Two-Neutron Halo is Unveiled in F 29

Abstract

We report the measurement of reaction cross sections (σRex) of F27,29 with a carbon target at RIKEN. The unexpectedly large σRex and derived matter radius identify F29 as the heaviest two-neutron Borromean halo to date. The halo is attributed to neutrons occupying the 2p3/2 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 F27 but are challenged for F29.