Fission branch in 259Lr and confirmation of 258Lr and 259Lr mass assignments.

The mass assignments for the activities previously assigned to {sup 259}Lr and {sup 258}Lr have been confirmed. The {sup 259}Lr and {sup 258}Lr were produced by the {sup 248}Cm({sup 15}N,4{ital n}) and {sup 248}Cm({sup 15}N,5{ital n}) reactions, respectively. The Lr {alpha} activities were measured directly using our rotating wheel system. Mass assignments were made by an {alpha}-daughter recoil catcher technique in which the {sup 255}Md and {sup 254}Md daughters were caught on aluminum foils placed directly above the rotating wheel. The {alpha} decay of the 20.07-h {sup 255}Fm and the 3.24-h {sup 254}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 {alpha} activities. We have determined the half-lives for {sup 259}Lr and {sup 258}Lr to be 6.34{sub {minus}0.42}{sup +0.46} and 3.92{sub {minus}0.31}{sup +0.35} s, respectively. The spontaneous-fission branch in {sup 259}Lr was measured to be 23{plus minus}2%. The spontaneous-fission mass distribution appears to be broadly symmetric and the average total kinetic energy is estimated to be 200{plus minus}10 MeV.