RETARDATION OF B(E2,01+-]21+) RATES IN SR-90 96 AND STRONG SUBSHELL CLOSURE EFFECTS IN THE A-APPROXIMATELY-100 REGION

Lifetimes of low-lying levels in Sr-90, Sr-92, Sr-94, Sr-96 were measured using a recently developed beta-gamma-gamma fast timing method. The deduced B(E2; 0(1)+ --> 2(1)+) rates which fill the N = 52-58 gap in the known B(E2) strengths for Sr-78-100, are exceptionally low, approximately 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 Sr-88-96 and Zr-90-98 nuclei, which form a region of the lowest B(E2) values (for nuclei with A > 56) second only to Pb-204-210. 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 Q(0) 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 is suggests a "phase change" in Sr collectivity.