Low-lying structures of exotic sc isotopes and the evolution of the N = 34 subshell closure

Recent investigations of exotic nuclei with N = 32 and 34 have highlighted the presence of sizable subshell closures at these neutron numbers that are absent in stable isotones. Indeed, the development of the shell gap at N = 32 is now well established from studies along the calcium, titanium, and chromium isotopic chains and, more recently, below the Z = 20 core in potassium and argon isotones. The onset of a new subshell closure at N = 34 was reported in ⁵⁴Ca owing to the relatively high energy of its first 2⁺ state. On the theoretical side, the development of these neutron subshell gaps has been discussed, for example, in the framework of tensor-force-driven shell evolution; as protons are removed from the πf₇₂ orbital, the vf5₂ state becomes progressively less bound and shifts up in energy relative to the vp3₂-vp1₂ spin-orbit partners. However, it was also reported that no significant N = 34 subshell gap exists in titanium, despite the fact that an inversion of the vf₅₂ and vp1₂ orbitals has been noted. Thus, the strength of the N = 34 subshell closure in the scandium isotopes, which lie between calcium and titanium, provides additional insight on the migration of the vf₅₂ orbital in exotic nuclei. In the present work, the low-lying structures of the neutron-rich isotopes ⁵⁴Sc, ⁵⁵Sc, and ⁵⁶Sc - investigated using in-beam γ-ray spectroscopy with fast radioactive projectiles - will be presented, and the evolution of the N = 34 subshell closure will be further examined. The results will be compared to modern shell-model calculations applied within the pf shell.