Fission Barrier of Superheavy Nuclei and Persistence of Shell Effects at High Spin: Cases ofNo254andTh220
Physical Review Letters
We report on the first measurement of the fission barrier height in a heavy shell-stabilized nucleus. The fission barrier height of 254 No is measured to be B f ¼ 6.0 AE 0.5 MeV at spin 15ℏ and, by extrapolation, B f ¼ 6.6 AE 0.9 MeV at spin 0ℏ. This information is deduced from the measured distribution of entry points in the excitation energy versus spin plane. The same measurement is performed for 220 Th and only a lower limit of the fission barrier height can be determined: B f ðIÞ > 8 MeV.
... : B f ðIÞ > 8 MeV. Comparisons with theoretical fission barriers test theories that predict properties of superheavy elements. Superheavy elements (SHE) owe their existence to quantum shell effects, which create a sizable barrier against fission. Without such shell-stabilizing effects, i.e., with only the liquid-drop component, the fission barrier in a heavy nuclide such as 254 102 No would be small, 0.9 MeV [1,2], and the spontaneous fission lifetime of 254 No would be 13 orders of magnitude shorter than observed  . The height of the fission barrier (defined as the energy difference of the ground and saddle states) determines the stability of the SHE against fission and is one factor affecting its production cross section. SHE are generally produced in fusion-evaporation reactions. As the hot compound nucleus cools by neutron emission, its survival against fission is governed by the barrier height. In fact, the high fission barriers predicted around Z ¼ 114 and N ¼ 184  are thought to be responsible for reversing at Z ¼ 112 the trend of decreasing evaporationresidue cross sections as a function of atomic number Z, resulting in a maximum effect at Z ¼ 114-115 . (Other factors could also play a role in decreasing the fission probability .) Clearly, a determination of the fission barrier is an important goal for experiment. Measurements to study SHE are challenging as the production rates can be as low as a few nuclei per month. The cross sections to produce deformed transfermium nuclei, which are stabilized by the same shell energy, are larger and studies of fission barrier properties become possible. 254 No is the best candidate for such a study as it has the highest cross section (although still small at around 1 μb) of the heaviest nuclei and the structure of its excited states is quite well known          . A previous attempt at determining the height of its fission barrier yielded a lower limit of ∼5 MeV for spins up to 22ℏ , which did not allow for differentiating between the available theoretical predictions. Indeed, density functional theory (DFT) calculations based on the Gogny D1S and Skyrme interactions give a barrier height between 6 and 12.6 MeV      . Calculations based on the macroscopicmicroscopic model give a lower value of 6.8 MeV [4, 22] . In this Letter, we present the first measurement of the barrier height B f for 254 No, which becomes the heaviest nuclide for which the barrier has been measured, the previous being PRL 113, 262505 (2014) P H Y S I C A L