Comparative Study of Langevin and Random Walk Models for Nuclear Fission in the Overdamped Regime

TL;DR

This study compares Langevin and Metropolis random walk models for nuclear fission, confirming their equivalence in overdamped regimes and analyzing their differences across actinides, with implications for modeling fission fragment distributions.

Contribution

It demonstrates the correspondence between Langevin dynamics and Metropolis random walk in the overdamped limit and explores their differences in predicting fission fragment distributions.

Findings

Metropolis walk matches Langevin in overdamped regime for lighter actinides.

Deviations appear for heavier actinides, with Langevin showing symmetric fission components.

Quantum corrections to temperature influence zero-point fluctuation effects.

Abstract

We present a comparative study of Langevin dynamics and a Metropolis random walk model applied to thermal neutron-induced fission of $^{229}$Th, $^{235}$U, $^{239}$Pu, $^{245}$Cm, $^{249}$Cf, and $^{255}$Fm. Both methods are implemented within an identical four-dimensional Fourier-over-Spheroid framework, using potential energy surfaces derived from the macroscopic-microscopic model. We show that the Metropolis walk corresponds to the overdamped limit of the Langevin equations and confirm this correspondence numerically by Langevin calculations performed in the strongly damped regime and with quantum corrections to the random force switched off. Under these conditions, the two approaches produce essentially identical mass distributions for the lighter actinides. Systematic deviations develop for the heavier actinides, where the Langevin dynamics yields a non-negligible symmetric fission component absent in the random walk results. We trace this difference to the kinematic structure of the Metropolis sampling and to the residual inertial dynamics retained in the Langevin framework. A parallel comparison of Langevin calculations with and without the quantum-corrected effective temperature $T^*$ isolates the contribution of zero-point fluctuations and suggests that their standard phenomenological treatment may overestimate their impact in certain cases. Both approaches qualitatively reproduce the asymmetric peak positions and their systematic evolution across the actinide chain, while a common quantitative limitation -- the narrowness of the predicted distributions -- points to the role of higher-dimensional deformation modes not included in the present parametrization.