Speaker
Description
A particular interest in contemporary nuclear structure research is the impact of the local shell structure on the proton-neutron mixing in low-energy quadrupole states at the onset of collectivity near neutron shell closures. In near-spherical nuclei, the two simplest quadrupole-collective excitations can be understood as a mixture of the collective $2^+$ proton and $2^+$ neutron excitations. The symmetric (isoscalar) coupling appears as the $2_1^+$ state while the antisymmetric (isovector) one forms the so-called $2_{1,ms}^+$ state with mixed proton-neutron symmetry. Based on the evolution of these states in the $N=80$ isotonic chain it has been suggested that the properties of the mixed-symmetry states are sensitive to the underlying subshell structure. In particular, the observed fragmentation of the $2_{1,ms}^+$ of $^{138}$Ce has been explained as due to the absence of a mechanism dubbed shell stabilization [1]. This then requires contributions from active proton configuration in both, the $1g_{7/2}$ and $2d_{5/2}$ proton orbitals, and thus leads to fragmentation of low-lying quadrupole phonon excitations at $Z=58$.
In order to examine further the effect of shell stbailization of the MSSs it is necessary to quantitaively identify and study the properties of these states in the next heavy $N=80$ isotones beyond $Z=58$ - $^{140}$Nd and $^{142}$Sm. This was the main goal of the IS546 experiment run in October 2017. Preliminary results from the experiment for both isotones will be shown and discussed.
