Speaker
Description
The impact of the local shell structure on the proton-neutron mixing in low-energy quadrupole states is of particular interest in contemporary nuclear structure research. In vibrational nuclei, the two most generic quadrupole-collective excitations are a mixture of the collective $2^+$ proton and $2^+$ neutron excitations. The symmetric (isoscalar) coupling appears as the lowest-lying $2^+$ state while the partly antisymmetric (isovector) one forms the so-called mixed-symmetry $2^+_\mathrm{1,ms}$ state. Due to the evolution of the $2^+_\mathrm{1,ms}$ states in the $N=80$ isotonic chain, the properties of the mixed-symmetry states seem to be sensitive to the underlying sub-shell structure. In the $N=80$ isotones $^{132}$Te, $^{134}$Xe, $^{136}$Ba, isolated $2^+_\mathrm{1,ms}$ states have been identified by our group. In contrast, a sudden fragmentation of the $2^+_\mathrm{1,ms}$ state of $^{138}$Ce has been observed due to the lack of a mechanism called valence-shell stabilization at the proton g$_{7/2}$ sub-shell closure at $Z=58$ [1].
Results of the projectile Coulomb-excitation experiment (IS 546) at HIE-ISOLDE using Miniball will be presented. The measured B($M1;2^+_i\to2^+_1$) strengths distribution of the $N=80$ isotone $^{140}$Nd clarifies the properties of the $2^+_\mathrm{1,ms}$ state at $Z=60$ [2].
[1] G. Rainovski et al., Phys. Rev. Lett. 96, 122501 (2006)
[2] R. Kern et al., Phys. Rev. C 102, 041304(R) (2020)
