Two-Neutron Halo is Unveiled in 29F

We report the measurement of reaction cross sections (sigma(ex)(R)) of F-27,F-29 with a carbon target at RIKEN. The unexpectedly large sigma(ex)(R) and derived matter radius identify F-29 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 F-27 but are challenged for F-29.