A method for evaluating the transmission properties of ultracold-neutron guides

A method for evaluating the properties of guides for ultracold neutrons (UCN) has been developed as an extension of the common technique for measuring UCN losses in the sample contained between two diaphragms. With two measurements at both ends of the guide one can evaluate both, the loss probability μ per wall collision and the probability f for diffuse scattering from the wall. A 3.4 m long aluminum tube coated with a double layer of stainless steel (thickness 500 nm) and beryllium (thickness 250 nm), proposed as a UCN guide for the Mini-D2 UCN source at the FRM-II reactor at Garching, was tested. We obtained μ=(1.1±0.1)×10-3 and f=0.18±0.04. Systematic errors and a possible improvement of the method are discussed.