Shape evolution in self-conjugate nuclei, and the transitional nucleus 68Se

We report on the first measurement of the absolute transition strength B(E-2; 0(1)(+) -> 2(1)(+)) in the self-conjugate nucleus Se-68. It is found that the 0(1)(+) -> 2(1)(+) transition displays a strength similar to that for the triaxial Ge-64 nucleus, in sharp contrast to the much stronger collectivity observed for the oblate Kr-72 nucleus. Shape evolution along the N = Z line from zinc to strontium is analyzed through beyond-mean-field calculations using the D1S force. Over this narrow mass region the nuclear structure shows clear-cut transition from prolate to oblate and back to prolate, with Se-68 standing as pivotal between triaxial and oblate deformations.