Nuclear Scission and Quantum Localization

Abstract

We examine nuclear scission within a fully quantum-mechanical microscopic framework, focusing on the non-local aspects of the theory. Using 240 Pu hot fission as an example, we discuss the identification of the fragments and the calculation of their kinetic, excitation, and interaction energies, through the localization of the orbital wave functions. We show that the "disentanglement" of the fragment wave functions is essential to the quantum-mechanical definition of scission and the calculation of physical observables. Finally, we discuss the fragments' pre-scission excitation mechanisms and give a non-adiabatic description of their evolution beyond scission.Nuclear scission, the process wherein a nucleus breaks into two or more fragments, poses a fundamental challenge to quantum many-body theory: scission implies a separation of the nucleus into independent fragments, while the Pauli exclusion principle introduces a persistent correlation between the fragments, no matter how far apart they are. The objective of this paper is to resolve this paradox by disentangling the fragments in a fully quantum-mechanical description that is consistent with experimental data. In addition to shedding light on fundamental aspects of many-body physics, a microscopic theory of scission is needed to make reliable predictions of fission-fragment properties, such as their excitation and kinetic energies, and their shapes. In particular, we revisit in a microscopic approach the question of the energy partition between light and heavy fragments which was addressed in a recent letter [1] within a statistical-mechanic treatment. While many technical challenges remain in the 70-year quest to develop a predictive theory of fission, understanding scission, remains a formidable conceptual obstacle to such a theory.Previous descriptions of scission have always been formulated within the context of a nuclear density, with an identifiable neck joining two pre-fragments. The neck ruptures at some point along its length, and all the matter to one side or the other of the rupture is relegated to the corresponding fragment. Despite its usefulness, this is ultimately a classical view of scission. In 1959 [2], this picture was used to qualitatively account for the different observed mass divisions in fission and the well-known "sawtooth" shape of the average neutronmultiplicity distribution. Later on, a more quantitative description of the nuclear shape was introduced [3], and scission was equated with a vanishing neck size. This criterion was later improved [4] by requiring that scission occurs when the Coulomb repulsion exceeds the attractive nuclear force between the fragments. Nörenberg [5] took a step toward a more microscopic description using a molecular model of fission calculated in a two-center Hartree-Fock+BCS approach. Bonneau et al.[6] used separate microscopic calculations of each fragment and a phenomenological nuclear interaction between them to define a scission criterion based on the ratio of their mutual nuclear and Coulomb ener...

Nuclear energy density optimization: Large deformations

“…correction between the two prefragments could amount to several MeV near scission [41][42][43]. Moreover, properly computing this relative contribution is difficult, as it reflects the degree of entanglement between prefragments [43,44]. Time-dependent Hartree-Fock calculations of low-energy heavy-ion reactions are even more problematic, as there is currently no solution to the discontinuity of the c.o.m.…”

Fission fragment charge and mass distributions inPu239(n,f)in the adiabatic nuclear energy density functional theory

“…At scission, the fragments are still strongly entangled. This point was first mentioned in [53], where a technique to disentangle the fragments based on a unitary transformation of the quasi-particles was introduced; see also [42] for details. If this operation is not performed, we can expect exchanges of nucleons between the two pre-fragments and therefore a broadening of the raw fission yields.…”

Fission fragment mass yield deduced from density distribution in the pre-scission configuration

“…[23,24] that the structure and basic properties of the nascent fragments are preliminary determined in the pre-scission configuration of fission-ing nucleus. In the pre-scission point two fragments are already created in the form of two nuclei connected by a neck.…”