Fully microscopic scission-point model to predict fission fragment observables

Abstract

We present an upgraded version of the SPY model, called SPY2 for version 2 of the scission point yield, to estimate mainly the yields and the kinetic energy distributions of fission fragments: The theoretical framework is similar to our previous version, i.e., a statistical scission point model, but this version is based on fully microscopic nuclear ingredients describing the fragments properties at the scission point. These include the static properties of some 7000 nuclei at 120 axial quadrupole deformations, such as binding energies, proton densities, single-particle level schemes, and states densities, coherently calculated within the constrained Hartree-Fock-Bogoliubov model on the basis of the Skyrme BSk27 interaction. The use of microscopic ingredients has been extended to the proton density distribution and the nuclear states densities. Considering realistic proton densities of fragments allows us to improve the definition of the scission point as well as the prediction of the kinetic energy distribution and to link the kinetic energy to the diffuseness of the fragments' proton density. New microscopic nuclear states densities improve the general coherence of the model, in particular regarding the inclusion of the odd-even pairing effect. In this updated SPY2 version, the calculation of the fission yields and kinetic energy distributions is significantly improved and found to be in relatively good agreement with experiments, at least qualitatively. A detailed study is performed for three well known fissioning systems, namely, thermal neutron induced fission of 235 U and 239 Pu and spontaneous fission of 252 Cf. A systematic analysis of the fission mode as well as mean fragments deformation and total kinetic energies has been performed for some 2000 fissioning nuclei with 78 Z 110 lying between the proton and neutron drip lines.

Nuclear Fission Dynamics: Past, Present, Needs, and Future

“…The collective motion is thus expected to be strongly overdamped. From saddle-to-scission the nucleus behaves as a very viscous fluid, the role of collective inertia is strongly suppressed, and the trajectories follow predominantly the direction of the steepest descent with the terminal velocity determined by the balance between the friction and the driving conservative forces, see for the assumption of the overdamped Brownian motion model [44][45][46][47][48][49] and partially to the scission-point model [50][51][52][53]. In both these phenomenological models it is assumed that the preformed FFs are in statistical equilibrium and that the collective energy flow is either vanishing or very small.…”

“…This behavior is apparently confirmed indirectly by experiments. In Langevin or Fokker-Planck [60][61][62][63][64][65], TDGCM [24,66], and scission-point [50][51][52][53] models the calculation of the FFs yields consider only a very limited range of nuclear shapes. In particular in such simulations one never introduces the octupole FF moments.…”

β -delayed fission of isomers in Bi188

“…I, 188 Pb lies in the transitional region between nuclei with symmetric and asymmetric FFMDs, which makes the theoretical description of FFMD challenging. The fully microscopic scission-point model, known as SPY [44], predicts a symmetric FFMD for 188 Pb (Fig. 11) using the same D1M binding energies.…”

Light element ($$Z=1,2$$) production from spontaneous ternary fission of $$^{252}$$Cf

“…We assume that the relevant fragment distribution, including the two large fragments as well as the light clusters or correlations, are already formed at the scission point, as also known from the scission point yield (SPY) model, see, e.g., [36][37][38][39][40] for recent related work. Hartree-Fock-Bogoliubov and related mean-field calculations have been performed for fission.…”