Abstract
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 radio-frequency 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 for the first time. Nuclear decay spectroscopy of is performed for the first time, revealing its half-life, -to--decay branching ratio, and -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.
10 More- Received 22 October 2014
DOI:https://doi.org/10.1103/PhysRevX.5.011018
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Published by the American Physical Society
Popular Summary
A significant branch of modern accelerator physics focuses on the production of new elements and their isotopes for fundamental research and medical applications. CERN ISOLDE’s Laser Ion Source and Trap (LIST) combines the high selectivity of laser ionization techniques, where specific colors can be tuned to choose a specific element, with a special geometry that inhibits impurities from exiting the ion source. However, contamination is a serious problem when attempting to isolate new elements and their isotopes. We show that, using the LIST, the contamination fraction of francium () can be suppressed by over a factor of a thousand compared with previous studies.
We study neutron-rich polonium derived from impinging protons on a uranium-carbide target with the aim of obtaining a pure sample with minimal contamination. The laser ionization occurring in the LIST employs a radio-frequency quadrupole ion guide that functions in one of two modes: (i) a mode in which the ions are simply guided to the extraction region, downstream of the LIST, and (ii) a high-selectivity mode, in which only ions created in the LIST are extracted. The latter mode results in a laser ionization efficiency drop of a factor of 20 but a high increase in selectivity, which yields access to new isotopes. The LIST contains two circular electrodes, and the ions are guided along the LIST’s axis by a potential gradient between the two electrodes; the resulting ion beam has energies ranging up to 60 keV. Unlike other setups, the laser ionization in the LIST is spatially separated from other ionization mechanisms, which accounts for the high purity of the resulting polonium. We determine the branching ratio of alpha and beta decay for to be approximately 28%. We additionally study the hyperfine structure of Po using atomic spectroscopy and confirm that the use of LIST does not result in any systematic effects.
Our results pave the way for studies of very exotic nuclei found in exploding stars and the production of alpha-emitting radioisotopes for cancer treatment.