We study the spatial structure of four valence neutrons in the ground state of He8 and C18 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 (1p3/2)4 and [(1p3/2)2(p1/2)2] configurations in the ground state wave function of He8 nucleus is 34.9% and 23.7%, respectively. This is consistent with the recent experimental finding with the He8(p,t)He6 reaction, that is, the ground state wave function of He8 deviates significantly from the pure (1p3/2)4 structure.
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