Sub-shell closure and shape coexistence in the transitional nucleus Zr98

In the rapid shape change from spherical to deformed nuclei in the Z = Zr-40 isotopic chain, recent work has identified shape coexistence in Zr-96. Between Zr-96 and the strongly deformed Zr-100, Zr-98 is expected to also exhibit coexistence of nuclear shapes. The degree of mixing between different configurations is mainly determined by the nucleon-nucleon interactions. For nuclear model predictions, experimental constraints are needed, but they are barely available for Zr-98. To study low-lying transitions in Zr-98, a Coulomb excitation experiment was conducted at the Argonne Tandem-Linac Accelerator System (ATLAS) facility using a Zr-98 beam extracted from the Californium Rare Ion Breeder Upgrade (CARIBU) ion source and Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) for gamma-ray spectroscopy coupled to the compact heavy ion counter (CHICO2) for ion detection. This paper reports on the first decisive deduction of the B(E2;2(1)(+)-> 0(1)(+)) transition strength in Zr-98 and on its interpretation.