Zakopane 2026 Conference on Nuclear Physics

The region near the $Z=28$ and $N=28$ shell closures provides a crucial testing ground for nuclear structure models. In particular, the neutron-deficient cobalt isotopic chain ($Z=27$) offers insights into the robustness of these magic numbers. At the $N=Z$ line, studies of the self-conjugate nucleus $^{54}$Co ($N=Z=27$) and its isomer provide essential information to investigate proton-neutron pairing correlations. Theoretical models predict a significant change in the mean-square charge radius for the ground state of self-conjugate nuclei compared to the isomeric state. While experiments on $^{38}$K, $^{42}$Sc, and $^{50}$Mn confirm this general trend, current models still underestimate the actual size of the effect, which highlights the need for new experimental data [1]. Additionally, precise nuclear charge radii for the superallowed $\beta$-emitter $^{54}$Co can be used to reduce uncertainties in $\mathcal{F}t$ value calculations, contributing to tests of the Standard Model and the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix [2].

To resolve this lack of experimental data, high-resolution collinear laser spectroscopy on neutron-deficient Co isotopes was performed at the IGISOL facility, University of Jyväskylä, Finland. This experimental approach allows the determination of ground- and isomeric nuclear properties by measuring the isotope shift and hyperfine structure of electronic transitions [3]. These measurements will improve our understanding of structural evolution across the $N=28$ shell closure, ultimately paving the way towards future campaigns targeting the proton emitter $^{53}$Co. This contribution provides an overview of these measurements, focusing on the extracted magnetic moments and isotope shifts. Finally, the first empirical estimates of the mean-square charge radii, obtained by comparing the cobalt data to regional systematic trends, will be presented.

References
[1] Á. Koszorús et al., "Proton-neutron pairing correlations in the self-conjugate nucleus $^{42}$Sc," Physics Letters B, 819, 136439 (2021).
[2] J. C. Hardy and I. S. Towner, "Superallowed $0^+ \rightarrow 0^+$ nuclear $\beta$ decays: 2014 critical survey, with precise results for $V_{ud}$ and CKM unitarity," Phys. Rev. C, 91, 025501 (2015).
[3] Á. Koszorús et al., "Nuclear structure studies by collinear laser spectroscopy," The European Physical Journal A, 60(1), 20 (2024).