Search

Liquid metals, such as sodium (Na), lead (Pb) and lead–bismuth eutectic (Pb–Bi), are considered as potential coolants for the fast spectrum nuclear reactors of the next generation. Intensive studies have being performed in different countries aiming at better understanding of their properties needed for design and safety analysis of nuclear installations. This article gives a brief review of the compilations and recommendations developed for the main thermophysical properties of Na, Pb, and Pb–Bi. The properties of theses coolants were measured in many laboratories, but mainly at the normal atmospheric pressure and at relatively low temperatures (except for Na). In general, the reliability of data is satisfactory; however, a large uncertainty still exists for some properties of Pb and Pb–Bi. A set of correlations for estimation of the main properties of Na, Pb, and Pb–Bi in the typical temperature ranges is proposed. For prognosis of their thermodynamic properties at high pressures and temperatures, relevant equations of state (EOS), based on the proven physical models and available experimental data, are indicated. The critical parameters for Pb and Pb–Bi are not well defined yet, and this hinders the EOS validation.

International audience; In this article, the fluid forces and the dynamics of a flexible clamped-clamped cylinder in turbulent axial flow are computed numerically. In the presented numerical model, there is no need to tune parameters for each specific case or to obtain coefficients from experiments. The results are compared with the dynamics measured in experiments available in literature. The specific case studied here consists of a silicone cylinder mounted in axial water flow. Computationally it is found that the cylinder loses stability first by buckling. The threshold for buckling is in quantitative agreement with experimental results and weakly-nonlinear theory. At higher flow speed a fluttering motion is predicted, in agreement with experimental results. It is also shown that even a small misalignment between the flow and the structure can have a significant impact on the dynamical behavior. To provide insight in the results of these fluid-structure interaction simulations, forces are computed on rigid inclined and curved cylinders, showing the existence of two different flow regimes. Furthermore it is shown that the inlet turbulence state has a non-negligible effect on these forces and thus on the dynamics of the cylinder.

In the context of the forthcoming next generation of Radioactive Ion Beams (RIBs) facilities based on an Isotope Separation On Line (ISOL) method, the development of production targets capable of dissipating the high power deposited by the primary beam is a major challenge. The concept of a high-power target based on a liquid Pb–Bi loop incorporating a heat-exchanger and a diffusion chamber was proposed within EURISOL DS and is being developed within the LIEBE 1 project. Due to the non-static character of the target, specific hydrodynamics issues are of concern. In this paper, these issues are studied mostly based on three-dimensional (3D) Computational Fluid Dynamics analysis of the flow of the Lead Bismuth Eutectic (LBE) target, resulting in optimized designs. The concept and hydrodynamic challenges of generating RIBs from a liquid-metal-loop target irradiated with a high-power primary beam are presented. The optimization of the target design has been conducted keeping in mind the need for a fast and efficient release of short-lived isotopes. This study shows that approximately 100 ms after the proton pulse the irradiated liquid-metal is entirely and uniformly evacuated from the irradiation volume and spread in a shower of small droplets (100-μm radii), in order to reduce the diffusion length of isotopes. Solutions to deal with the typical cavitation risk due to the presence of low-pressure zones in the liquid have also been found and simulated.

Previous thermal hydraulic experiments with flowing lead bismuth eutectic (LBE) in the MYRRHA/XT-ADS windowless spallation target mock-up installed at the THEADES loop at the Karlsruher Institut fur Technologie have shown that the LBE flow in the target mock-up behaves as a vacuum pump. An experimental setup is built and tested at the THEADES loop for quantification of this effect. In this paper, experimental evidence shows that the vacuum effect of flowing LBE at 200 °C, under MYRRHA like geometry and nominal LBE flow conditions, leads to an equivalent vacuum pumping speed for nitrogen gas of the order of 1 l/s. This is sufficient to reduce the vacuum pressure in the target zone below 10 −3 mbar.

The objective of the European 6th framework Integrated Project (IP) EUROTRANS (EUROpean Research Programme for the TRANSmutation of High Level Nuclear Waste in an Accelerator Driven System) is to demonstrate the feasibility of transmutation of high level nuclear waste using subcritical Accelerator Driven Systems (ADS). The spallation target represents the most challenging new component in an ADS since it is the component coupling the accelerator and the nuclear core and is subjected to very high thermal load in a high radiation field. In this document the thermal hydraulic activities which led to reliable design rules for a windowless target are presented and the status of the heavy liquid metal target mock-up experiment at the KArlsruhe Liquid metal LAboratory (KALLA) are reported.

