NUCLEAR-FISSION OF NEUTRON-DEFICIENT PROTOACTINIUM NUCLIDES

Fragment velocity, kinetic energy, mass yield, and element yield distr ibutions in the fission of neutron-deficient Pa isotopes produced in t he reactions of O-16 and O-18 on Bi-209 have been measured at incident beam energies near and above the Coulomb barriers by the time-of-flig ht and radiochemical methods. An asymmetric mass-division component ha s been observed. Measured fission cross sections were compared with th e results of statistical model calculations which take into account tw o fission barrier heights for symmetric and asymmetric yields. The fis sion barrier height deduced for the asymmetric fission is found slight ly lower than that for the symmetric one. The difference between the t wo barrier heights in the fission of the present protactinium nuclides (N similar to 135) is considerably smaller than that in the neutron-r ich nuclide of Pa-233 (N similar to 142), indicating that the differen ce sensitively depends on the neutron number of the fissioning nuclide .