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

The first gamma-ray spectroscopy of Ar-52, with the neutron number N = 34, was measured using the K-53(p, 2p) one-proton removal reaction at similar to 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 Ca-54, 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 Ar-52.