Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The exclusive deuteron break-up reaction is analyzed within a covariant approach based on the Bethe-Salpeter equation with realistic meson-exchange interaction. Relativistic effects in the cross section, tensor analyzing power and polarization transfer are investigated in explicit form. Results of numerical calculations are presented for kinematical conditions in forthcoming p + D reactions at COSY.

A rate equation approach is applied for the description of the self-organization (layering) phenomenon predicted in recent computer experiments. This layering was observed in finite precipitate systems during annealing and is caused by Ostwald ripening. We demonstrate that the layering is triggered by local inhomogenities in the spatial distribution of precipitate sizes and should not be treated in terms of the frequently considered "spinodal
decomposition" approach. The change of the spatial profile of precipitate size starts from the boundary of the system and occurs within a "reaction shell" having a thickness of the order of the diffusional screening
length. During annealing this "reaction shell" shifts progressively into the system, leaving behind layers of precipitates. The layering is shown to occur only in sufficiently large systems with characteristic dimensions of at least several diffusional screening lengths. The reason for the weak sensitivity of interlayer distance to variations of system parameters is elucidated.

One of the most critical problems in practical applications of spectrum adjustment in pressure vessel dosimetry is the insufficient knowledge of the input spectrum covariance matrix. Usually very crude approximations for this covariance matrix are used in adjustment procedures and, moreover, it is not clear to which extent these approximations influence the final results. Therefore it appeared useful to calculate this matrix as carefully as possible for a typical system and in positions for which a sufficient amount of activation measurements and precise transport calculations was available. The paper presents and discusses results of such calculations for a VVER-1000 type reactor. They base on the one hand on sensitivity coefficients estimated in transport calculations by means of the ANISN code and on the other hand on cross section covariance data taken from the Obninsk ABBN-93 data library. The results of spectrum adjustments based on the estimated covariance matrix are compared with results based on formerly used matrices. The obtained differences are discussed.

In 1995 at the integral test facility ISB-VVER in Elektrogorsk near Moscow natural circulation experiments were performed, which were scientifically accompanied by the Forschungszentrum Rossendorf. These experiments were the first of this kind at a test facility, which models VVER-1000 thermalhydraulics. Using the code ATHLET which is being developed by "Gesellschaft für Anlagen- und Reaktorsicherheit", pre- and posttest calculations were done to determine the thermalhydraulic events to be expected and to define and tune the boundary conditions of the test. The conditions found for natural circulation instabilities and cold leg loop seal clearing could be confirmed by the tests. Besides the thermalhydraulic standard measuring system, the facility was equipped with needle shaped conductivity probes for measuring the local void fractions.

Three dimensional Monte Carlo calculations considering all known geometrical and composition details were used for the determination not only of fluence integrals but also of fluence spectra with good statistical accuracy. This was possible by the application of own specially developed codes (TRAMO) and successfully modifying variance reduction on the basis of the weight window method (TRAWEI). Modern multigroup data sets were applied. For the real representation of fission sources the burn up and power history also for the fuel rods within the fuel assemblies was considered.
The investigation was accomplished for the irradiation period seven with a special low leakage loading of the reactor. During this period Russian and German detectors were irradiated in the ex-vessel cavity and analysed by advanced gamma spectrometric methods.
The reaction rates obtained with the calculated fluence spectra were compared with activation reaction rates measured in the ex-vessel cavity. Furthermore the calculated spectra were adjusted to the experimental reaction rates taking into account the variances and correlations of calculated input spectra, detector sensitivities and measurements. Using the adjustment code COSA2 based on the maximum likelihood method best estimate fluence spectra and fluence integrals could be provided.

Plasma immersion ion implantation was used to implant
oxygen ions into silicon with applied voltage pulses of -40 kV and 2.5
ms lenght. Positive ions, O2+ and O+, with a continuous energy distribution
between 0 and 40 keV were implanted. Between 3x104 and 3x105 pulses, corresponding
to nominal doses from 2x1016 to 2x1017/cm2 were used. The resulting oxygen
depth profiles were measured with elastic recoil detection analysis (ERDA)
using 13.4 MeV a particles. Rutherford backscattering was used to determine
possible co-implanted contaminations. The obtained depth profiles were
simulated using a linear supersposition of calculated single-energy profiles.
The results obtained for the energy distribution of the incident ions are
compared with calculations obtained from a theoretical model, and the agreement
is very good. The incident flux is found to be composed of 34(5)% O2+ and
66(5)% O+ ions with an Fe contamination of ca. 0.5%.

