Transition from mass asymmetry to symmetry in the spontaneous fission of Fm isotopes

Abstract

The observed transition in the spontaneous fission mass distribution from asymmetry in the lighter Fm isotopes to symmetry in the heavier Fm isotopes is studied on the basis of the potential energy surfaces (PES) and fragment shell effects in the two-center shell model. To that end, we have calculated the PES for ${}_{}{}^{256}{}_{}{}^{}\mathrm{Fm}$ and compared them with our previous PES results for other Fm isotopes. Our calculations show that the lighter Fm isotopes (up to ${}_{}{}^{256}{}_{}{}^{}\mathrm{Fm}$) should fission asymmetrically, while ${}_{}{}^{258}{}_{}{}^{}\mathrm{Fm}$ and other heavier Fm isotopes should fission symmetrically. This is due to the fact that the shell corrections in these nuclei are found to increase with increasing mass number and decreasing neck radius, particularly in the region beyond the second barrier. This, in turn, is a consequence of strong shell effects in fragment nuclei near the doubly closed-shell ${}_{50}{}^{132}{}_{}{}^{}\mathrm{Sn}_{82}^{}{}_{}{}^{}$. The results supporting these conclusions are shown.