We measured the mass and kinetic-energy partitioning in the spontaneous fission of ${}_{}{}^{258}{}_{}{}^{}\mathrm{Fm}$, ${}_{}{}^{259}{}_{}{}^{}\mathrm{Md}$, ${}_{}{}^{260}{}_{}{}^{}\mathrm{Md}$, ${}_{}{}^{258}{}_{}{}^{}\mathrm{No}$, and ${}_{}{}^{260}{}_{}{}^{}]$. All fissioned with mass distributions that were symmetric. Total-kinetic-energy distributions peaked near either 200 or 235 MeV. Surprisingly, these energy distributions were skewed upward or downward from the peak in each case, except for ${}_{}{}^{260}{}_{}{}^{}]$, indicating a composite of two energy distributions. We interpret this as a mixture of liquid-drop-like and fragment-shell directed symmetric fission, although theory had not anticipated this phenomenon.

• Received 23 December 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.313