Deformed shell stabilities influencing the fragment formation in sub-lead nuclei

Abstract

The fragment mass distributions from fission in the sub-lead nuclei gave traces of the co-existence of symmetric and asymmetric fission modes. To investigate the major drivers behind the observed asymmetry, we have accumulated the fission data of 19 neutron-deficient lead island nuclei from the last few years and compared them with the corresponding predictions made by the GEF (General description of fission observables) model. Consequently, both experimental and theoretical analysis performed to extract asymmetric fission properties led to similar results, showing the deformed proton shell gaps, at Z = 34 for light fragments and Z = 42, 44, and 46 for heavier fragments, associated with large quadrupole deformations as the possible origin for the asymmetric split for all isotopes ranging from 178−198Hg, 176−186Pt, and 179−191Au. These observations contradicted the earlier theoretical report where neutron numbers were declared to play a leading role in deciding asymmetric fission. This disagreement motivated us to get better insight by extending the examination over a large domain around Pb covering Hg, Pt, Au, Pb, and Po nuclides. Although protons in the light fragments are still spotted to be constant and influenced by the deformed shell gaps at Z = 34, protons in the heavier fragments are found increasing with rising ZCN and gave evidence of negligible influence from Z = 42, 44, and 46 shell gaps. Interestingly, the excellent compatibility of GEF results with the experimental outcomes proved it an efficient model for extracting fragment properties. However, it could only reproduce the experimental symmetric fission fraction (SFF) within the 30–52 MeV energy window. The GEF's inadequate predictability of the multichance fission contributions might be considered as one of the possible reasons behind the observed disagreements between the GEF and experimental SFF above 52 MeV. Moreover, the other reason might be the inadequate modeling of the evolution of the fission fragment mass/charge distribution with excitation energy.