A 1.5-s spontaneous fission activity has been produced by irradiating ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$ with 16-MeV tritons. On the basis of formation cross sections, fission half-life systematics, and the identification of other possible products, this 1.5-s activity has been attributed to ${}_{}{}^{259}{}_{}{}^{}\mathrm{Fm}$ formed by the reaction ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$($t$,$p$)${}_{}{}^{259}{}_{}{}^{}\mathrm{Fm}$. ${}_{}{}^{259}{}_{}{}^{}\mathrm{Fm}$ is the heaviest known isotope of Fm and has more neutrons than any other nuclide thus far identified. This measurement of the spontaneous fission of ${}_{}{}^{259}{}_{}{}^{}\mathrm{Fm}$ is the first to show a narrow, predominantly symmetric, mass division from spontaneous fission. It is accompanied by a very high kinetic energy, the most probable total kinetic energy being 242±6 MeV. These features show a marked acceleration in the trend toward more symmetric mass division and higher total kinetic energies than have been observed previously for the Fm isotopes as the mass increased.

NUCLEAR REACTIONS, RADIOACTIVITY, FISSION ${}_{}{}^{259}{}_{}{}^{}\mathrm{Fm}$, (SF); measured fragment-fragment $E$, $T_{\frac{1}{2}}$; deduced fragment mass distribution; ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$($t$,$p$), estimated $\sigma$.

• Received 17 July 1979

DOI:https://doi.org/10.1103/PhysRevC.21.966