II-10 Pastina formation in low density nucleonic matter – a mechanism for ternary fission

Approximately 0.3 % of binary fission decays observed in the spontaneous or thermal neutron induced fission of a heavy nucleus are accompanied by emission of an energetic light particle or fragment in a direction perpendicular to the axis defined by the separating massive fragments. Considerable theoretical and experimental effort has been directed towards understanding this type of ternary fission. Generally, statistical or dynamical only models have had limited success and have been unable to explain such key experimental results as the high yield of scission tritons relative to scission protons and the nonobservation of He. Recently, Lestone proposed a model in which a statistical evaporation of the ternary particle is moderated by time dependent emission barriers that evolve as the fissioning nucleus approaches the scission point [1]. Parameterizing the neck radius, the range of the nuclear force, temperature, time, and emission barrier height provided a good reproduction of isotopic yields for Z ≤ 6 and reasonable predictions for Z > 6. The experimental results of Koester et al. provide the most comprehensive data available for ternary fission yields [2]. For this study we focus on the data for the Pu(nth,f) reaction [2]. These experimental data include measured yields per fission event for 42 isotopes. In addition, 17 upper limits are also reported for yields of other isotopes. For our initial approach to modeling the yield data we employed the stellar nucleo-synthesis statistical equilibrium calculation (NSEC) of Meyer et al [3] to determine the relative yields of the constituent species. The key assumption of nuclear statistical equilibrium is that the chemical potential μ(Z, A) is governed by the equation