C. J. Zeissler

Objective: Expand electron and ion radionuclide microanalysis capabilities three ways: (1) develop pre-screening method for samples 106 times more massive than can be analyzed by electron and ion beam methods, (2) preconcentrate radionuclides in samples prior to ion and electron microscopy, and (3) provide a means to associate chemical and morphological information with radionuclides at concentration levels significantly below the detection limits of conventional beam and spectrometry methods.

Problem: Electron microscopy with x-ray spectrometry and ion beam instruments have an unfortunate limitation: a measurement session to survey a particle dispersion is limited to a total sample mass in the microgram range. Also, detection limits for x-ray spectrometry require that an element be present at a concentration of at least 0.1 weight percent in an analyzed particle. Many radionuclides at quantities much less than 0.1 weight percent (e.g., only a few thousand atoms in a gram of particulate matter) can be responsible for significant levels of radioactivity. This can exceed the detection capabilities of electron and ion microscopies by many orders of magnitude, so nuclear counting techniques are traditionally used instead, and chemical compound and microstructural information are not utilized.

Approach: Particulate samples of up to several grams in size are autoradiographed by a photostimulated luminescence phosphor system. Sample subdivision methods are incorporated for preconcentration and for particle isolation. These methods provide a bridge between differing detection limitations, and results from nuclear spectrometry and microscopy instruments may be correlated to yield full sample characterization.

Results, Impact, and Future Plans: Screening and preconcentration of 235U, 137Cs, and 60Co samples have been successfully applied. Up to three grams of particulate material may be analyzed for radioactivity per run. The autoradiography detection limit is generally in the 0.02 mBq to 2 mBq range, depending on the radioactivity decay mode. This corresponds to levels that are far below the detection limits of electron and ion probe methods (e.g., 2 mBq 60Co or 137Cs corresponds to 105 to 106 atoms, i.e., tens to hundreds of attograms). Particle isolation has been accomplished for 137Cs and 60Co samples, enabling correlation of nuclear measurements (radionuclide identification and quantification) with energy-dispersive x-ray spectrometry (chemical form) and microscopy (e.g., physical features), even when the radionuclides are present at levels that are many orders of magnitude less than the detection levels achievable by unassisted microscopy. Detection limit determination and particle isolation of uranium is planned for FY 2001. This work is carried out under the CSTL program Chemical Characterization of Materials.

Publications:
(1) Zeissler, C. J., et al., "Radioactive Particle Analysis by Digital Autoradiography," Journal of Radioanalytical and Nuclear Chemistry (accepted 2000).
(2) Zeissler, C. J., "IAEA Sample Preparation SOP," Report to the International Atomic Energy Agency, November 1999.

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