In this final design report the general layout and the optics of the beamline ROBL is described. The beamline is buildt by the Research Center Rossendorf at the bending magnet source BM20 of the European Synchrotron Radiation Facility (ESRF), Grenoble. The design criteria for the beamline optics and the adopted solutions are reported in detail. Some performance data of the beamline as resolution, focussing and intensity are modelled by the ray-tracing code SHADOW. Furthermore tha layout of the vacuum system is described.

We studied high dose implantation of tungsten into 6H-silicon carbide in order to synthesize an electrically conductive layer. Implantation was performed at 200 keV with a dose of 1.2×10(17) W(+)cm(-2) at temperatures between 200°C and 400°C. The influence of implantation temperature on the distribution of W in SiC was investigated and compared to results obtained earlier from room temperature (RT) and 500°C implants. Rutherford backscattering spectrometry (RBS) was employed to study the structure and composition of the implanted layers. Implantation at temperatures between RT and 300°C did not influence the depth distribution of C, Si and W. The W depth profile shows a conventional Gaussian shape. Implanting at higher temperatures led to a more confined W rich layer in the SiC. This confinement is explained by Ostwald ripening which is enabled during implantation at temperatures above 300°C. The depth of the implantation induced damage decreases slightly with increasing implantation temperature, except for 400°C implantation. The amount of damage, however, is significantly reduced only for implantation at 500°C.

The semi-magic nucleus ⁸⁹Y has been investigated in a (gamma,gamma')
experiment at an endpoint energy of the bremsstrahlung of E₀=
7 MeV. The scattered photons have been detected with a EUROBALL Cluster
detector. The observed excitations are discussed in the framework of the
shell model and a coupling of the unpaired proton to milti-phonon structures
in the doubly even neighbours. Transitions above 4.7 MeV are considered
to have E1 character. Around 6.3 MeV an unusually large concentration of
E1 strength is found. Its origin is likely to correspond to similar structures
in the ⁸⁸Sr and ⁹⁰Zr isotones which can be interpreted
to result from the constructive interference of strong two-phonon amplitudes
with weak admixtures from the low-energy tail of the giant dipole resonance.

The report gives an overview on the scientific work of the Institute of Safety Research in 1995.

The accelerator mass spectrometry (AMS) facility at the
3 MV Tandetron in Rossendorf has been applied for quantitative depth profiling
of deuterium and tritium in samples cut from the vessels walls of the fusion
experiment ASDEX-Upgrade in Garching. The tritium originates from D(d,p)T
fusion reaction in the plasma and it is implanted in the vessel walls together
with deuterium atoms and ion from the plasma. The high deuterium concentrations
in the samples were measured by a conventional SIMS mode. The concentrations
in the surface layers down to the analysing depth of about 25 mm are for
deuterium in the range of up to 3x1018 to 8x1021 atoms/cm3, corresponding
to a deuterium retention of 2.5x1016 to 2.5x1018 atoms/cm2 in a surface
layer £ 25 mm, and for tritium in the range form 1011 to 5x1015 atoms/cm3,
corresponding to a tritium retention of 3.0x1010 to 3.5x1012 atoms/cm2
in a surface layer £25 mm. The estimated total amount of tritium
in the vessel walls is in agreement with the total number of neutrons produced
in D(d,n)3He reaction.

Acoustic waves induced by an intensity-modulated focused
ion beam (FIB) have been measured. The experiments were performed with
Ga+ ions of 35 keV at a current of 3 nA and variable chopping frequency
up to 10 MHz. The acoustic signals were detected by means of a piezoelectric
sensor with integrated pre-amplifier. A dependence on the sample material
was found. The results suggest that the ion-acoustic effect can be utilized
for imaging and material characterization of surface and subsurface structures
in FIB systems.

Acoustic waves induced by an intensity modulated focused ion beam (FIB) have been measured. The experiments were performed with Ga+ ions of 35 keV at a current of 3 nA and variable chopping frequency up to 10 MHz. The acoustic signals were detected by means of a piezoelectric sensor with integrated pre-amplifier. Frequency and position sensitivity of the sensor has been measured by laser excitation. Ion acoustic measurements have been carried out at the resonance frequencies of the sensor. A dependence on the sample material was found. The results show that the ion-acoustic effect can be utilized for imaging and material analysis in FIB systems.

