Cross-section and neutron-emission data from heavy-ion fusion-fission reactions are consistent with the fission of fully equilibrated systems with fission lifetime estimates obtained via a Kramers-modified statistical model which takes into account the collective motion of the system about the ground state, the temperature dependence of the location and height of fission transition points, and the orientation degree of freedom. If the "standard" techniques for calculating fission lifetimes are used, then the calculated excitation-energy dependence of fission lifetimes is incorrect. We see no evidence to suggest that the nuclear viscosity has a temperature dependence. The strong increase in the nuclear viscosity above a temperature of ~1.3 MeV deduced by others is an artifact generated by an inadequate fission model. DOI: PACS number(s) : 24.75.+i, 24.60.Ky, 25.70. ER fus ER fus f J J J J J ⎛ − + = J J J J T

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The angular distributions of fission fragments have been measured over a range of near-and sub-barrier energies for reactions involving 7 Li, 11 B, 12 C and 16 O projectiles on 232 Th, 235,236,238 U and 237 Np 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.

Frescission charged-particle multiplicities, following fusion of ^^^'i6'''''^0Er+^^Si, have been measured. The multipHcities at the lowest bombarding energies limit the statistical model level density parameters. More importantly, the a data restrict the time spent near equilibrium and suggest evaporation occurs predominantly from larger deformations.

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The energy spectra of 344, 779, 964, 1086/1112 and 1408 keV y rays scattered through 5, 7, 10 and 15" by targets of AI, Cu, MO, Sn, Ta and Pb have been measured using a Ge(Li) detector. The Compton peaks are closely fitted by a convolution of an instrumental lineshape and a theoretical lineshape computed using the relativistic impulse model of bound Compton scattering. Integrated energy spectra yield incoherent scattering functions in very close agreement with theoretical values taken from existing tabulations based upon the Waller-Hartree form factor model and with values obtained by direct numerical integration of the relativistic impulse model lineshapes.

The influence of the ground-state spin of fusing nuclei on the subsequent fission fragment angular distributions has been investigated through realistic quantum-mechanical calculations. The domain of applicability of an approximate formula is found for spherical nuclei. For deformed nuclei, a generalization of the standard transition state model expression for the fission fragment angular distribution is presented. This predicts dramatic changes in the shape and anisotropy of the distribution, for reactions of deformed nuclei with large ground-state spin.