  Precision optical spectroscopy of exotic ions reveals accurate information
about nuclear properties such as charge radii and magnetic and quadrupole
moments. Thorium ions exhibit unique nuclear properties with high relevance for
testing symmetries of nature. We report loading and trapping of single
$^{232}$Th$^+$ ions in a linear Paul trap, embedded into and sympathetically
cooled by small crystals of trapped $^{40}$Ca$^+$ ions. Trapped Th ions are
identified in a non-destructive manner from the voids in the laser-induced Ca
fluorescence pattern emitted by the crystal, and alternatively, by means of a
time-of-flight signal when extracting ions from the Paul trap and steering them
into an external detector. We have loaded and handled a total of 231 individual
Th ions. We reach a time-of-flight detection efficiency of $\gtrsim 95\, \%$,
consistent with the quantum efficiency of the detector. The sympathetic cooling
technique is expected to be applicable for other isotopes and various charge
states of Th e.g., for future studies of $^{229m}$Th.
