  The values of the antineutron-nucleus scattering lengths, and in particular
their imaginary parts, are needed to evaluate the feasibility of using neutron
mirrors in laboratory experiments to search for neutron-antineutron
oscillations. We analyze existing experimental and theoretical constraints on
these values with emphasis on low $A$ nuclei and use the results to suggest
materials for the neutron/antineutron guide and to evaluate the systematic
uncertainties in estimating the neutron-antineutron oscillation time. As an
example we discuss a scenario for a future neutron-antineutron oscillation
experiment proposed for the European Spallation Source. We also suggest future
experiments which can provide a better determination of the values of
antineutron-nuclei scattering lengths.
