First observation of the competing fission modes in the neutron-deficient sub-lead region

TL;DR

This paper reports the first experimental observation of multimodal fission in the neutron-deficient sub-lead region, revealing both asymmetric and symmetric fission modes in $^{178}$Pt with theoretical support.

Contribution

It provides the first evidence of multimodal fission in the sub-lead region and compares experimental data with nuclear density functional theory predictions.

Findings

Dominance of asymmetric mass splits in fragment yields

Symmetric mode contributes about 1/3 of fission events

The most probable asymmetric mass split is approximately 79/99

Abstract

Fragment mass distributions from fission of excited compound nucleus $^{178}$Pt have been deduced from the measured fragment velocities. The $^{178}$Pt nucleus was created at the JAEA tandem facility in a complete fusion reaction $^{36}$Ar + $^{142}$Nd, at beam energies of 155, 170 and 180 MeV. The data are indicative of a mixture of the mass-asymmetric and mass-symmetric fission modes associated with higher and lower total kinetic energies of the fragments, respectively. The measured fragment yields are dominated by asymmetric mass splits, with the symmetric mode contributing at the level of $\approx1/3$. This constitutes the first observation of a multimodal fission in the sub-lead region. Most probable experimental fragment-mass split of the asymmetric mode, $A_{L}/A_{H}\approx 79/99$, is well reproduced by nuclear density functional theory using the UNEDF1-HFB and D1S potentials. The symmetric mode is associated by theory with very elongated fission fragments, which is consistent with the observed total kinetic energy/fragment mass correlation.