Two methods were evaluated for determining rare earths in plutonium: (1) For the lighter rare earths (<i>Z</i>≦66), or low concentrations of the heavier rare earths, an adjacent rare earth was added as a carrier and also as an internal standard, the rare earths were separated from plutonium by fluoride precipitation, and the measured intensity ratios for the sample and for solutions having known concentrations were compared. The <i>L</i>β1 x-rays were measured for the lighter rare earths and the <i>L</i>α1 x rays for the remaining lanthanides. (2) For the heavier rare earths (<i>Z</i>>66), the <i>L</i>α1 x-ray intensities were measured from a nitric acid solution of the sample and compared to intensities obtained for solutions having known concentrations. The minimum concentrations that could be measured with a relative standard deviation no greater than 4% by the separation internal standard method varied from approximately 0.5% for lanthanum to 0.01% for lutetium. The direct measurement of x-ray intensity was much less sensitive. Applicability of the methods was shown by successful analyses of plutonium alloys containing dysprosium, thulium, or lutetium.

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