Systematics of 21+ states in semimagic nuclei

We propose a simple systematics of low-lying 21+ energy levels and electromagnetic transitions in semimagic isotopic chains Z=28,50,82 and isotonic chains N=28,50,82,126. To this purpose we use a two-level pairing plus quadrupole Hamiltonian, within the spherical quasiparticle random phase approximation (QRPA). We derive a simple relation connecting the 21+ energy with the pairing gap and quadrupole-quadupole (QQ) interaction strength. It turns out that the systematics of energy levels and B(E2) values predicted by this simple model is fulfilled with a reasonable accuracy by all available experimental data. This systematics suggests that not only active nucleons but also those filling closed shells play an important role.