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The lifetimes of the first 2 + states in the neutron-deficient 76,78 Sr isotopes were measured using a unique combination of the γ-ray line-shape method and two-step nucleon exchange reactions at intermediate energies. The transition rates for the 2 + states were determined to be B(E2;2 + →0 + ) = 2220(270) e 2 fm 4 for 76 Sr and 1800(250) e 2 fm 4 for 78 Sr, corresponding to large deformation of β2 = 0.45(3) for 76 Sr and 0.40(3) for 78 Sr. The present data provide experimental evidence for mutually enhanced collectivity that occurs at N = Z = 38. The systematic behavior of the excitation energies and B(E2) values indicates a signature of shape coexistence in 76 Sr, characterizing 76 Sr as one of most deformed nuclei with an unusually reduced E(4 + )/E(2 + ) ratio.

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.