UCN transport simulation in solid deuterium crystals
  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. 
Yu. N. Pokotilovski
nucl-ex