Influence of proton and neutron deformed shells on the asymmetric fission of thorium isotopes

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 Ac-219 up to Np-238. Our results confirm that the main asymmetric fission mode around the heavier uranium isotopes is indeed characterized by an average atomic number around (Z(H)) = 54 in the heavy fission fragments. However, they also unambiguously show a stabilization effect in the light fission fragments around (N-L) = 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.

Automated summary

The average number of protons and neutrons in the primary fission fragments at scission is determined for 16 fissioning isotopes. The research confirms that the main asymmetric fission mode near heavy uranium isotopes is characterized by an average atomic number around 54 in the heavy fission fragments. However, it also shows a stabilization effect in the light fission fragments around 52-54 neutrons in the neutron-deficient thorium and actinium isotopes, indicating the significance of the deformed proton and neutron shell closures around 54 in the nuclear fission process.