One of the major challenges that our society faces is the increasing demand for energy in general and electricity in particular. During the last century our energy supply was based on fossil fuels. Nowadays, we are confronted with decreasing hydrocarbon reserves and excessive CO2 emissions. At the same time renewable energy sources cannot satisfy the complete demand. For this reason the European Union, Japan, the United States, Korea, Russia, China, India, and other countries recognize that nuclear energy needs to be part of the “energy basket” of the future.

The objective of the European 6th framework project EUROTRANS is to demonstrate the technical feasibility of transmutation of high level nuclear waste using accelerator driven systems. Within this objective the design of a European experimental ADS should demonstrate the technical feasibilities to transmute a sizeable amount of waste and to operate an ADS safely. This ADS will be a subcritical reactor system having liquid lead–bismuth eutectic as coolant. The liquid lead–bismuth eutectic is also intended to serve as target material for the spallation reaction which forms a crucial part to the subcritical reactor core. Since lead–bismuth eutectic is used as core coolant and spallation material, knowledge of its thermal hydraulic behaviour is essential. Within the DEMETRA domain of the EUROTRANS project, basic thermal hydraulic studies in order to support the design and safety analysis of XT-ADS components and the development of measurement techniques have been started.

Turbulence-induced vibration is typically considered as a type of vibration with one-way coupling between the fluid flow and the structural motion: the turbulence creates an incident force field on the structure, but the structural displacement does not influence the turbulence. It is however challenging to measure the turbulence forcing function experimentally. In this article, the forcing function in annular flow is computed by means of Large-eddy simulations. The pressure spectrum is applied to the inner cylinder and the resulting vibration is computed. It is shown that the commonly used multiplication hypothesis does not hold for the present results. The computed spectrum showed an upper limit to the coherence length. The results of these computations are compared to experimental results available in literature and to semi-empirical models. The predicted displacements compared well with experimental results.

Using the Low-Temperature Nuclear Orientation (LTNO) technique one can study various interesting properties of atomic nuclei and nuclear decay which can be deduced from the measurements of the angular distributions of charged particles emitted during the decay. However, the use of particle detectors working in conditions of LTNO devices (which are generally not available commercially) is a necessary precondition for the realization of these experiments. Planar HPGe detectors for detection of charged particles at “liquid helium” temperatures were developed and produced at NPI Řež. Relatively simple technology using vacuum evaporation and diffusion was employed. The performance of detectors at low temperatures was tested and their characteristics measured in a testing cryostat before using them in real experiments. The HPGe detectors were extensively used in a whole range of LTNO experiments with various physical objectives — in offline (IKS Leuven) as well as online (CERN-ISOLDE, Louvain-la-Neuve — LISOL) experiments. In frame of the project “Meson-Exchange Enhancement of First-Forbidden Beta Transitions in the Lead Region”, the measurements of angular distribution of emitted β-particles allowed to determine experimentally the “meson-exchange currents” contribution to the β-decay. In the project “Isospin Mixing in N∼Z nuclei”, the isospin-forbidden β-transitions of the nuclei in region ( A =50–100) were studied in order to obtain information on the isospin structure of the nuclear states. A new project looking for the possible presence of the tensor currents contribution to the β-decay is being prepared for the CERN-ISOLDE facility.

An experimental set-up is described for the precise measurement of the recoil energy spectrum of the daughter ions from nuclear beta decay. The experiment is called WITCH, short for Weak Interaction Trap for CHarged particles, and is set up at the ISOLDE facility at CERN. The principle of the experiment and its realization are explained as well as the main physics goal. A cloud of radioactive ions stored in a Penning trap serves as the source for the WITCH experiment, leading to the minimization of scattering and energy loss of the decay products. The energy spectrum of the recoiling daughter ions from the $\beta$--decays in this ion cloud will be measured with a retardation spectrometer. The principal aim of the WITCH experiment is to study the electroweak interaction by determining the beta--neutrino angular correlation in nuclear $\beta$--decay from the shape of this recoil energy spectrum. This will be the first time that the recoil energy spectrum of the daughter ions from $\beta$--decay can be measured for a wide variety of isotopes, independent of their specific properties.

