Transmission measurements of guides for ultra-cold neutrons using UCN capture activation analysis of vanadium

The efficient transport of ultra-cold neutrons (UCN) from their source to the experimental site is a major issue for various kinds of precision experiments. Neutron guides often have to transport the UCN several tens of meters with acceptable losses. In order to qualify the guides, their UCN transmission properties have to be determined with high precision. For this purpose we have developed a novel method. The transmitted UCN were absorbed at the end of the guide in a vanadium disc producing the β-emitter 52 V (half life 3.74 min). The intensity of the 1434 keV γ-ray following the β-decay was measured. UCN guides of non-magnetic nickel alloys made by the replication technique were studied. They show a high Fermi pseudopotential VF for UCN and a low surface roughness. For these guides the transmission per meter was determined with a relative error of ±0.6%, resulting in values above 0.95/m. By an absolute calibration of the γ-ray detection system we also deduced the absolute value of the UCN current absorbed in the vanadium plate. Possible applications of this method are discussed.