How Robust is the N = 34 Subshell Closure? First Spectroscopy of Ar 52

The first γ-ray spectroscopy of 52Ar, with the neutron number N = 34, was measured using the 53K(p, 2p) one-proton removal reaction at ~210 MeV/u at the RIBF facility. The 2+1 excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with N > 20. This result is the first experimental signature of the persistence of the N = 34 subshell closure beyond 54Ca, i.e., below the magic proton number Z = 20. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured 2+1 systematics of neutron-rich Ar isotopes, and support a N = 34 subshell closure in 52Ar.