The WITCH experiment will measure the recoil energy spectrum of the daughter ions in beta-decay. The main parts of the experiment are two Penning traps and a subsequent retardation spectrometer. The beta-decays take place in the ion cloud in the decay trap. Since the ion cloud is in vacuum and due to the cylindrical structure of the trap, the recoiling daughter ions can leave the cloud and the trap without any significant energy loss and can be energy analyzed in the retardation spectrometer. The WITCH experiment is set up foremost to study the electroweak interaction by measuring the beta-v angular correlation in nuclear beta-decay which can be inferred from the shape of the energy spectrum of the recoil ions. In the beginning the experiment will focus on pure Fermi decays which will allow to search for additional scalar coupling in the weak interaction. Since Penning traps have no restrictions regarding the element to be trapped the most suitable isotope can be picked for this purpose. The WITCH experiment is presently being set up at ISOLDE. Status and perspectives of the experiment will be presented in the following. (C) 2002 Elsevier Science B.V. All rights reserved.

Nearly perpendicular magnetic hyperfine fields have been observed for the first time in the Ag ``spacers'' of Fe/Ag multilayers using low temperature nuclear orientation of ${}^{110}{\mathrm{Ag}}^{m}$ at 6 mK. At the same time, vibrating sample magnetometry measurements at temperatures down to 4 K have shown the magnetic anisotropy of the Fe to be in plane. The direction of the Ag hyperfine field is thus noncollinear (nearly orthogonal) to the Fe anisotropy. These results are compared with full potential linearized augmented plane wave calculations using the wien97 code.

Low-temperature nuclear orientation experiments on radioactive ${}^{56}\mathrm{Mn}$ ions implanted into insulating, antiferromagnetic crystals of ${\mathrm{MnCl}}_{2}\ensuremath{\cdot}4{\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{CoCl}}_{2}\ensuremath{\cdot}6{\mathrm{H}}_{2}\mathrm{O}$ are reported. In ${\mathrm{MnCl}}_{2}\ensuremath{\cdot}4{\mathrm{H}}_{2}\mathrm{O},$ comparison of the $\ensuremath{\gamma}$-ray anisotropy of the ${}^{56}\mathrm{Mn}$ nuclei with that of ${}^{54}\mathrm{Mn},$ doped into the sample during growth, showed that both the ${}^{56}\mathrm{Mn}$ and ${}^{54}\mathrm{Mn}$ spins felt a very similar hyperfine field. The site occupancy factor in a simple, two-site model was deduced to be ${0.96}_{\ensuremath{-}0.07}^{+0.04}.$ In ${\mathrm{CoCl}}_{2}\ensuremath{\cdot}6{\mathrm{H}}_{2}\mathrm{O},$ the average hyperfine field for the implanted ${}^{56}\mathrm{Mn}$ was significantly less than that for ${}^{54}\mathrm{Mn}$ and corresponded to $f=0.53\ifmmode\pm\else\textpm\fi{}0.10.$

The presence of isospin mixing into the ground state of 52 Mn was studied via anisotropic positron emission from nuclei. With this method the isospin forbidden Fermi-component in the Gamow–Teller dominated β decay was determined. It is shown that sample purity and the control of positron scattering is of vital importance. Comparison between theory and experiment shows that shell model calculations of the isospin mixing probability deviate by a factor three to seven from experiment. For more recent Hartree–Fock-RPA based calculations the difference is over two orders of magnitude.

Two experiments to search for new physics beyond the standard model for electroweak interactions by measuring correlations between different spin and momentum vectors in nuclear β-decay are discussed. In the first experiment the correlation between the emission asymmetry and the longitudinal polarisation of positrons emitted by polarised nuclei is determined. This type of measurement is sensitive to the presence of right-handed currents but also to possible scalar and tensor-type currents in the weak interaction. The aim of the second experiment is to determine the βν-correlation in β-decay by measuring the energy spectrum of the recoil ions, using a Penning trap and a retardation spectrometer. In this case the focus is on the search for scalar currents in the weak interaction. The results of the experiments presented here are complementary to results from experiments in muon decay and at high-energy colliders.

