An isomeric state of ${}_{}{}^{222}{}_{}{}^{}\mathrm{Ac}$ with a half-life of 66 ± 3 sec has been discovered in bombardments of lead with ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ ions. The isomer decays by $\alpha$-particle emission and has a complex spectrum with major $\alpha$-particle groups at 6.46 (2%), 6.71 (8%), 6.75 (15%), 6.81 (27%), 6.84 (10%), 6.89 (15%), 6.97 (8%), and 7.00 MeV (15%). Evidence for the decay of the isomeric state both by isomeric transition (<10%) and by electron capture (∼1%) is presented. The half-life of ${}_{}{}^{218}{}_{}{}^{}\mathrm{Fr}$ was measured to be 0.7 ± 0.6 msec.

• Received 30 September 1971

DOI:https://doi.org/10.1103/PhysRevC.5.942