Review on the progress in nuclear fission

An overview is given on some of the main advances in experimental methods, experimental results and theoretical models and ideas of the last years in the field of nuclear fission. New experimental approaches extended the availability of fissioning systems considerably and 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 and some unexpected structure in the mass distributions in the fission of systems around Z = 80 to 84 as well as an extended systematics of the odd-even effect in fission fragment Z distributions have been measured [A. N. Andreyev et al., Rep. Progr. Phys. 81 (2018) 016301]. Three classes of theoretical descriptions of fission presently appear to be the most promising or the most successful ones: Self-consistent quantum-mechanical models fully consider the quantum-mechanical features of the fission process. Intense efforts are presently made to develop suitable theoretical tools [N. Schunck, L. M. Robledo, Rep. Prog. Phys. 79 (2016) 116301] for modeling the non-equilibrium, large-amplitude collective motion leading to fission. Stochastic, essentially classical models provide a fully developed technical framework. The main features of the fission-fragment mass distribution are well reproduced from mercury to fermium and beyond [P. M\"oller, J. Randrup, Phys. Rev. C 91 (2015) 044316]. In an alternative semi-empirical approach [K.-H. Schmidt et al., Nucl. Data Sheets 131 (2016) 107], considerable progress in describing the fission observables has been achieved by combining several general theoretical concepts. This new approach reveals a high degree of regularity and allows calculating high-quality data with a unique parameter set for a large number of systems and an extended excitation-energy range.