Discovery of 260Md and the decay properties of 258Fm, /sup 258m,g/Md, and 259Md

We have discovered a new neutron-rich isotope, 260Md, from 18O and 22Ne bombardments of 254Es. We observed a spontaneous-fission (SF) activity with a 32-day half-life in electromagnetically separated mass-260 fractions from these bombardments and we have measured the mass and kinetic energy distributions of this SF activity. The mass distribution was symmetric with the principal energy peak at 234-MeV total kinetic energy (TKE), similar to previous observations for heavy Fm isotopes. Surprisingly, we also observed a smaller symmetric component with 195-MeV TKE. We interpret these two peaks in the TKE distribution as arising from two types of fission in the same nucleus, or bimodal fission. The observed fission activity may be either from the SF decay of 260Md or 260Fm which would arise from electron capture (EC) decay of 260Md. We have eliminated the possible β- decay of 260Md by measuring β--SF time correlations for the decay of 260Md and we plan to determine if 260Md decays by EC by measuring time correlations between Fm x-rays and SF events. We also measured properties for heavy Fm and Md isotopes which include: (1) more accurate cross sections for the neutron-rich Md isotopes which we use to predict the production rates of yet undiscovered nuclides; (2) improved half-live measurements for /sup 258m,g/Md and 259Md; (3) confirmation of the EC decay of /sup 258m/Md by measuring Fm x-rays preceding the SF decay of 258Fm; and (4) very substantially improved mass and TKE distributions for the SF decay of 258Fm and 259Md