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Nuclear fission of heavy (actinide) nuclei results predominantly in asymmetric mass-splits. Without quantum shells, which can give extra binding energy to these mass-asymmetric shapes, the nuclei would fission symmetrically. The strongest shell effects are in spherical nuclei, so naturally, the spherical "doubly-magic" 132Sn nucleus, was expected to play a major role.
However, systematic studies of fission have shown that the heavy fragments are distributed around Z=52 to 56, indicating that 132Sn is not the only driver. Reconciling the strong spherical shell effects at Z=50 with the different Z values of fission fragments observed in nature has been a longstanding puzzle. Here, we show that the final mass asymmetry of the fragments is determined by the extra stability of octupole (pear-shaped) deformations which have been recently found experimentally around 144Ba (Z=56), one of the very few nuclei with shell-stabilized octupole deformation. Using a modern quantum many-body model of superfluid fission dynamics, we found that heavy fission fragments are produced predominantly with 52-56 protons, associated with significant octupole deformation acquired on the way to fission. These octupole shapes favoring asymmetric fission are induced by deformed shells at Z=52 and 56 [1]. In contrast, spherical "magic" nuclei are very resistant to octupole deformation, which hinders their production as fission fragments.
These findings also explain surprising recent observations of asymmetric fission of lighter than lead nuclei. Such as the discovery that 180Hg fission is mass asymmetric instead of being symmetric with two semi-magic 90Zr fragments [2]. To test the universality of the octupole effect on fission, we investigate with the constraint Hartree-Fock + BCS approach the effect of quadrupole and octupole deformations on the fission asymmetry of elements around 180Hg. The density at the scission as well as the neutron localisation function from which quantum shell signatures can be investigated show clearly an octupole deformation of the fragments[3].
[1] G. Scamps and C. Simenel, Nature 564, 382–385 (2018).
[2] A. N. Andreyev, Phys. Rev. Lett. 105, 252502 (2010).
[3] G. Scamps and C. Simenel, Effect of shell structure on the fission of sub-lead nuclei, Phys. Rev. C 100, 041602(R) (2019).
