Abstract
The strength of the $N$ = 28 magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of $^{46-48}$Ar, performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. The new mass values are up to 90 times more precise than previous measurements. While they suggest the persistence of the $N$ = 28 shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth of spectroscopic data and theoretical investigations performed with the \emph{SDPF-U} phenomenological shell model interaction. Our results are also compared with \emph{ab initio} calculations using the Valence Space In-Medium Similarity Renormalization Group and the Self-Consistent Green's Function approaches. Both calculations provide a very good account of mass systematics at and around $Z$ = 18 and, generally, a consistent description of the physics in this region. This combined analysis indicates that $^{46}$Ar is the transition between the closed-shell $^{48}$Ca and collective $^{44}$S.
Highlights
Just as the experimental evidence for “magic” proton and neutron numbers was instrumental for laying a basic foundation of nuclear theory [1,2], the observation of their demise in exotic nuclear systems [3] was pivotal for the establishment of the modern understanding of nuclear structure and the mechanisms driving its evolution far from β stability [4,5,6]
The new mass values are up to 90 times more precise than previous measurements. While they suggest the persistence of the N = 28 shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth of spectroscopic data and theoretical investigations performed with the SDPF-U phenomenological shell model interaction
The disappearance of the magic N = 20 shell closure was likewise evidenced through mass measurements of exotic sodium (Z = 11) isotopes [3], for which the binding energy normally reduced beyond a shell closure was found to increase due to deformation
Summary
Just as the experimental evidence for “magic” proton and neutron numbers was instrumental for laying a basic foundation of nuclear theory [1,2], the observation of their demise in exotic nuclear systems [3] was pivotal for the establishment of the modern understanding of nuclear structure and the mechanisms driving its evolution far from β stability [4,5,6]. Open-shell medium-mass nuclei provide important benchmarks for rapidly developing nuclear ab initio methods and modern theories of nuclear interactions based on chiral effective field theory In this context argon isotopes offer a complementary picture to the calcium chain that constitutes a traditional testing ground. One such approach, the valencespace formulation of the in-medium similarity renormalization group (VS-IMSRG) [41,42,43,44,45], opened ab initio theories to essentially all nuclei accessible to the nuclear shell model, including fully open-shell exotic systems. We present results from SCGF calculations of open-shell isotopes around the calcium chain using the recently derived NN + 3N(lnl) chiral Hamiltonian [52]
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