Studies of the N=60 shape transition; states in ⁹⁸Sr populated via β-decay

The evolution of ground-state shapes usually proceeds smoothly, however for Sr and Zr nuclei at N=60 there is an abrupt shape transition. The dramatic onset of deformation in ¹⁰⁰Zr was recently well reproduced in state-of-the-art Monte Carlo Shell Model calculations [1], which also predict that the same deformed configuration may coexist at higher excitation energies in the lighter Zr isotopes. The excited state spectrum of ⁹⁸Sr possesses very low-lying 2₁⁺ and 0₂⁺ states, just as in ¹⁰⁰Zr (see, e.g., Ref. [2]). Unfortunately, information on higher-lying configurations in ⁹⁸Sr is sparse, and, for example, there are no firmly assigned 0⁺ states above the 0₂⁺ level. In an experiment at the TRIUMF-ISAC facility, we have studied the β-decay of ⁹⁸Rb to ⁹⁸Sr. The high-statistics of the data obtained permitted the determination of branching ratios for very weak transitions important for assigning band structures. Additionally, γ-γ angular correlation measurements were used for both spin assignments and mixing ratios. Firm spin assignments have been made for additional excited 0⁺ and 2⁺ states, with the 0₃⁺ band suggested up to spin 4. These results will be placed in the context of the systematics [2] in the Sr isotopes, and the similarities in the structures of ⁹⁸Sr and ¹⁰⁰Zr will be discussed.

[1] T. Togashi, Y. Tsunoda, T. Otsuka, and N. Shimizu, Phys. Rev. Lett. 117, 172502 (2016)

[2] P.E. Garrett, M. Zielinska, and E. Clément, Prog. Part. Nucl. Phys. 124, 103931 (2022)