Search for fission from a long-lived isomer in ${}^{250}\mathrm{No}$ and evidence of a second isomer

In the present work, a $K$ isomeric state in ${}^{250}\mathrm{No}$, which is more stable against fission than the ground state, was experimentally studied. The aim was to measure the fission branch of this isomeric state. In total, 780 fission events attributed to the decay of ${}^{250}\mathrm{No}$ were detected. Among them 133 cases were attributed to the ground-state decay with a half-life of 4.0(4) $\mu \mathrm{s}$, which was populated by the deexcitation of the isomeric state via electromagnetic transitions with a half-life of 23(4) $\mu \mathrm{s}$. In addition, in two more cases, this long-lived isomeric state was populated in the deexcitation of a hitherto unknown, yet higher-lying and short-lived isomeric state with a half-life of $0.7_{-0.3}^{+1.4}\mu \mathrm{s}$. No direct fission from the long-lived isomeric state, i.e., with a lifetime of longer than 40 $\mu \mathrm{s}$, was identified. This results in an upper limit of 0.035 for the branching ratio for fission. This is a significantly more strict limit than the previously known value of 0.5. Nonobservation of fission branching of the long-lived isomer is discussed relative to theoretical predictions and within various semiempirical ways, which resulted in an attribution of a lower limit of ${10}^4$ for the fission-hindrance factor, caused by the $K$ quantum number. The presences of multiple high-$K$ isomeric states seemingly is a widespread phenomenon in deformed heavy nuclei.

Figure 1

Time distributions of radioactive decays of species associated with ${}^{250}\mathrm{No}$ produced in the ${}^{48}\mathrm{Ca}+{}^{204}\mathrm{Pb}$ reaction. Time distributions of fission events relative to their implantation signals: (a) all ER-FI, (b) ER-FI events without CE, and (c) ER-FI events with CE (i.e., ER-CE-FI). The number of identified events of each type is given parentheses. Time distributions of (d) CEs relative to preceding ER-signal and (e) FI's relative to CEs, which were observed as ER-CE-FI sequences. Curves represent the calculated time distributions of events according to Ref. [27]. Half-lives extracted from each time distribution are given.

Figure 2

An example of traces of the implantation signal (ER) correlated with decays of a long-lived isomeric state and ground state in ${}^{250}\mathrm{No}$. Signals from $X$ and $Y$ strips are shown in (a) and (b), respectively. Insets show the expanded view of traces in time regions, where low-energy electron signals attributed to decay of an isomeric state were detected.

Figure 3

Energy spectrum of CE signals found in the ER-CE-FI sequences.

Figure 4

$X$-strip traces of two ER-${\mathrm{CE}}_1-{\mathrm{CE}}_2$-FI events. Insets show close look of regions where CEs were detected. Energies of CEs are given.

Figure 5

Correlation times of the ${\mathrm{CE}}_1, {\mathrm{CE}}_2$, and FI events relative to the preceding events. Curves represent the calculated time distributions of each pair according to Ref. [27]. Shaded curves correspond to the calculated time distributions of the long-lived isomeric state and the ground state. See text for details.

Figure 6

An example of a trace ($X$ strip) of a FI event, where the CE was detected prior to the FI signal. The inset shows the expanded view of the trace in the time region where the CE signal was detected.

Figure 7

The time distribution of eight FI events relative to the preceding CE signal in their traces. The time-density function curve for the 4.0-$\mu \mathrm{s}$ activity, which describes the time distribution of ER-FI events without CE shown in Fig. 1, is given, and its shaded region corresponds to $\approx 50\%$ probability nof observing the FI events with correlation times up to $\approx 4.7\mu \mathrm{s}$. See text for details.