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 $^{204}\mathrm{Pb}$$($$^{48}\mathrm{Ca}$$,\mathit{xn})$$^{252\ensuremath{-}x}\mathrm{No}$ reactions. Isotopically pure ($g99.7$%) $^{204}\mathrm{Pb}$ targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (${t}_{1/2}=3.{7}_{\ensuremath{-}0.8}^{+1.1}$ and ${43}_{\ensuremath{-}15}^{+22}$ \ensuremath{\mu}s) were deduced from a total of 158 fission events. Both decays originate from $^{250}\mathrm{No}$ 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 \ensuremath{\alpha} branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity.