Fission Barrier of Superheavy Nuclei and Persistence of Shell Effects at High Spin: Cases ofNo254andTh220

We report on the first measurement of the fission barrier height in a heavy shell-stabilized nucleus. The fission barrier height of No-254 is measured to be B-f = 6.0 +/- 0.5 MeV at spin 15 (h) over bar and, by extrapolation, B-f = 6.6 +/- 0.9 MeV at spin 0 (h) over bar. This information is deduced from the measured distribution of entry points in the excitation energy versus spin plane. The same measurement is performed for Th-220 and only a lower limit of the fission barrier height can be determined: B-f (I) > 8 MeV. Comparisons with theoretical fission barriers test theories that predict properties of superheavy elements.