12.3-min 256Cf and 43-min 258Md and systematics of the spontaneous fission propertiesof heavy nuclides

The new isotope 12.3-min 256Cf was produced via the 254Cf(t,p) reaction, and a new 43-min isomer of 258Md was produced via the 255Es(α,n) reaction. The fragment mass and kinetic energy distributions from the spontaneous fission of 256Cf 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 258Md 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 258Fm 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 259Fm but show a sharp increase in symmetric mass division and total kinetic energy compared to 257Fm and the lighter Fm isotopes. No such abrupt change in properties was observed for 256Cf, which, like 258Fm, 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