Mean values of the number of protons and neutrons of the primary fission fragments at scission are determined for the asymmetric fission of 16 fissioning isotopes, from ${}^{219}\mathrm{Ac}$ up to ${}^{238}\mathrm{Np}$. Our results confirm that the main asymmetric fission mode around the heavier uranium isotopes is indeed characterized by an average atomic number around $\langle Z_{\mathrm{H}}\rangle =54$ in the heavy fission fragments. However, they also unambiguously show a stabilization effect in the light fission fragments around $\langle N_{\mathrm{L}}\rangle =52–54$ in the neutron-deficient thorium and actinium isotopes. This is a clear signature that these deformed proton and neutron shell closures around 54 play a major role in the nuclear fission process. The evolution along the thorium chain shows that the neutron shell appears to be dominant in the asymmetric fission of the lighter thorium isotopes, in contrast to the heavier thorium isotopes for which the stabilization originates from the proton shell.

• Received 12 June 2020
• Revised 13 June 2022
• Accepted 8 August 2022

DOI:https://doi.org/10.1103/PhysRevC.106.024618