  The formation of a dineutron in the nucleus $^{11}$Li is found to be
localized to the surface region. The experiment measured the intrinsic momentum
of the struck neutron in $^{11}$Li via the $(p,pn)$ knockout reaction at 246
MeV/nucleon. The correlation angle between the two neutrons is, for the first
time, measured as a function of the intrinsic neutron momentum. A comparison
with reaction calculations reveals the localization of the dineutron at
$r\sim3.6$ fm. The results also support the density dependence of dineutron
formation as deduced from Hartree-Fock-Bogoliubov calculations for nuclear
matter.
