[en] The purpose of this paper is to obtain the flight range of Th₂₃₄ atoms which are recoil atoms due to the alpha decay of uranium oxide. The thin sources of uranium were produced by electrodeposition method. Enriched, natural and depleted uranium sources were provided. Recoil atoms from the sources were collected with aluminium plates, and the collected activities were measured with a low background beta counter. The decay of Th₂₃₄ was observed, and effective collection efficiency (Ft-value) from the uranium sources with various thickness was estimated. The flight range of Th₂₃₄ in uranium oxide was able to be obtained experimentally. The experimental values were compared with the values calculated by the LSS formula. Agreement between the experimental and the theoretical values was good. Then, mica was used as the collector plates to observe alpha recoil tracks in the mica. The recoil tracks were observed, but some discrepancy between the observed and estimated densities of tracks was seen. Further studies are in progress. (Kato, T.)