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±2.9 MeV for ${}_{}{}^{255}{}_{}{}^{}\mathrm{Fm}$ and 182.1±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±170 b for ${}_{}{}^{255}{}_{}{}^{}\mathrm{Fm}$ and 4800 ± 250 b for ${}_{}{}^{251}{}_{}{}^{}\mathrm{Cf}$.

[NUCLEAR REACTIONS, FISSION ${}_{}{}^{255}{}_{}{}^{}\mathrm{Fm}\mathrm{}(n,f)\mathrm{}$, ${}_{}{}^{251}{}_{}{}^{}\mathrm{Cf}\mathrm{}(n,f)\mathrm{}$, $E=0.025\mathrm{}$ eV; measured $\sigma$, fragment $E$; deduced fragment masses.]

• Received 9 July 1974

DOI:https://doi.org/10.1103/PhysRevC.9.399