Fe-Al show interesting physical properties and offer some
special industrial applications. There are phase transitions combined with
changes in the magnetic behaviour. Another interesting fact is the excellent
oxidation and corrosion resistance of iron aluminides even at high temperatures.
Thin Fe-Al layers can be produced in different ways. Ion beam methods enable
to produce surface layers on bulk material modifying the initial properties
completely. The properties depend strongly on the phase structure induced
by the preparation process. 57-Fe Mössbauer spectroscopy, and in the
case of surface layers conversion electron Mössbauer spectroscopy
is very suitable to identify this phase structure. In the present work,
it is described for Al implanted Fe layers. Depending on implantation dose
and energy both magnetic and non-magnetic phases can be produced. Due to
the inhomogeneous distribution of Al in the Fe target a layer structure
of different phases can be created. Moreover, due to double implantation
an Fe-Si-Al alloy can be prepared

Abstract
The mixing of the doorway components of a giant resonance due to the interaction via the common decay channels influences significantly the distribution of the multipole strength and the energy spectrum of the decay products of the giant resonance. The photoemission turns out to be most sensitive to the overlapping of the doorway states. At high excitation energies, the interference between the doorway states leads to a restructuring towards lower energies and apparent quenching of the dipole strength.

Abstract
Using a phenomenological schematic model of multipole giant resonances we consider the effects of overlapping of their doorway components. The conccpt of the partial widths of a giant resonance becomes ambiguous when the escape widths get comparable with the spacings between the components. In such a case, the partial widths determined in terms of the K- and S-matrices differ from each othcr. The mixing of the doorway components due to the interaction via the common decay channels influences significantly their multipole strengths, widths and positions in energy.

The Tilted Axis Crankiug theory is applied to the model of two particles coupled to a triaxial rotor. Cornparirig with the exact quantal solutions, the interpretation and quality of the mean field approximations studied. Conditions are discussed when the axis of rotation lies inside or outside of the principal planes of the triaxial density distribution. The planar solutions represent ∆ I = 1 bands, whereas the aplanar solutions represent pairs of identical ∆ I = 1 bands with the same parity. The two bands differ by the chirality of the principal axes with respect to the angular momentum vector. The transition from planar to chiral solutions is evident in both the quantal and the mean field calculations. Its physical origin is discussed.

The tribological and microstructural properties of gas-pressure
sintered Si3N4 ceramics after chromium implantation have been investigated
in dependence on implantation energy. The fluence was kept constant at
1017 ions/cm2. The wear of the modified material is reduced for a load
of 2 N independent of ion energy. This behaviour is accompanied by a slight
incerase of the friction coefficient. At higher loads only high energy
implantations result in an improved wear behaviour. The microstructural
investigations show that no new phase are formed due to implantation. For
all energies a part of the surface becomes amorphous. For smaller energies
this amorphous layer reaches up to the surface whereas at higher energies
it is covered by still crystalline but damaged material. The amorphization
of the surface is accompanied by a large surface swelling which incerases
with implantation energy. We discuss the relationship between the microstructural
features and the observed wear behaviour.

Limitations of the pulse-shape discrimination (PSD) technique - a promising method to identify the charged particIes stopped in planar Si-detectors - have been investigated. The particle resolution turned out to be basically determined by resistivity fluctuations in the buk silicon which cause the charge-collection time to depend on the point of impact. Detector maps showing these fluctuations have been measured and are discussed. Furthermore we present a simple method to test the performance of detectors with respect to PSD. Another limitation of the PSD technique is the finite energy threshold for particle identification. This threshold is caused by an unexpected decrease of the total charge-collection time for ions with a short range, in spite of the fact that the particle tracks are located in a region of very low electric field.

Iron and magnesium are insoluble elements with each other
and there is no phase diagram. However, it is possible to produce artificial
alloys by ion implantation, in this study by iron implantation into magnesium.
Samples are investigated by conversion electron Mössbauer spectroscopy,
Auger electron spectroscopy and x-ray diffraction. While at low doses gaussian
shaped iron profiles and paramagnetic doublets as Mössbauer spectra
are obtained, the iron concentration reaches at the highest dose 90 at.-%
in maximum and the Mössbauer spectrum shows a dominant ferromagnetic
fraction. The x-ray diffraction pattern let conclude that a dilated a-iron
lattice is formed. Microhardness of all samples is clearly increased due
to the implantation.

