Bimodal symmetric fission observed in the heaviest elements

Abstract We measured the mass and kinetic energy partitioning in the spontaneous fission (SF) of 258 Fm, 259 Md, 260 Md, 258 No and 260 [104]. All these fissions have mass distributions that are symmetric. Total kinetic energy distributions peaked near either 200 or 235 MeV. Surprisingly, the total kinetic energy distributions were skewed upward or downward from the peak energy in each case, except for 260 [104], indicating a composite of two energy distributions. Such unique distributions have not been observed in the low-energy fission or SF of other nuclides. We interpret this as a mixture of liquid-drop-like and fragment-shell directed fission, although theory had not predicted this phenomenon.