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 $^{255}\mathrm{Es}$($\ensuremath{\alpha}$,$n$) 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\ifmmode\pm\else\textpm\fi{}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-\ensuremath{\mu}s $^{258}\mathrm{Fm}$ daughter. The observed narrow, symmetric mass distribution and the most probable total kinetic energy of 238\ifmmode\pm\else\textpm\fi{}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.RADIOACTIVITY, FISSION $^{256}\mathrm{Cf}$ (SF); measured ${T}_{\frac{1}{2}}$, fragment-fragment coin; deduced TKE, mass distribution. $^{258}\mathrm{Md}$; Measured ${T}_{\frac{1}{2}}$; EC decay to $^{258}\mathrm{Fm}$ (SF); measured fragment-fragment coin; deduced TKE, mass distribution.

Fission probabilities are presented for a series of Pu, Cm, and Cf nuclei with neutron numbers, $N$, in the range of 150-154. Results show that the fission thresholds are slowly varing with $N$ and $Z$ for $N=150\ensuremath{-}153$ but there is a strong decrease in the threshold for $N=154$. For $^{252}\mathrm{Cf}$ ($N=154$) the results indicate broad, strong resonances at \ensuremath{\sim} 5.0 and 5.4 MeV and suggest the breakup of the first axially asymmetric saddle into two barriers with large $\ensuremath{\hbar}\ensuremath{\omega}$.

Recent progress in the experimental determination of fission barrier and scission properties is reviewed. The importance of deformed nuclear shells in influencing fission decay rates in actinide nuclei is demonstrated. The dramatic influence of shape symmetries on fission rates and evidence for increasingly complex barrier shapes is presented. Recent data and interpretation suggest double and triple peaked fission barriers and multiple parallel paths in the saddle point region. These effects are generally understood in terras of modulations of a simple liquid drop fission barrier by energy deviations of ~ 1-3 MeV due to the extra stability of some deformed nuclear configurations (i.e., shell effects). The scission properties (mass and total kinetic energy distributions) for actinide nuclei are also reviewed, and it is demonstrated that nuclear shells in the nascent fragments near scission are also important. The most striking effect of this type is in the fermium region where fission undergoes a sudden transition from asymmetric to symmetric mass division as the result of approaching the doubly magic configuration of two 132Sn nuclei which have extraordinary binding energies. Various recent and proposed measurements to try to isolate shell and dynamic effects in the fragment distributions are also discussed.

Fission probabilities of heavy (mass 245-251) actinides have been determined using previously described techniques. Empirical average $\frac{{\ensuremath{\Gamma}}_{n}}{{\ensuremath{\Gamma}}_{f}}$ values and fission barrier parameters deduced from statistical model fits to the results are compared with previous results for lighter mass actinides. Odd-$A$ Bk and Es nuclei exhibit a strong dip in ${P}_{f}$ above the neutron binding energy, followed by a gradual increase with increasing excitation energy. We are not able to reproduce this trend with statistical model calculations.NUCLEAR REACTIONS, FISSION Measured fission probability in $^{244\ensuremath{-}245}\mathrm{Am}$, $^{245\ensuremath{-}249}\mathrm{Bk}$, and $^{249\ensuremath{-}251}\mathrm{Es}$. Determined fission barrier heights.

The fission probability was measured for a series of actinide nuclei as a function of the excitation enery using ($^{3}\mathrm{He},df$) and ($^{3}\mathrm{He},tf$) reactions. From these data, $\frac{{\ensuremath{\Gamma}}_{n}}{{\ensuremath{\Gamma}}_{f}}$ was determined from threshold up to \ensuremath{\sim} 12 MeV of excitation energy, and fitted by evaporation calculations which do not contain any arbitrary normalization factors. For heavier actinides, these fits are possible only if one assumes the fission process to proceed through a first saddle point which is not axially symmetric. These results and calculations also reproduce previously known empirical trends of average values of $\frac{{\ensuremath{\Gamma}}_{n}}{{\ensuremath{\Gamma}}_{f}}$, as a function of $A$ and $Z$.NUCLEAR REACTIONS, FISSION Measured fission probability in $^{230\ensuremath{-}233}\mathrm{Pa}$, $^{231\ensuremath{-}232}\mathrm{U}$, $^{233\ensuremath{-}239}\mathrm{Np}$, $^{237\ensuremath{-}238}\mathrm{Pu}$, $^{239\ensuremath{-}243}\mathrm{Am}$, $^{241\ensuremath{-}244}\mathrm{Cm}$. Determined barrier heights.