  (short version) Accurate determination of the coherent static structure
factor of disordered materials containing proton nuclei is prohibitively
difficult by neutron diffraction, due to the large incoherent cross section of
$^1$H. This notorious problem has set severe obstacles to the structure
determination of hydrogenous materials up to now, via introducing large
uncertainties into neutron diffraction data processing. Here we present the
first accurate separate measurements, using polarized neutron diffraction, of
the coherent and incoherent contributions to the total static structure factor
of 5 mixtures of light and heavy water, over an unprecedentedly wide momentum
transfer range. The structure factors of H$_2$O and D$_2$O mixtures derived in
this work may signify the beginning of a new era in the structure determination
of hydrogenous materials, using neutron diffraction.
