Sympathetic cooling of trapped Th3+ alpha-recoil ions for laser spectroscopy

Abstract

Sympathetic cooling of Th${}^{3+}$ ions is demonstrated in an experiment where ${}^{229}$Th and ${}^{230}$Th are extracted from uranium recoil ion sources and are confined in a linear Paul trap together with laser-cooled ${}^{88}$Sr${}^{+}$ ions. Because of their similar charge-to-mass ratios the ions are closely coupled and arrange themselves in two-species Coulomb crystals, containing up to a few tens of Th${}^{3+}$ ions. To show the suitability of the sympathetically cooled Th${}^{3+}$ ions for high-resolution laser spectroscopy, the absolute frequencies and isotope shifts of 5F${}_{5/2}$\,$\to$\,6D${}_{5/2}$ and 5F${}_{7/2}$\,$\to$\,6D${}_{5/2}$ transitions of ${}^{230}$Th${}^{3+}$ have been measured. The system is developed for hyperfine spectroscopy of electronic transitions of nuclear ground and isomeric states in ${}^{229}$Th${}^{3+}$.

8 pages, 8 figures