Formation and distribution of fragments in the spontaneous fission of 240Pu

Abstract

Background: Fission is a fundamental decay mode of heavy atomic nuclei. The prevalent theoretical approach is based on mean-field theory and its extensions, where fission is modeled as a large amplitude motion of a nucleus in a multi-dimensional collective space. One of the important observables characterizing fission is the charge and mass distribution of fission fragments.Purpose: The goal of this paper is to better understand the structure of fission fragment distributions by investigating the competition between the static structure of the collective manifold and stochastic dynamics. In particular, we study the characteristics of the tails of yield distributions, which correspond to very asymmetric fission into a very heavy and a very light fragment. Methods:We use the stochastic Langevin framework to simulate the nuclear evolution after the system tunnels through the multi-dimensional potential barrier. For a representative sample of different initial configurations along the outer turning-point line, we define effective fission paths by computing a large number of Langevin trajectories. We extract the relative contribution of each such path to the fragment distribution. We then use nucleon localization functions along effective fission pathways to analyze the characteristics of prefragments at pre-scission configurations.Results: We find that non-Newtonian Langevin trajectories, strongly impacted by the random force, produce the tails of the fission fragment distribution of 240 Pu. The prefragments deduced from nucleon localizations are formed early and change little as the nucleus evolves towards scission. On the other hand, the system contains many nucleons that are not localized in the prefragments, even near the scission point. Such nucleons are rapidly distributed at scission to form the final fragments. Fission prefragments extracted from direct integration of the density and from the localization functions typically differ by more than 30 nucleons, even near scission.Conclusions: Our study shows that only theoretical models of fission that account for some form of dissipative/stochastic dynamics can give an accurate description of the structure of fragment distributions. In particular, it should be nearly impossible to predict the tails of these distributions within the standard formulation of timedependent density functional theory. At the same time, the large number of non-localized nucleons during fission suggests that adiabatic approaches, where the interplay between intrinsic excitations and collective dynamics is neglected, are ill-suited to describe fission fragment properties, in particular their excitation energy.Introduction -A better understanding of nuclear fission is essential for different branches of basic sciences and applications. Fission governs the existence and stability of heavy and superheavy elements [1][2][3]. In nuclear astrophysics, fission rates and the related fission fragment distributions are key inputs to investigate the origin of elements heavier than iron [4][5][6]. ...

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

“…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.…”

Heavy-ion collisions and fission dynamics with the time-dependent Hartree–Fock theory and its extensions

“…It was also recognized as an essential ingredient to investigations of the evolution from saddle to scission in fissioning nuclei with time-dependent microscopic approaches [169]. One of the most important contributions of pairing to self-consistent mean-field calculations is the ability of the system to allow for level crossings, which results in fragments establishing their identity between the saddle and scission points [166,167,[170][171][172][173]. Pairing is also expected to play a role as a residual interaction in dynamical calculations of heavy-ion collisions, giving flexibility to the system to attain more compact shapes in fusion, influence transfer and breakup, or lowering the effect of spherical magic shells and open other magic numbers for final fragment formation in fission and quasifission studies [174].…”

Effect of shell structure on the fission of sub-lead nuclei

“…The spherical shell model developed by Goeppert Mayer in 1950 [5] explains the extra stability of nuclei with so-called "magic" numbers of protons and neutrons associated with fully occupied quantum shells (analogous to noble gas in atomic physics). Closed shells have then been naturally invoked as possible drivers to asymmetric fission [6][7][8][9][10], energetically favouring the formation of fragments with (doubly)magic clusters such as 132 50 Sn 82 . Neutron deformed shell effects in fission fragments with N ≈ 88 neutrons [11] as well as in the fissioning nucleus [12] have also been invoked.…”