Fission properties of the 1.5-s spontaneous fission activity produced in bombardment ofCm248withO18

We have measured the mass and kinetic-energy distributions of fragments from the spontaneous fission of a 1.5-s activity produced in bombardments of ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cm}$ with 95-MeV ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ ions. Its spontaneous fission decay exhibits a very symmetric, narrow (full width at half maximum = 12 mass units) mass distribution, a very high total kinetic energy of 234±2 MeV, and increasing total kinetic energy with increasingly symmetric mass division. Based on its half-life and the similarity of its fission properties to the unique fission properties so far only observed for ${}_{}{}^{258}{}_{}{}^{}\mathrm{Fm}$ and ${}_{}{}^{259}{}_{}{}^{}\mathrm{Fm}$, the most likely assignment of this activity is to the known ${}_{}{}^{259}{}_{}{}^{}\mathrm{Fm}$. However, assignment to some as yet undiscovered neutron-rich heavy element isotope such as ${}_{}{}^{260}{}_{}{}^{}\mathrm{Md}$ cannot be unequivocally excluded.