Improvement in the Determination of the Radium Content of Low-Grade Radium-Barium Salts

Improved Gamma-Ray Method of Determining the Radium Content of Low-grade Radium Salts.---Because of absorption of the rays by the salt, the present gamma-ray method is rather inaccurate for concentrations less than ${10}^{\ensuremath{-}5}$, and for concentrations above ${10}^{\ensuremath{-}7}$ the emanation method is too sensitive. To bridge the gap, the authors have modified the first method by adopting a shallow container with two curved sides, each concentric with the cylindrical string electrometer used to measure the rays, and only a small fraction of the radius apart. This shape has the effect of making the mean absorption due to the salt small and also very nearly equal to half the maximum absorption. The procedure is to take readings with the container in position (full of the salt to be measured) first alone, then with a small radium tube of known strength placed first just in front and then just in back of the container. The ratio of the first reading to the mean of the other two gives the ratio of the strength of the unknown to the sum of the strengths of the known and unknown within one per cent. The method is both quicker and more accurate than the emanation method for concentrations between ${10}^{\ensuremath{-}5}$ and ${10}^{\ensuremath{-}7}$. The container used was made of tin plate and was 10 cm. by 10 cm. by 2 cm. thick, the radii being 30 and 32 cm.