A First Glimpse at the Shell Structure beyond $^{54}$Ca: Spectroscopy of $^{55}$K, $^{55}$Ca, and $^{57}$Ca

Abstract

States in the N = 35 and 37 isotopes 55,57 Ca have been populated by direct proton-induced nucleon removal reactions from 56,58 Sc and 56 Ca beams at the RIBF. In addition, the (p, 2p) quasi-free single-proton removal reaction from 56 Ca was studied. Excited states in 55 K, 55 Ca, and 57 Ca were established for the first time via in-beam γ-ray spectroscopy. Results for the proton and neutron removal reactions from 56 Ca to states in 55 K and 55 Ca for the level energies, excited state lifetimes, and exclusive cross sections agree well with state-of-the-art theoretical calculations using different approaches. The observation of a short-lived state in 57 Ca suggests a transition in the calcium isotopic chain from single-particle dominated states at N = 35 to collective excitations at N = 37.