Iron implanted and subsequently annealed n-type Si(111) was
studied by conversion electron Mössbauer spectroscopy for phase analysis
and Auger electron spectroscopy for sputter depth profiling and element mapping.
During implantation (200 keV, 3 x 1017 cm-2, 350 °C) a mixture of ß-
and a-FeSi2 and after the subsequent annealing (900 °C for 18 h and
1150 °C for 1 h) a complete transition to the ß- and a-phase can
be detected. The as-implanted profile has Gaussian shape which becomes broad
during annealing at 900 °C to a plateau-like profile and shows only
a slight broadening and depth depending fluctuations of the iron concentration
after the 1150 °C annealing. With scanning Auger electron spectroscopy
the lateral iron and silicon distribution were investigated show for the
sample annealed at 900 °C large separated ß-FeSi2 precipitates
which grow due to the process of Ostwald ripening. At 1150 °C additionally
coalescence of the precipitates occur and a wide extended penetration a-FeSi2
network structure is formed.

The production of K⁺ and π⁺ mesons and protons in pBe collisions at T _p =2.9 GeV has been studied at the ITEP proton synchrotron. Ejectiles with a momentum of p=545 MeV/c were observed under an emission angle theta=17°. The detectors which have been developed for the identification of kaons out of a six orders of magnitude more intense background of pions and protons are described. A cross-section ration d²σ_K⁺ /dωdp: d²σ_π⁺ /dωdp: d²σ _{p ⁺} /dωdp of (1±0.34):(85±1):(31±1) has been measured. Normalization with existing pion data yields an invariant differential cross section E∙d³σ_K⁺ /d³ p=(3.1±1.2) mb GeV^-2c³sr^-1 and a total cross section of σ_tot(pBe) =(3.7±1.5) mb. These cross sections are compared with existing data and theoretical predictions. The A dependence of K⁺ production in the few-GeV range is analyzed.

Neutron particle-hole states in the doubly magic nucleus ²⁰⁸Pb
were investigated using a Euroball Cluster detector and five usual HPGe
detectors at the Osiris cube spectrometer in Cologne. 126 gamma-ray transitions
with energies up to 7.2 MeV could be detected, 14 of those for the first
time. The states were excited with the ²⁰⁸Pb (p,p'gamma)²⁰⁸Pb
reaction via isobaric analogue resonances in ²⁰⁹Bi at E_p=16.5,
17, 17.45, 17.5 and 18.5 MeV and through the subcoulomb ²⁰⁷Pb(d,pgamma)²⁰⁸Pb
reaction at E_d=10 MeV. For most of the excited states assignments
for the particle and/or the hole component of the configuration could be
made, 22 of these are new.

Characterization of Submicron Particles in an Environmentally-Relevant Water: Backwater From a Sanitary Landfill on a Uranium Mine Tailing at Freital, Saxony. A strategy for characterizing submicron particles in environmental waters based on filtration, centrifugation and photon correlation spectroscopy (PCS) is proposed. Filtration and centrifugation are not used here as methods for particle characterization but serve merely as techniques of a rough separation of the sample. They are applied as cautiously and non-invasively as possible and are only aimed at separating the sample sufficiently for measurements by primarily PCS. The separation should unmask small particles
that scatter little light and divide the sample into populations of particles that have monomodal or bimodal size distributions. In this way, the strategy optimizes subsequent particle-size determinations by PCS and minimizes the influence of sample preparation on the samples. A further essential feature of this strategy is the parallel application of complementary methods for both sample preparation and particle characterization (filtration/centrifugation, PCS/SEM, gravimetry/ICP-MS etc.). This facilitates the validation of results and increases the probability for recognizing artifacts. Based on our experimental experience, a guideline for PCS measurements on low-concentration colloids with the BI 90/Correlator BI 9000 AT system from Brookhaven Instruments Corp. was developed. This guideline is aimed at obtaining autocorrelation functions of sufficiently good photon counting statistics which is crucial in PCS on colloidal solutions poor in particles. A measuring campaign on an environmentally-relevant water (backwater from a sanitary landfill on a uranium mine tailing at Freital, Saxony) showed the presence of about 1 to 2 mg/l of submicron particles having a particle size of 30 to 300 nm and of about 5 mg/l of suspended matter (>1000 nm).

