Strong dineutron correlation in ⁸He and ¹⁸C

We study the spatial structure of four valence neutrons in the ground state of ⁸He and ¹⁸C nuclei using a core+4n model. For this purpose, we employ a density-dependent contact interaction among the valence neutrons, and solve the five-body Hamiltonian in the Hartree-Fock-Bogoliubov (HFB) approximation. We show that two neutrons with the coupled spin of S=0 exhibit a strong dineutron correlation around the surface of these nuclei, whereas the correlation between the two dineutrons is much weaker. Our calculation indicates that the probability of the (1p_3/2)⁴ and [(1p_3/2)²(p_1/2)²] configurations in the ground state wave function of ⁸He nucleus is 34.9% and 23.7%, respectively. This is consistent with the recent experimental finding with the ⁸He(p,t)⁶He reaction, that is, the ground state wave function of ⁸He deviates significantly from the pure (1p_3/2)⁴ structure.