Introduction

A phenomenon known as bimodal fission is related to the remarkable properties of spontaneous fission observed in several fermium and transfermium nuclei, e.g., $^{258,259}$Fm, $^{259,260}$Md, and $^{258,262}$No [1,2,3,4]. In these systems, a sharp transition takes place from an asymmetric mass division in, e.g., $^{256}$Fm and $^{256}$No to a symmetric split in, e.g., $^{258}$Fm and $^{258}$No. Furthermore, the total kinetic energy (TKE) distribution of the fission fragments appears to be composed of two Gaussians with the maxima near 200 and 233 MeV. It was postulated (Refs. [3,5,6,7]) that the higher-energy fission mode corresponds to a scission configuration associated with two touching nearly spherical fragments, with the maximum of Coulomb repulsion, whereas the lower-energy mode can be associated with more elongated fragments. Moreover, the higher-energy mode consistently produces narrow and symmetric mass distributions, while the mass distributions of fragments with lower TKEs are much broader and sometimes asymmetric [3].

In this work we discuss total binding energies and mass hexadecapole moments calculated along the static fission paths of $^{256}$Fm, $^{258}$Fm, and $^{260}$Fm. In Ref. [8] we studied the associated collective inertia. Here, the main focus is on differences in spontaneous fission properties found in this region of heavy fermium isotopes.