Spontaneous fission properties of 252,254No and 256,258[104] and the disappearance of the outer fission barrier

We have measured the mass and total kinetic energy distributions from the spontaneous fission of 252No, 254No, 256[104] and 258[104]. The results, in combination with earlier measurements for 256No, 258No and 262No, show a sharp transition from asymmetrical mass division in 256No to symmetrical division for 258No and 262No. However, all isotopes of element 104, including 260[104], appear to yield broadly symmetrical mass distributions. The total kinetic energies around 200 MeV for the element 104 isotopes indicate that they fission by the low-energy mode of bimodal fission. Recent calculations of static potential energy surfaces including higher-order asymmetric deformations suggest that the outer fission barrier is well below the ground state in energy, which accounts for the observance of the symmetric mass division. ((orig.))