Particle-size problem in the assay of plutonium in field soil samples

The hot particle problem in plutonium-contaminated soil is believed to be associated with the nature of random particle distribution. One result is that either relatively large-sized samples or relatively large numbers of smaller samples are necessary in order to accurately assay the relatively low levels of radioactivity present. For cleanup levels now being recommended, using the best of known field monitoring support equipment available, practicality may dictate that sampling precision must be as low as +-100%. An estimate was made of sample sizes needed for given accuracy based on the number of Pu particles per sample. One conclusion is that a concentration level of 239Pu in a very small sample aliquot is without practical meaning. It may record zero, but another aliquot from the same lot may have a high concentration. Variation in some field samples from the Nevada Test Site was determined by counting different samples for 241Am inasmuch as this nuclide is commonly used for rapid field detection of plutonium. Results indicated that the 241Am may not be present, at least in part, in particles. Rather, it was more uniformly distributed in the aged material involved. Information such as this has bearing on the practical aspects of cleaning up Pu-contaminated areas, especially to levels of a very few nCi/g soil or fractions of nCi/g soil