Standardization of 90Sr+ 90Y by Means of a Chemical Separation

Precipitation of strontium in a 90Sr + 90Y solution by fuming nitric acid offers an opportunity of preparing sources in which, at first, one of the two activities is much weaker than the other. If the initially weaker activity is known, the other can be calculated from the count rates measured at two suitably chosen times. A procedure is developed which quantitatively connects the specific activity of the mother solution with that of the solution dispensed onto the source mounts. The main difficulty is encountered in determining the initial 90Y activity of the 90Sr-enriched sources. Since a high degree of separation can hardly be achieved in a single precipitation, the 90Y content of the supernate has to be determined as well. If the 90Y is distributed uniformly after the precipitation, the corresponding 90Y activity retained by the precipitate can be calculated easily. Yet the 90Y concentration sometimes deviated significantly from uniformity. A way of partially circumventing this difficulty is pointed out. Exact formulae are derived expressing the activities in terms of the count rates observed at different times. A full account of the various sources of error is given; the spread of the results obtained is compatible with that expected. The specific activity of the mother solution is calculated with 15 sources prepared from four independent precipitations. The mother solution had already been calibrated by 24 laboratories taking part in an international comparison organized by the Bipm in 1964. The agreement is well within the limits of error. Although the separation method described here is too laborious to be used in routine work, it merits some attention as an independent method of standardizing a pure β-emitter by β-counting alone, without using any extrapolation. The accuracy reached compares favourably with that of currently used methods. The separation method may be superior to others when a 90Sr + 90Y solution with a high carrier content is to be standardized. (author)