Two-neutron correlations in microscopic wave functions of 6He, 8He, and 12C

Two-neutron densities obtained from microscopic wave functions of He-6 and He-8 are investigated to reveal dineutron correlations. In particular, the comparison of the two-neutron density with the product of one-neutron densities is useful for a quantitative discussion of dineutron correlations. The calculations show that the S = 0 spatial two-neutron correlation increases at the surface of He-6(0(1)(+)) and He-8(0(2)(+)). The enhancement is remarkable in the He-6(0(1)(+)) ground state but not as prominent in the He-8(0(1)(+)) ground state. Configuration mixing of many Slater determinants is essential to describe the dineutron correlations. Two-neutron densities in C-12 wave functions with a-cluster structures are also studied.