Pseudospin Symmetry and Microscopic Origin of Shape Coexistence in the Ni 78 Region: A Hint from Lifetime Measurements

Lifetime measurements of excited states of the light N=52 isotones Kr88, Se86, and Ge84 have been performed, using the recoil distance Doppler shift method and VAMOS and AGATA spectrometers for particle identification and gamma spectroscopy, respectively. The reduced electric quadrupole transition probabilities B(E2;2+→0+) and B(E2;4+→2+) were obtained for the first time for the hard-to-reach Ge84. While the B(E2;2+→0+) values of Kr88, Se86 saturate the maximum quadrupole collectivity offered by the natural valence (3s, 2d, 1g7/2, 1h11/2) space of an inert Ni78 core, the value obtained for Ge84 largely exceeds it, suggesting that shape coexistence phenomena, previously reported at N''49, extend beyond N=50. The onset of collectivity at Z=32 is understood as due to a pseudo-SU(3) organization of the proton single-particle sequence reflecting a clear manifestation of pseudospin symmetry. It is realized that the latter provides actually reliable guidance for understanding the observed proton and neutron single particle structure in the whole medium-mass region, from Ni to Sn, pointing towards the important role of the isovector-vector ρ field in shell-structure evolution.