# Neutron skin and signature of the $N$ = 14 shell gap found from measured proton radii of $^{17-22}$N

## TL;DR

This study measures proton radii of nitrogen isotopes to reveal a thick neutron skin and evidence of an N=14 shell gap, highlighting the role of tensor interactions and comparing experimental data with advanced nuclear models.

## Contribution

First measurement of proton radii for nitrogen isotopes, revealing a neutron skin and shell gap at N=14, supported by shell model and ab initio calculations.

## Key findings

- Thick neutron skin observed in nitrogen isotopes.
- Signatures of neutron halo effects in 22N.
- Shell gap at N=14 supported by experimental and theoretical data.

## Abstract

A thick neutron skin emerges from the first determination of root mean square radii of the proton distributions for $^{17-22}$N from charge changing cross section measurements around 900$A$ MeV at GSI. Neutron halo effects are signaled for $^{22}$N from an increase in the proton and matter radii. The radii suggest an unconventional shell gap at $N$ = 14 arising from the attractive proton-neutron tensor interaction, in good agreement with shell model calculations. $Ab$ $initio$, in-medium similarity re-normalization group, calculations with a state-of-the-art chiral nucleon-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.