An extensive on-line nuclear orientation study of the angular distribution in favoured α-decay of Bi, At and Rn nuclei near the Z=82 and N=126 shell closures, performed at ISOLDE is presented. Surprisingly large αanisotropies were observed, showing systematic changes from one isotope to the next. Comparison with various theoretical models proves that anisotropic α-emission in favoured decays of near-spherical nuclei is determined by the structure of the decaying nucleus.

Near perpendicular magnetic hyperfine fields in 110mAg have been observed in Fe/Ag multilayers. These fields are studied by low temperature nuclear orientation (LTNO) in multilayers [Ag(x ML)/Fe(y ML)]20, with (x,y) monolayer (ML) values of (2,10), (3,9), (5,10) and (3,18). The 110mAg γ-ray anisotropy was measured as a function of applied magnetic field parallel to the multilayer. The average induced hyperfine field of Ag is significantly influenced by the quality of the multilayer as measured by X-ray diffraction.

We report on the status of the measurements of the longitudinal polarization of positrons emitted by polarized 107In and 12N nuclei. Present results yield a lower limit of 306 GeV/c2 (90 % CL) for the mass of a possible W gauge boson with predominantly right-handed couplings, if interpreted in the framework of the manifest left-right symmetric model. In addition, the helicity structure of the weak interaction is discussed on the basis of experimental results at low and intermediate energies in the leptonic, semileptonic and hadronic sectors. Whereas in the minimal manifest left-right symmetric extension of the Standard Model the results from collider experiments are superior to results from other experiments, experiments in the three sectors of the weak interaction yield complementary information on the helicity structure of the interaction if interpreted in general left-right symmetric models.

In this paper the modal characteristics of a flexible cylinder in turbulent axial flow are investigated with partitioned fluid-structure interaction simulations. In these simulations a computational fluid dynamics calculation to resolve the flow field is coupled with a computational structure mechanics calculation to compute the structural behavior. The cylinder is initially deformed according to an eigenmode in vacuo and then released. From this free vibration decay of the cylinder in the turbulent axial flow, modal characteristics are determined. To assess the accuracy of these calculations, the same configuration is computed as in an experiment with a solid brass cylinder mounted in a water-conveying pipe. The natural frequency appears to be relatively insensitive to an increase in flow velocity in this case. Both experiments and computations show the same trend of slightly decreasing natural frequency with increasing flow velocity. The damping, on the other hand, is very sensitive to the flow velocity. A change in flow velocity from 10m/s to 30 m/s results in a modal damping increase from 1.4% to 3.0%. Changes in molecular viscosity due to temperature differences had only a small effect on modal damping.

In this paper the vibration behavior of a flexible cylinder subjected to an axial flow is investigated numerically. Therefore a methodology is constructed, which relies entirely on fluid–structure interaction calculations. Consequently, no force coefficients are necessary for the numerical simulations. Two different cases are studied. The first case is a brass cylinder vibrating in an axial water flow. This calculation is compared to experiments in literature and the results agree well. The second case is a hollow steel tube, subjected to liquid lead–bismuth flow. Different flow boundary conditions are tested on this case. Each type of boundary conditions leads to a different confinement and results in different eigenfrequencies and modal damping ratios. Wherever appropriate, a comparison has been made with an existing theory. Generally, this linear theory and the simulations in this paper agree well on the frequency of a mode. With respect to damping, the agreement is highly dependent on the correlation used for the normal friction coefficients in the linear theory.

Low-temperature nuclear orientation has been used to study the magnetic behaviour of 54Mn and 57Co impurities in the AuFe alloy, below (14 at.% Fe-spin-glass state) and above (18 at.% Fe-re-entrant state) the percolation threshold. The anisotropy of the gamma -rays was monitored in the direction of the external magnetic field, as a function of the external magnetic field Bext up to 8.5 T and of the temperature in the range 4-40 mK. In Au86Fe14 we have found total misalignment of the Mn and Co spins when Bext to 0 and a rather easy alignment of Mn and Co with the application of Bext, which is typical for a spin-glass behaviour. On the other hand, both Mn and Co spins in Au82Fe18 show an alignment (mean value approximately 32 degrees ), even at Bext to 0, and are turned only slowly by Bext which corresponds to the re-entrant state. The difference in the Mn and Co results can be interpreted as an indication of the existence of local correlations between the Fe spins, supporting the necessity of using the finite-range model for the re-entrant state.

