ - 10¹⁰) K.

Die technische Überprüfung von bisher 30 netzgekoppelten Anlagen ergab keine gravierenden Mängel. Einzelne technische Schwächen von eingesetzten Komponenten wurden identifiziert. Zur energetischen Bewertung der Anlagen wurde ein PC-gestützter PV-Anlagen-Analysator entwickelt und erprobt. Ein optimales Verfahren zur Umrechnung gemessener PV-Generator-Kennlinie auf STC-Bedingungen existiert noch nicht.

The code DYN3D/M2 was developed for steady-state and transient analysis of reactor cores with hexagonal fuel assemblies. The new version DYN3DR contains a method for neutron kinetics solving the two group neutron diffusion equation by a nodal method for cartesian geometry. The thermal-hydraulic model FLOCAL simulating the two phase flow of coolant and the fuel rod hehaviour is used in the two versions. The fundamentals of the neutron kinetics are described. The accuracy of the code is demonstated by comparisons with the results of rod ejection benchmarks for PWR with rectangular fuel assemblies. The developed algorithm of neutron kinetics are suitable for using parallel processing. The speedup of neutronic calculation is demonstrated for a steady state solution of diffusion equation.

Asymmetry of the Dielectron Emission Rate in an
Isospin-Asymrnetric Pion Medium
T.I. GULANOVA,¹ A. I. TITOV
Bogoliubov Theoretical Laboratory, Joint Institute for Nuclear Research,
141980 Dubna, Russia
B. Kämpfer
Research Center Rossendorf, Institute for Nuclear and Hadron Physics,
PF 510119, 01314 Dresden, Germany
Institute for Theoretical Physics, Technical University Dresden,
Mommsenstr. 13, 01062 Dresden, Germany

Abstract

The dielectron emission by pion annihilation in an isospin-asymmetric pion gas at finite temperature is considered. Due to the differente between the longitudinal and tramverse parts of the in-medium p meson self-energy a specific asymmetry of the rates is caused for electron pairs with relative momenta perpendicular or parallel to the total pair momentum. This asymmetry may be considered as a sensitive signal of in-medium-modifications of the p properties.
¹Permanent- address: Physical and Technical Institute of Uzbek Academy of Sciences, 700084 Tashkent,
RepubIic Uzbekistan

Zusammenfassung
Dargestellt wird ein in sich geschlossenes 3D-Modell zur Berechnung der ortsabhängigen Übertragungsfunktionen für stochastische Schwingungen der Regel- und Brennelemente in Reaktoren vom Typ WWER-440 unter Berücksichtigung der heterogenen Brennstoffbeladung realer Spaltzonenkonfigurationen. Basis sind die nodalen Bilanz-Gleichungen zur Berechnung der stationären Neutronediußverteilung in der Eingruppen-Diffusionsnäherung für 3D-Nodes in Hexagonal-Z-Geometrie. Die Absorberteiie eines WWER- 440-Regelelements werden durch entsprechende Randbedingungen für die Diffusionsgleichun im neutronenphysikalischen Reaktormodell erfaßt. Die Modellierung der Rauschquellen erfolgt auf der Grundlage einer Sweigruppen-Diffusionsnäherung. Die Neutronenflußdichteschwankungen werden dabei durch Störungen bei der Kickstreuung thermischer Neuttonen in "Absorber-Nodes" bzw. durch stochastische Schwankungen der Zweigruppen-Wirkungsquerschiiitte an den radialen Nodegrenzen der 'Brennstoff-Nodes" beschrieben. Zur Berechnung der Ubertragungsfunktionen für die Schwankungen der Neutronenflußdichte werden die nodalen Bilanz-Gleichungen in der Prompt-Response-Approximation verwendet. Die Kopplung der Zweigruppen-Quellmodeiie für Brennstoff- und Absorberteil eines Regelelements an das nodale Eingruppen-Übertragungsmodel erfolgt im Sinne der modifizierten Eingruppen-Diffussionsnäherung.
Anhand numerischer Berechnungen werden wesentliche Charakteristika der Übertragungsfunktionen für
Regelelementschwingungen in Dwckwasserreaktoren vom Typ WWER-440 diskutiert.

The German 1000 roofs PV programme, in which more than 2000 roof-mounted grid connected PV systems are now operating, provides invaluable information on the operational performance of the installed PV systems, the PV users and their consumption behaviour. The Institut für Solarenergieforschung (ISFH) and the Forschungszentrum Rossendorf (FZR), responsible for the organization of the 1000 roofs programme in the Federal States of Lower Saxony and Saxony, have been analysing the monitored performance and consumption data of more than 300 PV systems operating since 1991. The purpose of this work is to examine the relationship between PV system yield and electricity consumption for the realized projects and to take advantage of the first-hand information on the PV users consumption behaviour for future PV applications.