Transition States at the Fission Barrier

The current knowledge of the transition states that a nucleus traverses en route to fission is reviewed, together with the relevant theory of fission-fragment anisotropy. Theoretical progress in understanding this kind of data in terms of nuclear superfluidity is summarized. The evidence indicates that nuclear pairing effects play an important role in determining the transition state spectrum. Recent (d, pf) experimental data are noted, and their statistical analysis in terms of a pairing Hamiltonian considered in the fixed-energy ensemble appropriate for finite nuclei is presented. The results indicate that such pairing effects lead in Pu240 to an energy gap in the transition spectrum nearly twice that which occurs at the ground state shape of the same nucleus. Such a result is quite significant from the point of view of nuclear many-body physics, suggesting that the nuclear surface plays an important role in the pairing of nucleons in the finite nuclei. The significance for nuclear fission is also broad. Such a large energy gap forces reassessment of certain detailed conjectures that have been put forward concerning fission widths and mass asymmetry associated with capture resonances of specific spin and parity. The new situation suggests that the even-even and odd-odd spin-parity correlation that rules in the low energy spectra of deformed even-even nuclei does not prevail in the transition state spectrum. A possible explanation in terms of low energy mass- asymmetric and bending vibrations of the fissioning nucleus is discussed. Possible experimental studies that could support or negate the validity of this viewpoint are mentioned. (author)