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. A 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 a 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.