Langevin description of fusion, deep-inelastic collisions and heavy-ion-induced fission

On stochastic approaches of nuclear dynamics

We describe the fissioning dynamics of ^{240}Pu from a configuration in the proximity of the outer fission barrier to full scission and the formation of the fragments within an implementation of density functional theory extended to superfluid systems and real-time dynamics. The fission fragments emerge with properties similar to those determined experimentally, while the fission dynamics appears to be quite complex, with many excited shape and pairing modes. The evolution is found to be much slower than previously expected, and the ultimate role of the collective inertia is found to be negligible in this fully nonadiabatic treatment of nuclear dynamics, where all collective degrees of freedom (CDOF) are included (unlike adiabatic treatments with a small number of CDOF).

Induced Fission of (240)Pu within a Real-Time Microscopic Framework.

Prompt fission neutron spectra of actinides

An overview is given on some of the main advances in the experimental methods, experimental results, theoretical models and ideas of the last few years in the field of nuclear fission. New approaches have considerably extended the availability of fissioning systems for the experimental study of nuclear fission, and have provided a full identification of all fission products in A and Z for the first time. In particular, the transition from symmetric to asymmetric fission around 226Th, some unexpected structures in the mass distributions in the fission of systems around Z  =  80-84, and an extended systematics of the odd-even effect in the fission fragment Z distributions have all been measured (Andreyev et al 2018 Rep. Prog. Phys. 81 016301). Three classes of model descriptions of fission presently appear to be the most promising or the most successful. Self-consistent quantum-mechanical models fully consider the quantum-mechanical features of the fission process. Intense efforts are presently being made to develop suitable theoretical tools (Schunck and Robledo 2016 Rep. Prog. Phys. 79 116301) for modeling the non-equilibrium, large-amplitude collective motion leading to fission. Stochastic models provide a fully developed technical framework. The main features of the fission-fragment mass distribution have been well reproduced from mercury to fermium and beyond (Moller and Randrup 2015 Phys. Rev. C 91 044316). However, limited computer resources still impose restrictions, for example, on the number of collective coordinates and on an elaborate description of the fission dynamics. In an alternative semi-empirical approach (Schmidt et al 2016 Nucl. Data Sheets 131 107), considerable progress in describing the fission observables has been achieved by combining several theoretical ideas, which are essentially well known. This approach exploits (i) the topological properties of a continuous function in multidimensional space, (ii) the separability of the influence of fragment shells and the macroscopic properties of the compound nucleus, (iii) the properties of a quantum oscillator coupled to a heat bath of other nuclear degrees of freedom, (iv) an early freeze-out of collective motion, and (v) the application of statistical mechanics for describing the thermalization of intrinsic excitations in the nascent fragments. This new approach reveals a high degree of regularity and allows the calculation of high-quality data that is relevant to nuclear technology without specifically adjusting the empirical data of individual systems.

https://iopscience.iop.org/article/10.1088/1361-6633/aacfa7/pdf

Review on the progress in nuclear fission-experimental methods and theoretical descriptions.

Fission dynamics of hot nuclei have been investigated using the two-dimensional Langevin equation. Including particle evaporation in the continuous limit, prescission multiplicities of neutrons, protons, and \ensuremath{\alpha} particles have been calculated. Both the calculated number of prescission neutrons and the average total kinetic energy of fission fragments are consistent with experimental values when one-body dissipation is assumed. Unusually strong hydrodynamical two-body viscosity also reproduces the experimental neutron multiplicity, but it significantly underestimates the average kinetic energy.

One-body dissipation in agreement with prescission neutrons and fragment kinetic energies.

Multi-dimensional Langevin approach to fission dynamics

Dynamical scission model

The partition between the light ($L$) and the heavy ($H$) fission fragments of the excitation energy available at scission is studied in the framework of the sudden approximation, i.e., under the assumption that the neck rupture and the absorption of the neck pieces by the fragments happen infinitely fast. We are dealing with a sudden transition between two different nuclear configurations (${\ensuremath{\alpha}}_{i}\ensuremath{\rightarrow}{\ensuremath{\alpha}}_{f}$) and we only need to know the two sets of neutron eigenstates involved. The accent in the present work is put on the dependence of this share of energy on the mass asymmetry ${A}_{L}/{A}_{H}$ of the primary fission fragments during the low-energy fission of ${}^{236}$U. In particular, for every fragment mass $A$ we estimate the scission neutron multiplicity ${\ensuremath{\nu}}_{\mathrm{sc}}$, the average energy cost for their release $\ensuremath{\langle}{E}_{\ensuremath{\nu}\mathrm{sc}}\ensuremath{\rangle}$, the primary fragments' excitation energy ${E}_{\mathrm{sc}}^{*}$, and the corresponding temperature ${T}_{\mathrm{sc}}$. The results are analyzed separately for each value of $\ensuremath{\Omega}$ (the projection of the angular momentum on the symmetry axis). As general trends, a decrease of ${E}_{\mathrm{sc}}^{*}$ (${T}_{\mathrm{sc}}$) and an increase of ${\ensuremath{\nu}}_{\mathrm{sc}}$ ($\ensuremath{\langle}{E}_{\ensuremath{\nu}\mathrm{sc}}\ensuremath{\rangle}$) with increasing $A$ were observed.

N. Carjan

Papers

One-body dissipation in agreement with prescission neutrons and fragment kinetic energies.

Multi-dimensional Langevin approach to fission dynamics

Dynamical scission model

Partition between the fission fragments of the excitation energy and of the neutron multiplicity at scission in low-energy fission

Effects of the Fission Fragments on the Angular Distribution of Scission Particles