Resonance ionization spectroscopy of actinium isotopes in a supersonic gas jet

Full text of publication follows. The study of exotic nuclei at the limits of nuclear existence by laser spectroscopy requires the application of a technique with high efficiency and high spectral resolution. Laser ionization in the low-temperature and low-density supersonic gas jet from a de Laval nozzle installed at the buffer gas cell exit, results in a great reduction of the collisional broadening and thereby in a significant improvement of the spectral resolution and the ionization efficiency. In-gas-jet laser spectroscopy studies on the neutron deficient actinium isotopes ^214,215Ac (at the N=126 shell closure) revealed a total spectral resolution down to ∼400 MHz, enabling a 25-fold improvement in the extracted isotope shifts and hyperfine A- and B- parameters in comparison to former in-gas-cell spectroscopy studies. Moreover, the results show that the total ionization efficiency in the gas jet is comparable to that in the gas cell (∼0.5 %) and can potentially be improved by at least one order of magnitude by optimizing the duty cycle. Combining these results with state-of-the-art atomic calculations, nuclear properties such as spins, magnetic dipole and electric quadrupole moments, and differences in mean charge radii have been obtained for a number of actinium nuclei around the N=126 neutron shell closure. A comparison to large-scale shell-model calculations provides new evidence to the extent of magicity in this region. Further characterization and optimization of the in-gas- jet technique is ongoing at the off-line IGLIS laboratory at KU Leuven, where the physical and technical limits of the technique are being explored. This will ensure the best performance in spectral resolution and ionization efficiency for the future IGLIS setups that will be coupled to the in-flight mass separator MARA, at the Department of Physics in the University of Jyvaeskylae, and to the Superconducting Separator Spectrometer (S3) at the new radioactive ion beam facility SPIRAL2 (GANIL). (author)