Mass Symmetry in the Spontaneous Fission of ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$

The kinetic energy distribution of coincident fragments from spontaneous fission of ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$ has been measured with silicon detectors. The data reveal two significant features: (1) The mass-yield distribution is markedly more symmetric than for slightly lighter nuclei (e.g., ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$, ${}_{}{}^{254}{}_{}{}^{}\mathrm{Cf}$); (2) the average total kinetic energy increases monotonically with the approach to symmetric fission.