Symmetry-restored two-centre self-consistent approach to fission with arbitrary deformations, orientations, and distance of fragments

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£1,011,375

Jan. 1, 2022

Dec. 31, 2023

The vision of this proposal is to bring into the physics of nuclear fission the most advanced theoretical ideas and computation. Since the discovery of fission almost eighty years ago, a wealth of experimental data has been accumulated. This has been accompanied by the development of an efficient phenomenology and microscopy of spontaneous and induced fission. However, almost all of these studies rely on assuming the adiabaticity and/or thermalisation of fission. Is the energy sufficiently low and time sufficiently long for these assumptions to hold? This project has the ambition to implement theoretical modelling of fission that will deliver definite answers to these challenges.

From the outside, fission looks like a simple process where a single drop of matter splits into two or more smaller drops. However, this is very misleading: a huge conceptual gap exists between the splitting of liquid drops and nuclear fission. Briefly, during the fission process, one composite quantum system splits into two or more composite quantum systems, and all properties of such a process crucially depend on quantum physics, which is not the case for the classical liquid drop. Here, nucleons move in correlated quantum orbitals that evolve into correlated quantum orbitals within the fission fragments. Altogether, in fission we find all the beauty and difficulty of a mesoscopic system. The fission happens in the border region between classical and quantal, large and small, and slow and fast phenomena. This is why it is so challenging and consequently provides an important subject of fundamental science research.

Currently, the methodology used for describing induced fission at varying excitation energies is in a very rudimentary state. The standard framework, dating all the way back to the pioneering work of Bohr&Wheeler in 1939, is that of a hot thermalized compound nucleus, which is created after resonant neutron capture. However, applying this concept to the other mechanisms of creating pre-fission states is not really the right way to proceed. Indeed, after the beta-decay of a precursor system, photon absorption, Coulomb excitation by a passing charge, or particle transfer, the nucleus ends up in a fairly well determined intermediate "doorway" state, which then fissions. Depending on the excitation energy and fission time scale, such an intermediate state may or may not have enough time to thermalize, and then the very concept of a compound nucleus becomes highly questionable.

A full research programme to challenge the main paradigms of the theory of fission would require a substantial investment in the workforce and resources. The current proposal aims to deliver one fundamental computational element of such a programme, namely, the computer code that would allow for solving the nuclear DFT equations within the symmetry-restored two-centre self-consistent approach to fission with arbitrary deformations, orientations, and distance of fragments. The main challenge here is to perform efficient fission calculations using novel nonlocal density functionals, which are in the centre of current developments of the nuclear DFT. Using this tool, in the future, we will be able to build the doorway states explicitly, by employing the high-energy vibrational limit of the time-dependent density functional theory (DFT), and then to follow the fission with coupling to such vibrations included. This full programme will redefine future research of fission. We cannot anymore rely on the old ideas and simplifications. At present, not a single prediction of fission lifetimes beyond the adiabatic limit exists. To show that this is actually possible to obtain, would be in itself an important breakthrough in research. To show that without adiabaticity or thermalisation the results are fundamentally and qualitatively different, would lead to an entirely new approach to future research.