The micro-estimation of uranium by the counting of alpha-particle tracks in photographic emulsions

Abstract

The microscopic examination of a Nuclear Research emulsion, after exposure to an alpha-particle source and subsequent development, reveals the presence of tracks consisting of closely-packed grains, which can be easily and efficiently counted at a magnification of 1000. The application of this Photographic Method to the quantitative estimation of low levels of alpha-particle activity is limited only by the amounts of alpha-particle emitting Impurities incorporated in the emulsion during manufacture, and by the fading of the latent track image. Attempts were made to estimate microgram amounts of uranium using this technique. The uptake of uranium from a uranyl acetate solution by the Ilford Type C2 Nuclear Research emulsion is influenced by the Solute concentration, the pH., the temperature and the time of immersion. Using controlled pHs., of 2 and 4, a standard immersion time of 30 minutes and an immersion bath temperature of 25.0ºc, it was found that the uptake of uranium was directly proportional to the. uranyl ion concentration for uranium concentrations between 10(^-3) g./ml. and 10(^-5) g./mi,, but at lower concentrations this was not the case. Consequently a more sensitive method was developed. The uranium to be estimated was deposited on the plane ground end of a glass rod, about 0.15 cm(^2) in cross-section, and this firmly clamped against a piece of emulsion for a measured time of exposure. After, development the whole area of emulsion occupied by the tracks was counted, and the amount of uranium determined from the track count. The emulsions used were loaded with borax prior to exposure to prevent the fading of the latent track image. Employing this External Source method 10(^-6) g, of uranium can be estimated with an accuracy of 3 per cent, and 10(^-8) g., with an estimated accuracy of 6 per cent.