Improvements in Xrf Specimen Preparation Using the Dried Residue Method: Gallium in Plutonium Metal

Preparing dry specimens from liquid samples for XRF analysis avoids introducing caustic or hazardous liquids into the instrument. Several modifications were made to a dried residue specimen preparation method for quantifying gallium in plutonium metal in order to improve the method accuracy and precision. Ion exchange chromatography was utilized to remove the plutonium prior to casting the dried residue specimens. This coupled with several other changes improved the method relative error from ~5% to less than 1%. These results are sufficient for routine sample analysis and are almost comparable to results from the established process using liquid specimens. However, the analysis of radioactive liquid specimens is unnecessary for quantifying the plutonium gallium content using this dried residue approach. INTRODUCTION Gallium is alloyed with plutonium in manufacturing nuclear weapons, and quantifying the gallium content accurately is an essential step in the manufacturing process. WDXRF is a proven method for quantifying the gallium. The established XRF specimen preparation method involves an aqueous dissolution process in which chromatography is implemented to remove the plutonium[1,2]. Recently, an alternate dried residue specimen casting process was developed to eliminate the need to analyze liquid radioactive specimens[2-4]. In the aqueous method, the eluted gallium solution from the chromatography step is analyzed by XRF using zinc as an internal standard. Although excellent precision and accuracy are achieved with this method, the liquid specimens are radioactive due to residual amounts of plutonium as well as trace americium and uranium. Thus, the potential exists for radioactive solution to leak inside the instrument. A common method for preconcentrating trace elements to improve XRF detection limits is to cast a sample solution in microliter sized spots that are then dried[5-10]. In the current work, a dried residue casting process was used to prepare dry specimens from liquid samples. A sample is dissolved and then cast in microliter sized drops on Kapton film. After the drops dry, the specimen is sealed inside a cup and analyzed. The primary advantage for this method is that no radioactive liquids are introduced into the instrument. Also, since no acidic solutions are analyzed, the instrument steel cups are not corroded from the acid vapors. Previous studies have demonstrated the viability of this dried residue process[2,4]. However, the average relative precision and accuracy achieved in quantifying gallium in a set of plutonium test samples was approximately 4% and 5% respectively[2]. These results were adequate for a quick, Copyright ©JCPDS International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 98