Year
2001
Abstract
Los Alamos National Laboratory has two identical shufflers that have been calibrated with a dozen U3O8 certified standards from 10 g 235U to 3600 g 235U. The shufflers are used to assay a wide variety of material types for their 235U contents. When the items differ greatly in chemical composition or shape from the U3O8 standards a bias is introduced because the calibration is not appropriate. Recently a new tool has been created to calculate shuffler count rates accurately, and this has been applied to generate bias correction factors. The tool has also been used to verify the masses and count rates of some uncertified U3O8 standards up to 8.0 kg of 235U which were used to provisionally extend the calibration beyond the 3.6 kg of 235U mass when a special need arose. Metallic uranium has significantly different neutronic properties from the U3O8 standards and measured count rates from metals are biased low when the U3O8 calibration is applied. The application of the calculational tool to generate bias corrrections for assorted metals will be described. The accuracy of the calculational tool was verified using highly enriched metal disk standards that could be stacked to form cylinders or put into spread arrays.