Systematic measurements of the Bohr-Weisskopf effect at ISOLDE

The " Bohr-Weisskopf " effect, or " hyperfine structure (hfs) anomaly ", which results from the effect of the distribution of nuclear magnetization on the electro-nuclear interaction, will be measured systematically at the PS Booster ISOLDE, first for a long chain of radioactive cesium isotopes, analogously to previous isotope shift and hfs studies. In addition to the direct measurement of magnetic moment values, the results are expected to provide independent data for testing nuclear wavefunctions, these will be of importance for interpreting systematic parity non-conservation experiments, complementary to the single isotope study which requires a high precision knowledge of the electron wavefunction. Substantial progress in these calculations has been achieved recently. Precision measurements of the hfs splittings and nuclear magnetic moments are required, with sensitivity adequate for the radioactive isotopes produced. A triple resonance atomic beam magnetic resonance apparatus with optical pumping state selection has been constructed. Detection of the beam atomic state is by laser-induced fluorescence or by magnetic focusing and mass spectrometry. The performance of the apparatus has been tested with stable K and Rb beams. Results obtained for g-values and hfs anomalies are in excellent agreement with published data, and show the technique to be well suited for the on-line experiments at ISOLDE. Nevertheless, the implantation of such an experiment at ISOLDE requires the neutralization of the ISOLDE ion beam and the production of a thermal atomic beam. This corresponds to a very delicate interfacing part of the instrument which posed some problems, but which are now near to being solved. For that, in order to study the properties of various types of neutralizer and improve on the observed efficiency, we have constructed an auxiliary ion source, this allowed us to demonstrate that a neutralization efficiency of 40~\% can be achieved using a simple current heated tantalum tube in which a smaller Yttrium tube was inserted. A more sophisticated neutralizer, an " orthotropic " atomic beam source, has been constructed. It is essentially a small heated chamber, with entrance and exit holes, that contains the small neutralizing element at a negative potential. The neutral atoms, after multiple bounces from the chamber walls, emerge through the exit hole. The ion component returns from the chamber walls to the neutralizer under the action of the field for recycling. This source is expected to increase the efficiency in the thermal neutral beam production; this will be tested very soon using a beam of stable Cs from ISOLDE. Furthermore, to improve the acceptance angle of the apparatus, and thus, the sensitivity; we added, just behind the neutralizer, a new six-pole permanent magnet. Tests on the whole apparatus are in progress.\\ \\