Two-neutron halo structure of F31 and a novel pairing antihalo effect

Background: A newly identified drip-line nucleus F-31 offers a unique opportunity to study the two-neutron (2n) correlation at the east shore of the island of inversion where the N = 28 shell closure is lost. Purpose: We aim to present the first three-body theoretical results for the radius and total reaction cross sections of F-31. This will further help to investigate how the pairing and the breakdown of the N = 28 shell closure influence the formation of the 2n-halo structure and the antihalo effect in this mass region. Methods: A F-29+n + n three-body system is described by the cluster orbital shell model, and its total reaction cross section is calculated by the Glauber theory. Results: Our three-body calculations predict 3.48-3.70 fm for the root-mean-square radius of F-31, which corresponds to the total reaction cross section of 1530 (1410) to 1640 (1500) mb for a carbon target at 240 (900) MeV/nucleon. The binding mechanism and halo formation in F-31 are discussed. Conclusions: The present study suggests a novel antihalo effect in this mass region: When the pairing overcomes the energy gap between the p(3/2) and f(7/2) orbits, the inversion of the occupation number of these orbits takes place, and it diminishes the 2n-halo structure.