A spontaneous-fission activity decaying with a half-life of 380 ± 60 μsec, identified as ${}_{}{}^{258}{}_{}{}^{}\mathrm{Fm}$, was discovered among the recoil products from the bombardment of ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$ with 12.5-MeV deuterons. The sharply decreasing spontaneous-fission lifetimes of nuclei with $N>\mathrm{}152\mathrm{}$ show a determined trend contrary to theoretical predictions. The exceedingly short half-life of ${}_{}{}^{258}{}_{}{}^{}\mathrm{Fm}$ has led us to conclude that even-even nuclei in the immediate region beyond $N=158\mathrm{}$ are becoming catastrophically unstable toward fission.

• Received 29 December 1970

DOI:https://doi.org/10.1103/PhysRevLett.26.523