Fission Barriers for Nuclei Lighter than Radium

Potential energy surfaces have been calculated for the mass range $180\le A\le 212$ by using the shell-correction method on the basis of a symmetric two-center shell model. All calculations have been performed both with a constant and with a surface-dependent pairing strength. The results show that the fission barriers calculated with the latter prescription are incompatible with experimental data. The calculations for a constant pairing strength, however, in which the shells at the saddle are calculated on the same basis as those at the ground state, agree reasonably well with experimental data of the Berkeley group. The shell corrections for these nuclei are analyzed separately in detail and are found to exhibit remarkable regular structures which can be explained by an interplay between the shells of the fragments and those of the fissioning nucleus. Finally, the existence of shape-isomeric and the treatment of the zero-point energy in these calculations are discussed.