  We have investigated the diffuse reflection probabilities of Replica guides
for ultra-cold neutrons (UCN) using the so-called helium method. For the first
time we could establish a consistent description of the diffuse reflection
mechanism for different lengths of the guide system. The transmission of the
guides is measured depending on the helium pressure inside of the guides. A
series of simulations was done to reproduce the experimental data. These
simulations showed that a diffuse reflection probability of $d = (3.0 \pm 0.5)
\cdot 10^{-2}$ sufficiently describes the experimental data.
