Determination of the extraction efficiency for ${}^{233}$U source $\alpha$-recoil ions from the MLL buffer-gas stopping cell

Following the $\alpha$ decay of ${}^{233}$U, ${}^{229}$Th recoil ions are shown to be extracted in a significant amount from the MLL buffer-gas stopping cell. The produced recoil ions and subsequent daughter nuclei are mass purified with the help of a customized quadrupole mass spectrometer. The combined extraction and mass-purification efficiency for ${}^{229}$Th${}^{3+}$ is determined via MCP-based measurements and via the direct detection of the ${}^{229}$Th $\alpha$ decay. A large value of $(10\pm 2)$% for the combined extraction and mass-purification efficiency of ${}^{229}$Th${}^{3+}$ is obtained at a mass resolution of about 1 u/e. In addition to ${}^{229}$Th, also other $\alpha$-recoil ions of the ${}^{233,232}$U decay chains are addressed.