The discovery of 260Md and the decay properties of 258Fm, 258m,gMd and 259Md☆

Abstract We have discovered a new neutron-rich isotope, 260 Md, from 18 O and 22 Ne bombardments of 254 Es. We observed a spontaneous-fission (SF) activity with a half-life of 32 days in electromagnetically separated fractions with mass number 260 from these bombardments and we measured the mass and kinetic energy distributions of this SF activity. The mass distribution was symmetric with the principal energy peak at a total kinetic energy (TKE) of 234 MeV, similar to previous observations for heavy fermium isotopes. Surprisingly, we also observed a smaller symmetric component with a TKE of 195 MeV. 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 260 Md or from 260 Fm which would arise from electron-capture (EC) decay of 260 Md. We have eliminated the possible β − decay of 260 Md by measuring β − -SF time correlations for the decay of 260 Md and we plan to determine whether 260 Md decays by EC by measuring time correlations between fermium X-rays and SF events. We also measured various properties of the heavy fermium and mendelevium isotopes and obtained 1. (1) more accurate cross-sections for the neutron-rich mendelevium isotopes which we use to predict the production rates of yet undiscovered nuclides, 2. (2) improved half-Me measurements for 258m,g Md and 259 Md, 3. (3) confirmation of the EC decay of 258m Md by measurement of the fermium X-rays preceding the SF decay of 258 Fm and 4. (4) very substantially improved mass and TKE distributions for the SF decay of 258 Fm and 259 Md.