The empirical shell gap revisited in light of recent high precision mass spectrometry data

We present an updated view on the phenomenon of mutually enhanced magicity based on the current experimental knowledge of atomic masses, including some recent precision measurements performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. We discuss the trends of the proton and neutron empirical shell gaps computed either in the standard approach, along chains corresponding to magic numbers of protons and neutrons, respectively, or along the neighbouring isotonic chains differing by two nucleon numbers. We show that in the latter case the empirical shell-gap trend is anti-correlated to the one observed along the magic-number chains. We perform a theoretical investigation of the origin of this feature by an analysis of the contributions from the monopole, pairing and quadrupole interactions, focusing on the phenomenon of mutually enhanced magicity. We emphasize the role of quadrupole correlations for explaining the full range of experimental information.