Offline trapping of ${}^{221}$Fr in a magneto-optical trap from implantation of an ${}^{225}$Ac ion beam

Abstract

We demonstrate a new technique to prepare an offline source of francium for trapping in a magneto-optical trap. Implanting a radioactive beam of ${}^{225}$Ac, $t_{1/2}=9.920(3)$ days, in a foil, allows use of the decay products, i.e.${}^{221}$Fr, $t_{1/2}=288.0(4)$ s. ${}^{221}$Fr is ejected from the foil by the $\hat{I}\pm$ decay of ${}^{225}$Ac. This technique is compatible with the online accumulation of a laser-cooled atomic francium sample for a series of planned parity non-conservation measurements at TRIUMF. We obtain a 34% release efficiency for ${}^{221}$Fr from the recoil source based on particle detector measurements. We find that laser cooling operation with the source is $8_{-5}^{+10}$ times less efficient than from a mass-separated ion beam of ${}^{221}$Fr in the current geometry. While the flux of this source is two to three orders of magnitude lower than typical francium beams from ISOL facilities, the source provides a longer-term supply of francium for offline studies.

15 pages, 5 figures, accepted for publication in JINST