On the origin of asymmetric fission of actinides

Research paper by Guillaume Scamps, Cédric Simenel

Indexed on: 10 Apr '18Published on: 10 Apr '18Published in: arXiv - Nuclear Theory


Background: Nuclear fission of most actinides near the valley of stability is dominated by asymmetric modes with a heavy fragment charge distribution centred around $Z\approx52-55$ protons. Although spherical shell effects in the $^{132}$Sn are often considered as a driver for the formation of the heavy fragment, a convincing explanation for the origin of actinide asymmetric fission has been lacking so far. Purpose: To propose an explanation based on the couplings between fission dynamics and octupole deformations in the fragments, with the octupole correlations in the $^{144}$Ba region as a driver to asymmetric split. Method: Constrained and time-dependent Hartree-Fock microscopic calculations with dynamical pairing correlations have been performed for a series of actinides to compute expectation values of proton and neutron numbers in the fission fragments, their total kinetic energy (TKE), and their octupole and quadrupole deformations. Results: Deformed shell gaps are present at $Z=52$ and 56 in $^{142}$Xe and are particularly stable against octupole deformation. Heavy fission fragments are almost exclusively produced with $\langle Z \rangle\approx52-56$ and a significant octupole deformation at scission. Fragments with $\langle Z \rangle\approx54-56$ are more deformed, leading to a smaller TKE than $\langle Z \rangle\approx52$ fragments, in good agreement with experimental observations. Conclusions: Asymmetric fission of actinides is induced by octupole softness in the $^{144}$Ba region and not by spherical shell effects in the $^{132}$Sn region, except in symmetric fission of fermium isotopes in which both fragments are close to Sn.