The mass assignments for the activities previously assigned to ${}_{}{}^{259}{}_{}{}^{}\mathrm{Lr}$ and ${}_{}{}^{258}{}_{}{}^{}\mathrm{Lr}$ have been confirmed. The ${}_{}{}^{259}{}_{}{}^{}\mathrm{Lr}$ and ${}_{}{}^{258}{}_{}{}^{}\mathrm{Lr}$ were produced by the ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cm}$${(}^{15}$N,4n) and ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cm}$${(}^{15}$N,5n) reactions, respectively. The Lr α activities were measured directly using our rotating wheel system. Mass assignments were made by an α-daughter recoil catcher technique in which the ${}_{}{}^{255}{}_{}{}^{}\mathrm{Md}$ and ${}_{}{}^{254}{}_{}{}^{}\mathrm{Md}$ daughters were caught on aluminum foils placed directly above the rotating wheel. The α decay of the 20.07-h ${}_{}{}^{255}{}_{}{}^{}\mathrm{Fm}$ and the 3.24-h ${}_{}{}^{254}{}_{}{}^{}\mathrm{Fm}$ granddaughter activities in these aluminum foils were then measured and correlations with the decay of the Lr parent activities were made based on the distribution of the Fm activities among the Al foils, which corresponded to the half-lives of the parent Lr α activities. We have determined the half-lives for ${}_{}{}^{259}{}_{}{}^{}\mathrm{Lr}$ and ${}_{}{}^{258}{}_{}{}^{}\mathrm{Lr}$ to be 6.${34}_{\mathrm{-}0.42}^{+0.46\mathrm{}}$ and 3.${92}_{\mathrm{-}0.31}^{+0.35\mathrm{}}$ s, respectively. The spontaneous-fission branch in ${}_{}{}^{259}{}_{}{}^{}\mathrm{Lr}$ was measured to be 23±2%. The spontaneous-fission mass distribution appears to be broadly symmetric and the average total kinetic energy is estimated to be 200±10 MeV.

• Received 17 April 1991

DOI:https://doi.org/10.1103/PhysRevC.45.1058