Decay modes of ${}^{250}\mathrm{No}$

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-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.

Figure 1

Decay spectrum (histogram) for ${}^{219}\mathrm{Th}$ created in the setup reaction and the associated fit (solid line).

Figure 2

Decay spectrum for all recoil–fission correlations observed in this experiment. The inset expands the first 50 μs. The solid line is the result of a two-component fit as discussed in the text. The dashed and dot-dashed lines represent the contributions of the individual decay components of half-lives $3.7$ and 43 μs, respectively. Note that the mean flight time through the FMA of 1.8 μs was added to the implant–decay correlation time before binning the data.

Figure 3

Mass numbers of the residues associated with spontaneous fission decays. (Left panel) Decay time versus mass. (Right panels) Projections to the mass axis for all decays (a), purely long decays (b), and primarily short decays (c). See text and Table for more details.

Figure 4

Systematics of spontaneous fission (SF) decay lifetimes. The logarithm of the SF partial half-life is given as a function of neutron and proton number for isotopes, isotones, and isobars in the mass 250 region. See text for discussion.

Figure 5

Logarithm of partial spontaneous fission lifetimes of even-even nuclei in the $N=152$ region.