Unified description of fission in fusion and spallation reactions

We present a statistical-model description of fission, in the framework of compound-nucleus decay, which is found to simultaneously reproduce data from both heavy-ion-induced fusion reactions and proton-induced spallation reactions at around 1 GeV. For the spallation reactions, the initial compound-nucleus population is predicted by the Li\`{e}ge Intranuclear Cascade Model. We are able to reproduce experimental fission probabilities and fission-fragment mass distributions in both reactions types with the same parameter sets. However, no unique parameter set was obtained for the fission probability. The introduction of fission transients can be offset by an increase of the ratio of level-density parameters for the saddle-point and ground-state configurations. Changes to the finite-range fission barriers could be offset by a scaling of the Bohr-Wheeler decay width as predicted by Kramers. The parameter sets presented allow accurate prediction of fission probabilities for excitation energies up to 300 MeV and spins up to 60 \hbar.Comment: 16 pages, 20 figures. Submitted to Phys. Rev.

Influence of entrance-channel magicity and isospin on quasi-fission

The role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the $^{40}$Ca+$^{208}$Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of $N/Z$ in the two fragments early in the collision. This transfer of nucleons breaks the shell effect, causing this reaction to behave more like a non-magic one in the competition between fusion and quasi-fission. Future measurements of fission in reactions with exotic beams should be able to test this idea with larger $N/Z$ asymmetries.Comment: accepted for publication in Physics Letters

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

Abstract : 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 approaches extended the availability of fissioning systems for experimental studies of nuclear fission 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 model 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 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]. However, the 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 [K.-H. Schmidt et al., Nucl. Data Sheets 131 (2016) 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 influences of fragment shells and macroscopic properties of the compound nucleus, (iii) the properties of a quantum oscillator coupled to the 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 calculating high-quality data that are relevant for nuclear technology without specific adjustment to empirical data of individual systems