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

Abstract

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 Th, 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 (Möller 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.

Quasifission Dynamics in Microscopic Theories

“…Of these cross-sections fusion-fission arises from an excited and equilibrated composite system and therefore peaked around equal mass breakup as calculated in a statistical approach [14,16,90,91,92]. On the other hand, quasifission, which is a faster process and thus not fully equilibrated, could also contribute to the equal breakup regime.…”

Nuclear Fission Dynamics: Past, Present, Needs, and Future

“…All the codes that model the prompt neutron and gamma emissions have been built in the assumption that the neutron emission proceeds after the full acceleration of the fission fragments [71,[89][90][91][92][93]. This assumption can have significant consequences, as at full acceleration the neutrons are maximally boosted.…”

“…We did not cover or mention all phenomenological models on the market and not all microscopically inspired theoretical frameworks, as this is not a review of such approaches, for which we recommend Refs. [92,102].…”

Neutron-induced fission: properties of prompt neutron andγrays as a function of incident energy

“…However, in addition to the average neutron multiplicity and spectrum calculated in the Los Alamos model, CGMF provides many other observables, like neutron multiplicity distribution, γ-ray observables (average multiplicity and spectrum, multiplicity distribution), and spatial and energy correlation between prompt neutrons and/or γ rays. Other codes that treat the de-excitation fission fragments via emission of neutrons and γ rays are available [3][4][5].…”