  We analyze spatial structure of the neutron Cooper pair in superfluid
low-density uniform matter by means of BCS calculations employing a bare force
and the effective Gogny interaction. It is shown that the Cooper pair exhibits
a strong spatial di-neutron correlation in a wide range of neutron density
$\rho/\rho_0\approx 10^{-4}-0.5$. This feature is related to the crossover
behavior between the pairing of the weak coupling BCS type and the
Bose-Einstein condensation of bound neutron pairs. We also show that the
zero-range delta interaction can describe the spatial structure of the neutron
Cooper pair if the density dependent interaction strength and the cut-off
energy are appropriately chosen. Parameterizations of the density-dependent
delta interaction satisfying this condition are discussed.
