The first $\gamma$-ray spectroscopy of ${}^{52}\mathrm{Ar}$, with the neutron number $N=34$, was measured using the ${}^{53}\mathrm{K}(p,2p)$ one-proton removal reaction at $\sim 210\mathrm{MeV}/\mathrm{u}$ 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 ${}^{54}\mathrm{Ca}$, 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 ${}^{52}\mathrm{Ar}$.

Exclusive cross sections and momentum distributions have been measured for quasifree one-neutron knockout reactions from a ${}^{54}\mathrm{Ca}$ beam striking on a liquid hydrogen target at $\sim 200\mathrm{MeV}/u$. A significantly larger cross section to the $p_{3/2}$ state compared to the $f_{5/2}$ state observed in the excitation of ${}^{53}\mathrm{Ca}$ provides direct evidence for the nature of the $N=34$ shell closure. This finding corroborates the arising of a new shell closure in neutron-rich calcium isotopes. The distorted-wave impulse approximation reaction formalism with shell model calculations using the effective GXPF1Bs interaction and ab initio calculations concur our experimental findings. Obtained transverse and parallel momentum distributions demonstrate the sensitivity of quasifree one-neutron knockout in inverse kinematics on a thick liquid hydrogen target with the reaction vertex reconstructed to final state spin-parity assignments.

We report on the first $\gamma$-ray spectroscopy of low-lying states in neutron-rich ${}^{98,100}\mathrm{Kr}$ isotopes obtained from ${}^{99,101}\mathrm{Rb}(p,2p)$ reactions at $\sim 220\mathrm{MeV}/\mathrm{nucleon}$. A reduction of the $2_1^{+}$ state energies beyond $N=60$ demonstrates a significant increase of deformation, shifted in neutron number compared to the sharper transition observed in strontium and zirconium isotopes. State-of-the-art beyond-mean-field calculations using the Gogny D1S interaction predict level energies in good agreement with experimental results. The identification of a low-lying ($0_2^{+}$, $2_2^{+}$) state in ${}^{98}\mathrm{Kr}$ provides the first experimental evidence of a competing configuration at low energy in neutron-rich krypton isotopes consistent with the oblate-prolate shape coexistence picture suggested by theory.