Background: The mechanism underlying asymmetric fission in the preactinide region remains incompletely understood. Recent experimental analyses of correlations between fragment mass and total kinetic energy (TKE) have demonstrated the efficacy of this approach in identifying fission modes.

Purpose: This study aims to characterize the fission modes of ${}^{188}\mathrm{Pt}$ and ${}^{190}\mathrm{Hg}$, which are in the same isotonic chain, and elucidate their origins through macro-microscopic potential energy surface analyses.

Method: The three-dimensional Langevin approach considering nucleus elongation, deformation, and mass asymmetry is applied to simulate fission dynamics, with the incorporation of angular momentum effects. The effects of multichance fission are incorporated using a combined GEF (2024/V1.1) and Langevin model. Fission modes are determined using two-dimensional multi-Gaussian fitting of mass-TKE correlations.

Results: Angular momentum inclusion narrows the fragment mass and TKE distributions in ${}^{190}\mathrm{Hg}$. Calculations reproduce the double-peak structure in ${}^{190}\mathrm{Hg}$ at low excitation energies. Both first-chance and multichance fission calculations exhibit similar characteristic features in the mass distributions. By applying a two-dimensional multi-Gaussian fit to the mass-TKE correlations, the fission modes of both nuclei are identified as two asymmetric standard modes, a symmetric superlong mode, and a symmetric liquid-drop mode. The liquid-drop mode strengthens with increasing excitation energy, while others diminish, consistent with the weakening of microscopic corrections. Comparative analysis of ${}^{187}\mathrm{Ir}, {}^{188}\mathrm{Pt}$, and ${}^{190}\mathrm{Hg}$ reveals stable proton numbers at $Z\approx$ 35 for one asymmetric standard mode. Potential energy surface and single-particle level analyses demonstrate the asymmetric valleys linked to this asymmetric mode originate at small elongations, driven by proton energy gaps at $Z$ = 35 or 36.

Conclusion: The fission of ${}^{188}\mathrm{Pt}$ and ${}^{190}\mathrm{Hg}$ is consistently described by multiple modes, including the asymmetric mode with $Z\approx$ 35-36. The corresponding asymmetric valley on the potential energy surface emerges early in the fission path.

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• Fission
• Low & intermediate energy heavy-ion reactions
• 190 ≤ A ≤ 219