Low-energy fission within the Lublin-Strasbourg drop and Yukawa folded model

The evolution of fragment mass distribution in low-energy fission is studied as a function of the composition of the fissioning system over a wide range, from Hg to Fm isotopes. The relative importance of symmetric and asymmetric fission modes is investigated within a static picture based on elaborate, four-dimensional potential energy spaces using the modified Funny-Hills deformation coordinates. The potential energy is computed with the macroscopic-microscopic method. The Lublin-Strasbourg drop model is used to calculate the macroscopic part of the potential energy, while the Yukawa folded single-particle potential with an improved Strutinsky method and the Bardeen-Cooper-Schrieffer theory are employed to evaluate the shell and pairing contributions to the macroscopic energy. From the analysis of the potential energy surfaces, equilibrium deformations and saddle-point shapes are determined. A coexistence of symmetric and asymmetric fission valleys is observed. The relative depth of these valleys depends on the fissioning nucleus. The valleys' competition is used to explain the evolution of the fission-fragment mass distribution, from asymmetric fission in Hg-180 to symmetric fission in Fm-260. Good qualitative agreement is found with the experiment.