Decay Modes Of No-250

The fragment mass analyzer at the ATLAS facility has been used to unambiguously identify the mass number associated with different decay modes of the nobelium isotopes produced via Pb-204(Ca-48,xn)No252-x reactions. Isotopically pure (> 99.7%) Pb-204 targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (t(1/2)=3.7(-0.8)(+1.1) and 43(-15)(+22) mu s) were deduced from a total of 158 fission events. Both decays originate from No-250 rather than from neighboring isotopes as previously suggested. The longer activity most likely corresponds to a K isomer in this nucleus. No conclusive evidence for an alpha branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity.