  Small angle neutron scattering (SANS) is used to measure the density of heavy
water contained in 1-D cylindrical pores of mesoporous silica material
MCM-41-S-15, with pores of diameter of 15+-1 A. In these pores the homogenous
nucleation process of bulk water at 235 K does not occur and the liquid can be
supercooled down to at least 160 K. The analysis of SANS data allows us to
determine the absolute value of the density of D2O as a function of
temperature. We observe a density minimum at 210+-5 K with a value of
1.041+-0.003 g/cm3. We show that the results are consistent with the
predictions of molecular dynamics simulations of supercooled bulk water. This
is the first experimental report of the existence of the density minimum in
supercooled water.
