Retardation of B(E2;01+→21+) rates in 90-96Sr and strong subshell closure effects in the A ≅ 100 region

Lifetimes of low-lying levels in 90,92,94,96Sr were measured using a recently developed βγγ fast timing method. The deduced B(E2; 01+→21+) rates which fill the N=52-58 gap in the known B(E2) strengths for 78-100Sr, are exceptionally low, ≅ 8 W.u., and provide evidence for a strong Z=38 subshell effect extending from N=50 to N=58. Furthermore, these rates establish a close similarity between 88-96Sr and 90-98Zr nuclei, which form a region of the lowest B(E2) values (for nuclei with A>56) second only to 204-210Pb. The Sr B(E2) values are compared with predictions based on various global, regional and local systematics as well as the results of laser spectroscopic measurements. The discrepancy with the latter is explained as due to octupole vibrational collectivity. Unique to the Q0 systematics in the Sr region is a sudden shape change from spherical to deformed at N=60. This transformation is so remarkably large and abrupt for Sr nuclei that it suggests a 'phase change' in Sr collectivity. (orig.)