Long-lived K isomer and enhanced gamma vibration in the neutron-rich nucleus Dy-172: Collectivity beyond double midshell

摘要

The level structure of Dy-172 has been investigated for the first time by means of decay spectroscopy following in-flight fission of a U-238 beam. A long-lived isomeric state with T-1/2 = 0.71(5) s and K-pi = 8(-) has been identified at 1278 keV, which decays to the ground-state and gamma-vibrational bands through hindered electromagnetic transitions, as well as to the daughter nucleus Ho-172 via allowed beta decays The robust nature of the K-pi = 8(-) isomer and the ground-state rotational band reveals an axially-symmetric structure for this nucleus. Meanwhile, the gamma-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 non-axial collectivity in this doubly mid-shell region. The underlying mechanism of enhanced gamma vibration is discussed in comparison with the deformed Quasiparticle Random-Phase Approximation based on a Skyrme energy-density functional. (C) 2016 The Author(s). Published by Elsevier B.V.