Das Windenergiepotential wird für komplexes Gelände im Binnenland mit zwie rechzeitökonomischen Modellen simuliert. Hierzu wird das in schwach gegliedertem Gelände Mitteleuropas erprobte und bewährte Programmpaket WAsP, sowie ein einfaches mesoskaliges Strömungsmodell (AIOLOS) verwendet. Die Ergebnisse der Simulation werden für ausgewählte Standorte verglichen und diskutiert. Die mit dem Modell WAsP berechnete Leistungsdichte, welche im Rahmen einer Windpotentialstudie für Zentralsachsen ermittelt wurde, wird dargestellt.

The impressive development of focused ion beam (FIB) systems
from the laboratory level to high performance industrial machines during
the last twenty years is briefly reported. The design and the functional
principle of a liquid metal ion source as well as a FIB column are described.
The main application fields of the FIB technology are discussed. More in
detail the Rossendorf FIB activities, like writing Co+ FIB implantation
in silicon to fabricate CoSi2 microstructures by ion beam synthesis and
the writing Ga+ FIB implantation to create a patterned etch stop during
a following anisotropic etching for micromechanical 3D structure fabrication
are presented.

New experiments in plasma immersion ion implantation (PIII)
have been performed to explore the extended operation regimes available
with pulsed plasmas. A pulsed extraction voltage and a synchronized pulsed
plasma show a further reduction of thermal load, more stable operation
and improved implantation results. It was even possible to run PIII in
a mode with pulsed plasma and a DC voltage; in this way omitting the costly
high voltage modulating unit. Time resolved measurements of plasma density
and electron temperature have been made to get information about the plasma
buildup and decay process. In all cases a space charge sheath forms around
the sample where the ions are accelerated. The extension of the sheath
has been measured using time resolved Langmuir probe measurements. Iron
and aluminium samples were implanted with nitrogen using a DC plasma and
pulsed extraction voltage, pulsed plasma and pulsed extraction voltage,
and pulsed plasma and a DC extraction voltage. Analysis showed that in
all cases nitrogen could be implanted.

Plasma immersion ion implantation was used to implant
oxygen ions into silicon with applied voltage pulses of 40 kV and 2.5 ms
total lenght. Positive ions from the plasma O2+ and O+, with a continuous
energy distribution between 0 and 40 keV were implanted with nominal doses
between 2x1016 and 2x1017/cm2. The resulting oxygen depth profiles were
measured with elastic recoil detection using 13.4 MeV a particles. The
obtained depth profiles were simulated using a linear superposition of
single-energy profiles calculated with TRIM, in order to determine relevant
parameters of the accelerated ions. The energy distribution of the incident
ions is derived from the results obtained and compared with theoretical
models. The agreement found is very good. The plasma found to be composed
of 35(8)% O2+ and 65(8)% O+ ions. An Fe contamination in the plasma is
observed using Rutherford backscattering.

Ion beam assisted deposition (IBAD) as a means of synthesizing
a tungsten carbide layer on top of SiC is investigated by Rutherford Backscattering
(RBS), X-ray diffraction (XRD) and four point probe measurements. Evaporation
of tungsten and subsequent implantation of carbon leads to a crystalline
layer of tungsten carbide whose resistivity is 1554 mWcm. At an implantation
dose of 5x1017C+cm-2 enhanced surface erosion occurs. Simultaneous evaporation
of tungsten and implantation of carbon procedures a layer of crystalline
W2C mixed with tungsten. The resistivity of this layer is 400 mWcm, i.e.
only a factor of 5 higher than that of single crystal W2C.

Die Ergebnisse der Solaranlage Waldblickschule Freital werden zusammenfassend dargestellt. Besonderes Augenmerk liegt auf der Einkopplung von solar erzeugter Wärme in ein Fernwärmenetz. Negativ wirken sich dabei sowohl hohe Netzverluste als auch hohe Rücklauftemperaturen aus. Die Untersuchungen ergaben, daß unter wirtschaftlichen Gesichtspunkten der solare Deckungsgrad in Fernwärmenetzen bei maximal 10% liegen sollte.

