[Paper Review] Real time description of fission

Ionel Stetcu, Aurel Bulgac+3 more|arXiv (Cornell University)|Oct 9, 2018

Nuclear physics research studies9 references4 citations

TL;DR

This paper presents a real-time, fully microscopic simulation of nuclear fission using the time-dependent superfluid local density approximation (TDSLDA), solving coupled nonlinear partial differential equations to track fission dynamics from saddle to full fragment separation. It reveals strong one-body dissipation and neutron emission during acceleration—key findings unattainable in phenomenological models—offering quantitative estimates of kinetic and excitation energies and neutron emission rates beyond scission.

ABSTRACT

Using the time-dependent superfluid local density approximation, the dynamics of fission is investigated in real time from just beyond the saddle to fully separated fragments. Simulations produced in this fully microscopic framework can help to assess the validity of the current approaches to fission, and to obtain estimate of fission observables. In this contribution, we concentrate on general aspects of fission dynamics.

Motivation & Objective

• To develop a fully microscopic, real-time description of nuclear fission beyond phenomenological approximations.
• To assess the validity of current fission models by simulating dynamics from saddle to full separation.
• To compute fission observables such as total kinetic energy (TKE), total excitation energy (TXE), and neutron emission that are inaccessible in standard phenomenological models.
• To investigate the role of one-body dissipation and collective modes in fission dynamics.
• To provide reliable theoretical predictions of fission observables as a function of excitation energy for use in applications.

Proposed method

• The time-dependent superfluid local density approximation (TDSLDA) is used, formally equivalent to time-dependent Hartree-Fock-Bogoliubov (TDHFB) theory.
• The method solves 500,000+ coupled nonlinear time-dependent partial differential equations (PDEs) on large 3D spatial lattices with high numerical accuracy.
• The approach uses Skyrme-type energy functionals to ensure locality and consistency with the Kohn-Sham framework.
• The time evolution of quasiparticle wavefunctions is governed by a time-dependent Hamiltonian including single-particle and pairing fields as functionals of densities.
• The system is initialized with a single generalized Slater determinant (HFB vacuum), and the dynamics are evolved from the outer saddle to full fragment separation.
• The method incorporates continuum effects, pairing dynamics, and controlled numerical approximations with negligible corrections.

Experimental results

Research questions

• RQ1 How does the fission dynamics evolve in real time from the outer saddle to full fragment separation?
• RQ2 What is the role of one-body dissipation in shaping the fission trajectory and energy dissipation?
• RQ3 Can neutron emission occur during the acceleration phase of fission fragments, and how many neutrons are emitted on average?
• RQ4 How is the excitation energy shared between fission fragments, and what is its impact on TKE and TXE?
• RQ5 To what extent do collective modes beyond 2–5 degrees of freedom contribute to fission dynamics?

Key findings

• The fission dynamics are strongly overdamped due to dominant one-body dissipation, causing trajectories to follow the steepest descent path.
• The collective flow energy remains very low throughout the fission process, up to the scission point.
• On average, more than 0.4 neutrons are emitted from scission to full acceleration, independent of the fission trajectory.
• The excitation energy is preferentially deposited into the heavy fragment, consistent with experimental observations of increased neutron emission from heavy fragments at higher incident neutron energies.
• The TDSLDA framework enables the computation of TKE, TXE, and neutron emission rates—quantities inaccessible in standard phenomenological models that assume full acceleration before emission.
• The method provides a unique capability to study excitation energy sharing and fragment spin distributions before neutron emission, with future work planned for these observables.