Results on the operation of a bolometer detector in high magnetic fields and at very low temperatures are presented. We tested a diamond absorber on a Ge thermistor in magnetic fields up to 8 Tesla and temperatures down to 10 mK using an55Fe X-ray and109Cd electron source. Heavily doped Ge shows a giant magnetoresistance effect. This was used to tune the thermistor impedance to an optimal value.

A new technique, based on relative measurements of the longitudinal polarization (P) of positrons emitted in two opoosite directions with respect to the nuclear polarization (R=${\mathit{P}}^{\mathrm{\ensuremath{-}}}$/${\mathit{P}}^{+}$), is used to provide constraints on extensions of the standard model (${\mathit{R}}_{0}$). The first result, i.e., R/${\mathit{R}}_{0}$=0.926\ifmmode\pm\else\textpm\fi{}0.041, was obtained with $^{107}\mathrm{In}$. Interpreted in the framework of left-right symmetric models and assuming no mixing, it provides a 90% C.L. lower limit of 225 GeV/${\mathit{c}}^{2}$ for the mass of a possible right-handed charged W gauge boson, reducing the mass range allowed by other semileptonic experiments.

The low-temperature nuclear orientation of54Mn in ferromagnetic Pt+5 at% Fe has been studied in the temperature range 4–50 mK andin the external magnetic field range 0.5–8.5 T. A considerable noncollinearity of Mn magnetic moments with respect to the bulk magnetization direction has been found in the wholeB ext range. A model based on the Heisenberg Hamiltonian in the first neighbour approximation is proposed, which fully describes our experimental data.

The first measurements of the longitudinal polarization of positrons emitted by polarized 107 In are reported. The positron polarization is deduced from the time-resolved decay spectrum of positronium hyperfine states. Our result is in agreement with the standard electro-weak model and provides a new lower limit for the mass of an eventual, predominantly right-handed charged W-gauge boson of 220 GeV/c 2 .

The low-temperature nuclear orientation of 54Mn in ferromagnetic Pt-10 at.% Fe has been studied in the temperature range 4-50 mK and in the external magnetic field range 0.5-8.5 T. Considerable noncollinearity of Mn magnetic moments with respect to the bulk magnetization direction has been found in the whole Bext range. A model based on the Heisenberg Hamiltonian in the first-neighbour approximation is proposed, which fully describes the experimental data. The authors found that the first-neighbor Mn-Fe exchange interaction is antiferromagnetic with the value JMn-Fe=-21(1) K, which lies in the range of the nearest-neighbor Mn-Mn or Fe-Fe exchange interaction parameters for Pt-based alloys.

A status report is given of the developments in Leuven concerning in-beam polarization of radioactive nuclei. Some properties of the setup are described and first test results are discussed.

A survey is given of particle emission from nuclei that have been spin oriented by cryogenical means. Experiments and recent developments with detectors in the low temperature environment and their on-line application are reviewed. The most recent results are mentioned. Some phenomena to be unraveled in future studies are pointed out.

Anisotropic α-emission from nuclei in the decay chain from223Ra to207Tl has been observed using low temperature nuclear orientation in combination with high resolution α-detection at 4.2 K. Information on partial wave amplitudes as well as hyperfine interactions has been extracted.

We report on the first measurements of the longitudinal polarization of positrons emitted by polarized nuclei, using cryogenically polarized107In. This so-called polarization-asymmetry correlation is very sensitive to the mass of a possible right-handed gauge boson, which is invoked by parity symmetric extensions of the standard V-A electroweak model to explain the experimentally observed strong violation of parity which, however, may not be complete. The positron polarization is deduced from the time-resolved decay spectrum of positronium hyperfine states. Our preliminary result points to a lower limit of about 210 GeV for the mass of an eventual right-handed charged W gauge boson. This result can still be improved.

The hyperfine field of NaFe has been measured using Low Temperature Nuclear Orientation of cold implanted24Na. The combination of both the γ-and β-anisotropies yields information on the implantation fraction of Na in Fe. Furthermore, an, upper limit for the relaxation time of Na in Fe could be deduced.

Dynamical nuclear polarization techniques are newly being applied at the Leuven Isotope Separator On-Line of the Cyclotron Research Center in Louvain-la-Neuve to polarize the intense radioactive ion beams which have become available. The experimental setup is described and first results are reported.

