Pairing Forces Govern Population of Doubly Magic 54Ca from Direct Reactions

Direct proton-knockout reactions of 55Sc at ∼ 220 MeV / nucleon were studied at the RIKEN Radioactive Isotope Beam Factory. Populated states of 54Ca were investigated through γ -ray and invariant-mass spectroscopy. Level energies were calculated from the nuclear shell model employing a phenomenological internucleon interaction. Theoretical cross sections to states were calculated from distorted-wave impulse approximation estimates multiplied by the shell model spectroscopic factors, which describe the wave function overlap of the 55Sc ground state with states in 54Ca . Despite the calculations showing a significant amplitude of excited neutron configurations in the ground-state of 55Sc , valence proton removals populated predominantly the ground state of 54Ca . This counterintuitive result is attributed to pairing effects leading to a dominance of the ground-state spectroscopic factor. Owing to the ubiquity of the pairing interaction, this argument should be generally applicable to direct knockout reactions from odd-even to even-even nuclei.