Speaker
Description
The evolution of shell structure around doubly-magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the ‘south-west’ region of $^{78}$Ni, the development of deformation might trigger a major shift in our understanding of explosive nucleosynthesis. To this end, new spectroscopic information on key close-lying nuclei is very valuable.
The isomeric and β decay of $^{75}$Co, with one proton- and two neutron-holes relative to $^{78}$Ni, were studied at the Radioactive Ion Beam Factory (RIBF) in the RIKEN Nishina Center. This very exotic nucleus was produced in in-flight fission reactions of $^{238}$U (@ 345 MeV/nucleon) and subsequently studied exploiting the state-of-the-art BigRIPS [1] and EURICA [2] setups.
In this contribution I will discuss the partial β-decay spectra for $^{75}$Ni and $^{74}$Ni obtained from these data, and report a new isomeric transition in $^{75}$Co [3]. The coexistence of spherical and deformed shapes in the region immediately below $^{78}$Ni will be discussed in terms of PFSDG-U shell-model calculations, which reproduce well the states observed in $^{75}$Ni, but predict the first 1/2$^{-}$ level, the prolate K = 1/2 bandhead, 1 MeV below the observed (1/2$^{-}$) state in $^{75}$Co.
References:
[1] N. Fukuda et al., Nucl. Instrum. Methods 317, 323 (2013)
[2] P.-A. Söderström et al., Nucl. Instrum. Methods 317, 649 (2013)
[3] S. Escrig et al., arXiv:2101.06246, nucl-ex (2021)
