Near- and sub-barrier fission fragment anisotropies and the failure of the statistical theory of fission decay rates

The angular distributions of fission fragments have been measured over a range of near- and sub-barrier energies for reactions involving 7Li, 11B, 12C and 16O projectiles on 232Th, 235,236,238U and 237Np targets. The discrepancies between our experimental fission anisotropies and the transition state model increase dramatically as the beam energy decreases through the region of the fusion barriers; decrease smoothly with projectile size with a fixed target; show no evidence of a discontinuous behaviour across the Businaro-Gallone ridge in the mass asymmetry degree of freedom; and, at sub-barrier energies, are strongly influenced by the ground-state spin of the targets. A good fit to measured fission anisotropies can be obtained if, immediately following fusion, the system has the K-state distribution of the entrance channel, and this initial distribution is broadened with time due to a coupling between the intrinsic and collective rotational degrees of freedom. If, at well above barrier energies, the entrance channel uniformly populates the K states for each J then the strength of the coupling between the intrinsic and the rotational degrees of freedom required to reproduce observed fission anisotropies leads to a limiting fission timescale of several 10-20 s. This limiting time is not due to the slowing of nuclear shape changes caused by the viscosity of heated nuclear matter, but is due to the finite time required to change the angle of the symmetry axis relative to the direction of the total spin. (author)