Structure of 55Sc and development of the N=34 subshell closure

Abstract

The low-lying structure of ${}^{55}$Sc has been investigated using in-beam $\gamma$-ray spectroscopy with the ${}^9$Be(${}^{56}$Ti,${}^{55}$Sc+$\gamma$)$X$ one-proton removal and ${}^9$Be(${}^{55}$Sc,${}^{55}$Sc+$\gamma$)$X$ inelastic-scattering reactions at the RIKEN Radioactive Isotope Beam Factory. Transitions with energies of 572(4), 695(5), 1539(10), 1730(20), 1854(27), 2091(19), 2452(26), and 3241(39) keV are reported, and a level scheme has been constructed using $\gamma \gamma$ coincidence relationships and $\gamma$-ray relative intensities. The results are compared to large-scale shell-model calculations in the $sd$-$pf$ model space, which account for positive-parity states from proton-hole cross-shell excitations, and to it ab initio shell-model calculations from the in-medium similarity renormalization group that includes three-nucleon forces explicitly. The results of proton-removal reaction theory with the eikonal model approach were adopted to aid identification of positive-parity states in the level scheme; experimental counterparts of theoretical $1/2_1^{+}$ and $3/2_1^{+}$ states are suggested from measured decay patterns. The energy of the first $3/2^{-}$ state, which is sensitive to the neutron shell gap at the Fermi surface, was determined. The result indicates a rapid weakening of the $N=34$ subshell closure in $pf$-shell nuclei at $Z>20$, even when only a single proton occupies the $\pi f_{7/2}$ orbital.

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• On the robustness of sub-shell closures: A high angular momentum analysis of the titanium isotopes
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