Evidence for Z=6 `magic number' in neutron-rich carbon isotopes

TL;DR

This paper provides experimental and theoretical evidence for a proton magic number at Z=6 in neutron-rich carbon isotopes, highlighting the role of spin-orbit interactions and advanced nuclear forces in nuclear structure.

Contribution

It presents the first experimental confirmation of Z=6 as a magic number in light, neutron-rich nuclei and demonstrates that modern ab initio calculations can accurately reproduce these features.

Findings

Evidence of Z=6 subshell closure in 13-20C

Agreement between experimental data and ab initio calculations

Insights into the role of chiral nuclear forces in nuclear structure

Abstract

The nuclear shell structure, which originates in the nearly independent motion of nucleons in an average potential, provides an important guide for our understanding of nuclear structure and the underlying nuclear forces. Its most remarkable fingerprint is the existence of the so-called `magic numbers' of protons and neutrons associated with extra stability. Although the introduction of a phenomenological spin-orbit (SO) coupling force in 1949 helped explain the nuclear magic numbers, its origins are still open questions. Here, we present experimental evidence for the smallest SO-originated magic number (subshell closure) at the proton number 6 in 13-20C obtained from systematic analysis of point-proton distribution radii, electromagnetic transition rates and atomic masses of light nuclei. Performing ab initio calculations on 14,15C, we show that the observed proton distribution radii and subshell closure can be explained by the state-of-the-art nuclear theory with chiral nucleon-nucleon and three-nucleon forces, which are rooted in the quantum chromodynamics.