D. Peterson, B. B. Back, R. V. F. Janssens, T. L. Khoo, C. J. Lister, D. Seweryniak, I. Ahmad, M. P. Carpenter, C. N. Davids, A. A. Hecht, C. L. Jiang, T. Lauritsen, X. Wang, S. Zhu, F. G. Kondev, A. Heinz, J. Qian, R. Winkler, P. Chowdhury, S. K. Tandel, and U. S. Tandel

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}$$,\mathrm{xn})$$252-x\mathrm{No}$ reactions. Isotopically pure ($>99.7$%) $204\mathrm{Pb}$ targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives ($t_{1/2}=3.7_{-0.8}^{+1.1}$ and ${43}_{-15}^{+22}$ μs) 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 α branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity.