Observation of mutually enhanced collectivity in self-conjugate 38 76 Sr 38

The lifetimes of the first 2${}^{+}$ states in the neutron-deficient ${}^{76,78}$Sr isotopes were measured using a unique combination of the $\\ensuremath{\\gamma}$-ray line-shape method and two-step nucleon exchange reactions at intermediate energies. The transition rates for the 2${}^{+}$ states were determined to be $B(\\mathrm{E}2;{2}^{+}\\ensuremath{\\rightarrow}{0}^{+})=2220(270)$ ${e}^{2}$fm${}^{4}$ for ${}^{76}$Sr and 1800(250) ${e}^{2}$fm${}^{4}$ for ${}^{78}$Sr, corresponding to large deformation of ${\\ensuremath{\\beta}}_{2}$ $=$ 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 $B(\\mathrm{E}2)$ 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.