12.3-minCf256and 43-minMd258and systematics of the spontaneous fission properties of heavy nuclides

The new isotope 12.3-min ${}_{}{}^{256}{}_{}{}^{}\mathrm{Cf}$ was produced via the ${}_{}{}^{254}{}_{}{}^{}\mathrm{Cf}$($t$,$p$) reaction, and a new 43-min isomer of ${}_{}{}^{258}{}_{}{}^{}\mathrm{Md}$ was produced via the ${}_{}{}^{255}{}_{}{}^{}\mathrm{Es}$($\alpha$,$n$) reaction. The fragment mass and kinetic energy distributions from the spontaneous fission of ${}_{}{}^{256}{}_{}{}^{}\mathrm{Cf}$ were found to be very similar to those from the spontaneous fission of lighter Cf isotopes. The mass division is primarily asymmetric, and the average total kinetic energy is 189.8±0.9 MeV. The 43-min ${}_{}{}^{258}{}_{}{}^{}\mathrm{Md}$ presumably decays by electron capture and provides an opportunity to study the mass and kinetic energy distributions from the spontaneous fission of the 380-μs ${}_{}{}^{258}{}_{}{}^{}\mathrm{Fm}$ daughter. The observed narrow, symmetric mass distribution and the most probable total kinetic energy of 238±3 MeV are similar to those reported for the spontaneous fission of ${}_{}{}^{259}{}_{}{}^{}\mathrm{Fm}$ but show a sharp increase in symmetric mass division and total kinetic energy compared to ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$ and the lighter Fm isotopes. No such abrupt change in properties was observed for ${}_{}{}^{256}{}_{}{}^{}\mathrm{Cf}$, which, like ${}_{}{}^{258}{}_{}{}^{}\mathrm{Fm}$, has 158 neutrons. The marked difference between the spontaneous fission properties of the heavier Fm isotopes and those of other spontaneously fissioning nuclides is compared to some theoretical predictions.