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A separation foil is a crucial component in superthermal ultracold neutron (UCN) sources. Its primary function is to prevent contamination from external gases. Formvar, owing to its ability to form extremely thin and mechanically stable films, enables UCN to pass via quantum tunneling and thus represents a promising candidate for next-generation separation foils.
In this work, Formvar foils with thicknesses ranging from 8 to 323 nm were fabricated. The neutron optical potential of Formvar was characterized by neutron reflectometry using the D17 instrument at ILL. Preliminary analysis indicates a neutron optical potential of approximately 22 neV. UCN transmission measurements were performed at ILL PF2. Under identical conditions, the 22 nm Formvar foil achieved a transmission of approximately 93%, compared compared with 58% for a 14 μm-thick aluminum foil.These results demonstrate the potential of ultrathin Formvar films as high-transmission separation foils for superthermal UCN sources.