Superfluid versus viscous descent from saddle to scission

The spectrum of mass yields from spontaneous fission and thermal neutron induced fission of even nucleides sometimes shows a fine structure, indicating a preference for doubly even mass divisions. This occurs only when the fissility parameter is relatively small. For a given nuclide, the fine structure is more pronounced when the total kinetic energy carried away by the fragments is larger than average. The preference for doubly even mass divisions is interpreted as the preservation of superfluidity, i.e. of the paired configuration, during the descent from saddle to scission. Conversely, loss of fine structure means excitation of quasiparticles during the descent. Alternative models for the saddle-to-scission transition are discussed in relation to the observed variance in the number of neutrons evaporated from a single fragment, under the constraint of a fixed total kinetic energy and a fixed mass ratio. It is concluded that fine structure in the mass yields may also occur, when the total kinetic energy is much lower than average.