ROLE OF THE 2ND BARRIER UPON MASS DIVISION IN THE SPONTANEOUS FISSIONOF THE HEAVIEST ELEMENTS

In the region where theorists had earlier predicted the disappearance of the outer fission barrier or its dropping below the ground state, t he mass and total kinetic-energy distributions from spontaneous fissio n of No-252, No-254, 256[104], and 258[104] have been measured. The re sults, in combination with earlier measurements for No-256, No-258, an d No-262, show a sharp transition from the asymmetrical mass division in No-256 to the symmetrical mass division for No-258 and No-262. Conv ersely, all isotopes of element 104, including 260[104], appear to yie ld broadly symmetrical mass distributions. The total kinetic energies around 200 MeV for the 104 isotopes indicate the fission by the low-en ergy mode of bimodal fission. Based on the hypothesis that the second barrier is responsible for the asymmetrical mass distributions, and wh en it disappears, for symmetrical ones, these observations for the iso topes of element 104 are in agreement with the 1976 calculations of th e heights of the second fission barrier relative to the ground state. Some recent calculations of static potential-energy surfaces and of ba rrier heights deduced from half lives for spontaneous fission indicate that the second barrier is from 0 to 2.9 MeV above the ground state f or the No and 104 isotopes. However, shape degrees of freedom have bee n limited in these calculations; therefore, they fail to provide reali stic heights for the outer fission barrier. For the few cases where hi gher-order asymmetrical deformations are included, this barrier height is well below the ground state and, for these nuclides, symmetric mas s division only is observed. Without more extensive calculations of po tential-energy surfaces for comparison with the findings, a firm concl usion about the role of the second barrier upon mass division in fissi on is impossible to obtain.