Investigations of proton-neutron correlations close to the drip line

Abstract

Proton-neutron correlations in nuclei above the $Z=50$ shell closure are investigated with the aim of understanding the behavior of the $2$${}^{+}$ and $4$${}^{+}$ states in Te and Xe isotopes, which remain at a rather constant energy as one approaches the shell closure at $N=50$. Our calculations reveal that standard quasiparticle random phase approximation calculations, involving a quadrupole-quadrupole (QQ) interaction with constant strengths, cannot explain this feature. It is found that to reproduce the experimental data within this model one has to include a variable proton-neutron interaction. It turns out that an increased proton-neutron QQ interaction increases the collectivity (i.e., $B(E2)$ values) when approaching the $N=50$ region, whereas an increased proton-neutron pairing interaction decreases the collectivity. We thus conclude that the ratio between the $B(E2)$ value and ${2}^{+}$ energy is a ``fingerprint'' of proton-neutron collectivity and it should be determined in future experiments concerning light Te isotopes. Based on this criterion, we conclude that the available experimental data indicate an enhanced proton-neutron pairing interaction by approaching doubly magic $Z=N=20$ and $Z=N=28$ regions.