P. Schuurmans

WITCH: a recoil spectrometer for weak interaction and nuclear physics studies

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 β-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 β-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 β-decay can be measured for a wide variety of isotopes, independent of their specific properties.

Fundamental weak interaction studies using polarised nuclei and ion traps

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.

WITCH: a recoil spectrometer for β-decay

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

Alpha-anisotropy studies of near spherical nuclei at ISOLDE

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.

Helicity structure in low- and intermediate-energy weak interaction

Abstract 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.

Anisotropic alpha-emission in the223Ra decay chain

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.

Particle-detection from oriented nuclei

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.

Perpendicular induced magnetic moments of Ag in Ag/Fe multilayers

Abstract We have studied the induced magnetic moments of Ag in Ag/Fe multilayers with low temperature nuclear orientation. The experiments show that there are induced magnetic moments in the Ag spacer and these induced magnetic moments do not lie in the plane of the multilayer for 2 and 4 ML of Ag at low external field.

Enhanced performance of a bolometric detector at mK temperatures in high magnetic fields

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.

Search for new physics in beta-neutrino correlations with the WITCH spectrometer

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