Speaker
Description
Stochastic transport models are a standard tool for describing fission fragment mass distributions, yet the multidimensional Langevin dynamics and the Metropolis random walk are rarely confronted directly within the same framework. We present such a comparison [1] for thermal neutron-induced fission of $^{229}$Th, $^{235}$U, $^{239}$Pu, $^{245}$Cm, $^{249}$Cf, and $^{255}$Fm. Both methods run on identical four-dimensional potential energy surfaces computed in the Fourier-over-Spheroid (FoS) shape parametrization within the macroscopic-microscopic model, ensuring full consistency of the nuclear structure input.
The Metropolis walk corresponds to the overdamped limit of the Langevin equations. We confirm this correspondence numerically: Langevin calculations in the strongly damped regime, with quantum corrections to the random force switched off, produce mass distributions essentially identical to the random walk for the lighter actinides. Systematic deviations develop for the heavier systems, where the Langevin dynamics yields a non-negligible symmetric fission component absent in the random walk. This difference traces back to the kinematic structure of the Metropolis sampling and 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. Both approaches reproduce the asymmetric peak positions and their systematic evolution across the actinide chain, while the common narrowness of the predicted distributions points to higher-dimensional deformation modes not included in the present calculations.
Finally, we briefly present preliminary random-walk results obtained on native five-dimensional potential energy surfaces of the Warsaw macroscopic-microscopic model for ten actinide systems --- the first dynamical calculations performed on Warsaw macroscopic-microscopic model surfaces in the FoS parametrization.
[1] A. Augustyn, T. Cap, M. Kowal, and K. Pomorski, submitted to Phys. Rev. C; arXiv:2605.00586