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

The low-lying structure of the neutron-rich nucleus Ar50 has been investigated at the Radioactive Isotope Beam Factory using in-beam γ-ray spectroscopy with Be9(Ca54,Ar50+γ)X, Be9(Sc55,Ar50+γ)X, and Be9(Ti56,Ar50+γ)X multinucleon removal reactions at ∼220MeV/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 41+→21+ 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 Ar50 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 Ar50 is similar in magnitude to those in Ca52 and Ti54 and, notably, predict an N=34 subshell closure in Ar52 that is larger than the one recently reported in Ca54.