Analysis of a method for extracting angularly collimated UCNs from a volume without losing the density inside

In order to extract ultracold neutrons (UCNs) with a restricted range of vertical velocity components from a super-thermal UCN source without significantly reducing UCN density inside, we propose to use a horizontal escape slit with a mirror on the bottom and a rough ceiling. The behaviour of UCNs in such a slit is modelled, using a Monte Carlo simulation both to check its feasibility for this method and to determine the characteristic parameters required. The slit is found to perform well and a good choice of the parameters is presented.