Symmetric fission in the neutron-induced fission of Fm255

The kinetic energy distributions of coincident fission fragments from the thermal-neutron-induced fission of $^{255}\mathrm{Fm}$ and $^{251}\mathrm{Cf}$ have been measured with phosphorus-diffused silicon detectors. The most probable values for the postneutron total kinetic energy are 192.5\ifmmode\pm\else\textpm\fi{}2.9 MeV for $^{255}\mathrm{Fm}$ and 182.1\ifmmode\pm\else\textpm\fi{}2.7 MeV for $^{251}\mathrm{Cf}$. Fragment mass distributions were calculated without applying neutron emission corrections. The resultant mass and kinetic energy distributions for $^{255}\mathrm{Fm}$ indicate a predominantly asymmetric mass division combined with appreciable symmetric fission. Fragment pairs near mass symmetry were found to be unusually energetic, which is a characteristic shared with symmetric fission in $^{257}\mathrm{Fm}$ and $^{258}\mathrm{Fm}$. These results are well described by the two-center model of fission. Thermal-neutron-induced fission cross sections were measured as 3400\ifmmode\pm\else\textpm\fi{}170 b for $^{255}\mathrm{Fm}$ and 4800 \ifmmode\pm\else\textpm\fi{} 250 b for $^{251}\mathrm{Cf}$.[NUCLEAR REACTIONS, FISSION $^{255}\mathrm{Fm}(n,f)$, $^{251}\mathrm{Cf}(n,f)$, $E=0.025$ eV; measured $\ensuremath{\sigma}$, fragment $E$; deduced fragment masses.]