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

Abstract

We report the measurement of reaction cross sections ($\ddot{I}{ƒ}_R^{\mathrm{ex}}$) of ${}^{27,29}$F with a carbon target at RIKEN. The unexpectedly large $\ddot{I}{ƒ}_R^{\mathrm{ex}}$ 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.

8 pages, 4 figures