Aluminum is commonly used as vacuum separation foil in neutron guides and detectors. Due to its low surface potential neutrons with angular velocities down to 3m/s are passing the foil. However, according to Steyerl  neutron transmission is also affected by the surface roughness of the passed foil. The transmission decreases towards slow neutrons with ~1/v², .
Hence many neutron experiments mostly need the slow neutrons I studied this behavior as side project in my thesis. Therefore, I have developed polishing techniques to achieve roughnesses for aluminium foils and neutron guides of Ra = (0.04 ± 0.01) μm and Rz = (0.10 ± 0.02) μm Steyerl, Effect of surface roughness on the total reflexion and transmission of slow neutrons. Zeitschrift für Physik, 254:169–188, April 1972