Neutron skin and signature of the N = 14 shell gap found from measured proton radii of [superscript 17−22] N

Abstract

A thick neutron skin emerges from the first determination of root mean square radii of the proton distributions for ^17&minus;22N from charge changing cross section measurements around 900A MeV at GSI. Neutron halo effects are signalled for ²²N from an increase in the proton and matter radii. The radii suggest an unconventional shell gap at <em>N</em> = 14 arising from the attractive proton&ndash;neutron tensor interaction, in good agreement with shell model calculations. <em>Ab initio</em>, in-medium similarity re-normalization group, calculations with a state-of-the-art chiral nucleon&ndash;nucleon and three-nucleon interaction reproduce well the data approaching the neutron drip-line isotopes but are challenged in explaining the complete isotopic trend of the radii.