Recently semiconductor particle detectors, operational at low temperatures (4 K), have been incorporated in the low temperature nuclear orientation apparatus at ISOLDE-CERN. First test experiments with standard room temperature preamplifier components revealed a resolution of 4 keV at a 160 keV conversion electron line. This technical development makes it possible to study the anisotropic emission of α- and β-particles, conversion electrons and X-rays from oriented nuclei at the nuclear orientation facility at ISOLDE in addition to the standard γ-detection. Furthermore, we discovered that at 4 K the Si detectors are automatically fully depleted.

A system to polarize nuclear beams in-flight has been built at the central beam line of the Leuven Isotope Separator On Line (LISOL) at the Cyclotron Research Center in Louvain-la-Neuve. Surface collisions at grazing incidence as well as the tilted foil method are being applied as polarizing techniques. These methods are generally applicable and overcome the limitations imposed by the low temperature in situ nuclear orientation technique. First polarization tests have been carried out with a 50 keV N-14+ ion beam: (i) transmitted through a 2-mu-g/cm2 tilted C-foil and (ii) scattered at about 2.5-degrees from a flat Si(100) single crystal. The atomic polarization of the ions was detected by measuring the circular polarization of the 500 nm NII 2s(2)2p3p3D-2s(2)2p3d3F-degrees and the 567 nm NII 2s(2)2p3s3P-2s(2)2p3p3D optical transitions. Values for S/I of 10.2(1.9)% (500 nm) were deduced for the tilted foil and 7.7(1.1)% (500 nm) and 6.9(1.7)% (567 nm) for the scattering at the Si-surface.

Recently, a set-up to polarize nuclei 'in flight' has been developed at the LISOL isotope separator in Louvain-laNeuve. It is constructed in such a way that two complementary methods can be used: Tilted Foil (TF) and Ion Beam Surface Interaction at Grazing Incidence (IBSIGI). Both methods rely on an atomic polarization transfer to the nucleus, via hyperfine interaction. The atomic polarization is obstained by an anisotropic ion beam-surface interaction. This development completes the already existing Low Temperature Nuclear Orientation set-up and expands considerably the possibilities to polarize short lived nuclei. In a first run atomic polarizations of 50 keV N-14-beams were observed. Values up to 10% were obtained.

This paper reports on a low-temperature nuclear orientation (LTNO) study of Fe/Ag multilayers [Fe( x ML)/Ag( y ML)] 20 with ( x, y ) values of (9, 3), (18, 3) and (10, 5). The samples were prepared by molecular beam epitaxy (MBE) and the quality of the multilayers was checked by X-ray diffraction. The γ-ray anisotropy of the neutron activated 110 m Ag was measured as a function of the applied magnetic field. It was found that, in low external applied fields, the induced Ag hyperfine fields are directed out of the plane of the multilayer.

International audience; The windowless target electron beam experimental irradiation (WEBEXPIR) program was set-up as part of the MYRRHA/XT-ADS R&D effort on the spallation target design to investigate the interaction of a proton beam with a liquid lead–bismuth eutectic (LBE) free surface. In particular, possible free surface distortion or shockwave effects in nominal conditions and during sudden beam on/off transient situations, as well as possible enhanced evaporation were assessed. An experiment was conceived at the IBA TT-1000 Rhodotron, where a 7 MeV electron beam was used to simulate the high power deposition at the MYRRHA/XT-ADS LBE free surface. The geometry and the LBE flow characteristics in the WEBEXPIR set-up were made as representative as possible of the actual situation in the MYRRHA/XT-ADS spallation target. Irradiation experiments were carried out at beam currents of up to 10 mA, corresponding to 40 times the nominal beam current necessary to reproduce the MYRRHA/XT-ADS conditions. Preliminary analyses show that the WEBEXPIR free surface flow was not disturbed by the interaction with the electron beam and that vacuum conditions stayed well within the design specifications.

The MYRRHA accelerator-driven system (ADS) is being developed in Mol, Belgium incorporating a windowless spallation target, as this is the only option within the constraints of the envisaged neutron source performance. In this paper the research programme and experimental set-up required for the R&D effort towards the conditioning of Pb-Bi eutectic for use in a windowless liquid-metal spallation target are presented. The experiences acquired commissioning the set-up, the current status of the experiment and first results are discussed.

