Fission Time Measurements: A New Probe into Superheavy Element Stability

Reaction mechanism analyses performed with a $4\pi$ detector for the systems ${}^{208}\mathrm{Pb}+\mathrm{Ge}$, ${}^{238}\mathrm{U}+\mathrm{Ni}$ and ${}^{238}\mathrm{U}+\mathrm{Ge}$, combined with analyses of the associated reaction time distributions, provide us with evidence for nuclei with $Z=120$ and 124 living longer than ${10}^{-18}\mathrm{s}$ and arising from highly excited compound nuclei. By contrast, the neutron deficient nuclei with $Z=114$ possibly formed in ${}^{208}\mathrm{Pb}+\mathrm{Ge}$ reactions have shorter lifetimes, close to or below the sensitivity limit of the experiment.

Figure 1

Atomic number versus energy measured at 20° (log scale) above the identification thresholds. The boxes labeled from (a) to (i) show the event selections for the dips of Fig. 2.

Figure 2

Blocking dips at 20°. The horizontal bars indicate the angular range of integration. The full curves correspond to simulations for $t_{\mathrm{reac}}\ll {10}^{-18}\mathrm{s}$ (see text for the other curves). The labels (a) to (i) refer to the event selections shown in Fig. 1.