Evolution of collectivity in 72Kr: evidence for rapid shape transition.

The transition rates from the yrast 2+ and 4+ states in the self-conjugate 72Kr nucleus were studied via lifetime measurements employing the GRETINA array with a novel application of the recoil-distance method. The large collectivity observed for the 4+→2+ transition suggests a prolate character of the excited states. The reduced collectivity previously reported for the 2+→0+ transition was confirmed. The irregular behavior of collectivity points to the occurrence of a rapid oblate-prolate shape transition in 72Kr, providing stringent tests for advanced theories to describe the shape coexistence and its evolution.