The presence of isospin mixing in the $T=5/2$ ground state of $^{71}\mathrm{As}$ was studied via anisotropic positron emission from oriented nuclei. A small isospin-forbidden Fermi component in the predominantly Gamow-Teller \ensuremath{\beta} decay was established, corresponding to an isospin mixing probability of $(13\ifmmode\pm\else\textpm\fi{}4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. The sign of the magnetic moment of $^{71}\mathrm{As}$ was determined to be positive.

In a series of experiments the hyperfine fields of Ag at the interface with Fe were measured in $\mathrm{Fe}∕\mathrm{Ag}$ multilayers using the low temperature nuclear orientation technique. The thickness of the Fe layers was chosen so that the in-plane shape anisotropy would dominate over the perpendicular surface anisotropy. However, for multilayers with a well ordered multilayer structure as measured by x-ray diffraction, the Ag magnetic hyperfine fields were found to be nearly perpendicular to the layer plane. The out-of-plane magnetic structure of the induced Ag magnetic moments has been also supported by vibrating sample magnetometry measurements on the same samples. In addition, the magnitude of the induced Ag hyperfine fields are significantly reduced by roughness. Calculations using WIEN97 confirm that the Ag hyperfine fields are strongly affected by the number of nearest Fe neighbors and their distance.

As a new nuclear physics application of low temperature detectors we propose the measurement of the beta asymmetry parameter for the allowed beta decay of 3H by means of low temperature nuclear orientation in high external fields. Since 3H is the lightest mirror nucleus these experiments are of particular interest in connection with the search for physics beyond the standard electroweak model. For an accurate measurement of the low energy beta particles, low thresholds and good energy resolution are equally important. Two types of detectors are discussed. The first type is based on the “standard” ionisation detectors and operates at 4.2 K. The low threshold is achieved by cooling the first stage of the preamplifier thereby reducing the electronic noise. More accurate results can be obtained with calorimetric detectors. We have shown that the use of bolometers in high magnetic fields is possible, with a very low threshold and a good energy resolution.

Behaviour of the Co magnetic moments in dilute PtFe alloys was studied in the temperature range of 5-50 mK and the external magnetic field range of 0-8.5 T using low-temperature nuclear orientation. Kondo behaviour of Co, known to exist in pure Pt, was found in the sample of lowest concentration only (Pt+0.22 at.% Fe). Large misalignment of the Co magnetic moments was seen, especially for alloys having Fe concentration slightly above the percolation limit 0.76 at.%. An anomaly in the hyperfine field value on the Co nucleus was also found in these alloys. >

Mass selective buffer gas cooling is a technique used for ions that are stored in a Penning trap. The technique can be applied to all elements and the mass resolving power achieved has proven to be sufficient to resolve isobars. When not only a few but 106 and more ions are stored at the same time, space charge starts to play a dominant role for the spatial distribution. In addition, the observed cyclotron frequency is shifted. This work investigates these effects by numerical calculations.

A setup has been constructed which implements the techniques of grazing surface scattering and of the tilted foil method to produce nuclear polarized beams at the Leuven Isotope Separator On Line (LISOL), which is connected to the cyclotron Cyclone at Louvain‐la‐Neuve. This development makes it possible to use the intense radioactive ion beams (RIB), which will be available soon in the framework of the RIB−project at Louvain‐la‐Neuve, for polarization experiments. Especially solid state studies using polarized probe nuclei are envisaged in the beginning. First experiments to test the setup have been carried out and the results are very promising. The specific features of the setup are discussed together with the results of the test experiments.

The experiment described here will search for deviations from the V-A structure of the standard electroweak model. It is based on measuring the recoil energy spectrum in nuclear beta decay which is determined by the electron-neutrino angular correlation. For pure Fermi decays this is exactly known in the standard model and any deviation will point to additional scalar interaction. The experiment consists of a Penning trap coupled to a retardation spectrometer to measure the energy of the recoiling daughter nuclei. The current status will be presented.

The WITCH (Weak Interaction Trap for CHarged particles) experiment is a retardation spectrometer coupled to a Penning trap and measures the beta-neutrino angular correlation via the shape of the recoil energy spectrum. The present form of the Standard Model describes weak processes in terms of vector and axial-vector type interactions, but the possible presence of scalar and tensor interactions is not yet ruled out. The main aim of this experiment is a test of the Standard Model for possible admixture of scalar and tensor currents. (C) 2002 Elsevier Science B.V. All rights reserved.