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A Laser Ion Source and Trap (LIST) for a thick-target, isotope-separation on-line facility has been implemented at CERN ISOLDE for the production of pure, laser-ionized, radioactive ion beams. It offers two modes of operation, either as an ion guide, which performs similarly to the standard ISOLDE resonance ionization laser ion source (RILIS), or as a more selective ion source, where surface-ionized ions from the hot ion-source cavity are repelled by an electrode, while laser ionization is done within a radiofrequency quadrupole ion guide. The first physics application of the LIST enables the suppression of francium contamination in ion beams of neutron-rich polonium isotopes at ISOLDE by more than 1000 with a reduction in laser-ionization efficiency of only 20. Resonance ionization spectroscopy is performed directly inside the LIST device, allowing the study of the hyperfine structure and isotope shift of 217 Po for the first time. Nuclear decay spectroscopy of 219 Po is performed for the first time, revealing its half-life, α-to-β-decay branching ratio, and α-particle energy. This experiment demonstrates the applicability of the LIST at radioactive ion-beam facilities for the production and study of pure beams of exotic isotopes.

At radioactive ion beam facilities like ISOLDE at CERN, a high purity of the element of interest in the ion beam is essential for most experiments on exotic nuclei. Due to its unique combination of high ionization efficiency and ultimate elemental selectivity, the Resonance Ionization Laser Ion Source, RILIS, has become the most frequently used ion source at ISOLDE and at the majority of similar facilities worldwide. However, isobaric contamination predominantly stemming from unspecific surface ionization may still introduce severe limitations. By applying the highly selective resonance ionization technique inside a radio-frequency quadrupole ion guide structure, the novel approach of the Laser Ion Source and Trap, LIST, suppresses surface ionized isobaric contaminants by an electrostatic repelling potential. Following extensive feasibility studies and off-line tests, the LIST device has been adapted and refined to match the stringent operational constraints and to survive the hostile environment of the ISOLDE front-end region enclosing the highly radioactive nuclear reaction target. The LIST operation was successfully demonstrated for the first time on-line at ISOLDE during two experiments, attesting its suitability for radioactive isotope production under routine conditions. Data of these on-line characterization measurements confirm a suppression of surface-ionized isobars by more than a factor of 1000 in accordance to off-line studies that were carried out for the preparation of the on-line experiments. During the first on-line test, the suppression was associated with an efficiency loss of not more than a factor of about 50 with respect to normal RILIS operation. These losses could be further reduced to only about 20 during the second run. Results of the off-line studies in comparison to the first on-line characterization data are discussed here

This work presents the development and testing of highly efficient excitation schemes for resonance ionization of palladium. To achieve the highest ionization efficiencies, a high-power, high repetition rate Ti:sapphire laser system was used and 2-step, 3-step and 4-step schemes were investigated and compared. Starting from different excited steps, the frequencies of the final ionization steps were tuned across the full accessible spectral range of the laser system, revealing several autoionizing Rydberg series, which converge towards the energetically higher lying state of the Pd+ ion ground state configuration. Through proper choice of these excitation steps, we developed a highly efficient, fully resonant 3-step excitation scheme, which lead to overall efficiencies of % and %, measured at two independent mass separator setups. To our knowledge, these are presently the highest efficiency values ever achieved with a resonance ionization laser ion source.

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