A method was developed for the determination of plutonium isotopic composition at the sub-picogram level. Investigations of mass spectral backgrounds and of means of concentrating and purifying sample plutonium were conducted. Isotopic measurements were made by alpha energy analysis and mass spectrometric analysis. A technique for evaluating mass spectral background with a plutonium spike enriched in the mass 239 isotope to 99.98 atom percent is described. A combined alpha energy-mass analysis procedure for the determination of isotopic composition on a single sample mount is reported. The data presented demonstrate that sources of environmentally occurring plutonium could be differentiated by isotopic composition analysis with the developed method even at sub-picogram levels.

We discuss the general approach, computerized data analysis methods, and results of measurements used to determine the isotopic composition of plutonium by gamma-ray spectroscopy. The simple techniques are designed to be applicable to samples of arbitrary size, geometry, age, chemical, and isotopic composition. The combination of the gamma spectroscopic measurement of isotopic composition coupled with calorimetric measurement of total sample power is shown to give a totally nondestructive determination of sample plutonium mass with a precision of 0.6% for 1000-g samples of PuO2 with 12% 24°Pu content. The precision of isotopic measurements depends upon many factors, including sample size, sample geometry, and isotopic content. Typical ranges are found to be 238Pu, 1 to 10%; 239Pu, 0.1 to 0.5%; 24°Pu, 2 to 5%; 241Pu, 0.3 to 0.7%; 242Pu (determined by isotopic correlation); and 241Am, 0.2 to 10%.

A method was developed for the determination of plutonium isotopic composition at the sub-picogram level. Investigations of mass spectral backgrounds and of means of concentrating and purifying sample plutonium were conducted. Isotopic measurements were made by alpha energy analysis and mass spectrometric analysis. A technique for evaluating mass spectral background with a plutonium spike enriched in the mass 239 isotope to 99.98 atom percent is described. A combined alpha energy-mass analysis procedure for the determination of isotopic composition on a single sample mount is reported. The data presented demonstrate that sources of environmentally occurring plutonium could be differentiated by isotopic composition analysis with the developed method even at sub-picogram levels.

Plutonium dioxide (PuO2) standards are often used both as heat standards and isotopic standards for calorimetric assay. Calorimetric assay is the combination of the power in watts measured in a calorimeter with the effective specific power (P{sub eff}) in watts/g Pu, determined either by nondestructive gamma-ray assay or by destructive mass spectrometry, to yield the total elemental plutonium mass in the sample. To use a PuO2 sample as a heat standard for calorimetry, one must determine both the plutonium mass and P{sub eff} with very small uncertainties and then calculate the sample watts from the known plutonium mass, specific powers, and isotopic composition. Well-characterized PuO2 standards have plutonium mass values determined by analytical chemistry with a precision and accuracy on the order of 0.1%-0.2% relative to the total mass of the sample. Mass spectrometry, typically used to determine the isotopic fractions of plutonium standards, is very accurate and precise for the major isotopes but is somewhat less precise for low-abundance isotopes. The characterization of the 241Am/Pu ratio in the standard is also of great importance because 241Am can contribute significantly to P{sub eff} and to the heat output of the standard. The determination of the 241Am/Pu ratio in a plutonium-bearing sample is a process that is less standardized than mass spectrometry. There are no certified reference materials (CRMs) traceable to the national measurement system for 241Am in plutonium, and routine analytical 241Am/Pu ratio measurements often exhibit uncertainties of several percent relative to the total plutonium or greater.

The Plutonium Inventory Verification Program at Mound Laboratory provides a nondestructive means of assaying bulk plutonium-bearing material. The assay is performed by combining the calorimetrically determined heat output of the sample and the relative abundances of the heat-producing isotopes. This report describes the method used for the nondestructive determination of plutonium-238, -240, -241 and americium-241 relative to plutonium-239 using gamma-ray spectroscopy for 93 percent plutonium-239 material. Comparison of chemical data on aliquots of samples to the nondestructive data shows accuracies of +-7 percent for 238Pu/239Pu, +-15 percent for 24°Pu/239Pu, +- 3 percent for 241Pu/239Pu, and +-7 percent for 241Am/239Pu.