Onset of collectivity for argon isotopes close to N=32

The shell closure at $N=32$ has been investigated by a first spectroscopy of the $N=31$ nucleus ⁴⁹Ar at the Radioactive Isotope Beam Factory. Using the ⁵⁰Ar(p,pn) reaction channel in inverse kinematics, ⁵⁰Ar projectiles at 217 MeV/nucleon impinged on a 150 mm long liquid hydrogen target, part of the MINOS device. Prompt deexcitation γ rays were measured with the NaI(Tl) array DALI2⁺. Reaction products were analyzed with the SAMURAI spectrometer, which allowed the measurement of the momentum distributions and angular momentum transfer. Data were compared to state-of-the-art theoretical predictions, including shell-model, energy-density functional, and ab initio calculations. An onset of collectivity is suggested besides the spherical configuration typical of a closed shell nucleus, such as for ⁵²Ca.