Mid Term Review

Mean field study of collective processes in low-energy nuclear reactions

Mr Patrick McGlynn
NPAA/FTP - ANU

Nuclear reactions are driven by a combination of single particle dynamics and collective processes. In low energy processes such as below barrier fusion and spontaneous fission, the nucleus undergoes coherent quantum tunnelling, which is poorly described by ordinary mean field techniques. Attempts to extend the mean field method using imaginary time paths appear promising at first, but eventually encounter problems owing to numerical stiffness. An alternative to extending the mean field method for description of low energy processes is to select processes which are well described by mean field approaches.

Nuclear fission, especially in heavy and superheavy nuclei, is heavily influenced by strong shell effects. The much more easily obtained quasifission reaction may probe some of the same shell effects, but in order to use quasifission to understand shell effects in fission, we need to understand shell effects in quasifission. To study this, several techniques have been used. Calculation of potential energy surfaces (PES) is important for understanding the driving processes behind fission. This was performed for a range of thorium nuclides and reflects the known competition between symmetric and asymmetric modes. The same techniques were used to calculate a potential energy surface for oganesson-294. Time-dependent Hartree-Fock (TDHF) software was used to simulate reactions forming oganesson-294 in different entrance channels, at energies chosen to produce mostly quasifission paths. These paths and their resulting fragments were compared with the fission PES. The comparison shows that the quasifission trajectories are strongly influenced towards the same modes as seen in the PES and suggests that quasifission can effectively probe shell effects in the fission of superheavy nuclei.

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