Fragment velocity, kinetic energy, mass yield, and element yield distributions in the fission of neutron-deficient Pa isotopes produced in the reactions of ${}^{16}$O and ${}^{18}$O on ${}^{209}$Bi have been measured at incident beam energies near and above the Coulomb barriers by the time-of-flight and radiochemical methods. An asymmetric mass-division component has been observed. Measured fission cross sections were compared with the results of statistical model calculations which take into account two fission barrier heights for symmetric and asymmetric yields. The fission barrier height deduced for the asymmetric fission is found slightly lower than that for the symmetric one. The difference between the two barrier heights in the fission of the present protactinium nuclides $\left(N\sim 135\right)$ is considerably smaller than that in the neutron-rich nuclide of ${}^{233}$Pa $\left(N\sim 142\right)$, indicating that the difference sensitively depends on the neutron number of the fissioning nuclide.

• Received 12 March 1997

DOI:https://doi.org/10.1103/PhysRevC.56.891