Observation of the competing fission modes in ¹⁷⁸ Pt

Fragment mass distributions from fission of the 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 ≈1/3. 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 ≈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.