Nuclear fission of heavy (actinide) nuclei results predominantly in asymmetric mass splits1. Without quantum shell effects, which can give extra binding energy to their mass-asymmetric shapes, these nuclei would fission symmetrically. The strongest shell effects appear in spherical nuclei, such as the spherical ‘doubly magic’ (that is, both its atomic and neutron numbers are ‘magic’ numbers) nucleus 132Sn, which contains 50 protons and 82 neutrons. However, a systematic study of fission2 has shown that heavy fission fragments have atomic numbers distributed around Z = 52 to Z = 56, indicating that the strong shell effects in 132Sn are not the only factor affecting actinide fission. Reconciling the strong spherical shell effects at Z = 50 with the different Z values of fission fragments observed in nature has been a longstanding puzzle3. Here we show that the final mass asymmetry of the fragments is also determined by the extra stability provided by octupole (pear-shaped) deformations, which have been recently confirmed experimentally around 144Ba (Z = 56)4,5, one of very few nuclei with shell-stabilized octupole deformation6. Using a quantum many-body model of superfluid fission dynamics7, we find that heavy fission fragments are produced predominantly with 52 to 56 protons, which is associated with substantial octupole deformation acquired on the way to fission. These octupole shapes, which favour asymmetric fission, are induced by deformed shells at Z = 52 and Z = 56. By contrast, spherical magic nuclei are very resistant to octupole deformation, which hinders their production as fission fragments. These findings may explain surprising observations of asymmetric fission in nuclei lighter than lead8.
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We thank B. Jurado, A. Chatillon and F. Farget for useful discussions at the early stage of this work. We are grateful to D. J. Hinde for continuous support to this project. We thank M. Caamaño for providing references to experimental data. B. Jurado and D. J. Hinde are also thanked for their careful reading of the manuscript. This work has been supported by the Australian Research Council under grant number DP160101254. The calculations were performed in part at the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government, in part using the COMA system at the CCS in the University of Tsukuba, which is supported by the HPCI Systems Research Projects (project hp180041), and using the Oakforest-PACS at the JCAHPC in Tokyo, which is supported in part by the Multidisciplinary Cooperative Research Program in CCS, University of Tsukuba.