  The extraction efficiency of ultracold neutrons from cryogenic moderators
depends critically on the neutron transparency of the moderator material.
  The Monte Carlo simulation of the probability of the UCN going out from
non-ideal (disordered) solid deuterium crystals has been performed.
  It was based on the use of the correlation function describing the density
fluctuations in a disordered material, the latter being inferred from the
measured very low neutron energy total cross sections for this material.
