Exploring shell effects in fission yields of neutron-deficient Th, Ac, and Ra isotopes near N=126

Abstract

Studies of nuclear fission over recent decades have led to a well-defined mapping of neutron and proton shell effects across the nuclear chart, particularly within the valley of stability. These shell effects play a crucial role in driving the asymmetric splitting of fissioning nuclei, as reflected in fission yields that are strongly influenced by spherical and deformed shell closures. In the actinide region, the existence of two primary fission modes, standard I and standard II, has been well-established. These fission modes are associated with the proton (neutron) shells at $Z=52$ ($N=82$) and $Z=56$ ($N=88$), respectively. Recently, a new proton shell around $Z=36$ has been found for the lighter fission fragments of pre-actinide nuclei. This discovery demonstrates that as we expand fission studies towards more exotic regions of the nuclear chart, new shell structures emerge. In this proposal, we aim to explore for the first time the most neutron-deficient isotopes of Th, Ac, and Ra. This region offers a unique opportunity to investigate stabilization effects around the spherical neutron shell $N=50$. To achieve this, we plan to use a primary beam of $^{238}$U at 1~GeV/u together with the Fragment Separator (FRS) to produce secondary beams of $^{213-216}$Th, $^{209-214}$Ac and $^{207-213}$Ra. For the investigation of the fission process, we will use the experimental methodology successfully applied in the S415, S438, and S455 experiments, being a continuation of those studies. Fission will be induced by electromagnetic-excitation (Coulex) reactions in inverse kinematics on an active target composed of Pb and C foils. The resulting fission fragments, together with emitted neutrons, will be measured using the R$^3$B experimental setup, which allows for complete kinematic measurements.