Nitrogen was implanted into four cylinders (height 37
mm and diameter 4, 6, 8, and 12 mm) covered with aluminium foil using plasma
immersion ion implantation at 30 kV. The homogeneity of the implantation,
i.e. the spatial variation of the implanted dose, was determined for selected
points on the top and the side walls of the cylinders with sputter depth
profiling using Auger electron spectroscopy. Calculations approximating
the dynamic sheath evolution with discrete sheath positions were performed
to obtain the variation of the implanted dose and angle of incidence. Good
agreement is found between the calculations and the experimental data.
The ion dose exhibits a maximum on the top and decreases on the side walls.
A second maximum is formed on the wall of the cylinder. The total dose
variation is of the order of 50 %.

In plasma immersion ion implantation the ions are accelerated
from the plasma to the target. The total current, as delivered by the high
voltage pulse generator, is only a qualitative reading of the ion flux
density on the target. The main reason for this is a large secondary electron
coefficient, strongly depending on the ion energy and the surface composition.
In this work direct time-resolved ion flux measurements are presented.
High voltage pulses of eU0 = 5, 10, 15 and 20 kV were applied to a spherical
target with a small orifice. The ions were collected in the high vacuum
region behind this orifice and their energy distribution function exhibits
a sharp peak at the nominal energy eU0 with the height decreasing during
the voltage pulse, in agreement with the theoretical predictions for a
collisionless transit through the plasma sheath.

Based on the analysis of a 43 years irradiation data serie (Dresden) sizing rules for stand alone PV systems are presented. Values of sizing irradiation and system autonomy are derived from the statistical analysis, taking into account realistic properties of the PV system components. These data seem to bee applicable to larger regions of Gemany and middle Europe. The sizing rules are compared with experimantal results and with results of simulations.

The further miniaturization of silicon micromechanical
structures in combination with the high developed microelectronic technology
at the mm- and sub-mm level will lead to a new generation of microdevices.
A modern technique to fabricate three dimensional micromechanical structures
is the combination of high concentration p+-doping by writing ion implantation
using a focused ion beam (FIB) and subsequent anisotropic and selective
wet chemical etching. FIB patterned and chemical etched 3D structures have
been fabricated using 35 keV Ga+-ion implantation and subsequent anisotropic
etching in KOH/H2O-solution. The Ga+-FIB patterned test structures were
characterized by scanning electron microscopy.

This paper presents computer simulation studies of boron
nitride using ab initio and empirical methods. Results of self-consistent
DFT-LDA calculations are shown, which were performed to characterize static
ground-state properties of the different BN modifications. With help of
these calculations the cubic phase is predicted as stable modification
under standard conditions. Furthermore an empirical interatomic potential
is introduced, which was parameterized by means of ab initio results and
allows a reliable description of structures and energies of Bn, Nm and
BnNm clusters and solid modifications. Finally, using this classical forcefield
a MD-simulation of N2 impact on a h-BN target is presented.

The present paper investigated time-resolved incident ion monitoring during PIII processes on the basis of high energy secondary electron measurements with scintillation technique. Taking into consideration the sheath geometry and a secondary electron emission coefficient of the target material, the experimental data were in a good agreement with theoretical predictions for dependences on target-voltage, pressure and plasma density.

A silicon nitride based ceramic was implanted with 150
keV titanium to a fluence of 1017 ions/cm2. Subsequently the samples were
implanted with oxygen using plasma immersion ion implantation (PIII) and
conventional beam-line implantation to stimulate the formation of lubricating
oxides in the surface. The wear is reduced for all implanted samples. The
friction coefficient is slightly higher than in the non-implanted case
for all implantation conditions investigated although oxide formation in
the wear track is suggested by detection of a relatively high amount of
oxygen in the wear track as indicated by energy-dispersive X-ray spectroscopy
(EDX). We discuss the possible mechanisms that lead to the observed tribological
behaviour.

Es wird ein Verfahren zur Messung der Massenfeuchte von in Rohrleitungen geführten Zweiphasenströmungen (gasförmig/flüssig) mit kleinem Massenstromanteil der flüssigen Phase (<30%) beschrieben, dadurch gekennzeichnet, dass die Positronen in die Strömung emittiert werden und die bei der Annihilation der in die Strömung emittierten Positronen entstehende Gamma-Strahlung in Abhängigkeit von der Entfernung des Annihilationsortes von der Positronenquelle gemessen wird und dadurch, dass die Form der Reichweitenverteilung der Positronen zur Bestimmung der Massenfeuchte verwendet wird.