Evidence for bimodal fission in the heaviest elements

Abstract

We have measured the mass and kinetic-energy partitioning in the spontaneous fission of five heavy nuclides: 258 [104], were measured on-line by a special rotating-wheel instru ment, while the others were determined off-line after mass separa tion.All fissioned with mass distributions that were symmetric. Total-kinetic-energy distributions peaked near either 200 or 235 MeV. Surprisingly, because only a single Gaussian energy distribution had been observed previously in actinide fission, these energy distri butions were skewed upward or downward from the peak in each case, except for 260 [104], indicating a composite of two energy distribut ions. We were able to fit accurately two Gaussian curves to the gross energy distributions from the four remaining nuclides. From the multiple TKE distributions and the shapes of the mass distribu tions, we conclude that there is a low-energy fission component with liquid-drop characteristics which is admixed with a much higherenergy component due to closed fragment shells. We now have further evidence for this conclusion from measurements of the neutron multi plicity in the spontaneous fission of 26 "Md.