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

The low-lying structure of Sc55 has been investigated using in-beam γ-ray spectroscopy with the Be9(Ti56,Sc55+γ)X one-proton removal and Be9(Sc55,Sc55+γ)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 γγ coincidence relationships and γ-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 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 1/21+ and 3/21+ 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 πf7/2 orbital.