Observation of Structures in the Mass-Tke Distributions of Fission Fragments in Spontaneous Fission

The number of prompt neutrons emitted in the fission event has been measured separately for each complementary fragment in coincidence with fragment mass and kinetic energies in spontaneous fission of Cf-252 and Cm-248. Two high efficient Gd-loaded liquid scintillator tanks were used for the neutron registration. Approximately 3.10(6) fission events coincident with prompt neutron emission have been accumulated for each isotope. Two-dimensional neutron multiplicity distributions corrected for efficiency, background and pile-up have been reconstructed for each value of the fission fragment mass and total kinetic energy (TKE). Based on these unfolded multiplicity distributions fragment mass and TKE distributions for specific numbers of emitted neutrons from each of the complementary fragments have been obtained. These distributions exhibit pronounced structures reflecting the fine structures in the potential-energy surface. Structures showing a periodicity of two-masses at the edges of the mass-TKE distributions corresponding to the odd-even effect of the neutron pairing in the fissioning nucleus have been observed for the first time at large values of the fragment total excitation energy.