Analysis of Naturally Produced Technetium and Plutonium in Geologic Materials

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• 作者：Paul Dixon ; David B. Curtis ; John Musgrave ; Fred Roensch ; Jeff Roach ; and Don Rokop
• 刊名：Analytical Chemistry
• 出版年：1997
• 出版时间：May 1, 1997
• 年：1997
• 卷：69
• 期：9
• 页码：1692 - 1699
• 全文大小：132K
• 年卷期：v.69,no.9(May 1, 1997)
• ISSN：1520-6882

文摘

In uncontaminated natural materials, plutonium andtechnetium exist exclusively as products (daughters) ofnuclear reactions in which uranium is the principalreactant (parent). Under conditions of chemicalstabilityover geologic periods of time, the relative abundances ofdaughter and parent elements are fixed by the rates ofnuclear reactions and the decay of the daughter radionuclide. The state of this nuclear secular equilibriumcondition is the primary basis of the geochemical studyof these elements in nature. Thus, it is criticalthatnuclear parent and daughter abundances are measuredin the same sample. We have developed a quantitativeprocedure for measuring subpicogram quantities of plutonium and technetium in gram quantities of geologicmatrices such as uranium ores. The procedure takesadvantage of the aggressive properties of sodium peroxide/hydroxide fusion to ensure complete dissolution andhomogenization of complex materials, the precision provided by isotope dilution techniques, and the extremesensitivity offered by thermal ionization mass spectrometry. Using this technique, a quantitative aliquot canberemoved for uranium analysis by isotope dilution thermalionization mass spectrometry or spectrometry. Although the application of the procedure is unique, theanalytical concepts may find more general application instudies of environmental contamination by nuclear materials. To assess the precision and accuracy of theanalytical results, blanks and standards were analyzedroutinely for a 1-year period to ensure quality control ofour sample analyses. The average technetium blank is5± 4 fg (n = 8), and that for plutonium is 0.17 ± 0.15pg(n = 7). Thus, the detection limit for technetium(definedas 3 times the standard deviation of the average blank) is11 fg, and that for plutonium is 0.44 pg. To assesstheprocedural precision, Canadian Reference Material BL-5was analyzed routinely with samples. The results ofsevenreplicate analyses for technetium in this standard reference material yield a technetium concentration of 59.0fg/g, with a remarkably small standard deviation of 0.6fg, 1.0% of the average value. The results of sixreplicateanalyses for the concentration of plutonium in BL-5 give1.012 pg/g, with an equally small standard deviation of0.016, 1.6% of the average value. No direct measureofaccuracy can be done on the technetium or plutoniumanalyses, because no standard reference material existsfor these elements. To help constrain the accuracy ofourmeasurements, equilibrium technetium/uranium andplutonium/uranium abundances were calculated usingthe nuclear reaction code MCNP. For technetium, suchcalculations are relatively insensitive to variations inmodel parameters, and measurements fall within a 21%high/low bias. For plutonium, the calculations areverysensitive to model parameters and hence inherently lessprecise. Indirectly, spike and isotope mixcalibrationsmade from weighted quantities of certified isotopes (bothtechnetium and plutonium) can be used to determine thebias of the measurement system for these elements.These calibrations show that the measurement systemisbiased by no more than ±1.5%.