Phase Transitions in Nuclear Fission. II

The nucleus immediately after the second saddle point in a dumbbell structure is described in terms of two nuclear clusters linked by quasiparticle exchange. The itinerant quasiparticles near the Fermi surface are designed to resolve the inconsistency between the alternative models of nuclear fission, the shell model and the cluster model. The effective residual interaction operative between quasiparticles comprises long-range and short-range parts, the former originating from the exchange coupling of the two forming clusters and the latter from the pairing interaction. The condensed state at low excitation energies is then characterized by the corresponding order parameters, mass asymmetry between two clusters and the energy gap in the single-quasiparticle excitation spectrum. Each order parameter displays a continuous behaviour at a transition point, indicating a phase transition of the second kind. In the condensed phase, short-range order carries on an active competition against long-range order. Under certain conditions there occurs saturation mass asymmetry in the ground state which leads to the production of the closed shell fragment. A discussion is devoted to the current-carrying state which may play a role in fission and furnish an explanation of the mass asymmetry.