We study the binary mass distribution for the recently predicted thermally fissile neutron-rich uranium and thorium nuclei using statistical model. The level density parameters needed for the study are evaluated from the excitation energies of temperature dependent relativistic mean field formalism. The excitation energy and the level density parameter for a given temperature are employed in the convolution integral method to obtain the probability of the particular fragmentation. As representative cases, we present the results for the binary yield of 250 U and 254 Th. The relative yields are presented for three different temperatures T = 1, 2 and 3 MeV.
The pre-scission particle multiplicities suggest a lifetime of ∼ 10 −20 s for the nuclear fission to occur which is in contrast to the fission lifetime ∼ 10 −18 s as predicted by atomic probe. This long standing ambiguity, arising due to the orders of magnitude differences among the fission lifetime measured from the nuclear and atomic probes, has been addressed within a dynamical model which includes the contributions from the nuclear shell effects. We show that, at lower excitation energies, these two probes decouples as the fissioning system survives for a long time without any particle evaporation. We also consider a wide range of reactions to study the impact of the excitation energy of compound nucleus on the fission dynamics in general. Our model predicts the average fission life time of superheavy nucleus 302 120, to be more than 10 −18 s which is in reasonable agreement with the recent experiments.
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