Speaker
Description
The region of neutron-rich nuclei around the $N=40$ subshell closure and the $Z=28$ proton shell is of particular interest for studying the evolution of nuclear structure, including shape coexistence, collectivity, and decay mechanisms far from stability. While $^{68}$Ni was initially considered a doubly magic nucleus, experimental studies of neighboring isotopes have revealed pronounced deformation effects and the coexistence of spherical, oblate, and prolate configurations. In this context, $\beta$ decay of cobalt isotopes provides a sensitive probe of these structural phenomena, as it populates a wide range of states in the daughter nuclei, including isomeric and neutron-unbound levels.
This work focuses on the $\beta$ decay of neutron-rich cobalt isotopes, in particular the two isomeric states in $^{68}$Co. The high-spin isomer ($J^{\pi}=7^{-}$, $T_{1/2}=0.23$ s) is associated with a $\pi f_{7/2}^{-1}$ configuration and is known to populate a limited set of high-spin states in $^{68}$Ni, de-exciting through a hindered $5^{-}$ isomer. In contrast, the low-spin isomer ($T_{1/2}=1.6$ s) has an uncertain spin assignment ($J^{\pi}=2^{+}$–$4^{+}$), and previous studies indicate that a significant fraction of the $\beta$-decay strength remains unobserved. This missing strength may correspond either to direct feeding of the $0^{+}$ ground state or to population of high-lying, possibly neutron-unbound states, which were not accessible with earlier experimental setups.
A key aspect of this study is the investigation of $\beta$-decay strength distribution and the competition between $\gamma$-ray emission and neutron emission above the neutron separation energy. Recent observations of $\gamma$ rays emitted from highly excited states challenge the conventional statistical model of $\beta$-delayed neutron emission, which assumes a compound nucleus with no memory of its formation. The coexistence of $\gamma$ decay and neutron emission, as well as discrepancies between neighboring isotopes, indicate that additional structural effects, such as reduced spectroscopic overlap, may play an important role.
Experimentally, the study was performed at the Facility for Rare Isotope Beams (FRIB) at Michigan State University using the FRIB Decay Station Initiator (FDSI). Neutron-rich cobalt isotopes were produced via in-flight fragmentation and delivered to a setup combining high-resolution $\gamma$-ray spectroscopy (HPGe and LaBr$_3$(Ce) detectors), the VANDLE neutron time-of-flight array, and the Modular Total Absorption Spectrometer (MTAS). This configuration enables high-efficiency detection of both $\gamma$ rays and neutrons, allowing for a comprehensive reconstruction of decay schemes and a reduction of the Pandemonium effect.
The work is in progress and expected results will provide a detailed characterization of $\beta$-decay feeding in $^{68}$Co, clarify the nature of its isomeric states, and contribute to resolving current discrepancies in the description of neutron–$\gamma$ competition.