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

Background: A newly identified drip-line nucleus $^{31}\mathrm{F}$ 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 $^{31}\mathrm{F}$. 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 $^{29}\mathrm{F}+n+n$ 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 $^{31}\mathrm{F}$, 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 $^{31}\mathrm{F}$ 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.