Role of the second barrier upon mass division in the spontaneous fission of 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, the mass and total kinetic-energy distributions from spontaneous fission of 252No, 254No, 256[104], and 258[104] have been measured. The results, in combination with earlier measurements for 256No, 258No, and 262No, show a sharp transition from the asymmetrical mass division in 256No to the symmetrical mass division for 258No and 262No. Conversely, all isotopes of element 104, including 260[104], appear to yield broadly symmetrical mass distributions. The total kinetic energies around 200 MeV for the 104 isotopes indicate the fission by the low-energy mode of bimodal fission. Based on the hypothesis that the second barrier is responsible for the asymmetrical mass distributions, and when it disappears, for symmetrical ones, these observations for the isotopes of element 104 are in agreement with the 1976 calculations of the heights of the second fission barrier relative to the ground state. Some recent calculations of static potential-energy surfaces and of barrier heights deduced from half lives for spontaneous fission indicate that the second barrier is from 0 to 2.9 MeV above the ground state for the No and 104 isotopes. However, shape degrees of freedom have been limited in these calculations; therefore, they fail to provide realistic heights for the outer fission barrier. For the few cases where higher-order asymmetrical deformations are included, this barrier height is well below the ground state and, for these nuclides, symmetrical mass division only is observed. Without more extensive calculations of potential-energy surfaces for comparison with the findings, a firm conclusion about the role of the second barrier upon mass division in fission is impossible to obtain