Recent results from EURICA at RIBF: decay spectroscopy of ¹⁷²Dy and ⁷⁰Co

Gamma-ray spectroscopy following the β decay is an effective tool for probing low-lying yrast and non-yrast states, from which key information on nuclear structure, such as shape transitions/coexistence and single-particle orbits, can be obtained. For the study of rare isotopes, especially when the nucleus of interest lies at the boundaries of availability for spectroscopic studies, isomeric decays are likely to be a more useful means than β decays to populate excited levels. The combined β-γ and isomeric-decay measurements at RIBF, which has the capability of providing the world's strongest RI (radioactive ion) beams, are at the forefront of exploration of exotic nuclei far from stability. Research opportunities for decay spectroscopy at RIBF have been expanded in the EURICA (EUROBALL-RIKEN Cluster Array) project. The main body of EURICA consists of 12 Cluster-type HPGe detectors, surrounding a highly segmented silicon stopper system named WAS3ABi. In addition to the normal EURICA setup, 18 LaBr₃(Ce) detectors and plastic scintillators are installed for the fast-timing measurement of γ and β rays, respectively. A wide range of unstable nuclei on the Segre chart are within the scope of the EURICA project. In this presentation, two recent topics from EURICA will be introduced. First, low-lying states of ¹⁷²Dy have been populated following the decay from a newly identified K^π = 8^- isomer. While the robust nature of the K isomer and the ground-state rotational band reveals an axially-symmetric structure for this nucleus, the γ- vibrational levels have been identified at unusually low excitation energy compared to the neighboring well-deformed nuclei, indicating the significance of the microscopic effect on the nonaxial collectivity in this doubly mid-shell region. Secondly, the level properties of ⁷⁰Co have been investigated by the β decay from ⁷⁰Fe and towards ⁷⁰Ni. The results are compared to the advanced Monte Carlo shell-model calculations, which predict strongly prolate-deformed shapes for the low-lying levels in ⁷⁰Co, including its ground state. It is revealed that the decay patterns in the neutron-rich A = 70 isobars from the new island of inversion to the Z = 28 closed-shell regime are governed significantly by the selectivity of nuclear shape. This document is composed of an abstract and the slides of the presentation. (author)