Abstract
Fragment mass distributions from fission of the excited compound nucleus 178Pt have been deduced from the measured fragment velocities. The 178Pt nucleus was created at the JAEA tandem facility in a complete fusion reaction 36Ar + 142Nd, 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, AL/AH≈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.
Highlights
Understanding of the nuclear fission process is important for many areas of fundamental science, technology, and medicine
For every identified fission event, velocities of coincident fission fragment (FF) were derived from the measured TOF values and TOF distances calculated with help of the multi-wire proportional counter (MWPC) coordinates
Table 1) – which match well the total kinetic energy (TKE) = 135.9 MeV value expected from the Viola systematics [48] – are indicative of less deformed scission configurations, whereas for the symmetric mode, highly elongated FF shapes are expected from its lower TKE values
Summary
Understanding of the nuclear fission process is important for many areas of fundamental science, technology, and medicine. According to the global survey of calculated FFMDs [26], a new extended region of asymmetric fission is expected in neutron-deficient Re–Pb isotopes with 98 N 116. It is separated from predominantly asymmetricallyfissioning actinides by a zone of symmetric fission around Ir–At in the vicinity of N ∼ 120–126 [9], whose properties were extensively investigated in the past In view of PES properties in the sub-lead region [13,15,16,17], an observation of a competition between fission modes will shed light on the nature of near-scission configurations of nuclei, which are some 60 nucleons lighter and greatly deficient in neutrons, as compared to actinides and transactinides. This Letter provides the first experimental demonstration of the existence of competing fission modes in sub-lead nuclei, by revealing the presence of asymmetric and symmetric fission modes through measurements of FFMDs from fission of 178Pt
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