Orateur
Description
Beta-decay studies provide a powerful and selective tool for investigating low-spin states in atomic nuclei, offering information that is complementary to, and often extends, that obtained from direct reaction studies.
In this work, we present a study of low-spin excited states in $^{46}$K, populated via the $\beta$ decay from the $0^{+}$ ground state of $^{46}$Ar. The nucleus $^{46}$K ($Z=19$, $N=27$) lies in the vicinity of the $Z=20$ and $N=28$ shell closures, a region that has attracted considerable attention due to the evolution of the proton shell gap between the $1d_{3/2}$ and $2s_{1/2}$ orbitals [1-3]. Recent transfer-reaction studies of the neighboring $^{46}$Ar [4] have shown that its ground state is largely characterized by a $\pi(2s_{1/2}^{0}1d_{3/2}^{4})$ configuration, making it a candidate for a bubble nucleus and for a possible new proton magic number at $Z=18$. In parallel, direct-reaction studies on potassium isotopes have explored the degree of mixing between proton configurations, driven by the near-degeneracy of of the $\pi(2s_{1/2})$ and $\pi(1d_{3/2})$ orbitals in this mass region, as well as cross-shell excitations through reactions such as $^{48}$Ca(d,$\alpha$),$^{48}$Ca(p,$^{3}$He) and $^{47}$K(d,t)$^{46}$K [5-7]. The latter reaction has been shown to preferentially populate states in $^{46}$K with a dominant $\pi(2s_{1/2}^{-1}1d_{3/2}^{3})$ character, reflecting the structure of the $^{47}$K ground state.
Within this framework, $\beta$-decay spectroscopy of $^{46}$K provides complementary and overlapping insights into its low-spin structure, populating states that are weakly or not observed in direct reactions while also probing configurations accessed by both mechanisms. Owing to the higher intensity of the radioactive ion beam and the use of the state-of-the-art AGATA spectrometer [8], an extended level scheme has been established compared to previous $\beta$-decay studies [9,10], together with new estimates of $\beta$-decay branching ratios. These results will provide additional constraints on spin and parity assignments and contribute to a more detailed characterization of the underlying nuclear wave functions. Combined with the additional data sets obtained at the GANIL facility--namely transfer reactions on $^{46}$Ar and $^{47}$K beams--this work allows us to establish a coherent picture of the proton shell evolution in the vicinity of the $Z=20$ and $N=28$ shell closures.
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[4] D. Brugnara, et al., arXiv preprint arXiv:2506.23228 (2025).
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