A two-neutron halo is unveiled in $^{29}$F

TL;DR

This study reveals that $^{29}$F exhibits a two-neutron halo structure, making it the heaviest known Borromean halo nucleus, challenging existing nuclear models and highlighting the role of neutron orbital occupation.

Contribution

The paper provides the first experimental evidence of a two-neutron halo in $^{29}$F and combines experimental data with advanced theoretical calculations to explain its structure.

Findings

$^{29}$F has an unexpectedly large reaction cross section.

$^{29}$F is identified as the heaviest two-neutron Borromean halo.

Shell model calculations support the halo interpretation.

Abstract

We report the measurement of reaction cross sections ($\sigma_R^{\rm ex}$) of $^{27,29}$F with a carbon target at RIKEN. The unexpectedly large $\sigma_R^{\rm ex}$ and derived matter radius identify $^{29}$F as the heaviest two-neutron Borromean halo to date. The halo is attributed to neutrons occupying the $2p_{3/2}$ orbital, thereby vanishing the shell closure associated with the neutron number $N = 20$. The results are explained by state-of-the-art shell model calculations. Coupled-cluster computations based on effective field theories of the strong nuclear force describe the matter radius of $^{27}$F but are challenged for $^{29}$F.