Bimodal symmetric fission observed in the heaviest elements

We measured the mass and kinetic-energy partitioning in the spontaneous fission of ²⁵⁸Fm, ²⁵⁹Md, ²⁶⁰Md, ²⁵⁸No, 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.