Evaporation Residue Cross Sections Measured Near the N = 126 Spherical Closed Shell in 45Sc- and 44Ca-Induced Reactions

Abstract

Evaporation residue cross sections were measured for shell-stabilized nuclides near the N = 126 closed shell in ^45Sc- and ^44Ca-induced reactions on the lanthanide targets ^156-^158, ^160Gd, ^159Tb, and ^162Dy. The experiments were performed at the Texas A&M University Cyclotron Institute, with the K500 cyclotron providing the accelerated beam. The Momentum Achromat Recoil Separator was used to separate the desired evaporation residues from the other reactions products. The evaporation residue cross sections ranged between 2.7 mb and 1 μb for the reactions, and the effects of the shell stabilization and the relative neutron content of the compound nucleus on the cross sections were examined. The cross sections in the ^45Sc-induced reactions were up to four orders of magnitude smaller than ^48Ca-induced reactions on the same targets due to the relative neutron-deficiency of ^45Sc. This observation suggests that ^45Sc would be a poor projectile for synthesizing superheavy elements.

The experimental data were analyzed within the framework of a theoretical model aimed at elucidating the major physical factors which determined the evaporation residue cross sections. The model describes the fusion-evaporation reaction as a series of three independent steps: capture, compound nucleus formation, and deexcitation into the cold evaporation residue. The primary factor in determining the evaporation residue cross sections was found to be the difference between the fission barrier and the neutron separation energy. The nuclear level density in the model was modified to incorporate the collective motion of the nucleons. The influence of collective effects suggests that cross sections for superheavy elements produced near the predicted N = 184 spherical closed shell may not be enhanced by the presence of the shell closure.