Bimodal symmetric fission observed in the heaviest elements

TLDR

The study measured mass and kinetic‑energy partitioning in the spontaneous fission of several heavy nuclei. The fission of these nuclei produced symmetric mass distributions with total‑kinetic‑energy peaks near 200 or 235 MeV, and the energy spectra were skewed, revealing a composite of two distributions that the authors interpret as a mixture of liquid‑drop‑like and fragment‑shell‑directed symmetric fission, a phenomenon not predicted by theory.

Abstract

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.