Sympathetic cooling of Th3+ ions is demonstrated in an experiment where 229Th and 230Th are extracted from uranium recoil-ion sources and are confined in a linear Paul trap together with laser-cooled 88Sr+ ions. Because of their similar charge-to-mass ratios the ions are closely coupled and arrange themselves into two-species Coulomb crystals containing up to a few tens of Th3+ ions. To show the suitability of the sympathetically cooled Th3+ ions for high-resolution laser spectroscopy, the absolute frequencies and isotope shifts of 5F5/2→6D5/2 and 5F7/2→6D5/2 transitions of 230Th3+ are measured. The system is developed for hyperfine spectroscopy of electronic transitions of nuclear ground and isomeric states in 229Th3+.

The work was carried out in cooperation with Johannes Gutenberg University Mainz and the GSI Helmholtz Centre for Heavy Ion Research, where the U-233 sources were produced. The results have now been published in the journal Physical Review A (https://journals.aps.org/pra/abstract/10.1103/PhysRevA.109.033116), where they were highlighted as an “Editor’s Suggestion”.