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This report presents a review of the methods and capabilities of nondestructive measurements of the isotopic composition of plutonium using gamma-ray spectroscopy. I. equilibrium with Pu as " Pu-U equilibrium" or "aged' About 99% of equilibrium is attained in 45 days after complete removal or separation of U from a sample. We call samples where Pu-U equilibrium 237 does not exist "freshly separated." For those samples, U cannot be used as 241 a measure of Pu. We also observe in Fig. 1 that both Am and U decay to the same 237 isotope, Np. Many of these decays populate the same excited states in 237 237 Np and give rise to identical gamma rays. Thus, most of the useful U 241 gamma rays have a contribution from the Am in the sample. The amount of this interference depends upon the particular gamma ray and how long the Am has grown into the sample. Figure 2 shows the relative contributions for iraportant U gamma rays where it has been assumed that all U and Am were
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