June 25, 2015 Preprint Open Access

Low-Lying Structure of ⁵⁰Ar and the N = 32 Subshell Closure

Steppenbeck, D.; et al.

The low-lying structure of the neutron-rich nucleus 50Ar has been investigated at the Radioactive Isotope Beam Factory using in-beam γ-ray spectroscopy with 9Be (54Ca, 50Ar+γ) X,9Be(55Sc,50Ar+γ)X, and 9Be(56Ti, 50Ar + γ)X multinucleon removal reactions at ∼220 MeV/u. A γ-ray peak at 1178(18) keV is reported and assigned as the transition from the first 2+ state to the 0+ ground state. A weaker, tentative line at 1582(38) keV is suggested as the 4+1→2+1 transition. The experimental results are compared to large-scale shell-model calculations performed in the sdpf model space using the SDPF-MU effective interaction with modifications based on recent experimental data for exotic calcium and potassium isotopes. The modified Hamiltonian provides a satisfactory description of the new experimental results for 50Ar and, more generally, reproduces the energy systematics of low-lying states in neutron-rich Ar isotopes rather well. The shell-model calculations indicate that the N=32 subshell gap in 50Ar is similar in magnitude to those in 52Ca and 54Ti and, notably, predict an N=34 subshell closure in 52Ar that is larger than the one recently reported in 54Ca.