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An improved method for the determination of 90 Sr and 91 Y in grass ashes and other materials was developed, based on a tributylphosphate (TBP) extraction of the radio-yttrium. The interference of 91 Y with the ^-measurement of the extracted 90 Y was studied and eliminated either by graphic subtraction of the 91 Y decay diagram from the total decay plot of the yttrium fraction or by energy discrimination of these two nuclides by Cerenkov-counting. Using the latter method, the analysis of one sample of ash takes six hours. Results of 90 Sr and 91 Y acivity concentrations in grass samples collected after the Chernobyl event and of 90 Sr in various IAEA reference materials are given.

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