Spontaneous fission ofFm259

Abstract

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.