Oblate stability of A É 110 nuclei near the r - process path

Even-even A'110 nuclei approaching the astrophysical r-process path have been investigated using both the cranked and the configuration-constrained shell models. The calculations show that, with increasing neutron number in the Z>40 nuclides, nuclear shapes evolve from prolate, through triaxial to oblate deformations. In contrast to other regions of the nuclear chart, pronounced oblate shapes dominate the collective rotation from ground states to very high spins (I;40), when N>70. The stability of the oblate shapes is due to the simultaneous upper-shell neutron and proton Fermi surfaces, reinforced by the rotation alignment behavior of both nucleon types. Configuration-constrained calculations predict the coexistence of well-deformed prolate and oblate multiquasiparticle ~isomeric! states.